1
AGREEMENT BETWEEN FOUR
COMMONLY USED TONOMETERS AND THEIR RELATION WITH CENTRAL CORNEAL THICKNESS IN SUBJECTS
WITH NORMAL AND HIGH INTRAOCULAR PRESSURE
DISSERTATION SUBMITTED TOWARDS FULFILLMENT OF THE RULES AND REGULATIONS FOR THE M.S. BRANCH III
OPHTHALMOLOGY EXAMINATION OF THE TAMILNADU DR. M.G.R. MEDICAL UNIVERSITY
TO BE HELD IN APRIL, 2015
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AGREEMENT BETWEEN FOUR
COMMONLY USED TONOMETERS AND THEIR RELATION WITH CENTRAL
CORNEAL THICKNESS IN SUBJECTS WITH NORMAL AND HIGH INTRAOCULAR
PRESSURE:
A CROSS-SECTIONAL STUDY
SUBMITTED BY
Dr. BIJINU MARY PHILIP CHRISTIAN MEDICAL COLLEGE VELLORE
DISSERTATION SUBMITTED TOWARDS FULFILLMENT OF THE RULES AND REGULATIONS FOR THE M.S. BRANCH III
OPHTHALMOLOGY EXAMINATION OF THE TAMILNADU DR. M.G.R. MEDICAL UNIVERSITY
TO BE HELD IN APRIL, 2015
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BONAFIDE CERTIFICATE
This is to certify that this dissertation titled “Agreement between four commonly used tonometers and their relation with central corneal thickness in patients with normal and high intraocular pressure: A cross-sectional study” done towards fulfillment of the requirements of the Tamil Nadu Dr MGR Medical University, Chennai for MS Branch III Ophthalmology examination to be conducted in April 2015, is the bonafide original work of Dr. Bijinu Mary Philip, Post Graduate student in Ophthalmology, Christian Medical College, Vellore.
Dr.Bijinu Mary Philip P.G.Registrar
M.S.Opthalmology Schell eye Hospital CMC Vellore
4
BONAFIDE CERTIFICATE
This is to certify that this dissertation titled “Agreement between four commonly used tonometers and their relation with central corneal thickness in patients with normal and high intraocular pressure: A cross-sectional study” done towards fulfillment of the requirements of the Tamil Nadu Dr MGR Medical University, Chennai for MS Branch III Ophthalmology examination to be conducted in April 2015, is the bonafide original work of Dr. Bijinu Mary Philip, Post Graduate student in Ophthalmology, Christian Medical College, Vellore.
Dr Lekha Mary Abraham; DO, DNB Professor and Guide
Department of Ophthalmology Christian Medical College Vellore-632001
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BONAFIDE CERTIFICATE
This is to certify that this dissertation titled “Agreement between four commonly used tonometers and its relation with central corneal thickness in subjects with normal and high intraocular pressure” done towards fulfillment of the requirements of the Tamil Nadu Dr MGR Medical University, Chennai for MS Branch III Ophthalmology examination to be conducted in April 2015, is the bonafide original work of Dr.Bijinu Mary Philip Post Graduate student in Ophthalmology, Christian Medical College, Vellore.
Dr Andrew Braganza;
M.S. Opthalmology
Professor and Head of the Department Department of Ophthalmology
Christian Medical College Vellore-632001
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ANTI PLAGAIRISM CERTIFICATE
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ACKNOWLEDGEMENTS
First of all, I would like to thank Almighty God for granting me the privilege to learn the science of Ophthalmology in this prestigious institution. I thank Him for all the blessings that he had given me in my life and I thank Him for being with me throughout the ups and downs in my life.
I am deeply indebted to Dr Lekha Mary Abraham, my Guide and my mentor who was always there to help me throughout my thesis work and I thank her for all the support and advice that she had given me throughout my course for my studies and for my life.
I am extremely grateful to both my co-guides Dr.Andrew Braganza and Dr. Arathi Simha R for their expertise advice and support for my thesis, giving valuable corrections and suggestions at the right time.
I would like to thank all the patients who were willing to take part in the study.
I thank all the Consultants and fellow Registrars for allowing their patients to be a part of this study.
I express my gratitude to our librarian Mr. Deenadayalan. D, for helping me to get all the necessary articles and books.
Special thanks go to Medical Records Officer, Mr. Anand Amirtaraj and his team for leading the patients to my OPD on time.
Sincere thanks to Dr.Dhipak Arthur for helping me to take the necessary pictures.
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I would like to thank my statisticians, Miss.Gowri and Dr.Visalatchi for helping me to do my statistical analysis.
I thank the Fluid Research for funding the study.
Above all, I would like to thank my family especially my parents , my husband and my son for all their support, advice and prayers.
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TABLE OF CONTENTS
S. No. Content Page No
1 INTRODUCTION...10
2 AIMS ...14
3 OBJECTIVES...16
4 LITERATURE REVIEW ...18
5 MATERIALS AND METHODS...49
6 RESULTS AND ANALYSIS ...58
7 DISCUSSION...100
8 SUMMARY...110
9 LIMITATIONS...112
10 CONCLUSIONS………...114
11 REFERENCES………. 116
12 ANNEXURE-1 IRB APPROVAL FORM...130
13 ANNEXURE II-COLOUR PLATES...131
14 ANNEXURE-II1 PATIENT PROFORMA...134
15 ANNEXURE IV-CONSENT FORMS……… 135
16 ANNEXURE V-DATA SHEET………141
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INTRODUCTION
11
INTRODUCTION
Glaucoma is a leading cause of blindness worldwide. It is defined as a chronic optic neuropathy characterized by optic nerve head damage and specific field defects corresponding to the changes in the disc. (1) Most types of glaucoma are asymptomatic till the late stages. Hence early detection ad appropriate treatment is critical. There are many factors that contribute to the pathogenesis of this potentially blinding, yet, treatable disease. The main factors, which are considered to be responsible for the damage to the retinal ganglion cells, include age, myopic refraction, intraocular pressure (IOP) and systemic vascular disorders. Of these, IOP is the only factor that can be modified to reduce the progression of glaucoma. ( 2)
In the last few decades, many new topical medications to reduce IOP have been introduced in the market. Hence the treatment of glaucoma in terms of reduction in IOP has become easier. Therefore accurate measurement of IOP is extremely critical in the diagnosis and management of glaucoma. IOP measurement should become a part of a complete ophthalmic evaluation for all patients who go to an ophthalmologist even if it is only for refraction and spectacles. Accurate and precise measurement of IOP is one of the crucial steps, not only in the diagnosis of glaucoma but also in the follow up and assessment of response to treatment.
Methods for measurement of IOP have come a long way from digital palpation to digital display. Palpatory method of checking IOP is now limited only to those patients with grossly scarred cornea where no other tonometry can measure IOP. Indentation
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tonometry using Schiotz tonometer has its limitations in routine ophthalmic practice.
Along with the advances of all other diagnostic modalities in treatment of glaucoma, intraocular pressure measurement also has advanced over time, with development of a myriad of tonometers, each of them supposedly compensating the limitations of others.
Though there are a number of new tonometers, which have been launched in the last few decades, Goldmann Applanation Tonometer (GAT) since its discovery in 1950 is still considered as the gold standard
Though GAT is the gold standard, its accuracy is affected by the thickness of cornea as GAT assumes a fixed central corneal thickness (CCT) in a population of widely varying CCT. (3) One may over diagnose glaucoma based on high IOP due to increase in CCT.
On the other hand, a high IOP may be missed due to underestimation of IOP in patients with thin cornea. This knowledge along with the inconvenience of the use of slitlamp in specific instances for checking IOP using GAT has lead to the emergence of other methods of tonometry. Various other methods such as Dynamic Contour Tonometer (DCT), Air puff noncontact tonometer (NCT) and Tonopen are routinely available for measurement of IOP. The IOP measured by these are supposed to be less affected by CCT. (4)
Most published studies concerning the effect of CCT and measured IOP relate to GAT.
(5) However, there is increasing evidence that IOP measured using other tonometers also may be affected by CCT. When using new tonometers with their added advantages, one needs to look at whether they measure up to the gold standard in terms of accuracy and precision. Hence it is important to see if Tonopen, DCT and NCT are similar to GAT in the measurement of IOP. It is also important to see if these instruments are in agreement
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in patients with high IOP. Given that one is really interested in the IOP in patients with glaucoma and that IOP is the only treatable factor, this becomes very important in monitoring response to treatment.
Few studies have compared two of these methods and their relation with CCT in normal subjects. (6) To the best of our knowledge there is no single study which has compared four different tonometers for their precision in measuring IOP in patients with normal and high IOP. Similarly it is rather uncommon to find a single study that looked at the effect of CCT on IOP using four different tonometers. In our study we have tried to look at how different the IOP is when measured using different tonometers and whether CCT has a role to play in the difference in IOP measured using these four tonometers.
14
AIMS
15
AIMS
1. To determine the agreement between four commonly used tonometers in measuring intraocular
pressure(IOP) in patients with high and normal IOP.
2. To study the relationship between the measured IOP and central corneal thickness using each of
these four tonometers.
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OBJECTIVES
17 OBJECTIVES:
1. To find the relative agreement of measured IOP recorded between four tonometers, viz; Goldmann Applanation Tonometer(GAT), Tonopen, Dynamic Countour Tonometer(DCT) and Air puff tonometer(NCT) in patients with high and normal IOP.
2. To determine how the central corneal thickness affects IOP measurement using each of these instruments.
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REVIEW OF LITERATURE
19
Review of literature
Glaucoma is a chronic optic neuropathy characterised by specific disc and field changes for which intraocular pressure (IOP) is the only modifiable risk factor. Glaucoma is usually asymptomatic till its end stage, when the prognosis is poor. Early intervention by using modalities to reduce IOP could potentially halt the progression of this blinding disease. Therefore accurate measurement of IOP is extremely critical in the diagnosis and management of glaucoma. IOP can be measured using various tonometers of which Goldmann Applanation Tonometer (GAT) is considered as the gold standard.
EPIDEMIOLOGY AND GLOBAL BURDEN OF GLAUCOMA
There are approximately 11.2 million persons aged 40 years and older with glaucoma in India(7). With increase in life expectancy, especially in developing countries like India, the disability adjusted life years due to glaucoma has increased more than 2 fold between the years 1990 and 2010. (8) Therefore by 2020 the number of people with glaucoma is expected to rise to 60 million. (9) These numbers reflect the urgent need of early detection and proper treatment of this disease, as timely management can halt progression.
For prevention of glaucoma related blindness accurate detection of the disease is the first step in the recommended strategy. (10) Several factors contribute to the damage of retinal ganglion cells with resultant visual loss in glaucoma, of which IOP is the only modifiable one.(2) Hence IOP measurement should be part of a routine ophthalmic examination. (10)
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INTRAOCULAR PRESSURE: RISK FACTOR FOR GLAUCOMA
In early 1990s through various population based surveys IOP was found to be associated with glaucoma, although not in the form of cause effect relation ship.Most important studies in this group are Baltimore Eye Survey (11)and Barbados Eye Study(12), where they found IOP as an important factor in glaucoma. Barbados eye study was a population based survey, done among residents of Barabados island to evaluate the risk factors for open angle glaucoma. Participants aged 40-84 years, underwent applanation tonometry, visual field testing, fundus photography, anthropometry and blood pressure testing. From the study they have concluded that age, male gender, high IOP and family history of glaucoma were associated with high incidence of open angle glaucoma.
The Baltimore eye survey (11) is another landmark study in history of glaucoma. It was also a population based study, among the residents of Baltimore.All subjects were above 40 years of age and underwent tonometry, perimetry, fundus photography and detailed opthalmologic and medical examination.High IOP, systemic hypertension, family history of glaucoma, and structure and organization of optic nerve head etc were identified as potential risk factors for glaucoma in this study.
However, the value of IOP reduction in halting the progression of glaucoma was disputed until the results of large randomized clinical trials of late 1990s became available.The two most important studies among these are Ocular hypertension treatment study
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(OHTS)(13) and Early manifest glaucoma trial(EMGT)(2).
OHTS (13) was a randomised clinical trial done in USA to determine the efficacy of topical medications to prevent or delay progression to open angle glaucoma in patients with ocular hypertension. Partcipants were of age between 40-80 years, with IOP of 24- 32 mmHg in one eye and 22-32mmHg in the other eye. All of them were randomized to either observation or administration of available topical ocular hypotensives. In the treatment group it was found that there was reduction of IOP by about 22.5%. At 5 year follow up, the cumulative risk of development of glaucoma was 4.5% in the treatment group, and 9.9%.in the observation group. Thus it was clearly demonstrated that reduction of IOP can halt the progression of optic nerve damage among ocular hypertensives.
EMGT (2) is another important landmark clinical trial in glaucoma.This was a randomised controlled trial to evaluate the effect of reduction of IOP in early and untreated glaucoma patients, to explore factors which lead to progression of optic nerve damage and also to study the natural history of disease. Subjects between 50-80 years of age with early glaucomatous field defects, were randomised to either treatment or no treatment with close follow up. The primary outcome that was measured was the duration needed to detect early signs of progression of disease. It was found that while it took an average of 66 months to show signs of progression in the treated group, the untreated group showed of signs of progression in 48 months. Although it was always known that IOP reduction either halted or reduced the rate of progression of glaucoma, the EMGT
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was the first scientific study which clearly demonstrated the importance of reduction of IOP in preventing glaucoma progression. Even though there are other risk factors implicated in the develpoment of glaucoma, their role has not been established in any of the randomised clinical trials.
WHAT IS NORMAL IOP?
Leydhecker and co-workers (14) have done Schiøtz tonometry on 10,000 normal individuals. In their study the pressure readings appeared to be in a normal fashion at first, but closer inspection of the data revealed that the distribution was not Gaussian, but skewed to the right, which means that we cannot define an upper limit for IOP by adding 2 or 3 standard deviations to the mean. The skewed distribution meant that abnormal IOP has to be defined empirically ie; an abnormal pressure is one that causes optic nerve damage, and it is specific for each individual.
The IOP is determined by the balance between the rate of aqueous secretion and aqueous outflow, which in turn depends on facility of outflow, resistance to outflow and episcleral venous pressure. All these factors are summed up in the Goldmann equation (6);
Po= F/C+ P v
where, PO is the IOP in the undisturbed eye in mm Hg, F is the aqueous formation in µl/min, C is the facility of outflow in µl/min/mmHg and Pv is the episcleral venous pressure in mmHg.
23 FACTORS AFFECTING IOP
Age: Most studies (15) show that IOP increases with increasing age. Although aqueous formation decreases with increase in age, increase in IOP with age may be due to decreased uveoscleral outflow and impaired outflow facility
Gender: Women tend to have higher IOP than men, especially after the age of 40.The Barbados Eye Study (12) showed that women were more likely to have high IOP without glaucomatous damage, whereas men were more likely to develop open angle glaucoma.
Heredity: There is a polygenic hereditary influence on IOP. Studies have shown that first-degree relatives of patients with open-angle glaucoma have higher IOP than the general population. (16,17)
Diurnal variation: IOP varies at an average of 3–6 mmHg in normal individuals during the day.(18) Diurnal variation of IOP follows a pattern, with the maximum pressure in the mid morning hours and the minimum pressure late at night or early in the morning.
Patients with glaucoma have much wider variation of IOP (19), with worse outcomes. So a single pressure taken at one point of time is not representative of the patient’s average IOP.
Postural variation: There is an increase of at least 6mm of Hg, when patient changes their position from sitting to supine position (20). This rise may be due to the increase in episcleral venous pressure while lying down and the difference markedly increases in patients with normal tension and open angle glaucoma.
24
Cardiovascular factors and systemic conditions: Normally, IOP fluctuates 1–3 mmHg in each cardiac cycle with variation in arterial pressure (21). The magnitude of this IOP fluctuation is related to the height of the ocular pressure and variation of arterial pressure.
Systemic hypertension and glaucoma show only a modest association. Beta blockers like atenolol, which are commonly used as antihypertensives can also decrease IOP.(22).
Though diabetics are noted to have higher IOP than general population, a decrease in IOP is seen during acute hypoglycemic attacks. (23) Hormones like glucocorticoids and growth hormone can cause increase in IOP, while progesterone and relaxin decrease IOP.(24)
Episcleral Venous pressure: Increased episcleral venous pressure is associated with increase in IOP and subsequent glaucoma. (25) Increase in IOP with Valsalva maneuver, and in people who play wind instruments, are related to this. Whole body, head down position as in sheershaasana, can cause increase in IOP.(25) This can also be attributed to increase in episcleral venous pressure.
Exercise: Exercise can decrease or increase IOP depending on the nature of the exercise.
Prolonged exercise leads to decrease in IOP. The postulated mechanisms for this are metabolic acidosis, hypocapnia and increase in blood lactate levels. (26) Heavy exercises such as weight lifting can cause increase in IOP probably due to increase in episcleral venous pressure.(25)
General Anaesthesia: Anaesthetics and sedatives can decrease IOP. IOP increases during excitatory phase of general anaesthesia and decreases as anaesthesia becomes longer and deeper.(28)
25
Refractive error: Several studies have noted high IOP in myopic individuals ie; IOP correlates with axial length.(27) .
METHODS FOR MEASURING INTRAOCULAR PRESSURE
Intraocular pressure can be measured using 3 methods.
1. Palpatory method: Here the examiner estimates IOP by the response of the eye to digital pressure. Palpation should be used only in limited circumstances because only gross alterations in IOP can be detected using this method. In special clinical situations like completely dislocated lens in the anterior chamber, or a total corneal scar, this method will give us an idea of whether the IOP is low or high. (29)
2. Manometry method: In this method intraocular pressure (IOP) is measured directly in a living eye using a manometric technique. (30) For this a needle is inserted into the anterior chamber through a self-sealing, beveled corneal puncture. The needle is connected to a fluid-filled tubing, and the height of the fluid in the tubing corresponds to IOP. Even though the direct method is the most accurate, its obvious clinical limitations necessitate other means for measuring IOP.(31)
3.Tonometry method: Tonometers are of two types,
a. Indentation tonometers b. Applanation tonometers.
With indentation we measure the amount of deformation or indentation of the globe in response to a standard weight applied to the cornea. On the other hand, with applanation
26
instruments, the clinician measures the force necessary to flatten a small, standard area of the cornea.
A. INDENTATION TONOMETERS
In indentation tonometry, IOP is estimated by measuring the deformation or indentation of the globe, after placing a known weight over the cornea (32). The shape of indentation by this tonometer is a truncated cone. The Schiøtz tonometer is the prototype for indentation tonometer.
Schiøtz tonometer
It consists of a metal plunger that slides through a hole in a concave metal footplate. The plunger is on a hammer device, which is connected to a needle that crosses a scale. The plunger, hammer, and needle together weigh about 5.5 g. By the addition of appropriate weights this can be increased to 7.5,10, and 15. The more the plunger indents the cornea, the lower the IOP.
Technique for measurement:
Here the patient lies supine and fixates on an overhead target. A drop of topical anesthetic, is instilled in each eye and the tonometer is placed directly over the eye perpendicular to the apex of the cornea. The measurement is noted to the nearest 0.25 scale units and is repeated until three consecutive readings agree within 0.5 scale units.
The average scale reading is converted to IOP in mmHg using a conversion chart. The Schiøtz tonometer is portable, inexpensive and easy to operate. It is reasonably accurate for a wide range of IOP. (33)
27 Sources of error in Indentation tonometer:
The accuracy of Schotz tonometer is based on the concept that all eyes respond in the same way to the external force of indentation. Factors affecting Schiotz tonometry readings are
1. Ocular Rigidity
Since the conversion tables for Schiotz tonometer are based on the Coefficient of ocular rigidity, the eyes that deviate significantly from this factor give false IOP measurements.
It shows falsely low IOP readings when scleral rigidity is low and vice versa. (34) 2. Blood volume alteration
The variability in expulsion of intraocular blood during indentation influences IOP. (35) 3. Corneal factor
If cornea is steeper or thicker, there is greater displacement of fluid and this will cause falsely high IOP reading.(36)
4. Moses effect
In eyes with low IOP, the cornea may mould into the space between the plunger and hole of Schiotz tonometer, pushing the plunger up and showing a falsely high reading.(37)
28
Electronic Schiøtz tonometer
It has a continuous recording of IOP that is used for tonography. It has a magnified scale, which enables easier detection of even small differences in IOP.
Impact–rebound tonometer
It is an updated version of an indentation tonometer, which has a very light, disposable, sterile probe, that is propelled forward onto the cornea by a solenoid (38). The time taken for the probe to return to its resting position and the characteristics of the rebound motion are indicative of the IOP. The time taken for the probe to return to its resting position is longer in eyes with lower IOP and faster in eyes with higher IOP. Because the probe is extremely light, with a very short time of contact, this can be used without topical anaesthesia and hence is useful in those patients who are allergic to topical anaesthetics.
The impact–rebound tonometer is comparable to the GAT in normals and post keratoplasty eyes (39). It has decreased accuracy in scarred corneas like GAT and it tends to correlate with central corneal thickness. (41) This tonometer can be used in large screening camps, (40) and also when patients are unable to be seated at the slit lamp (40).
The tonometer can be used by paramedical personnel with good reliability. (40)
B. APPLANATION TONOMETERS
In applanation tonometry, the force required to flatten a particular area of the sphere is determined. (42) The shape of deformation is simple flattening and since the shape is always constant, its relationship with IOP can be derived mathematically. Applanation
29
tonometry displaces 0.5microlitre of aqueous humor, which raises IOP by about 3%.
Since the volume displaced is very small, ocular rigidity should ideally not affect applanation tonometry.
Applanation tonometers are of two types, variable area tonometers and variable force tonometers.
Variable area tonometers measure the area of the cornea that is flattened by a known force. The prototype is Makalakovs tonometer
Variable force tonometers on the other hand, measure the force required to applanate a standard area of the corneal surface. The prototype is Goldmann applanation tonometer (GAT).
Maklakovs Tonometer
It is based on the concept that that IOP is estimated by measuring the area of cornea flattened by a known weight. (43) It has a dumbbell –shaped cylinder, with endplates of polished glass on either end. This end plate has diameter of 10mm. A set of four instruments is available, with weight 5,7.5,10 and 15gm. It also has a cross –action wire handle supplied to support the instrument on the cornea.
The patient is kept in supine position and the cornea anesthetized using topical anaesthetic. A layer of dye is applied to end plate and the instrument is allowed to rest on the cornea vertically for 1 second. It produces a white imprint on the end plate, which corresponds to the area of cornea that is flattened. The diameter of white area is measured with a transparent plastic measuring scale and the IOP is read from a conversion table.
30
Even though the volume displaced by this instrument is less than indentation, ocular rigidity plays a role here also. The conversion table has been prepared taking this factor also in to consideration.(44)
Goldmann Applanation Tonometer (GAT)
GAT is the international clinical gold standard for measuring IOP (1). The GAT determines the force necessary to applanate an area of the cornea which is 3.06 mm in diameter. It works on the principle called Imbert- Ficks law. This law states that, in a perfectly spherical, perfectly flexible, infinitely thin and dry sphere the external force applied against it will be the pressure inside the sphere multiplied by the area flattened by the external force.
But cornea is neither spherical nor dry nor is it perfectly thin or flexible. So Goldmann has modified this law by incorporating all these factors. By this he found that when the inner area of flattening is about 7.35mm2, the capillary force created by the tear film and the corneal rigidity will counter balance each other and the external force will be equal to the internal force.(3) This is achieved when the diameter of the external area is equal to 3.06mm which is used in GAT. Thus in a cornea of average thickness, if an area of the cornea with a diameter of 3.06 mm is applanated, the force necessary to change the shape of the cornea, is equal in magnitude and opposite in direction to the capillary attraction of the tear film for the tonometer head. Therefore, when the cornea is flattened, the force of the tonometer counterbalances and provides a measure of IOP. IOP in mmHg is equal to the force of the tonometer in grams multiplied by 10.
31 Description of the tonometer
The instrument is mounted on a slit lamp and the examiner’s view is directed through the centre of plastic biprism, which is used to applanate the cornea. Two beam splitting prisms in the applanating unit optically convert the circular area of corneal contact to semicircles. The prisms are adjusted so that the inner margins of semicircles overlap when cornea is applanated. The biprism is attached to the slit lamp using a rod, which contains a coil spring and a series of levers.
Technique
The cornea is anaesthetized using topical anesthetic and the tear film is stained using fluorescein on a strip, which is moistened using methylcellulose solution. The patient is asked to look straight ahead with the eyes open. The examiner then retracts the eyelids so that the tonometer head does not touch the lid margins and force the patient to blink.
Using cobalt blue light from slit lamp, the biprism is illuminated and is brought into gentle contact with apex of cornea. The fluorescent semicircles are viewed through the biprism and force against the cornea is adjusted till the inner edges overlap. The IOP is then read directly from a scale on the tonometer housing.
Sources of error in GAT
1. Inadequate fluorescein staining of the tear film causes an underestimation of IOP. This problem can be avoided if IOP is measured within the first minute or so after instilling fluorescein.
32
2. Elevation of eyes to more than 15° above the horizontal leads to overestimation of IOP.
3. Widening the palpebral aperture excessively causes overestimation of IOP.
4. IOP is affected by thickness of cornea also. In thick cornea IOP is overestimated and underestimated when cornea is thinner than normal.
5. If corneal astigmatism is greater than 3D, IOP is underestimated for with-the-rule astigmatism and overestimated for against the-rule astigmatism.
6. Repeated measurements of IOP can lead to underestimation
7. If the patient squeezes or the examiner exerts pressure on globe, IOP may be overestimated.
Calibration
GAT has to be calibrated according to the instructions given by the manufacturer, every month. If the tonometer is not calibrated with in 1 mmHg it should be repaired.
Disinfection
Since GAT is a contact method there is chance of spreading the infections such as viral conjunctivitis and sometimes, lethal infections like Hepatits B, Hepatitis C and HIV infection. Keeping the tip of the biprism in diluted sodium hypochlorite solution for 20 minutes can satisfactorily disinfect the applanation tip.
33
Perkins tonometer
The Perkins tonometer is more or less similar to GAT, except that it is portable and counter balanced. Since it is portable it can be used in operating room, bedside monitoring, and also in community screening.(45).Here the light comes from batteries and force is provided by a spring. Perkins tonometer tends to underestimate IOP and this underestimation increases as IOP increases.(46)
Draeger tonometer
The Draeger tonometer is similar to GAT, but uses a different biprism. The force is supplied by an electric motor. Draeger tonometer is portable and counterbalanced, so it can be used in a variety of positions and locations.
MacKay-Marg Tonometer
MacKay-Marg tonometer consists of a movable plunger, which is 1.5 mm in diameter which project out a little from a surrounding sleeve (47). The movements of the plunger are calculated with the help of a transducer and this is documented on a paper strip. The IOP and the corneal bending pressure exerted by the plunger and its supporting spring oppose each other when the device touches the cornea. At the point of applanation, this corneal bending pressure is transferred to the footplate, which is indicated by a notch in the pressure tracing. The height of the notch is the measure of IOP.
The MacKay-Marg tonometer measures IOP over a brief period of time and hence many readings taken should be averaged to minimize the effects of cardiac and respiratory cycles on IOP. Unlike GAT, this tonometer can be used in scarred corneas. The tip of the
34
tonometer is covers with a diposable plastic cover before measurement of IOP in each patient. Hence there is no transfer of infection. The tonometer is reasonably accurate even when used over soft contact lenses.
Tonopen
Tonopen is a small, handy, applanation tonometer that functions on the same principle as that of the MacKay-Marg tonometer.(48) It is found to be precise in common situations but not so accurate outside normal range.(49) The measurement of IOP using Tonopen is affected by corneal thickness like GAT because it is also an applanation device.
Tonopen is a useful device for bedside monitoring of IOP, measurement of IOP in patients under general anaesthesia, screening in outreach camps and school screening and for examination of children after sedation. However, in infants, tonopen seems to overestimate IOP and hence its use in management of congenital glaucoma is questionable.(50)
Unlike GAT, which depends on an optical end point, Tonopen has an electronic end point and therefore should theoretically be more precise in corneas with irregular surfaces.(51) A smooth surface on the irregular cornea can be chosen to measure the IOP because of its small applanating surface, since studies have shown that there is no difference in IOP when measured in the central and peripheral cornea in normal eyes using Tonopen. (52)
Instrument Description
This handheld tonometer has a strain gauge that creates an electrical signal when the footplate flattens the cornea. The diameter of the footplate is 1.5 mm. The force curves
35
are sensed by the built in single chip microprocessor. 4-10 readings are averaged to get the final digital display.
Technique
A disposable latex membrane is placed on the transducer of the Tonopen before measurement for each patient. After instilling topical anaesthetic drop in the eye, the probe tip is gently positioned on the patient’s cornea right in the centre. The tonometry is repeated until two readings with a confidence limit of 5% is achieved. An average of 2 readings are taken.
Pneumatic tonometer
The pneumatic tonometer consists of a sensing device that has a gas chamber covered by a silicone diaphragm. The gas pressure within the chamber is converted into an electrical signal by a transducer. This is recorded on a paper strip. There is an exhaust vent in the device through which gas in the chamber can escape. When the diaphragm touches the cornea, the gas vent reduces in size, and the pressure rises in the chamber. (53)
The pneumatic tonometer is used to measure IOP in eyes with scarred, and edematous corneas. Even though the pneumotonometer is subject to errors related to corneal thickness, it is less than that of GAT. This instrument is fairly accurate in normal eyes, but overestimates in low IOP and vice versa.(54) It is fairly accurate when used over therapeutic soft contact lenses.
36
Non–Contact Tonometer (NCT)
The NCT works by applanating the cornea by a jet of air which creates a constant force that momentarily deforms the cornea.(55) Hence there is no direct contact between the instrument and the patients’ cornea with the obvious advantage of preventing the transmission of infection from one patient to the next. However, the aerosol produced on the tear film by the air puff could contain microorganisms.(56)
NCT has three main parts.
a) Alignment system: It helps to align the patients’ cornea.
b) Optoelectronic applanation monitoring system: This consists of a transmitter, which allows a collimated beam of light to touch the cornea and a receiver which receives only the coaxial rays from the cornea.
c) Pneumatic system: This generates the puff of air.
Technique
Here the patient observes an internal target while the examiner aligns the patient’s cornea by superimposing the reflection of target on it in a stationary ring. When it is properly aligned, the operator presses the trigger, and a puff of air is directed onto the patient’s cornea and IOP is displayed on a digital readout.
NCT can be used for screening programs because it can be operated by non-medical personnel and does not require topical anesthesia. The IOP readings obtained using NCT seems to correlate well with GAT in IOP less than 20 mmHg.(57) Various studies have
37
shown under estimation of IOP with NCT as compared to GAT. (58) The NCT measures IOP over very short intervals and therefore it is important to take the average of a few readings. This instrument has an internal calibration system.
Ocular Response Analyzer
The Reichert Ocular Response Analyzer is a recent adaptation of NCT. It has an air puff tonometer that directs the air jet against the cornea and measures two pressures.
1.When the air jet flattens the cornea as the cornea is bent inward 2. When the air jet lessens in force and the cornea recovers.
The first is called the resting IOP. The difference between the first and the second applanation pressure is called corneal hysteresis.(59) Based on evaluation, this device gives 4 different parameters; Goldmann-correlated IOP, corneal-compensated IOP (IOPcc), corneal resistance factor (CRF) and corneal hysteresis.
Among these parameters, corneal hysterisis is considered an indicator of the viscoelastic properties of the cornea. The IOP as measured by the Ocular Response Analyzer (ORA) correlate well with GAT and is less dependent on central corneal thickness than GAT.
(60.)
Ocuton tonometer
The Ocuton is a hand-held tonometer. It also works on the applanation principle. It has a probe that is so light that it is barely felt, and hence no anaesthesia is needed prior to testing. It is used for home tonometry. It can be compared to GAT but tends to read
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higher than the GAT (61) especially in patients with thicker corneas. Studies have shown that Ocuton tonometer needs further methodological and technical refinement before it can be used routinely in clinical situations. However, because of its economical and practical advantages it can be used for screening glaucoma(62)
Dynamic contour tonometry (DCT)
This tonometer is based on the principle that by matching the contour of a sphere, the pressure on the outside equals the pressure on the inside.(63) DCT has a special, cylindrical, contour-matched pressure-sensing tip with a concave contact surface of 10mm radius. It matches with the surface contour of a tension free human cornea when intra-and extra ocular pressures are equal and no forces are acting perpendicularly on the cornea. When this contour match is reached, the integrated pressure sensor measures IOP, by eliminating corneal rigidity and adhesion forces and it is recorded digitally on the liquid crystal display (LCD). Unlike GAT, DCT measures real time IOP ie; actual measurement by DCT is pulsed. The bottom of this pulse curve is taken as IOP and displayed in digital display.
GAT usually measures the average difference between maximum and minimum pressure, but DCT reads the minimum. DCT also gives the difference between maximum and minimum pressure, which is called as ocular pulse amplitude (OPA). OPA is said to be an indirect indicator of ocular blood flow.
When DCT values were compared with manometry readings it was found that DCT readings are actually close to true IOP reading by manometry. (64) DCT values are found to be higher than GAT readings depending upon the level of IOP and corneal
39
biomechanical properties.(65) Unlike GAT, the DCT does not appear to be affected by corneal thickness. So it is a useful tool in situations where IOP measurements may be inaccurate due to abnormalities in corneal biomechanical properties. It is particularly useful in eyes with keratoconus, corneal edema and those that have undergone penetrating keratoplasty and refractive surgery. (66)
Pressure Phosphene Tonometer
The pressure phosphene tonometer (PPT) is a self tonometry device which was first described in 1998. It uses the entoptic phenomenon of pressure phosphene to evaluate IOP. (67) The PPT is applied perpendicular to the eyeball through the partially closed eyelid, and the applied pressure is increased gradually until the moment when the patient clearly perceives a well-formed phosphene . The device is then removed from the eyelid and IOP can be read from the dial.
The PPT has several advantages:
a) It is a noncontact tonometer.
b) PPT is not influenced by corneal biomechanical properties.
c) It insights into patient-specific, diurnal variations.
d) It also has good reproducibility when used by patients.
Most studies showed poor agreement between PPT and GAT, and it tends to underestimate IOP.(68)
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AGREEMENT BETWEEN DIFFERENT TONOMETERS
Tonopen vs GAT:
Tonopen measurements are comparable to GAT measurements in common clinical situations,(49) but it is not so in extreme situations. Several studies have shown that Tonopen has a tendency to overestimate low IOPs and underestimates high IOPs. (49) In a study by Bafa et al., (69) where they compared GAT and Tonopen in 90 eyes, with IOP, ranging from 6-28 mm Hg it was found that there was no statistically significant difference between both and agreement between both the instruments was good in 83%
of patients.
Iester et al., (70) compared Tonopen and GAT IOP in 3 different sub groups. They found that Tonopen underestimated lOP, when the GAT measurement exceeded 24 mmHg, thus proving that it is inaccurate for high IOP. But for low lOP values, the Tonopen did not show any difference compared to GAT.
Selvetat etal., (71),in their study showed that, inter method agreement between GAT and tonopen was poor and the mean of difference between GAT IOP and Tonpen IOP was 4.5±0.5 mm Hg. In their study they also noted that the Tonopen vs GAT differences did not show a tendency to vary in relation to IOP in contrast to other studies.
Bradfield et al.,(72) studied agreement between tonopen and GAT in normal adolescents and they found that, Tonopen underestimated IOP, when GAT IOP was <11 mm Hg and overestimated when GAT IOP >11 mm Hg . The 95% limits of agreement between the instruments was ± 6.4 mm Hg in the office setting.
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Bandophadyay et al., (73) in a population based survey of 203 subjects, found that the mean of the paired differences between IOP readings by the two instruments (Tonopen minus GAT) was 1 mm Hg with a standard deviation of 2.28. The Tonopen readings were within ±3 mm Hg of the GAT readings in 86.2% of eyes.
NCT Vs GAT
Several studies have shown fair agreement between GAT and NCT IOP readings. Mohan et al., (74) compared GAT with NCT and found that the mean of the difference in IOP between NCT and GAT in those with IOP below 18 mmHg was 0.99 ± 1.66 mmHg and 1.71 ± 1.88 mmHg in subjects with IOP >18 mmHg. It showed fair agreement between the two instruments at lower IOP range but poorer agreement at high IOP ranges. In a study by Masood Shah et al., (75) it was found that the mean IOP was statistically significantly different between the two tonometers.
Tonnu et al., (76) showed that NCT had a tendency to overestimate IOP at high IOP levels and underestimate at low IOP. In a study by Mosley et al., (77) where GAT and NCT were compared in different sub groups of IOP, it was found that at low pressures (<10 mmHg) the NCT overestimated IOP obtained with GAT, whereas at high pressures (>19 mmHg) there was underestimation. Between10 and 19 mmHg there was no significant difference between the readings obtained with either method.
Meera Nair et al., (78) showed that even though good agreement existed between GAT and NCT, the latter showed a tendency to overestimate IOP in the lower IOP range and underestimate in the normal and high IOP ranges. All these studies show that NCT is a
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useful and easy screening tool in a general ophthalmology clinic but not in subspecialty glaucoma clinic.
DCT vs GAT
In a study by Kaufmann et al., (79) where they compared GAT with DCT, in patients with normal IOP, it was found that DCT readings were almost in concordance with GAT, with DCT showing slightly higher IOP.
Schneider et al., (80) in their study of 100 patients with normal IOP, found that there was a clear agreement between both the tonometers (p<0.01). Here again, DCT showed a tendency to overestimate IOP slightly. In most of the studies, (81) DCT and GAT readings found to correlate well. However, DCT showed slightly higher IOP in normotensives and lower IOP in patients with high IOP.
Barleon et al., (82) studied the agreement between both instruments in high and normal IOP groups, and they found good overall correlation between both instruments, but the agreement between them in high IOP groups was poor.
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CENTRAL CORNEAL THICKNESS AND ITS EFFECT ON IOP USING DIFFERENT TONOMETERS
Central corneal thickness (CCT) is one of the most important confounding factors that affect measurement of IOP using most of the commercially available tonometers. In the Rotterdam study(83), it was found that the mean CCT in normal subjects varies between 537-554microns
Factors affecting CCT
a) Race: Different studies have shown that the black population has thinner CCT values than their white counter parts. (83)
b) Age: CCT is found to increase in infancy till 2-4 years of age, (84) at which time they reach the adult value. In adults, CCT is found to decrease, at least in some individuals.
This decrease is more in glaucoma patients than normal population. (83)
c) Corneal refractive surgeries: After all corneal refractive procedures the cornea is artificially thinned. (85) So it is important to repeat CCT in glaucoma patients who underwent any of these surgical procedures.
d) Diurnal variation and hormonal factors: CCT found to be highest in the morning and progressively decreases throughout the day. Increase in CCT during pregnancy demonstrates the influence of hormones like estrogen on CCT. (86) Some disease like diabetes also has influence on CCT, with diabetics having higher CCT. (87)
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Measurement of CCT
CCT measurement is an integral part of diagnosis and management of glaucoma.
Different methods are available for measuring CCT.
Ultrasound Pachymeter
This is currently the most reliable and commonly used pachymeter. The ultrasound pachymeter has a probe, which touches the cornea gently, which sends out an ultrasound signal that ultimately returns to the probe, for analysis of corneal thickness. Ultrasound measurements can be affected by corneal hydration.
Optical Pachymeter
Optical techniques measures CCT using a device attached to slit lamp. Here the observer aligns the anterior and endothelial surface of cornea by image doubling. The corneal thickness is then estimated using an equation based on refractive index and radius of curvature of anterior surface of cornea. These variables and the examiner-dependent nature of the technique are the major disadvantages of this modality
Orbscan System
It is a noncontact method, which scans the anterior segment to provide maps that will give CCT as well as anterior and posterior topography measurement. It may underestimate values in patients with corneal haze especially after LASIK.
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Anterior Segment Optical Coherence Tomography
ASOCT uses high-resolution images to get central and peripheral corneal thickness. It is found to show consistently lower values than ultrasound pachymetry. (88).
EFFECT OF CCT ON MEASUREMENT OF IOP USING DIFFERENT TONOMETERS
GAT and CCT
GAT works on principle of modified Imberts-Ficks law. In their modification of this law Goldmann and Schimidt(3) assumed the average CCT to be 520 microns, while acknowledging that variations in CCT values will affect the GAT reading. When GAT IOP was compared with IOP obtained by manometry method, it was found that there is significant discrepancy between both, and this difference was due to CCT. (89). CCT measurement has become very important after OHTS, which showed that in ocular hypertensives, those with thinner corneas are at a risk of progression to glaucoma. (12).
GAT overestimates IOP in thicker corneas and underestimated in thinner corneas.
Ehler et al., (90) found that the average increase in IOP was about 0.7 mmHg for every 10 microns increase in CCT from a mean CCT of 520 microns. This CCT dependent inaccuracy of GAT readings is extremely important in cases with irregular corneas as in keratoconus and also after different refractive surgeries. In eyes with keratoconus, GAT IOP is found to be lower than true IOP, by about 5.3 ± true IOP. Ismail et al., (91) found
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that in eyes that had undergone penetrating keratoplasty, GAT measurements may be less precise than non-applanation tonometry.
Meyenberg et al., (92) suggested that GAT could slightly underestimate IOP in post keratoplasty eyes (3.1±2.5 mm). Most of the studies showed that GAT is not so effective as other applanation tonometers in post keratoplasty eyes, and that it tends to under- estimate the values. There seems to be an agreement on the apparent IOP-lowering effect of the different modalities of refractive surgeries. Kirwan et al., (93) found that the mean GAT IOP decreased 3.7±2.3 mmHg following LASIK, and this decrease was observed following LASEK also.
It should be noted that IOP is not truly decreased after these procedures, but rather it is underestimated as a result of changes in the corneal structure ie.,decreased thickness, presence of fluid or scar tissue. Attempts have been made to derive specific formulas to calculate the influence of CCT on IOP measurement, but there is no consensus about its use in clinical practice. (94).The simple concept that IOP is being over or underestimated depending on these variables should be kept in mind when estimating the subject’s IOP in clinical setting
Tonopen and CCT
Tonopen also depends on the same principle of applanation as GAT and therefore it is also affected by CCT. In a study by Veena et al., (95) comparing the accuracy of different tonometrs in eyes which have undergone keratoplasty, it was found that tonopen
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IOP was less affected by CCT than GAT. This is probably related to the lesser area of applanation in Tonopen as compared to the biprism in GAT.
Archana et al., (96) looked at the effect of CCT on IOP determined using tonopen, GAT and ocular blood flow tonometer and found that tonopen is least affected by CCT.
Mollan et al., (97)evaluated the efficacy of four different tonometers in eyes with keratoconus and found that in post keratoplasty eyes, GAT and Tonopen showed a good agreement (mean difference 0.14 mmHg) and Tonopen IOP was largely independent of CCT.
NCT and CCT.
Gupta et al., (98) studied the effect of CCT on NCT and GAT in glaucomatous eyes and found that NCT was more affected by corneal thickness (0.4 mmHg/10 µ) while GAT was least affected by corneal thickness (0.3 mmHg/10µ). The difference however, was not statistically significant (p = 0.42). The study also concluded that the effect of CCT on NCT is significantly higher with thicker corneas.
A study by Petit et al., (99) in normal subjects found that NCT is more affected by CCT than all other commonly used tonometers. With regard to the influence of corneal properties, it is likely that NCT is more influenced by CCT than GAT.(100) In thinner corneas, there seems to better correlation between the tonometers, while in thicker corneas, non-contact tonometry systematically yields higher readings than GAT.
48 DCT and CCT
DCT is designed to be largely independent of most of the corneal biomechanical properties. Boehm et al., (101), compared DCT IOP with manometrically obtained IOP and found that the DCT IOP was closely related to true IOP and the difference between the two was dependent on CCT. Though IOP was influenced by CCT, its clinical relevance was considered insignificant.
In a study by Mathiass et al., (102), where the effect of CCT on DCT in open angle glaucoma patients was studied, both DCT and GAT were dependant on CCT to almost the same degree. Milla et al., (103) found a good agreement between GAT and DCT when CCT was between 540-545 microns.
In a study by Schneider et al., (80) where they tried to find the influence of CCT on GAT and DCT IOP readings, and found that CCT and DCT were not correlated (p<0.71),but GAT was significantly influenced by CCT(p<0.03). Petit et al., (99) also showed a weak correlation between CCT and DCT and it was not statistically significant. (r=0.160, p=0.079).
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MATERIALS AND METHODS
50 STUDY DESIGN
This is a cross sectional/ observational hospital based study in Indian eyes.
PATIENT SELECTION
Subjects were chosen from among those patients who came for an ophthalmic evaluation to the outpatient department, provided they fulfill the inclusion and exclusion criteria as given below.
Inclusion Criteria :
1. Patients more than 18 years of age.
2. Willingness to participate in the study
3. Normal IOP subjects: Patients with IOP 10 to 24 mm Hg 4. High IOP subjects: Patients with IOP > 24 mmHg Exclusion Criteria:
1. Any corneal pathology, including corneal oedema which prevents accurate measurement of IOP using GAT and DCT.
2. Patients with corneal astigmatism more than 2 dioptres as assessed by retinoscopy, and in patients with hazy media as determined using keratometry.
3. Patients who have undergone any intraocular surgery within the last 6 weeks.
4. Patients who are cannot co-operate for IOP measurement due to posture-related or physical problems.
5. Patients with active inflammation such as conjunctivitis and uveitis.
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Institutional Review Board (IRB) approval:
The study protocol was approved by the IRB which constituted of members outside the institution as per ICMR guidelines required for any study conducted in the institution.
Ethical committee approval was also obtained in the same way.
METHODOLOGY
All subjects who fulfilled the inclusion and exclusion criteria as given above were recruited for the study after getting an informed consent using the consent form that was approved by the IRB and in compliance with 1964 declaration of Helsinki
a. Best corrected visual acuity(BCVA): BCVA was determined by doing an objective retinoscopy and subjective refraction using a Snellen chart at 6 meters.
b. Slit lamp biomicroscopy: All patients underwent complete ophthalmic examination using a using slit lamp and a dilated fundus examination using a 78 D lens.
c. Indirect ophthalmoscopic examination-All patients underwent indirect opthalmoscopic examination using a 20D to look for peripheral retinal lesions.
d. IOP measurement-The principal investigator performed all IOP measurements using NCT, GAT, DCT and Tonopen in that order. While IOP was being measured, the readings were documented by an optometrist in order to prevent an intra observer bias. All measurements were taken in the above mentioned
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sequence giving at least a 5 minute interval between each measurement. Two IOP readings were taken for each tonometer and averaged. One hour after the IOP measurements, CCT was measured using an ultrasound pachymeter by an optometrist who did this measurement for all subjects included in the study.
TECHNIQUE OF MEASUREMENT WITH EACH TONOMETER
Goldmann Applanation Tonometry:
Haag-Streit International (SWISS MADE - AT 900) slit lamp with applnation tonometer mounted on it was used for measurement. After explaining the procedure, the patient was positioned comfortably on the slit lamp. The prism tip which is kept in the disinfecting solution, was rinsed in saline, wiped and dried before applying to eye. After keeping the prism in correct position, the tension knob was set at graduation mark of 1. Proparacaine was instilled into the lower fornix to anesthetize the cornea. Tear film was stained using fluorescein on a strip which was moistened using diluted methyl cellulose solution. The patient was asked to look straight ahead with both eyes open. The eyelid is retracted so that the tonometer head does not touch the eyelid and force the patient to blink. With biprism illuminated by a cobalt blue light from slit lamp, it is brought into gentle contact with apex of cornea. The fluorescent semicircles are viewed through the biprism and force against the cornea is adjusted on the tension knob till the inner edges overlap. The IOP is then read directly from a scale on the tonometer housing.
53 Tonopen
We used a (REICHERT TONOPEN-XL: ANNEXURE:2,PICTURE 6) hand held tonometer for our study. This handheld tonometer has a strain gauge that creates an electrical signal as the footplate flattens the cornea. A built in single chip microprocessor senses the proper force curves and averages 4-10 readings to get the final digital display. A disposable latex membrane was placed on the transducer of the Tonopen for each patient before measurement.
After instilling topical proparacine drop in the eye, the probe tip was gently positioned on the patient’s cornea right in the centre. The tonometry was repeated until two readings with a confidence limit of 5% is achieved. An average of two of these readings was taken .
Non –Contact Tonometer
We used NIDEK (NIDEK-TONOREF –II:ANNEXURE 2;PICTURE NO;3) NCT for this study. This instrument is mounted on a table and it is a combination of both automated refractometer and NCT. After explaining the procedure, the patients was positioned comfortably.
They were instructed to observe an internal target projected within the instrument. While they were observing the target, the operator aligned the cornea by superimposing the reflection of the target from the patient’s cornea on to a stationery ring. When it was properly aligned a puff of air was automatically directed onto the patient’s cornea.
Once initiated, the puff of force increased until the cornea is applanated over a predetermined area. The tonometer then translated the applanation force into a measure of IOP.
IOP was displayed on a digital screen. Average of 2 readings was used.
54 Dynamic Contour Tonometry:
The Pascal DCT ( Ziemer S Ophthalmology, SMT Swiss Micro technology 2005 ANNEXURE 2:PICTURE No;1 ), is an accessory device which is installed into the optical axis of slit lamps. The Pascal DCT was mounted on a Haag Striet slit lamp(HAAG-STREET INTERNATIONAL,SWISS MADE). Sterile Sensor cap was fitted to the Sensor Tip prior to the measurements and disposed after single use.
All measurements were done using the standard technique. A drop of 0.5% Proparacaine, was instilled to the patient’s inferior fornix to anaesthetize the cornea.. No fluorescein was used. The patient was requested to keep the eye wide open and to look straight. The examiner aligned the end of the sensor tip to the apex of the patient’s cornea.
The Pascal unit was switched on by turning the blue knob in clock wise direction.
Looking through left ocular, the slit lamp is advanced until the surface of the sensor tip touched the cornea. The area where the sensor tip touched, would appear as a darker area on the cornea. Regular continuous oscillating sound was heard when the sensor tip had established correct contact and alignment with the cornea. After counting about five to seven wave forms of the oscillating sound, the sensor tip was removed away from the patient’s cornea.
The machine would now compute IOP and OPA from the pressure curve, and the results (IOP, OPA and QF-quality factor) displayed on the LCD screen. Two measurements of quality factor either 1 or 2 were taken for each patient and averaged.
55 Central corneal thickness Measurement
CCT is measured using an ultrasonic pachymeter(TOMEY-SP,2000)for CCT measurement. After anesthetizing the cornea with topical proparacaine 0.5% drops, the patient was asked to look straight ahead. The pachymeter probe(ANNEXURE 2:PICTURE No:4) was placed gently on the centre of the cornea. Two consecutive readings were taken for each subject and averaged
Sample size calculation
Sample size was calculated using the below mentioned test
Agreement - Single Group - Continuous outcome-ICC (Testing against Population value)
Sample reliability value 0.5
Population reliability value 0.7
Power (1- beta) % 80
Alpha error (%) 1
1 or 2 sided 2
Number of replicates 4
Required sample size 60
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From the above table it is found that the required sample size to show an agreement of about 0.7 is around 60 subjects, with 80% power and 5% level of significance using the four tonometers.
;
Formula used for calculating sample size is given above Data collection and analysis
All data were entered in clinical research form and analyzed.Descriptive variables were analyzed using mean and standard deviation.Paired t test was done to find the difference between each instruments.Pearsons correlation and concordance correlation coeeficient were done to find the correlation between each instruments. 95% limits of agreement was
57
done to find the agreement between each instrument and GAT,and Bland-Altmann plots were used to depict the same.Intraclass correlation was done to find agreement wherever possible.Linear regression analysis was done to find the influence of CCT on IOP measured by each instrument.
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RESULTS AND ANALYSIS
59
RESULTS AND ANALYSIS
This study was conducted to look at the agreement between four commonly used tonometers; with specific advantages for each of these over GAT. We calculated sample size based on published data that showed a good agreement between GAT and NCT using intra class correlation. Our sample size was 60 patients in each group. We recruited 65 patients in the normal IOP category. In the high IOP group on the other hand, we were able to recruit only 42 patients who fulfilled the inclusion and exclusion criteria.
Out of the 65 normal patients studied 31 were males and 34 were females. This is depicted in Table 1 and Graph 1. Out of 42 patients with high IOP, there were 29 males and 13 females as shown in Table 2 and Graph 2.
Table 1: Gender Distribution in the Normal IOP Group
GENDER FREQUENCY PERCENTAGE
Male 31 48
Female 34 52
Total 65 100
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Table 2: Gender Distribution in the High IOP Group
GENDER FREQUENCY PERCENTAGE
Male 29 69
Female 13 31
Total 42 100
Graph 1: Gender Distribution in the normal IOP group
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Graph 2: Gender Distribution in the high IOP group
The mean age of the patients were similar in both the groups; 57.6years in the normal IOP group and 55.5years in the high IOP group. The age wise distribution of patients in the normal IOP group and the high IOP group are given in Table 3, Graph 3 and Table 4, Graph 4 respectively.
