ORIGINAL ARTICLE
Year : 1998 | Volume
: 46 | Issue : 2 | Page : 81--86
Prevalence of primary glaucoma in an urban South Indian population
A Jacob, R Thomas, SP Koshi, A Braganza, J Muliyil
Schell Eye Hospital, Christian Medical College, Vellore, India
Correspondence Address:
A Jacob
Schell Eye Hospital, Christian Medical College, Vellore
India
Abstract
Glaucoma is fast emerging as a major cause of blindness in India. In order to estimate the prevalence of primary open angle glaucoma (POAG) and primary angle closure glaucoma (PACG) in an urban South Indian population, we examined 972 individuals aged 30-60 years, chosen using a cluster sampling technique from 12 census blocks of Vellore town. They underwent a complete ocular examination, including applanation tonometry and gonioscopy, at the Medical College Hospital. Characteristic field defects on automated perimetry was a diagnostic requisite for POAG. Prevalence (95% CI) of POAG, PACG, and ocular hypertension were 4.1 (0.08-8.1), 43.2 (30.14-56.3), and 30.8 (19.8-41.9) per 1,000, respectively. All the PACG cases detected were of the chronic type. Hitherto unavailable community-based information on primary glaucoma in our study population indicates that PACG is about five times as common as POAG.
How to cite this article:
Jacob A, Thomas R, Koshi S P, Braganza A, Muliyil J. Prevalence of primary glaucoma in an urban South Indian population.Indian J Ophthalmol 1998;46:81-86
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Jacob A, Thomas R, Koshi S P, Braganza A, Muliyil J. Prevalence of primary glaucoma in an urban South Indian population. Indian J Ophthalmol [serial online] 1998 [cited 2023 May 29 ];46:81-86
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Full Text
The World Health Organization has estimated that India has a 1% prevalence of blindness.[1] Of the estimated 8.9 million blind in India, 12.8% are due to glaucoma. The problem is expected to reach alarming proportions by the turn of the century.[2] While there are excellent population-based data available from the West,[3][4][5][6][7][8][9] such data from South Asia, especially India, are lacking. Most available information in this region is hospital-based,[10][11][12][13][14] and as highlighted in recent publications,[2] actual prevalence figures of disease are still not available.
Racial variations in the prevalence of primary glaucoma are well known.[15] Amongst the Caucasian races, 75-95% of primary glaucoma is open angle glaucoma (POAG).[16] These figures are almost reversed amongst Eskimos and East Asians where primary angle-closure glaucoma (PACG) constitutes 80-90% of primary glaucoma.[10],[14]
The Vellore Eye Survey (VES) was designed to obtain epidemiological data on ocular morbidity including primary glaucomas (POAG and PACG) in a South Indian population. The VES intends to prospectively study a subset of this population with periodic eye examinations in order to generate incidence data. To the best of our knowledge, it is the first comprehensive study of the prevalence (and risk factors) for eye diseases using modern examination and diagnostic methods on a random sample of an Indian population. The purpose of this report is to present the VES methods and data on the prevalence of POAG and PACG.
Subjects and Methods
Vellore town is situated about 140 km south-west of Madras in the state of Tamil Nadu. According to the 1991 national census the total population was about 300,000. Anthropologically, the population of Vellore is mostly Dravidian. The religion is predominantly Hindu; Muslims constitute about 20% and Christians about 10% of the population(Census of India, 1991).
In 1993, the departments of Community Health and Cardiology of the Christian Medical College and Hospital (CMC&H), Vellore in collaboration with the Indian Council of Medical Research (ICMR) initiated a study on the prevalence of coronary heart disease and its known risk factors (anthropometric, dietary, socioeconomic, lifestyle-related, biochemical and clinical) amongst the urban population of Vellore town and its surrounding villages. The urban component of the ICMR study used a sampling frame that consisted of all the census wards of Vellore town; 20 clusters were selected using a cluster sampling technique from the sampling frame, representing all the geographic regions of Vellore town.[17]
In November-December 1994, the Department of Ophthalmology in collaboration with the Department of Community Health, CMC&H, randomly identified an urban cohort of 12 of the above 20 clusters, consisting of a total population of 5,697 people. 1,932 individuals in the age group of 30-60 years were identified from this population and visited by field workers who invited and motivated them to participate in this prospective study. Those who failed to keep the first appointment were visited again and revisited as required, in order to encourage participation. Apart from free examination and treatment at the Eye Hospital no incentive was offered for participation in the study.
Data on occupation, past medical and eye history were obtained in an interviewer-administered questionnaire. The participants were interviewed in person in their native language by one of three optometrists who were trained to follow a standardized questionnaire.
Visual acuity and refraction were performed by experienced optometrists. A comprehensive ocular examination including slit lamp biomicroscopy, applanation tonometry, gonioscopy, dilated pupil indirect ophthalmoscopy as well as stereo-biomicroscopy of the disc and macula (using a non-contact +78 D lens) was performed in a standard manner by one of two ophthalmologists participating in the study. Both had trained in the department for a minimum of 2 years following their ophthalmic qualification, of which at least 1 year was spent in the glaucoma clinic. The examination procedure was standardized prior to the study. Some components of the examination could not be carried out on patients who refused such examination, had had surgery on the eye, those with corneal scars, or those with acute infection.
Gonioscopy was performed in a routine manner using the Goldmann two-mirror lens. Occludability was assessed using dim ambient and slit lamp illumination with the patient looking straight ahead; care was taken to ensure that the slit beam did not encroach upon the pupillary area during this phase. Next, the slit beam height and illumination were increased and the patient instructed to look in the direction of the mirror and the angle manipulated open to look for synechiae. If this manoeuvre was unsuccessful or there was a doubt about the presence of synechiae, indentation gonioscopy was performed using a Sussmann four-mirror lens.
Gonioscopic grading was based on the angle structures actually visualized [Annexure:I]; inter-observer agreement for this had been quantified in an earlier study performed at this hospital.[18] A fairly similar method has been used in other population-based studies.[19],[20],[21] Grade III was considered narrow; Grade II and less were considered closed. In all cases a forced choice was made regarding occludability of the angle.[18]
The cup-disc ratio (CDR) was recorded in its widest axis and glaucomatous features[22] looked for and recorded using stereo-biomicroscopy with a 78-dioptre lens. The disc features considered suggestive of glaucoma are given in [Annexure:II].
An automated visual field (Humphrey Field Analyzer 30-2 program) was scheduled on the following indications: intraocular pressure (IOP) >21 mmHg, CDR ≥0.7 in either eye, difference in CDR >0.2 between the two eyes and /or the presence of glaucomatous features. Unreliable fields[23],[24] were repeated during another session.[24] If an advanced field defect precluded a successful 30-2 examination, the macular program or a 10-2 program was used. Patients who did not keep the field appointment were reminded by mail and later visited by the field workers. The study protocol required a reliable field on all patients.
The diagnosis of glaucoma was confirmed by a glaucoma specialist on the basis of the clinical details and established glaucomatous visual field defects[25] in each eye. For the purpose of this study, primary open angle glaucoma (POAG) was defined as the presence of elevated IOP and /or glaucomatous disc changes in the presence of typical glaucomatous field defects,[25] an open angle on gonioscopy and no evidence of a secondary cause. The diagnosis of ocular hypertension was made based on an IOP >21 mmHg, absence of glaucomatous disc features, no demonstrable glaucomatous field defects and open angles with no evidence of a secondary cause.
Primary angle closure glaucoma (PACG) was considered under the following headings:
(i) Acute primary angle closure glaucoma was defined as an acute painful red eye with a history of loss of vision, dilated pupils, raised intraocular pressure and closed angles on gonioscopy in the absence of any secondary cause.
(ii) Chronic primary angle closure glaucoma (CACG) was considered to be either appositional or synechial. Chronic appositional angle closure was diagnosed in the presence of raised intraocular pressure and closed angles on gonioscopy, in the absence of peripheral anterior synechiae. These angles could always be opened on "manipulation" with the two-mirror gonioscope or by indentation gonioscopy as described earlier. Chronic synechial angle closure was diagnosed in the presence of peripheral anterior synechiae[26] with occludable or closed angles on gonioscopy, even in the absence of raised intraocular pressure. The presence of glaucomatous field defects or optic disc changes were not considered mandatory for the diagnosis of angle closure glaucoma. Secondary causes of angle closure were excluded.
Results
Of the 1932 total eligible population in the age group of 30-60 years, 1521 could be contacted by our field workers and were invited to participate in the study. 972 persons (50.3% of eligible population; 63.9% of the invited population) presented themselves for examination at the base hospital.
While the responders did not significantly vary from the non-responders with regard to age, socio-economic status or educational background, the response among the men was poor in comparison to that amongst the women [Table:1]. This difference in response was significantly different (p<0.001, chi-square test).
IOP recordings were available in 947 (97.4%) of the right eyes and 946 (97.3%) of the left eyes. The mean IOP in both the right and left eye was 15.48 mmHg (standard deviation 3.57 right eye, 3.59 left eye). IOP was greater than 21 mmHg in 62 eyes of 40 persons.
Of the 1,749 eyes examined, 10.35% were considered occludable on gonioscopy. 61 (8.5%) of the 717 eyes of males were considered occludable. In females, 120 (11.6%) of 1032 eyes were considered occludable. 63 persons (6.5% of population, 21 males and 42 females) had occludable angles in both eyes. The fellow eyes of the persons with occludable angles in one eye, had a low gonioscopic grading (indicating narrow angles) but were not labeled as occludable.
Peripheral anterior synechiae were seen in 24 (2.5%) of the right eyes and 23 (2.4%) in left eyes. Peripheral anterior synechiae were bilateral in 14 eyes and unilateral in 19. The fellow eyes of those with unilateral synechiae had narrow angles and were considered occludable.
The mean CDR in males was 0.32 in both eyes and was 0.30 in both eyes of females. One or more disc features suggestive of glaucoma were seen in 118 eyes of 80 (8.2%) subjects. The CDR was ≥0.7 in 65 eyes of 45 (4.6%) people [Table:2]. Cup-disc asymmetry >0.2 was seen in 82 subjects (8.4%).
Based on the criteria defined earlier, a total of 169 (17.94%) individuals were given an appointment to undergo a Humphrey field analysis. Only 82 (48.5%) of these subjects underwent field tests as required by the study protocol. In these patients a disc with glaucomatous features was seen in 85 eyes of 56 patients; a CDR ≥0.7 in 26 eyes of 18 patients and a CDR asymmetry of >0.2 between the eyes was seen in 26 patients. IOP >21mm Hg was present in 26 eyes of 16 patients.
Nine of the 82 subjects who underwent the field examination were diagnosed to have glaucomatous field changes. Of these, 4 had were classified as POAG; 5 had features of angle closure glaucoma with characteristic field and disc changes and were therefore classified as chronic PACG (3 synechial and 2 appositional).
Chronic synechial angle closure was diagnosed in 33 patients. Both eyes had peripheral anterior synechiae in 14 individuals; in 10 only the right eye was involved and in 9 the left eye was involved. Chronic appositional closure was diagnosed in 9 patients (5 right eyes and 4 left eyes). There was no significant difference between the two sexes for the prevalence of PACG [Table:3]; p=0.06, chi-square test). The fellow eyes of all patients with unilateral angle closure glaucoma (synechial or appositional) had narrow occludable angles.
Ocular hypertension was diagnosed in both eyes of 16 persons, and one eye of 14 others.
In summary, PACG in its various forms was diagnosed in 42 persons [Table:4]. POAG was diagnosed in 4 persons. There were 30 ocular hypertensives. Only one subject had been previously diagnosed to have glaucoma (CACG). The diagnosis had been made elsewhere prior to the study, and the patient was on treatment with topical pilocarpine.
Discussion
Reliable population-based data on the prevalence of glaucoma in India are scarce. Besides IOP and disc features, demonstration of a characteristic field defect using a full threshold strategy on automated perimetry is required to detect cases of POAG.[23] Similarly, gonioscopy is mandatory to confirm the diagnosis of PACG.[10],[27],[28] To the best of our knowledge there has been no population-based study incorporating these criteria for the diagnosis of glaucoma in India. We have reported the prevalence of glaucoma from the first such study in India using these criteria.
The prevalence of POAG (0.41%) is lower than that reported elsewhere. This is probably explained by the age of our study population, selected essentially to gather incidence data during follow up examinations. In other studies, the prevalence in similar age groups is close to ours.[6] It is likely that the prevalence would be higher with inclusion of the older age groups.
The major finding of interest in the present study is the prevalence of PACG. Hospital-based data from India had suggested that PACG was at least as common as POAG.[10],[11] The prevalence of PACG (4.32%) that we found was several times that of POAG; far more than suspected or anticipated based on hospital attendance data.[10],[11],[12] Most population based studies have performed gonioscopy only on those subjects suspected to have shallow chambers on the basis of the flashlight and or the van Herick tests.[29] These tests have low sensitivity and are poor predictors of angle closure glaucoma.[18]
This high prevalence in the present report can be explained by the comprehensive examination protocol that included gonioscopy for all subjects. A study from South Africa in a population of predominantly East Asian descent that used a similar protocol also found an "unexpectedly" high prevalence of chronic angle closure glaucoma.[20] More recently, a high prevalence of angle closure glaucoma was reported from Mongolia.[30] While racial differences in prevalence could play a role, it seems only logical to expect a higher detection rate using gonioscopy, which is the appropriate test for diagnosis of angle closure and therefore, our findings are likely to reflect the true prevalence rate in the population.
POAG has hitherto been considered to be the predominant type of primary glaucoma and generally leads to blindness without any symptoms. The possible explanation for the hospital based 1:1 ratio of POAG to PACG in South Asia was the symptomatic nature of acute PACG necessitating hospital visits. However, despite the high prevalence of chronic PACG, we did not find any acute PACG. Moreover, only one of our chronic closures had been diagnosed as glaucoma prior to recruitment in the study. Interestingly, in the South African study only 30% of their chronic PACG patients had a history suggestive of PACG.[20] This lack of symptoms with chronic PACG in our community is worrying and is underscored by the predominance of synechial closure. At least five times the number of POAG patients in the study population have asymptomatic, treatable but potentially blinding chronic angle closure.
The response rate in our study was only 50.3% of the eligible subjects. Though this compares unfavourably with the response in the Framingham and Baltimore studies, 67% and 79%, respectively,[31],[32] it compares well with the response in a large Japanese study[33] where the response rate was only 38.5%, and a recent study from Taiwan[21] where the response rate was as low as 10.3%. The low response rate is probably explained by the stressful and time consuming nature of a comprehensive eye examination conducted in a hospital setting. Free examination (and treatment if required) were the only incentives for participation. Except for sex, the responders did not in any way differ from the non-responders; the low response rate is therefore unlikely to have affected the results.
It could be argued that subjects with PACG suspected an ocular problem and preferentially attended the study. However, as chronic closure (there were no acute angle closures) in this population was almost always asymptomatic, this criticism is probably unjustified.
Significantly more females than males responded to the study. This causes a problem in estimating the true prevalence of PACG, classically considered to be commoner in females. However, despite their higher response rate we did not find a significantly higher prevalence of PACG amongst females [Table:3]. The notion that PACG occurs more commonly in females may not hold true for our population, especially for chronic ACG. This requires to be substantiated by further population based studies.[34]
A major limitation of our study is the poor response rate. This was especially significant in the case of the patients invited to undergo field testing. As a typical field defect was mandatory for the diagnosis, this would have probably affected the final estimate of the prevalence of POAG. Since only 48.5% of those invited underwent the HFA testing, the estimate of the prevalence of POAG is probably an underestimate. We attempted to project the possible number of field defects (POAG) amongst those who did not undergo the HFA test, applying a sub-group specific rate, by using the observed prevalence of raised IOP, glaucomatous features of the disc, and the cup:disc ratio among those subjects that had not taken the field test. Unfortunately, the numbers involved were small, and we could not obtain a precise estimate. However it may be reasonable to apply the prevalence of field defects in those who did undergo field testing to those who did not and assume that the true prevalence of POAG may be about twice our estimate. If that is so, in the study population, angle closure would still be five times as common as POAG.
It is apparent that only a small proportion of patients with occludable angles or even peripheral anterior synechiae (while at risk for acute angle closure) actually progress to glaucomatous disc of field changes; our current classification of angle closure 'glaucoma' probably needs to be altered to reflect this. Even if we were to consider that field defects are the only important event in any glaucoma, about half the patients with established glaucomatous field defects in this population (5 of 9) had angle closure.
A recent report predicts that by the turn of the century India will have 5.59 million people suffering from POAG and a similar number from PACG.[2] These data are a projection of available hospital based statistics.
Our data could also be used to project the problem for the entire country. In our study population, the 30-60 year olds constituted about one-third of the total population. Assuming that the rest of India's 900 millions has a similar age distribution, the 30-60 year olds will constitute about 300 million people. If the prevalence of PACG is indeed 43.2 per 1,000 population, we would expect 12,960,000 to be affected by the disease. If our prevalence of POAG (4.1 per 1,000 population) is also true, we would expect 1,230,000 to be affected by the disease in the 30-60 year age group. This is a total of about 14 million affected by primary glaucoma. This figure has been arrived at by other workers, but the ratio of POAG and PACG have been very different.[2] If we include the older age groups, this figure is likely to be much higher, especially for POAG. On the other hand, the elderly population with occludable angles may undergo cataract surgery, which would then almost eliminate the risk of PACG. Significant racial variations around the country are likely to negate such projections, but till similar data are available for the rest of the country, this approximation is all that is possible.
We report the results of the first population-based study from India that has used modern examination techniques for the detection of glaucoma. Earlier studies did not utilize gonioscopy or automated perimetry, the currently accepted standards for the diagnosis of PACG and POAG. Despite a low response rate of only 50.3%, our study suggests a hitherto unsuspected high prevalence of chronic PACG; if confirmed by other investigators it has ominous portents for health planners in our country. Population-based data, collected in a valid manner, are urgently required from other parts of the country in order to plan strategies to prevent the potential blindness associated with glaucoma. The availability of effective prophylactic treatment for PACG makes this all the more compelling.
Acknowledgments
The authors would like to thank Ms. Velankanni and Mrs. Alice Pandian of the CHTC Computer Cell for their help with the data entry and analysis.
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