The Future of Glaucoma in Asia (continued)

Racial Variations in Angle Closure Glaucoma

It is becoming clear that racial differences in angle configurations play a role in the pathogenesis of PACG.⁴ The prevalence of this disease in East Asians is approximately 1% and accounts for 50% of all glaucomas in this population. PACG is therefore probably the most widespread type of glaucoma worldwide because of the large population in East Asia.^22,23

The underlying mechanism for the high incidence of angle closure glaucoma among Asians and blacks compared with Caucasians is not clear.²⁴ However, the thickness and rigidity of the iris in these groups have been postulated as possible causes. Oh et al found that the iris joined the ciliary body more anteriorly in Asians than in blacks or Caucasians (p = 0.03) [Table 4], which may predispose to progressive formation of PAS.²⁴ In addition, the irides of Asians had a greater tendency to a concave figuration of the peripheral iris compared with the other 2 groups (Table 5). These authors sug-gested that there may be a direct relationship between the anatomical configuration of the anterior chamber angle, particularly the position of iris insertion, and the comparatively high rate of angle closure glaucoma among Asians.

Table 4. Position of iris insertion after indentation gonioscopy in different racial groups.

Table 5. Peripheral curvature of the iris in different racial groups.

Glaucoma is a major cause of blindness in India.²⁵ Indeed, 12.8% of the 8.9 million blind people in India are blind due to glaucoma, and angle closure glaucoma con-stitutes 45.9% of all primary adult glaucomas.²⁶ The Vellore Eye Survey, performed in southern India, found that PACG was significantly more common than primary open angle glaucoma (POAG), with a prevalence of 43.2 for PACG and 4.1 for POAG per 1,000 population, respectively.²⁵ However, the reason for this high rate is probably due to the case definition used for this study.

Dandona et al performed an assessment of the prevalence and features of angle closure glaucoma in an urban population in South India.²⁷ Among people aged 40 years or older, 1.08% had manifest PACG and 2.21% had occludable angles, which is substantially less than that reported in Mongolia. Interestingly, only one-third of patients with PACG had been previously diagnosed, and 41.7% were blind in one or both eyes due to PACG. Projecting these results to the estimated 288 million urban population in India suggests that 2.4 million people in urban India may have PACG or are at risk for the disease due to the presence of occludable angles.

A survey of visual morbidity in Mongolia found that 35% of the blindness in the population aged 40 years or older was due to glaucoma, and was similar to that due to cataract (36%).²⁸ A study to classify glaucoma according to mechanism among 942 individuals in Mongolia found that the prevalence of PACG was 1.4% and that of gonioscopically occludable angles was 6.4%, while the prevalence of POAG was only 0.5%.

A Singapore study found a higher rate of hospital admissions for PACG among Chinese in Singapore than for Malays or Indians (Table 6).²⁹ The rates did not differ significantly between Malays and Indians in this study, and were similar to those seen in Thailand, another ethnic Southeast Asian population. Meanwhile, Nguyen et al found that Vietnamese people living in the USA had a higher prevalence of narrow angles and a greater risk of angle closure than Caucasians.³⁰

Table 6. Mean annual admission rate for primary angle closure glaucoma per 100,000 population for different ethnic groups in Singapore between 1993 and 1997.

These studies are of particular interest since Mongolia is believed to be the genetic centre of the Sino-Mongoloid race, with migration occurring among the neighbouring regions of North and East Asia, and to Alaska, Canada, and Greenland.³¹ The disparity in prevalence of PACG between the Inuit populations and Caucasians has been attributed to the shallow anterior chamber found among the Inuit,³¹ which is also seen among the East Asian populations.²⁴

Conclusion

In the 1998 Dohlman Lecture, Spaeth described glaucoma as a process in which there are stages of deterioration, the final stage being total blindness and incapacitation due to loss of visual function. This hypothesis led to the premise that glaucoma is a process or group of processes characterised by progressive damage to ocular tissues.³² In describing the problems of defining glaucoma, Spaeth questioned whether there is any one thing that applies to everybody with glaucoma and noted that not all patients pass through every stage of glaucoma a sudden rise in IOP may cause damage to many different tissues in the eye, while a slow increase in IOP may only damage the optic nerve and its connections. Therefore, rather than defining glaucoma, Spaeth set out a series of stages from objective and subjective normality through symptomatic damage to incapacitation (Table 7).

Table 7. Stages of glaucoma according toSpaeth.

From a practical point of view, this can be helpful in treating patients on the basis of what is happening to that individual, rather than on the basis of comparison to the 'normal' population.