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• Relevance of angle closure in Asians
  Published on: 22 March 2016

• Published on: 22 March 2016

  Relevance of angle closure in Asians

  • , Ophthalmologist
  • Other Contributors:
    • R Thomas, J Muliyil, R George

  Editor,
  In their article Foster et al have addressed an issue of relevance to developing countries that have a high prevalence of angle closure. The likelihood ratio of a positive test using results that we have reported (sensitivity 61.9% and specificity 89.3%) is approximately 6 which is similar to the LR for the authors' 15% cutoff. Invocation of pupillary block to explain the discrepancy seems r...

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  Editor,
  In their article Foster et al have addressed an issue of relevance to developing countries that have a high prevalence of angle closure. The likelihood ratio of a positive test using results that we have reported (sensitivity 61.9% and specificity 89.3%) is approximately 6 which is similar to the LR for the authors' 15% cutoff. Invocation of pupillary block to explain the discrepancy seems reasonable. Twenty-seven percent of our cases do not open following a laser PI2. Technically these patients had plateau iris that could be missed by the limbal chamber depth (LCD). If we were to exclude such cases our LCD sensitivity would be more similar to the authors'.

  While our results are similar, we would apply them differently. The authors consider LCD to have two major applications: opportunistic screening in ill-equipped clinics and population based screening. In both situations they emphasize the need for a high sensitivity, accepting a "certain number" of false positives.

  In a developing country, screening in an ill-equipped clinic is quite similar to population based screening. Persons who are test positive will have to travel long distances to seek expert help at centers that are "better." While the goal of screening is identification of pathology (or eyes at risk), false positives can be a major logistical problem. Six percent of Mongolian eyes were occludable on gonioscopy but only 5-10% of these might actually go on to closure. The positive predictive value of LCD for the detection of occludable angles is reasonably high, but referral based on LCD alone (which includes false positives) would account for 20% of the population. As denial of treatment, in this case to the majority and that too after travelling long distances, is undesirable, one option might be to treat all referred occludable angles. In that case, based on LCD alone, one in five Mongolians would need to travel to obtain a laser iridotomy that is not necessarily innocuous and one that is probably not needed in the first place. The same logic applies to population based screening but involves even more false positives.

  In both situations we would argue for a test with a high specificity and settle for reasonable sensitivity. The sensitivity and specificity of LCD, especially for established ACG is indeed high, but is the specificity high enough? As table 4 shows the positive predictive value is actually lower for angle closure and angle closure glaucoma as compared to occludable angles. This of course is a function of prevalence, and further emphasizes the need for a high specificity. More so in the case of occludable angles than established ACG. On the other hand the negative predictive value of LCD is good for most grades (especially grade I). Hence, at least in a busy clinic situation, a negative LCD can be used to rule out occludable angles and established angle closure.

  A positive LCD on its own doesn't mean very much. However, LCD (LCD greater than or equal to 5%) can be used in conjunction with raised IOP. This would increase the specificity to about 99% (Specificity for both tests positive = [1-(1-specificity of the first test)(1- specificity of the second test)]. The sensitivity would decrease as the product of the two test sensitivities but will mainly affect detection of occludable angles, not established angle closure glaucoma.

  If an appropriate test is not available, perhaps we should not screen at all. Certainly with the long-term goal of improving the quality of ophthalmology, teaching gonioscopy to ophthalmologists in developing countries might be a better option. While gonioscopy itself cannot predict which eyes will go into closure, it is the current gold standard; ophthalmologists in developing countries could be taught this technique with as much ease (or difficulty) as the registrars who used it for the study.

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  Conflict of Interest:
  None declared.

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