Screening for angle-closure glaucoma

proven difficult to replicate; the test did not give the same combination of high sensitivity and specificity for detection of angle-closure in a rural population in Taiwan.6

Axial anterior chamber depth (ACD) measurement

Measurement of the axial anterior chamber depth (ACD) can be performed in a noninvasive manner, and is relatively quick and easy to perform on a large number of subjects, compared with gonioscopy. It has therefore been proposed as a suitable screening test for angle-closure. In a study performed in Mongolia,7 axial ACD measurement using optical pachymetry gave a sensitivity of 85% and a specificity of 84% at a screening cutoff of less than 2.22 mm for detecting occludable angles. Similar results have been reported by other investigators. In a community setting in Greenland,8 optical ACD measurement resulted in a sensitivity of 86% and a specificity of 88%. In rural Taiwan, ultrasonographic ACD measurement in combination with refractive status provided a sensitivity of 84% and a specificity of 83%.

However, in the Chinese population in Singapore,9 axial ACD measurement yielded sensitivity and specificity of 75.6% and 73.7%, respectively. In this study, the definitions of disease, the age-and sex-standardized prevalence of occludable angles and angle closure were identical with those of the study conducted in Mogolian population, which gave higher sensitivity and specificity figures.7 It has been suggested that this discrepancy is attributable to the difference in relationship between axial ACD and rate of peripheral anterior synechia (PAS) in different populations. In Mongolian people, PAS were very uncommon in eyes with an ACD > 2.5 mm. Below this level, the proportion of eyes with PAS increased steeply. In contrast, there is a gradual, incremental increase in the rate of PAS across the entire range of ACD in Singaporeans, thereby, in subjects with deeper ACD, the rate of PAS exceeded that seen in Mongolians.10 This difference, together with the discrepancy in the performance of axial ACD measurement between the two studies supports the suggestions that non-pupillary block mechanisms may play a significant role in the pathogenesis of angle-closure in some groups of East Asian people.11,12

Limbal chamber depth (LCD) measurement

The limbal chamber depth (LCD) is usually assessed using the van Herick technique.13 In this method, a thin slit-beam is focused on the cornea and anterior chamber perpendicular to the temporal limbus. An optical section viewed from the nasal aspect at an angle of 60-degrees. A grading of the peripheral ACD is expressed as a proportion of corneal thickness. A grade-I angle corresponds approximately to a peripheral chamber depth (PCD) < 1/4 corneal thickness (CT), a grade II-angle to PCD of 1/4 CT, a grade III-angle to PCD of ¼ to ½ CT, and a grade IV-angle to PCD ≥ one CT.

Recently, a modification of this grading was developed14 and has been tested for mass screening.9,14 This method employed the same technique, in which the slit lamp is used to examine the peripheral ACD, and describe this as a proportion of peripheral corneal thickness. The modification differs by having seven grades (0%, 5%, 15%, 25%, 40%, 75%, and ≥ 100%) instead of the traditional four grades for the van Herick test.

In a recent population-based study of Chinese residents of Singapore,9 the modified LCD measurement performed well in the detection of occludable angles. A a screening threshold of LCD ≤ 15% (equivalent to the traditional ‘grade 1 [PCD <1/4 corneal thickness]’) gave a sensitivity and specificity of 83% and 88% respectively, with a positive predictive value of 45.3% in the 50-year old and older population, which means that approximately one in two of those who were abnormal on screening would have an occludable angle. In this study, axial ACD was also measured using ultrasound. The performance of ACD as a screening tool was not as good as that of LCD estimation. The modified LCD grading scheme showed similar results in a previous study performed in a Mongolian population,14 yielding a sensitivity of 84% and specificity of 86% for detection of occludable angles.

Slit-lamp grading of the mid-peripheral angle

More recently, a new method of assessing the anterior chamber angle was developed by Hong et al.15 This method assesses the mid-peripheral angle. Using a slit-lamp, a very narrow, vertical beam of light, is projected to create a line connecting the limbus at 6 o’clock to the pupil margin. The beam is off-set at an angle of 30 degrees from the sagittal plane. On the inferior surface of the iris, the point at which the thickness of the cornea (CT) is equal to the depth of the anterior chamber (ACD) is identified. The distance from the corneal limbus is described in terms of the corneal thickness (CT). For example, Grade 2 indicates the distance from point at which ACD = CT to the corneal limbus equals twice the corneal thickness. The greatest advantage of Hong’s grading method is that it can actually measure the chamber ‘angle’ (not ‘depth’) using the concept of arctangent. The second advantage is that the system is not affected by the corneal thickness, in contrast to Van Herick’s grading, which is directly affected by the corneal thickness. Another advantage is that Hong’s method can be used to measure the anterior chamber angles in all quadrants.

It has been found that Hong’s grading score is significantly correlated with the Van Herick limbal chamber depth grades and Scheie gonioscopic grading. Furthermore, this method showed a better, or at least equal, efficacy to differentiate between people with positive or negative dark-room prone-position test (DRPT) compared to Van Herick or gonioscopic grading methods.15

So far, the efficacy of this test for screening PAC has not been tested in a popu- lation-based study. However, the simplicity and its ability to differentiate DRPT

(+) and DRPT (-) patients implicates the usefulness of this test as a screening tool for PAC.

Postscreening management

Once angle closure has been detected, laser iridotomy should be performed, as a safe and effective prophylactic treatment to prevent progression to ACG. Laser iridotomy was shown to be effective in widening the drainage angle and reducing elevated IOP in Mongolian people.16

Prospect

The high prevalence of primary ACG in Greenland Inuit has been well documented. In this population, the proportion of blindness due to ACG was reduced from 64% in 1962 to 9% over 37 years following a systemic program of ACD measurement as well as Van Herick testing in older Inuit, followed up by gonioscopy, and if necessary, prophylactic iridectomy or laser iridostomy. These data strongly suggest that early detection of occludable angles and prophylactic laser iridotomy are probably effective methods in the prevention of ACG, at least in some populations.

However, this view may not be directly applicable to other populations, particularly in China or South-East Asia. In southern China, a non-pupil-block mechanism may account for a considerable proportion of angle-closure. Wang et al.12 stated that 55% of all angle-closure in China is caused by combined pupil-block and non-pu- pil-block mechanisms, with only 38% being attributable to pupil block alone. More recently, researchers in Guangzhou reported considerable variation in the thickness of the iris, its level and angle of insertion, and the position of the iris insertion with respect to the ciliary body as assessed by ultrasound biomicroscopy.17 Some types of plateau iris are associated with forward rotation of the ciliary body, but in Chinese eyes, some are more related to the innate structure of the iris. Another peculiarly Asian characteristic is that of a very thick iris thrown into prominent circumferential folds. Considering these situations, it becomes clear that conventional YAG laser iridotomy is unlikely to prevent progression of angle-closure. For some of these structural variations, other preventive treatments will need to be devised such as broad surgical iridectomies or laser iridoplasty.

Detection may present similar obstacles as treatment. Measurement of axial ACD by itself will not be an adequate screening test for angle-closure. UBM technology, Scheimpflug photography,and optical coherence tomography (OCT) are increasingly being used examine the living architecture of the anterior chamber. It may be possible to adapt these high-tech imaging devices to become screening tools for large numbers.

In Mongolia, YAG peripheral iridotomy was effective in preventing progression of PAC.16 Almost half of those with established PACG required no further treatment to control the intraocular pressure. On the basis of this previous work, a randomized controlled trial of screening for angle closure was started several years ago by Nolan and Mongolian colleagues.18 A total of 4725 people aged 50 years and above were randomized to an intervention or a control arm. Existing glaucoma was detected and all optic discs were photographed before the two arms continued. In the intervention arm, 685 subjects who were screened out by measurement of ACD then progressed to a full ophthalmic examination with gonioscopy. Of these, 156 were treated with laser iridotomy. The follow-up has been completed. The result of this trial will determine if ACG can be prevented and whether a nationwide screening program of the older population for angle-closure should be recommended for Mongolia.

Available evidence suggests that ACG is preventable. Early detection of primary angle-closure appears to be a realistic objective in future prevention of blindness programs in Asia. However, there are considerable variations in the pathogenesis of ACG in different populations. Hence, mechanisms causing angle-closure, potential screening tools, and effectiveness of prophylactic iridotomy will need to be determined for each population at risk.

References

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