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Who needs an iridotomy?
  1. DAVID S FRIEDMAN
  1. Wilmer Eye Institute, Johns Hopkins University, Baltimore, MD 21287, USA

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    Angle closure glaucoma (ACG) is one of the leading causes of global blindness. Recent population based research on Chinese subjects in Singapore and a southern Indian population found high rates of ACG among those populations.12 Close to 2% of individuals over the age of 40 were found to have ACG in these studies. Given that almost half of the world's population lives in China and India, millions of individuals are at risk of ACG and may benefit from better screening strategies to identify them before glaucoma develops.13

    However, the decision to perform a laser peripheral iridotomy (LPI) on a patient with a narrow angle is often highly subjective. What is an “occludable” angle? If one can see trabecular meshwork is the patient “safe.” If one cannot, is the patient at significant risk? What proportion of the angle needs to be visible? What should be done in the developing world setting where an ACG suspect is unlikely to receive a second eye examination in the near future?

    A recent World Health Organization (WHO) sponsored meeting on glaucoma blindness worldwide proposed that a new nomenclature be employed for describing individuals with ACG. Those with 90 degrees of angle in which the trabecular meshwork is visible who have glaucomatous optic nerve damage have “primary ACG,” while those with similarly closed angles and elevated eye pressure or peripheral anterior synechiae (PAS) have “primary angle closure.” The termglaucoma is only used for those with glaucomatous optic neuropathy and visual field loss. A third, important category of patients is those with narrow angles as described above with no evidence of glaucoma or damage to the angle (that is, no elevated IOP or peripheral anterior synechiae). These individuals are simply described as having “narrow angles,” not having angle closure glaucoma. How to manage this large group of asymptomatic individuals is not well documented in the literature and poses a major public health problem if screening programmes are to be undertaken in the developing world.

    Another source of confusion when discussing ACG is the co-mingling of acute and chronic angle closure. Acute ACG has classic signs and symptoms and is typically relieved by laser iridotomy. Several studies have found from 15% to 45% of all ACG is acute.1-5 The majority of cases are therefore chronic ACG, which may behave quite differently from acute ACG. It is not clear if laser iridotomy is effective at stopping the progression of chronic ACG once it starts.

    Prophylactic laser iridotomy has been proved to be beneficial for individuals who have suffered a monocular attack ofacute ACG. Lowe documented in the 1950s that when the contralateral eye of individuals suffering acute ACG was treated either with nothing or with pilocarpine once or twice a day, 50% developed acute attacks over a period of up to 25 years.6 Conversely, only one of 54 individuals treated with prophylactic surgical iridectomy during this same time developed an acute attack. Edwards reported a high likelihood of developing an acute attack in the untreated fellow eye in the first month.7 Snow also documented high rates of attacks in untreated contralateral eyes.8 Clearly, contralateral eyes of patients with acute ACG are at significant risk for an acute attack and iridectomy virtually eliminates this risk.

    The study by Lowe described above reported the effectiveness of surgical iridectomy in preventing acute attacks in second eyes. Flecket al randomised individuals to surgical or laser iridectomy and obtained similar results.9 A recent retrospective study of patients undergoing laser peripheral iridotomy in the contralateral eye in Singapore also found excellent results with this treatment.10

    The literature is less clear about the natural history of untreated individuals who have gonioscopically narrow angles. Wilensky and colleagues enrolled 129 mostly European derived subjects with “occludable” angles and central anterior chamber depth (measured by optical pachymetry) less than 2.0 mm in a prospective study.11 Patients were recruited over a 5 year period at five separate centres. Eight patients (6.2%) developed acute angle closure glaucoma (AACG) and 17 (13.2%) developed either appositional closure or peripheral anterior synechiae in at least 0.5 clock hours of the superior quadrant (median follow up 3 years). Dark room prone provocative testing did not consistently predict who would develop angle closure during follow up. This study in European derived individuals clearly demonstrates that a combined screening strategy using anterior chamber depth and gonioscopy had a low positive predictive value for the development of acute attacks.

    Alsbirk examined 75 Greenland Eskimos with shallow central anterior chamber depth and a van Herick score of two or less 10 years after a baseline examination.12 He had performed gonioscopy on 69 of these individuals at the baseline. Of the 20 individuals felt to have “occludable” angles at baseline, seven (35%) developed ACG, as opposed to four of 49 (8%) felt to be non-occludable. However, of these 11 cases, two were acute attacks and only one other had peripheral anterior synechiae associated with elevated eye pressure. The remainder had either intermittent symptoms or synechiae on gonioscopy. Furthermore, this population has a very high risk of ACG, with 1% of individuals over the age of 60 blind from this disease.

    These two articles sum up the English literature on how well a physician seeing a gonioscopically narrow angle predicts the later development of ACG in untreated eyes. Only European derived individuals being followed by glaucoma specialists and Greenland Eskimos being examined by a single highly trained observer were studied. No data are published in the English literature on Asian populations followed longitudinally. How should clinicians and public health officials behave when identifying individuals with narrow angles who have limited access to healthcare services given the paucity of data?

    Theoretically, laser peripheral iridotomy should prevent the onset of chronic ACG as well as acute ACG, although once chronic ACG is established, limited evidence suggests that laser peripheral iridotomy may be insufficient to control intraocular pressure.1314Chronic ACG may be associated with higher rates of blindness than is typically found with open angle glaucoma.1 This has led some to advocate more aggressive screening so that early laser peripheral iridotomy can be performed in individuals with “high risk” of either acute ACG or chronic ACG.

    While laser peripheral iridotomy appears relatively harmless, one cannot be certain of the long term safety of this procedure. laser peripheral iridotomy disrupts the natural flow of aqueous in the eye and results in significant increase in lens-iris contact.15 Theoretically, this may predispose to a more rapid development of cataract since less aqueous is in contact with the lens epithelium. Several studies have attempted to look at this issue, but follow up has been short, no lens grading system was used, and no acceptable control groups were studied.1617 Focal lenticular opacities seen after argon laser peripheral iridotomy are said not to progress, but once again, follow up has been short in published reports. The plausibility of laser peripheral iridotomy being able to cause cataract is supported by the strong evidence in the literature that trabeculectomy can do just this in glaucoma patients.18

    Laser energy delivered at the time of treatment as well as the altered fluidics of the eye may have other ramifications including the hastening of corneal endothelial cell dysfunction. Once again, researchers looking at specular microscopy have only studied small numbers of individuals at relatively short follow up.1619 Argon laser peripheral iridotomy has been consistently reported to cause localised transient corneal oedema,1720 with rare case reports of corneal decompensation.21 One study documented a higher rate of endothelial cell loss after argon laser peripheral iridotomy than after YAG laser peripheral iridotomy.16 Another potential complication of laser peripheral iridotomy is the development of posterior synechiae following laser iridotomy.17 Posterior synechiae can both limit vision in dim environments and make later cataract surgery more challenging.

    While these complications seem minor when compared with the risk of an acute attack, they are in fact of significant concern for individuals contemplating glaucoma control programmes in developing countries with high prevalence of narrow angles. Visually significant complications that affect as few as 5% of those treated can be devastating to such programmes. If laser peripheral iridotomy hastens cataract significantly, one could cause more blindness with widespread screening and iridotomy treatment than one prevents in developing countries where cataract services are not universally available.

    Fifteen per cent of Mongolian women over 50 years of age have occludable angles as defined by an observer being unable to see pigmented trabecular meshwork for more than 90 degrees on gonioscopy with a Goldmann lens. A similar proportion of Chinese residents of Singapore were found to have occludable angles in a recently completed population based survey. Assuming similar rates among older women in China, then about 25 million Chinese women and 12 million men will be eligible for prophylactic laser peripheral iridotomy in the year 2010. The numbers may be even higher in India. A complication that occurs in only 5% of those treated will adversely impact close to two million individuals assuming that all are identified and treated.

    Before embarking on large scale screening campaigns and investing resources in expanding prophylactic treatment of these individuals, data need to be collected on the natural history of treated and untreated narrow angles. Research should be conducted to identify factors at baseline that predict who, with narrow angles, is at greatest risk of developing either acute or chronic angle closure glaucoma and, ultimately, significant loss of visual field and central vision. Once such factors are identified, more appropriate screening and treatment recommendations can be made so that resources and treatments are appropriately allocated.

    References

    Video Reports (www.bjophthalmol.com)

    • Capsule staining and mature cataracts: a comparison of indocyanine green and trypan blue dyes. D F Chang

    • Pearls for implanting the Staar toric IOL. D F Chang

    • An intraocular steroid delivery system for cataract surgery. D F Chang

    • Evaluation of leucocyte dynamics in mouse retinal circulation with scanning laser ophthalmoscopy. Heping Xu, A Manivannan, Garry Daniels, Janet Liversidge, Peter F Sharp, John V Forrester, Isabel J Crane

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