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Editors Selection IGR 16-4

Basic Research: Outflow facility in PDS

Comment by Robert Ritch & Syril Dorairaj on:

26334 Aqueous humor dynamics in pigment dispersion syndrome, Toris CB; Haecker NR; Teasley LA et al., Archives of Ophthalmology, 2010; 128: 1115-1118

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The main objective of this prospective, comparative case control study by Toris et al. (1269) was to investigate aqueous humor dynamics in PDS patients with or without elevated IOP and compare the results with those of patients with ocular hypertension (OHT) and normal controls. In this well-designed study with mathematically correct methodology, the data is well presented and interpreted. It is a fairly straightforward study with reasonable conclusions regarding the pathogenesis of elevated IOP in PDS. The authors conclude that elevated IOP is caused by reduced outflow facility and that the major factor accounting for elevated IOP in patients with PDS is increased resistance to flow across the trabecular meshwork (decreased outflow facility). This differs from OHT without PDS, in which reduced uveoscleral outflow has been documented.

Deriving anterior chamber volume, the authors concluded that the PDS-OHT group had a significantly larger mean volume than the OHT group. They attributed this difference to anterior segment structural abnormalities in PDS, such as a relatively larger iris, greater iris insertion-trabecular meshwork distance and a more posterior iris insertion. Anterior chamber volume changes could have been more accurately and reliably determined with UBM/ASOCT/Pentacam rather than an approximate derivation from ACD. The authors could also have discussed the role of the iris configuration/concavity and the mechanism of reverse pupillary block in contributing to increased anterior chamber volume in the PDS-OHT group.

Though there was no significant difference among the groups in mean age, which ranged from 40.9 to 44.8 years, the PDS-OHT group with 40.9 years had the youngest mean age. PDS occurs more frequently in young myopes, and a larger sample size might have provided a significant difference between the groups. It would have been interesting if the authors had mentioned (Table 1) other potentially bias-inducing confounding parameters, such as sex, refractive error, axial length. As in another study by the same group on exfoliation syndrome, these entities are characterized by physical blockage of the trabecular meshwork by extraneous material and the major barrier to outflow facility lies within this structure.

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