Optimum size of iridotomy in uveitis.

The failure of neodymium-doped yttrium aluminium garnet (Nd:YAG) peripheral iridotomy (PI) to prevent and relieve primary acute angle-closure glaucoma (AACG) has been well documented in the literature and has been attributed to an inadequate size of PI. In cases of uveitis and iris bombe-associated AACG the failure rate is significantly greater, in the region of 40–61%. We present a case of a recurrence of AACG in a uveitic patient despite having a patent PI. We believe a much larger PI is required to prevent recurrent episodes of AACG in a uveitic eye. Therefore, we constructed and applied a mathematical model to determine the optimal size of iridotomy and to help understand and modify treatment options. A 22-year-old female presented to the eye casualty with a 1 day history of a severely painful left eye, headache, nausea and vomiting. Her vision was counting fingers in the left eye and 6/6 in the right (Figs 1a,2a,2b). She attended 3 weeks go prior to this presentation with a similar episode, treated with Nd:YAG PI. She has a history of left chronic anterior uveitis resulting in raised intraocular pressure (IOP), which remained stable following insertion of Ahmed valve and cataract surgery, 5 months prior to her presentation. Medical therapy was initiated for AACG and a further Nd:YAG laser PI was performed reducing the IOP from 58 to 28 mmHg. The PI reduced the degree of iris bombe but did not resolve the occlusion of the drainage angle (Figs 1b,2c). Therefore, the next day she underwent a left surgical iridectomy (Figs 1c,2d). Twelve months on, she has remained stable with an IOP of 12 mmHg on no antiglaucoma medication with a visual acuity (VA) of 6/9. A mathematical model was constructed to determine the optimal size of PI required in patients with uveitis related iris bombe and angle closure (Fig. 3a). To mimic the posterior synechiae (PS), the inner edge of the iris was assumed to be adhered to the lens, preventing the flow of aqueous between the posterior and anterior chambers. As fluid accumulates in the posterior chamber, this drives a pressure difference (ΔP) across the iris and causes it to deform. The PI formed in the iris is modelled as a small cylindrical aperture of the radius r. For the system to be in equilibrium, the liquid flow through the PI must be matched exactly by the production flow Q of aqueous. Assuming the flux of liquid through the PI can be approximated by Poiseuille’s law, it emerges that the transiris pressure difference can be written as ΔP = 8ηhQ/(π r), where η is the viscosity of aqueous. This formula was used to determine the optimum radius r of the PI. Using the model parameters (Table 1), three typical examples of the iris shape for differing ΔP are shown in Figure 3b; as ΔP increases, the iris bulges