|Spongy iris may cause angle closure glaucoma|
|Thursday, 01 July 2010|
Aptel F, Denis P. Optical coherence tomography quantitative analysis of iris volume changes after pharmacologic mydriasis. Ophthalmology; January 2010: 3-10.
In a nutshell
The authors from Lyon, France, describe a method using anterior segment OCT for estimating iris volume and to quantify changes induced by pharmacologic mydriasis in narrow angle eyes predisposed to angle closure. They studied 30 persons who had acute angle closure glaucoma (ACG) in one eye, and measured iris volume changes in fellow eyes (all had a prophylactic laser peripheral iridotomy) and compared them with normal open angle eyes. Thirty minutes after instillation of 1% tropicamide, mean iris volume increased significantly in the fellow eyes from 44.94 +/- 2.1mm³ to 49.92+/- 2.9mm³; whereas it decreased in open angle eyes from 44.29+/- 3.9mm³ to 37.88 +/- 2.2mm³. Similar changes were observed after instillation of phenylephrine 10%. A transfer of extracellular fluid from the iris stroma to the anterior chamber was advanced to explain this anatomic change. In narrow angle eyes iris volume increases linearly with increasing pupil diameter; for each 1mm dilation, the volume increases by 13.4mm³.
Primary angle closure is a leading cause of blindness worldwide with a higher prevalence among Asians, especially Chinese. Relative pupil block is the main known causative mechanism. It tends to develop in eyes that are anatomically smaller and are predisposed to ACG when the proximity between the lens and the posterior face of the iris generates an increase in aqueous flow resistance, forcing the iris to bow anteriorly. However, only 1 in 10 develop acute angle closure. New anterior chamber assessment techniques allows observation of iris shortening, increased thickness and convexity; a patent laser iridotomy eliminates pupil block. All the patients in this study were white; this feature may differ across ethnicities. The change at first may seem surprising as one would assume that the iris volume decreases after pupil dilation because of tissue retraction. Eyes which had suffered an acute attack were not studied because of consecutive iris atrophy and loss of sphincter pupillae, which would have distorted the analysis.
Joseph A Coleiro