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H. A. Quigley, D. M. Silver, R. J. Plyler, D. S. Friedman; The Iris Loses Half Its Volume During Pupil Dilation: A New Risk Factor for Angle Closure Glaucoma. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5100. doi: https://doi.org/.
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Theoretical models of the anterior segment suggest that the iris has greater area (volume) when the pupil is small than after pupil dilation. If the iris loses volume at larger pupil size, and if eyes differ in the degree to which the iris volume changes, this parameter could be a measurable, predictive risk factor for angle closure glaucoma (ACG).
Thirty-nine patients at the Wilmer Glaucoma service (20 open angle suspects or patients (OAG), 19 angle closure suspects or patients (ACG)) underwent Visante anterior segment OCT imaging under 3 conditions (maximum pupil constriction to light, maximum physiological dilation in dark, and after pharmacological dilation), as well as gonioscopy, IOL Master axial length measurement, and slit lamp examination. The total iris cross-sectional area was measured nasally and temporally with custom software, 3 times in each of 4 meridians (intersession reproducibility 4%). The principal outcome variable was change in iris area per mm pupil diameter increase. Patients with past cataract surgery were excluded; eyes with laser iridotomy or trabeculectomy were included.
Overall iris area decreased 10.2 + 2.5% with each mm increase in pupil diameter, linear regression showing a significant relationship (pupil diameter (mm) = 8.79 iris area (mm2) + 0.28, R2 = 0.83, p=0.0001). ACG subjects had similar iris area to OAG subjects in bright light (3.11 + 0.50 vs 3.22 + 0.40 sq mm, n.s., adjusted for baseline pupil diameter). More severely affected ACG eyes tended to have less iris area change per mm pupil dilation.
Iris volume loss could occur from either decrease in blood volume or loss of extracellular water. Estimates of the intralumenal, iris vascular space in human eyes show that it is insufficient to account for the magnitude of volume change observed. The iris stroma has a variable fluid content that freely exchanges with the aqueous humor and could decrease rapidly (like a squeezed sponge) on pupil dilation. The speed and extent of this exchange probably determines the change in iris stromal area (volume) when the pupil enlarges, avoiding angle closure in most eyes. Eyes with smaller dimensions combined with poor iris fluid exchange would be expected to retain more iris volume on physiological pupil dilation and be at greater risk to suffer angle closure.The iris loses 50% of its volume with 5 mm of pupil dilation. Further studies of this newly recognized iris volume change as a predictor of ACG may be useful.
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