Iris cross-sectional area decreases with pupil dilation and its dynamic behavior is a risk factor in angle closure

J Glaucoma. 2009 Mar;18(3):173-9. doi: 10.1097/IJG.0b013e31818624ce.

Abstract

Purpose: To estimate the change in iris cross-sectional (CS) area with pupil dilation using anterior segment optical coherence tomography comparing eyes with angle closure (AC) to open angle glaucoma (OAG).

Methods: Sixty-five patients from the Wilmer Glaucoma service, 36 with definite or suspected OAG and 29 with definite or suspected AC, underwent anterior segment optical coherence tomography imaging under 3 conditions (pupil constriction to light, physiologic dilation in the dark, and after pharmacologic dilation). The nasal and temporal iris CS areas were measured with custom software, 3 times in each of 4 meridians. The principal outcome variables were iris CS area and change in iris CS area/mm pupil diameter change. The relation of these parameters to potential variables that would influence iris area was estimated by multivariate regression.

Results: CS area was smaller in eyes with larger pupil diameter, those that had undergone trabeculectomy, and those of European-derived persons (P<0.05 for all in a univariate analysis). In a multivariate model with CS area as the dependent variable, larger pupil diameter (with a 0.19 mm decrease in CS area for each 1 mm of pupil enlargement, P=0.0002), and trabeculectomy remained significant factors. In a second multivariate model, AC irides had less change in CS area/mm pupil enlargement than OAG or OAG suspects (P=0.01). Change in iris CS area was essentially complete in 5 seconds (n=10 eyes).

Conclusions: The iris loses nearly half its volume from a pupil diameter of 3 to 7 mm, probably by eliminating extracellular fluid. Smaller iris CS area change with physiologic pupil dilation is a potential risk factor for AC. Dynamic iris CS area change deserves testing as a prospective indicator of AC.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Female
  • Glaucoma, Angle-Closure / physiopathology*
  • Glaucoma, Open-Angle / physiopathology
  • Gonioscopy
  • Humans
  • Intraocular Pressure
  • Iris / physiopathology*
  • Light
  • Male
  • Middle Aged
  • Mydriatics / administration & dosage
  • Ocular Hypertension / physiopathology
  • Pupil / drug effects
  • Pupil / physiology*
  • Pupil / radiation effects
  • Risk Factors
  • Tomography, Optical Coherence
  • Tonometry, Ocular

Substances

  • Mydriatics