Slit lamp-simulated oblique flashlight test in the detection of narrow angles in Chinese eyes: the Liwan eye study

Abstract

Purpose: To assess a modified slit lamp-simulated oblique flashlight test in the identification of persons with suspected angle closure.

Methods: Standard oblique flashlight test (SOFT) and a modified slit lamp-simulated flashlight test (SSFT) were performed on participants identified as primary angle closure suspects and controls from a population-based study. SOFT graded the iris shadow on the nasal iris as shallow, medium, and deep. SSFT measured the length of iris shadow and corneal diameter by a slit lamp graticule eyepiece after standardizing the illumination parallel to the iris plane.

Results: SOFT yielded sensitivity and specificity of 76.3% and 80.7% for the detection of eyes with two or more quadrants of pigmented trabecular meshwork not observed on static gonioscopy. Proportions of subjects in the "shallow" category increased from 9.6% in Shaffer angle width grade 4 to 82.3% in grade 0 and were 72.1% in those with steep, 56.7% in those with plateau, and 13.9% in those with regular iris profiles. SSFT yielded 84.8% sensitivity and 76.7% specificity using a cutoff of 0.18 for the ratio between iris shadow length and corneal diameter. This ratio was monotonically associated with the axial anterior chamber depth, and it was significantly greater in eyes with Shaffer angle width grade lower than 2 and significantly less in eyes with Shaffer angle width grades 3 and 4. Reproducibility of SSFT was high (paired t-test; P > 0.05).

Conclusions: More accurate measurement by controlling illumination and following a more precise method to measure shadow formation may improve the performance characteristics of this test. However, this method does not appear appropriate for community-based screening because of its relatively low specificity.