Can the ISNT Rule Be Applied To Patients of African Ancestry

Abstract

Abstract: : Purpose: The ISNT rule (normal disc rim thickness: Inferior > Superior> Nasal> Temporal) is widely used in evaluation of the optic nerve head. The purpose of this study is to examine the validity of the ISNT rule in black and white subjects. Methods: All patients underwent a complete eye examination including automated achromatic perimetry (AAP), simultaneous stereo disc photos, HRT– II and OCT–3 optic nerve imaging. Normal subjects had no evidence of glaucoma or ocular hypertension and normal visual fields. Open angle glaucoma patients had glaucomatous optic neuropathy and an associated visual field defect. One eye was randomly enrolled. The ISNT rule was assessed by masked evaluation of stereo photos, HRT– II rim area in four sectors, and OCT–3 rim length measured at the 3, 6, 9 and 12 o’clock positions. Results: Forty–seven normal subjects (24 black and 23 white) and 48 open angle glaucoma patients (18 black and 30 white) were enrolled. Clinical evaluation of disc photos revealed that the ISNT rule was intact in 38/47 (80.85%) normal eyes with no significant difference between blacks and whites (P=0.46, Fisher’s Exact Test), compared to 2/18 (11.1%) black and 12/30 (40%) white glaucoma patients (P<0.0001 and P=0.0006 for black and white patients respectively). Automated assessment of the ISNT rule in normals with commercially available HRT– II and OCT–3 software was inconsistent with the clinical assessment (intact ISNT rule: HRT–II, 25.53%; OCT–3, 15.55%). Conclusions: The ISNT rule is clinically applicable to normal subjects of African ancestry and is disobeyed in glaucoma patients. Automated optic nerve topography using HRT–II and OCT–3 was not consistent with the clinical assessment. New software needs to be developed if the ISNT rule is to be applied to imaging technologies.

Keywords: optic disc • imaging/image analysis: clinical • clinical (human) or epidemiologic studies: risk factor assessment