Dandona L, Nanda A
Public Health Ophthalmology Service, L.V. Prasad Eye Institute, Hyderabad, India.
Indian J Ophthalmol. 1998 Jun;46(2):93-6.
Blindness is usually defined by visual acuity criteria. Patients with markedly constricted visual fields are visually impaired even if they have good visual acuity. To our knowledge, no standardised criteria exist to determine the extent of constriction for fields done with the currently used automated static perimetry. The purpose of this study was to suggest a simple method to do so which would help in determining blindness due to field constriction. We reviewed a number of constricted visual fields obtained with Humphrey automated static perimetry. The central 30 degrees field was divided into six concentric zones. By trial and error, we devised criteria for defining visual field constriction based on absolute loss of sensitivity (< or = 0 dB) and relative loss of sensitivity (< or = 5 dB). We suggest that if a zone has at least 75% test points < or = 0 dB and no point > 10 dB, it be considered to have absolute loss of sensitivity for the purpose of defining visual field blindness. Two exceptions to this are also suggested to prevent this criterion from becoming too rigid. Examples are shown to demonstrate application of these criteria in defining blindness due to visual field constriction to < 10 degrees as suggested by the World Health Organization. Standardised determination of visual field constriction with automated perimetry could be useful in more accurate estimation of blindness in surveys, as well as in assessing eligibility for being classified as blind for legal benefits.
