Visual function during acute hypoglycaemia.

Hypoglycaemia symptoms are of particular interest in view of the importance of hypoglycaemia unawareness. Visual symptoms arising during acute hypoglycaemia may be the result of metabolic disturbances in the visual pathways within the central nervous system or impairment of the refractive apparatus of the eye and binocular function. This study investigated the effect of hypoglycaemia upon visual acuity, binocular interaction and contrast sensitivity. Various aspects of visual function were examined in ten normal subjects before, during and after acute insulin-induced hypoglycaemia. A simultaneous study of hypoglycaemic symptoms enabled us to relate the objective findings to the symptoms as reported by the subjects. Snellen visual acuity, fusion and stereopsis were not affected by hypoglycaemia. Five subjects noted visual disturbance. Eight developed significant impairment of contrast sensitivity, which closely matched the lowered blood glucose concentration. These observations suggest that hypoglycaemic visual symptoms are due to neuroglycopenia of central visual pathways rather than changes within the refractive apparatus or abnormality of binocular function. Although these symptoms are not a constant feature of the hypoglycaemic state, subtle impairment of visual function occurs in most cases. Contrast sensitivity testing can be useful for assessment of fine changes in visual function.