Pulfrich’s phenomenon in optic nerve hypoplasia

Abstract

Optic nerve hypoplasia (ONH) is a congenital deficiency of retinal ganglion cells and their axons that form the optic nerve. The condition is associated with visual deficits ranging from no light perception in severe cases, to vision within normal range in mild cases of ONH. We report here a case of mild ONH with normal vision and with visual symptoms relating to a spontaneously occurring Pulfrich effect. A twelve-year-old girl presented with typical visual symptoms (veering to the right when walking, bumping into things) associated with the spontaneous Pulfrich effect. When asked to follow a bob suspended from a simple rigid pendulum swinging in the frontal plane, a clockwise ellipse was reported, indicative of a left-sided Pulfrich effect. This effect was neutralised by an 85% transmission ophthalmic tint placed before the right eye. The inter-ocular latency difference responsible for the illusion was calculated to be 0.88 +/- 0.55 ms. Fundus biometry showed an asymmetry in R and L optic nerve dimensions, the left being smaller than the right. Vision was normal in each eye, as were visual fields, colour vision and stereopsis. The affected eye showed normal flash and pattern VEPs and ERGs, except for a borderline reduction of the pattern ERG (PERG) to very small checks. A case of mild ONH with normal visual function in each eye resulted in a small inter-ocular delay associated with errors in visual perception consistent with a spontaneous Pulfrich effect. Wearing a pair of glasses with the right lens tinted eliminated these difficulties.