The first time I came across a patient with latent nystagmus was during my clinical training year. An ophthalmologist came to talk to me about a patient I had refracted earlier that day. He told me the patient’s visual acuity as measured by him was much better than that measured by me.

He asked me if I’d used an occluder when I’d measured the monocular visual acuity to which I responded yes, as I did with all the patients I refracted.

He told me that the patient had latent nystagmus which was only apparent when one eye was occluded. The latent nystagmoid eye movement reduced the visual acuity below the actual level. I hadn’t noticed the latent nystagmus during the refraction. He also said that when measuring monocular visual acuity for a patient with latent nystagmus that I should use a high powered trial lens in front of the eye I wasn’t measuring.

It was the lack of light entering the occluded eye that caused the nystagmoid eye movements. He left but not before saying ‘Don’t they teach you anything in optometry school?’. Latent nystagmus is a benign, early onset conjugate horizontal jerk-nystagmus often associated with congenital esotropia and dissociated vertical deviation.

It only becomes apparent during occlusion of one eye. It is not apparent when the person has both eyes open and neither is occluded. The direction of the movement is toward the non-occluded eye, and therefore the direction of movement changes depending on which eye is occluded.

A harsh lesson but one I’ve never forgotten. I always look for nystagmus and if it’s present I don’t occlude but blur.