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A Case of Different Null Zones for Distance and Near Fixation

Correspondence: Requests for reprints should be addressed to: Stephen P. Kraft, M.D., The Hospital for Sick Children, 555 University Ave., Toronto, Ontario, Canada M5G 1X8. e-mail: stephen.kraft{at}sickkids.ca

A compensatory head posture (CHP) is a common clinical feature of congenital motor nystagmus. It is usually caused by an eccentric null zone, which is the eye position where the nystagmus intensity (amplitude x frequency) is least. A patient adopts the posture to maximize the binocular visual acuity. Occasionally, a patient may have more than one null zone, leading to the adoption of different CHPs at various times.

A 10-year-old boy with congenital motor nystagmus and orthophoria, and with good corrected vision in both eyes, presented with a face turn that had been noted since infancy. For distance fixation, he consistently adopted a left face turn due to a null zone in right gaze. For near fixation, he adopted a right face turn due to a null zone in left gaze. Eye movement recordings confirmed the different locations of the null zones for distance and near fixation.

After a trial of base-out prisms to stimulate convergence, which eliminated his head posture at both positions, he underwent artificial divergence surgery. He has had a satisfactory result for 18 months after surgery with a satisfactory head posture and a well-controlled exophoria.

Measures to induce convergence, with prisms and then surgery, can be an effective strategy to correct the head postures caused by two different null zones in a patient with congenital motor nystagmus.

Key words: congenital nystagmus, eye movement recordings, head posture, strabismus surgery