Stereopsis disorders in patients with congenital nystagmus

To investigate the stereopsis functions in the cases of congenital nystagmus (CN) and determine the relations between nystagmus, visual acuity (VA) and stereoacuity (SA).

The random-dot stereograms was used to examine the patients' SA, crossed and uncrossed disparity. The oscillation types of the eyeballs were determined under the naked eyes cooperating with the electronystagmorgraphy (ENG) examination. VA was measured at 5 m and 30 cm with distance and near International Standard Charts. The patient was permitted to see the target with his compensatory head posture.

Of the 57 patients tested, only 8 cases have normal stereopsis including normal SA, crossed and uncrossed disparity, which are all the jerk nystagmus. Forty-one have abnormal SA including 19 stereoblind, 18 abnormal stereopsis and 4 abnormal SA only. Serious damages of stereopsis exist in most of the pendular and more than half of the jerk nystagmus. The relations between stereopsis and VA are that (1) the better the VA is, the less the stereopsis is damaged, and the SA improved as VA is improved, i.e. SA is directly proportional to VA, but is nonlinear. It can be expressed as an S-shaped function; (2) the stereopsis damages may appear in normal VA in distance (6 cases) or normal VA at 30 cm, which we can not explain; (3) stereopsis is more marked with near VA than with VA in distance; (4) normal SA (SA < or = 60") is not achieved by any subject at monocular or binocular vision less than 0.4, stereoblind appears as soon as VA is under 0.1.

VA and/or SA can be damaged by the abnormality of CN. Both VA and SA can be apparently affected by the oscillation types although VA is demonstrated to affect SA.