Head Position-Dependent Changes in Ocular Torsion in Skew Deviation

Abstract

Purpose:: Skew deviation is a vertical strabismus that is often associated with abnormal ocular torsion and head tilt. It has been attributed to imbalance of vestibulo-ocular reflex (VOR) projections from the utricles in the inner ears to ocular motoneurons, but direct evidence is lacking. The utricles lie roughly in the horizontal plane, and they normally detect static positions (tilts) of the head. We postulate that if skew deviation is caused by imbalance of the utriculo-ocular reflex, the abnormal torsion might be head position-dependent. The purpose of this study is to investigate whether ocular torsion differs in upright vs supine position in skew deviation, and to compare the findings in trochlear nerve palsy

Methods:: Seven patients with skew deviation and five patients with unilateral trochlear nerve palsy were recruited. While sitting upright, double Maddox rod lenses were placed before each eye while the patient fixated on a light. They were instructed to rotate the lenses until the lines became horizontal and parallel. The experiments were repeated in supine position. A torsion change index (TCI) was calculated as (T_up - T_supine)/ T_up where T_up is torsion in upright and T_supine is torsion in supine position. TCI was calculated for both the hyper- and hypotropic eyes in patients with skew deviation or trochlear nerve palsy, and compared using ANOVAs.

Results:: In skew deviation, the ocular torsion decreased substantially or disappeared completely in supine position, whereas in trochlear nerve palsy, the torsion did not change or increased slightly from upright to supine position. The mean TCI was 0.86 for the hypotropic and 0.92 for the hypertropic eye in skew deviation, whereas the mean TCI was 0.00 for the hypotropic and -1.45 for the hypertropic eye in trochlear nerve palsy (p=0.000).

Conclusions:: The head position-dependent change in ocular torsion provides evidence that skew deviation is caused by imbalance of the utriculo-ocular reflex. The finding that trochlear nerve palsy does not exhibit such change provides a "fourth step" which could be used clinically, in addition to the three-step test: ocular torsion that differs in upright vs supine position indicates skew deviation, whereas torsion that does not change significantly between upright and supine indicates trochlear nerve palsy.

Keywords: strabismus: diagnosis and detection • eye movements • neuro-ophthalmology: diagnosis