Static ocular counterroll reflex in skew deviation

Abstract

Objective: The static ocular counterroll (OCR) reflex generates partially compensatory torsional eye movements during head roll. It is mediated by the utricle in the inner ear. Skew deviation is a vertical strabismus thought to be caused by imbalance in the utriculo-ocular pathway. We hypothesized that if skew deviation is indeed caused by damage to this reflex pathway, patients with skew deviation would show abnormal OCR.

Methods: Eighteen patients with skew deviation caused by brainstem or cerebellar lesions and 18 normal participants viewed a target at 1 m. Ocular responses to static passive head roll-tilts of approximately 20° were recorded using search coils. Static OCR gain was calculated as the change in torsional eye position divided by the change in head position during sustained head roll. Perception of the subjective visual vertical (SVV) was also measured.

Results: Group mean OCR gain was reduced by 45% in patients. At an individual level, OCR gains were asymmetric between eyes and between torsional directions in 90% of patients. In addition, the hypotropic eye incyclotorting gain was lower than the hypertropic eye excyclotorting gain during head roll toward the hypotropic eye in 94% of patients. No consistent pattern of gain asymmetry was found during head roll toward the hypertropic eye. The SVV was tilted toward the hypotropic eye.

Conclusion: Static OCR gain is significantly reduced in skew deviation. Interocular and directional gain asymmetries are also prevalent. The asymmetries provide further evidence that disruption of the utriculo-ocular pathway is a mechanism for skew deviation.

Figure 1. Representative eye movement tracings during head roll

Representative tracings of clockwise head roll (i.e., toward the right shoulder) from 0° to approximately 20°, and the corresponding partially compensatory torsional eye movements (ocular counterroll) of the right eye in a normal participant (A) and of the hypertropic eye in a representative patient (B). Positive y-axis values = clockwise; negative y-axis values = counterclockwise. The position measurements were made before and a minimum of 6 s after the head roll when the torsional values were stable as shown by the shaded region.

Figure 2. Mean ocular counterroll gains for controls (n = 18) and patients (n = 18) by eye during incyclotorsion and excyclotorsion

Error bars represent the SEM. Hyper and hypo eyes = hypertropic and hypotropic eyes in patients (right and left eyes in normal participants). Incyclo = incyclotorsion; excyclo = excyclotorsion.

Figure 3. Normalized ocular counterroll gain differences of individual patient

Normalized gain differences of each individual patient were plotted against the mean and 95% confidence interval (CI) for 18 controls to show the interocular (A, B) and directional (C, D) gain asymmetries in patients. (A) Hypertropic eye excyclotorting gain vs hypotropic eye incyclotorting gain during head roll toward the hypotropic eye; positive values indicate that the former gain was higher than the latter, while negative values indicate the latter gain was higher than the former. (B) Hypertropic eye incyclotorting gain vs hypotropic eye excyclotorting gain during head roll toward the hypertropic eye. (C) Incyclotorting vs excyclotorting gains of the hypertropic eye. (D) Incyclotorting vs excyclotorting gains of the hypotropic eye. Hyper and hypo eyes = hypertropic and hypotropic eyes in patients (right and left eyes in normal participants). Incyclo = incyclotorsion; excyclo = excyclotorsion.