Effects of earth-fixed vs head-fixed targets on static ocular counterroll

Effect of small head tilt on ocular fundus image: Consideration of proper head positioning for ocular fundus scanning

Head tilt and resultant ocular cyclotorsion can influence the results of ophthalmologic examinations. Thus, proper head positioning during fundus scanning has been emphasized. However, there is no perfect method to control the head tilt and little is known about the effect of small head tilts. In this study, we investigated the effect of minimal head tilt on the ocular cyclotorsion which we cannot easily detect.Forty-seven participants without ophthalmologic or vestibular abnormalities were recruited as normal subjects. Their faces were positioned at the desired head tilt using a customized adjustable head tilter and facial and fundus photographs of both the left and right eyes were taken in the upright neutral position; as well as at rightward and leftward head tilts of 2°, 4°, and 6°. The actual head tilt was determined using the facial photographs by measuring the slope of a line that intersected the corneal reflexes of both eyes. Rotational changes in the fundus images were recorded and the correlation of these changes with the degree of head tilt was determined.The degree of head tilt was significantly correlated with rotational changes in the fundus images from both the right and left eyes (P < 0.001; right eye: R = 0.897, left eye: R = 0.899). The mean relative compensations for head tilt, mediated by the ocular counterrolling reflex, were 0.376 ± 0.255 in the right eye (range: -0.02 to 1.0), and 0.350 ± 0.263 in the left eye (range: -0.03 to 1.0), and exhibited a significant negative correlation with head tilt (P < 0.05). The mean relative compensation of the right eye did not differ significantly from that of the left eye (P = 0.380), but the value did vary widely among individuals and within individuals.Even very small head tilt was partially and variably compensated for, and caused significant rotation in the fundus image. We concluded that proper head positioning does not guarantee the minimal ocular cyclotorsion change of the eyes and image-adjusting technique would be a better solution for minimizing errors from ocular cyclotorsion changes.

New understanding on the contribution of the central otolithic system to eye movement and skew deviation

The otolith organs consist of the utricle and saccule. The utricle mediates the utriculo-ocular reflex by detecting horizontal head translation and static head tilt. Skew deviation is a vertical strabismus caused by imbalance of the utriculo-ocular reflex pathway and is commonly caused by lesions in the brainstem or cerebellum. It is associated with abnormal utriculo-ocular reflexes including asymmetric reduction of the translational vestibulo-ocular and ocular counterroll responses. Skew deviation is also associated with head position-dependent changes in ocular torsion and vertical strabismus. The reduction in ocular torsion and vertical strabismus when changing from an upright to supine position in skew deviation allows us to devise a new bedside 'upright-supine test' to differentiate skew deviation from fourth nerve palsy and other causes of vertical strabismus.

Ocular torsion and vertical misalignment

PURPOSE

This article considers vertical misalignment and torsion of the eyes that arise from disorders of vestibulo-ocular reflex (VOR) pathways.

RECENT FINDINGS

Infarction of the nodulus is one of the causes of skew deviation, a vertical strabismus accompanied by torsion of the eyes and tilt of the subjective visual vertical. Vertical components of childhood strabismus may arise from dysgenesis of vestibular projections in the brainstem. If vertical misalignment decreases greatly in the supine position compared to the erect poison one may conclude that skew deviation rather than a fourth nerve palsy is responsible for the strabismus. Impairment of the horizontal translational VOR in patients with skew has provided further evidence that imbalance in the otolith-ocular pathway, caused by disruption of projections from the utricle to motor neurons, is the mechanism of skew deviation. Bedside examination showing a normal horizontal head impulse test, direction-changing nystagmus in eccentric gaze, or skew deviation is sensitive and specific for stroke. Skew deviation can identify stroke when an abnormal head impulse test falsely suggests a peripheral vestiblular lesion. Reduction in gain and failure of adaptation to vision of the torsional angular VOR in head positions away from the erect signify fundamental differences in the central organization of convergence of semicircular canal and otolith signals, when compared to the horizontal and vertical angular VORs.

SUMMARY

The effects of head position and of lesions of the nodulus are assuming increasing importance in analysis and differential diagnosis of vertical misalignment and torsion of the eyes.

Understanding skew deviation and a new clinical test to differentiate it from trochlear nerve palsy

Skew deviation is a vertical strabismus caused by a supranuclear lesion in the posterior fossa. Because skew deviation may clinically mimic trochlear nerve palsy, it is sometimes difficult to differentiate the 2 conditions. In this review we compare the clinical presentations of skew deviation and trochlear nerve palsy and examine the pathophysiology that underlies skew deviation. We then describe a novel clinical test-the upright-supine test-to differentiate skew deviation from trochlear nerve palsy: a vertical deviation that decreases by > or =50% from the upright to supine position suggests skew deviation and warrants investigation for a lesion in the posterior fossa as the cause of vertical diplopia.