Skew deviations distinguished from oblique muscle palsy

Nashville, TN-Knowledge of skew deviations and their management has changed dramatically in recent years. Skew deviations occur more commonly than previously suspected and result from lesions in the vertical vestibular ocular reflex pathway.

Skew deviations can be differentiated from oblique muscle palsy by attention to the direction of the cyclotorsion in the hypertropic eye, according to Sean P. Donahue, MD, PhD.

"Skew deviation and its management have changed quite a bit in the last 15 years. During my residency, a diagnosis of skew deviation was one of exclusion. At that time, it was considered a vertical misalignment that was typically comitant. Now we know that skew deviation is much more common than we previously thought," said Dr. Donahue, associate professor of ophthalmology, neurology, and pediatrics, Vanderbilt University Medical Center, Nashville.

Understanding skew deviation

To understand skew deviation best, Dr. Donahue noted, it is necessary to understand the vestibular ocular projections. Three semicircular canals (anterior, posterior, and horizontal) respond to acceleration and provide innervation that maintains the eyes on a target despite head movement. The otolith apparatus provides tonic innervation and maintains the eye position when the head is turned and tilted. The anterior canal projects to the superior rectus muscle and the interior oblique muscle; the posterior canal projects to the superior oblique and the inferior rectus muscle.

"When the head is tilted to the right, both the anterior and posterior canal projections are stimulated (the superior oblique and superior rectus muscles and the inferior oblique and inferior rectus) and inhibit the yoke projections below, which causes the ocular counter-rolling reflex. When the head is tilted to the left, the left eye rises and there is incyclotorsion and the right eye falls and there is excyclotorsion. This is the normal phenomenon that keeps the eyes straight. In skew deviation, this is abnormal," he explained.

Ocular tilt reaction is a type of skew deviation. It is characterized by vertical deviation, conjugate ocular torsion (both eyes rotated in the same direction), and a head tilt in the direction of the hypotrophic eye, which is excyclorotated. The hypertrophic eye is incyclorotated. The result is that the patient and the eyes are rotated in the same direction of tilt.

This is the key to differentiating skew deviation from oblique muscle palsy. The wrong eye is rotated in the wrong direction, Dr. Donahue emphasized.

Dr. Donahue and his associates reported five such patients in Archives of Ophthalmology (1999;117:347-352). All patients seemed to have oblique muscle palsy but all had paradoxical torsion. Dr. Donahue and his colleagues hypothesized that skew deviation occurs as the result of asymmetric damage to the anterior semicircular canal projections.

"This ramps up the oblique muscles and causes incyclotorsion of the eye and a compensatory head tilt," he said.

He suggested that skew deviations may be much more common than previously suggested. Patients with brainstem injuries are now being examined in ophthalmology clinics, whereas previously they were institutionalized.

Surgical, non-surgical therapy

Surgical and non-surgical treatments are available for skew deviation. Dr. Donahue prefers non-surgical approaches initially.

"Most of these cases are relatively comitant. If the patient is complaining of double vision, paste-on prisms can be used on spectacles. In most cases, even though there is torsion, the torsional fusional amplitudes are sufficient to handle the torsion. If surgery is necessary, superior rectus recessions are effective for correcting the vertical deviation and reduce the torsion. If the hypertrophic eye is excyclorotated, an inferior rectus recession is also effective," Dr. Donahue advised.

He also pointed out that in his experience the abnormal head position characteristic of skew deviation is not corrected by placing the eyes orthotropic and eliminating the double vision, which may be due to residual ocular torsion. Another noteworthy observation is that even though the patients remain trophic, they are not bothered as much by the double vision compared with patients with normal vision. This may be the result of the severity of their head injury, which prevents them from being bothered by double vision.