A-pattern strabismus with overdepression in adduction: a special type of bilateral skew deviation?

Skew deviation is an acquired vertical ocular misalignment caused by damage to the prenuclear vestibular inputs to the ocular motor nuclei. A-pattern strabismus often has bilaterally symmetric vertical incomitance and overdepression in adduction (superior oblique overaction) and can be associated with developmental delay, cerebral palsy, hydrocephalus, spina bifida, or posterior fossa or other brainstem disease. The purpose of this study is to describe the ocular motility and torsion findings in patients with A-pattern strabismus and bilateral overdepression in adduction (superior oblique muscle overaction) and to propose a possible brainstem mechanism underlying these observations.

Most of the 13 patients identified had other neurologic abnormalities, including spina bifida, hydrocephalus, perinatal stroke, or global delay. Only 2 patients had vertical ocular misalignment in primary gaze. Of the 13, 7 had incomitant vertical tropias during lateral gaze, and 12 had bilateral incyclotorsion documented on fundus examination. Despite having bilateral overdepression in adduction (superior oblique overaction), 11 of the 13 had no difference in vertical ocular misalignment with alternating head tilt rather than reversing hypotropias as would be expected from primary oblique dysfunction. The findings are consistent with damage to the utricular pathways corresponding to the anterior semicircular canal and a resulting posterior canal predominance to the extraocular muscle subnuclei that creates increased tonus to the depressors, bilaterally.

A-pattern strabismus may, in some cases, represent a special form of skew deviation. The ocular motility and clinical findings are consistent with bilateral damage to the utricular pathways corresponding to the anterior semicircular canals rather than bilateral primary superior oblique muscle overaction.