Three dimensions of skew deviation

• skew deviation

Skew means set, placed, or running obliquely; slanting.1 For a century, skew deviation has been the descriptive term for an acquired vertical deviation that slants the interpupillary axis to an oblique orientation.2,3 This condition has traditionally connoted a severe and debilitating neurological injury within the posterior fossa.4, Subsequent investigation has refined our understanding of this disorder and expanded the boundaries of this diagnosis.5 Working over the past two decades, the promethean team of Brandt and Dieterich have established that skew deviation has a clinical localising value within the posterior fossa6,7; that it is a component of the ocular tilt reaction (a triad of skew deviation, binocular torsion, head tilt)7–9; and that it is associated with a tilt in the subjective visual vertical.8–11 These studies have supplanted our basic descriptive definition of skew deviation with a mechanistic understanding of this complex disorder.

The underlying mechanism for skew deviation involves a unilateral lesion that inhibits (or occasionally stimulates) central otolithic pathways which run from the medulla to the mesencephalon.10,11 Since a physiological imbalance in central graviceptive tone is normally produced by a head or body tilt in the roll plane, this lesion produces the sensation of body tilt and evokes the same stimulus as is evoked by the Bielschowsky head tilt test.12 By activating prenuclear input to the superior rectus and superior oblique muscles of the ipsilateral eye, and the inferior rectus and oblique muscles of the contralateral eye, and by simultaneously inhibiting their antagonists, a cyclovertical divergence is produced, with intorsion of the higher eye, extorsion of the lower eye, and a corrective head tilt toward the side of the …