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Skew Deviation
Definition
Skew deviation is an acquired vertical misalignment of the eyes (vertical strabismus) caused by a supranuclear lesion disrupting the graviceptive vestibulo-ocular (utriculo-ocular) pathway, which normally balances vestibular tone in the roll plane.1) It reflects damage anywhere along the pathway from the peripheral otolith organs (utricle) through the vestibular nuclei, medial longitudinal fasciculus, and midbrain integration centers, most often in the brainstem, thalamus, or cerebellum.2) The vertical deviation may be comitant (same in all gaze positions, ~64% of cases) or incomitant, and amplitude is typically ≤5 prism diopters (range 1–30 PD).3)
Key Clinical Features and Localization
- Frequently occurs as part of the ocular tilt reaction (OTR) — the synkinetic triad of skew deviation, ocular torsion (cyclotorsion), and head tilt toward the shoulder of the hypotropic eye.
- The hypertropic eye shows fundus incyclotorsion and the hypotropic eye excyclotorsion — distinguishing it from fourth (trochlear) nerve palsy, in which the hypertropic eye is typically excyclotorted.
- Lesions rostral to the midpons cause contralateral head tilt; lesions in the utricle, caudal brainstem, or cerebellum cause ipsilateral head tilt.
- Usually accompanied by other neurologic signs (nystagmus, gaze paresis, ataxia, saccadic pursuit, internuclear ophthalmoplegia) in ~85% of patients; ischemic stroke is the leading cause (~52%).
Distinguishing from Fourth Nerve Palsy
- Upright-supine test: vertical misalignment decreases by ≥50% when moving from upright to supine in skew deviation (sensitivity ~80%, specificity 100%), but not in trochlear palsy.
- Ocular torsion direction and comitancy further help differentiate; the Parks-Bielschowsky 3-step test can be positive in either condition and is insufficient alone.
- As a component of the HINTS exam (Head-Impulse–Nystagmus–Test-of-Skew), skew is regarded as a central sign in acute vestibular syndrome and can prompt stroke evaluation.
Treatment Options
The overarching principle is to treat the underlying cause (e.g., stroke, demyelination, tumor, hemorrhage) and refer to neurology and/or otolaryngology, since skew deviation is often a marker of serious posterior fossa pathology. Many cases resolve spontaneously — resolution documented in ~42% of patients, usually within 3 months but sometimes up to 12 months — so an initial period of observation with symptomatic management is reasonable.
- Prisms (first-line symptomatic therapy): Ground-in or temporary Fresnel prisms alleviate diplopia, particularly with smaller vertical deviations. In one series, prism spectacles relieved persistent diplopia in 62% (68/110) of patients; success was not predicted by cause, amount of misalignment, or degree of incomitance. Useful as a bridge while awaiting possible recovery.
- Botulinum toxin: May be considered for cases that do not resolve.
- Strabismus (cyclovertical muscle) surgery: Reserved for stable, non-resolving deviations, especially larger deviations or those with significant ocular torsion. Goals are individualized — vertical diplopia, torsional diplopia, tilt of perceived visual vertical, and head tilt.
- Other measures: Occlusion (patching one eye) can relieve intractable diplopia when other options fail.
Because accompanying neurologic deficits — especially ataxia — persist in ~44% of patients and may be more disabling than the diplopia itself, management should address the broader neurologic picture, not the ocular misalignment alone.