Skew deviation demonstrated on alternate cover testing, in which one eye depresses and intorts while the other elevates and extorts, is best explained by:

• A A trochlear nerve palsy
• B Unilateral vestibular hypofunction confined to the labyrinth
• C Superior semicircular canal dehiscence
• D An imbalance in the otolith-ocular reflex pathways from a brainstem or cerebellar lesion ✓

Explanation

Skew deviation results from disruption of the utricular (otolith) inputs to the ocular motor nuclei within the brainstem, classically in lateral medullary infarction or cerebellar lesions, producing the ocular tilt reaction with head tilt and ocular torsion. A fourth nerve palsy gives isolated hypertropia worsening on contralateral gaze without the full tilt reaction, killing option A. Pure labyrinthine hypofunction does not produce sustained vertical ocular misalignment, killing option B.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.