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Posterior canal predominance in bilateral skew deviation
  1. SEAN P DONAHUE
  1. Vanderbilt University School of Medicine
  2. Nashville TN 37232–8808, USA
  3. Department of Ophthalmology, University of Arkansas for Medical Science, Little Rock, AR, USA
  1. MICHAEL C BRODSKY
  1. Vanderbilt University School of Medicine
  2. Nashville TN 37232–8808, USA
  3. Department of Ophthalmology, University of Arkansas for Medical Science, Little Rock, AR, USA

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    Editor,—We were excited to see the recent case report of Park et al regarding the 30 year old man with horizontal locked-in syndrome and disconjugate gaze.1 We were intrigued by the description of his eye movements on attempted horizontal gaze, whereas “when the patient was asked to look to the right side, the right eye moved upward with intorsion, and at the same time, left eye moved downward and extorsion . . . when the patient was asked to look to the left side, . . . the left eye moved upward with intorsion whereas the right eye moved downward with extorsion.” Magnetic resonance imaging revealed a large ventral pontine infarct. The authors postulated that the lesion caused a disturbance in the neural integration of prenuclear inputs to the interstitial nucleus of Cajal.

    We believe we can refine further their mechanism for this observed disconjugate gaze based on the anatomy of the vestibular ocular reflect pathways, as it is probably a type of bilateral skew deviation. Each semicircular canal provides excitatory innervation to an extraocular muscle and its contralateral yoke, and inhibitory innervation to the corresponding antagonist extraocular muscles.2 The otolithic pathways are less well understood but are believed to follow the same pathways as the semicircular canal pathways.3 Each anterior semicircular canal provides excitatory innervation to the ipsilateral superior rectus and the contralateral inferior oblique muscle, while inhibiting the yoke ipsilateral inferior rectus and contralateral superior oblique muscle. Unilateral injury to these vestibular-ocular pathways produces classic skew deviation with hypertropia of one eye in all fields of gaze, whereas bilateral injury produces alternating hypertropia in side gaze. Bilateral damage to anterior canal pathways causes a posterior canal predominance with bilateral tonic downgaze.4

    Theoretically, bilateral damage to the otolithic-ocular pathways corresponding to those of the anterior semicircular canal should produce the motility disturbance described in the patient reported by Park et al. The disinhibition resulting from such damage would produce posterior canal predominance, and increase tonus to all four depressors (both inferior recti and both superior obliques). Since the vertical action of the superior oblique is more prominent in adduction, the abducting eye should have a relative hypertropia on side gaze (alternating skew on lateral gaze). Likewise, because the torsional action of the superior oblique is more prominent in abduction, dynamic intorsional movements of the hypertropic eye would be seen on attempted abduction.

    In this scenario, fundus examination should demonstrate bilateral intorsion in primary position, and detailed motility measurements would show an A-pattern. However, these findings would have been difficult to detect in this patient who could not elevate the eyes above the midline. We believe that bilateral injury to the same pathways may be responsible for A-pattern strabismus and bilateral superior oblique overaction seen in some patients with posterior fossa disease.5

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