A patient sustains a lesion at the right pontine tegmentum affecting the abducens nucleus. The expected eye movement deficit would be:
- A Right horizontal gaze palsy (both eyes unable to look right) ✓
- B Right lateral rectus palsy only
- C Internuclear ophthalmoplegia affecting left adduction
- D Vertical gaze palsy with convergence retraction nystagmus
Correct answer: A. Right horizontal gaze palsy (both eyes unable to look right)
Explanation
The abducens nucleus in the pons contains not only motor neurons to the ipsilateral lateral rectus but also interneurons whose axons cross the midline in the medial longitudinal fasciculus (MLF) to innervate the contralateral medial rectus via CN III. Therefore, a nuclear-level CN VI lesion disrupts conjugate horizontal gaze to the ipsilateral side, causing a complete ipsilateral horizontal gaze palsy. A fascicular CN VI lesion only causes ipsilateral lateral rectus palsy. Internuclear ophthalmoplegia results from an MLF lesion, not the nucleus itself.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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