A 58-year-old hypertensive man develops horizontal diplopia. Examination shows complete failure of conjugate left gaze and, on attempted right gaze, failure of adduction of the left eye with nystagmus of the abducting right eye. Where is the lesion?

• A Left paramedian pontine reticular formation and left medial longitudinal fasciculus ✓
• B Right abducens nucleus and right medial longitudinal fasciculus
• C Left oculomotor nucleus in the midbrain
• D Bilateral medial longitudinal fasciculi only

Explanation

The combination of an ipsilateral horizontal gaze palsy (from destruction of the PPRF or abducens nucleus) with an ipsilateral internuclear ophthalmoplegia (MLF lesion) is the one-and-a-half syndrome, almost always in the pons. The only remaining horizontal movement is abduction of the contralateral eye. Bilateral MLF lesions alone would give bilateral INO without gaze palsy, and a single oculomotor nucleus lesion cannot abolish conjugate gaze to one side.

Reference: Snell's Clinical Neuroanatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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