A 58-year-old hypertensive man develops horizontal diplopia. On attempted right gaze, neither eye can abduct or adduct; only the left eye can adduct on left gaze, and it does so with abducting nystagmus of the right eye. Where is the lesion?

• A Bilateral medial longitudinal fasciculi in the midbrain
• B Left abducens nucleus in the pons
• C Right paramedian pontine reticular formation combined with the adjacent right medial longitudinal fasciculus ✓
• D Right oculomotor nucleus in the midbrain

Explanation

This is the one-and-a-half syndrome: an ipsilateral horizontal gaze palsy (the 'one') plus an ipsilateral internuclear ophthalmoplegia (the 'half'). The only remaining horizontal movement is abduction of the contralateral eye, often with nystagmus. Both components arise from a single pontine lesion involving the ipsilateral PPRF or sixth nerve nucleus together with the adjacent ipsilateral MLF. An isolated abducens nucleus lesion would spare all adduction movements, which is not seen here.

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

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