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One-and-a-Half Syndrome — SCE Neurology MCQ

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HardCerebrovascular DiseaseOne-and-a-Half SyndromeSCE Neurology

A 55-year-old man presents with horizontal diplopia. Examination shows complete inability to adduct the right eye on leftward gaze (right INO). He also has complete inability to abduct the right eye on rightward gaze (right conjugate gaze palsy). He has left-beating nystagmus on leftward gaze. Where is the lesion?

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Correct answer: D — Right MLF and right CN VI nucleus/PPRF — one-and-a-half syndrome

The one-and-a-half syndrome results from a unilateral pontine lesion affecting both the ipsilateral CN VI nucleus (or PPRF) AND the ipsilateral MLF. This causes: (1) ipsilateral conjugate horizontal gaze palsy (the 'one' — cannot look to the right with either eye) AND (2) ipsilateral internuclear ophthalmoplegia (the 'half' — right eye cannot adduct on leftward gaze). The only horizontal eye movement preserved is abduction of the contralateral (left) eye, which shows nystagmus. A: MLF alone causes INO without gaze palsy. C: Left MLF would cause left adduction failure. D: Bilateral MLF causes bilateral INO. E: CN III lesion causes down-and-out eye with ptosis.

Reference: Clinical Neuroanatomy