A 55-year-old woman with brainstem stroke shows: on attempted right gaze, the right eye cannot adduct and the left eye cannot abduct; on attempted left gaze, only the left eye abducts normally with nystagmus. The lesion involves which structures?

• A Left PPRF and left MLF
• B Right abducens nucleus alone
• C Bilateral medial rectus subnuclei of the third nerve
• D Right PPRF and right MLF together in the pons ✓

Explanation

The one-and-a-half syndrome results from a pontine lesion destroying both the ipsilateral PPRF (or abducens nucleus) and the ipsilateral MLF. With a RIGHT-sided lesion, right gaze is abolished completely (the 'one'), and on left gaze only left abduction survives, since right adduction fails via the damaged MLF (the remaining 'half'). An isolated abducens nucleus lesion abolishes conjugate gaze toward that side only but leaves adduction intact, ruling out option B.

Reference: Walsh and Hoyt's Clinical Neuro-Ophthalmology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.