On attempted right gaze, a patient's right eye cannot move at all horizontally while the left eye abducts normally. On attempted left gaze, both eyes move fully except that the right eye fails to adduct. The lesion responsible involves which structures and where?

• A Left PPRF and left MLF in the dorsal pons
• B Bilateral medial rectus subnuclei of the third nerve in the midbrain
• C Right abducens nucleus or PPRF plus the right MLF in the dorsal pons ✓
• D Right abducens nerve fascicle at the ventral pons

Explanation

The one-and-a-half syndrome combines a conjugate gaze palsy toward one side (the 'one') with an ipsilateral internuclear ophthalmoplegia (the 'half'), leaving only abduction of the contralateral eye. It requires a single lesion in the dorsal pontine tegmentum damaging the PPRF or abducens nucleus and the adjacent ipsilateral MLF. In this patient only left eye abduction survives, so the lesion is on the right. The commonest causes are paramedian pontine infarction and demyelination.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.