A 62-year-old hypertensive man develops horizontal diplopia. Examination shows complete failure of conjugate gaze towards the right, and on attempted left gaze there is adduction failure of the right eye with normal abduction of the left eye. The lesion is located in the:

• A Left paramedian pontine reticular formation
• B Right medial longitudinal fasciculus alone
• C Right abducens nucleus
• D Right paramedian pontine reticular formation plus adjacent right medial longitudinal fasciculus ✓

Explanation

This is the one-and-a-half syndrome: an ipsilateral PPRF (or abducens internuclear neurons) lesion abolishes ipsilateral conjugate gaze, and an adjacent ipsilateral MLF lesion abolishes adduction of the ipsilateral eye on contralateral gaze. Only contralateral abduction survives, hence 'one and a half'. An isolated MLF lesion gives only an INO with preserved conjugate gaze, excluding option B.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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