A 48-year-old man with a brainstem stroke has complete horizontal gaze palsy of both eyes to the right, and on attempted left gaze the left eye abducts with nystagmus but the right eye cannot adduct. Vertical movements and convergence are intact. The lesion involves:
- A Right paramedian pontine reticular formation or right sixth nerve nucleus together with the right medial longitudinal fasciculus ✓
- B Left paramedian pontine reticular formation alone
- C Bilateral medial longitudinal fasciculi only
- D Right oculomotor nucleus and right abducens nucleus
Correct answer: A. Right paramedian pontine reticular formation or right sixth nerve nucleus together with the right medial longitudinal fasciculus
Explanation
This is the one-and-a-half syndrome: a lesion of the ipsilateral PPRF or abducens nucleus abolishes all conjugate gaze toward that side (the 'one'), and involvement of the ipsilateral MLF abolishes adduction of the ipsilateral eye on gaze away (the 'half'). The only surviving horizontal movement is abduction of the contralateral eye. Bilateral MLF lesions give bilateral INO with intact conjugate gaze, so option C cannot explain the right gaze palsy.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.