A 45-year-old woman presents with bilateral ptosis, loss of vertical gaze, and convergence retraction nystagmus on attempted upgaze, with preserved horizontal eye movements. Which structure is damaged?
- A Rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and posterior commissure ✓
- B Abducens nucleus in the pons
- C Left paramedian pontine reticular formation (PPRF)
- D Oculomotor nucleus in the midbrain
Correct answer: A. Rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and posterior commissure
Explanation
The constellation of bilateral upgaze palsy, convergence-retraction nystagmus (from co-activation of horizontal rectus muscles on attempted upgaze), and light-near dissociation is the dorsal midbrain (Parinaud) syndrome. The key structures are the posterior commissure (mediating upgaze via the riMLF-interstitial nucleus of Cajal pathway) and the riMLF itself. Isolated oculomotor nucleus damage causes ipsilateral upgaze palsy but not this full syndrome. PPRF lesions impair ipsilateral horizontal gaze. Abducens nucleus lesions cause ipsilateral horizontal gaze palsy.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.