A 35-year-old man with an apex lung tumour presents with partial ptosis of the right eye. The ptosis is mild, and the lid droop worsens in darkness. The anatomical basis is interruption of sympathetic supply to which structure?
- A Superior tarsal (Muller) muscle ✓
- B Levator palpebrae superioris
- C Orbicularis oculi
- D Inferior tarsal muscle
Correct answer: A. Superior tarsal (Muller) muscle
Explanation
The superior tarsal (Muller) muscle arises from the undersurface of levator palpebrae superioris and inserts on the upper border of the tarsal plate. It is smooth muscle supplied by cervical sympathetic fibres travelling with the ophthalmic artery and contributes roughly 2 mm of lid elevation. Loss of this supply in Horner syndrome causes mild ptosis that worsens in darkness when sympathetic tone normally rises. Levator palpebrae is striated muscle supplied by the oculomotor nerve, and orbicularis oculi closure is unaffected here.
Reference: Snell's Clinical Anatomy by Regions, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.