Following severe head trauma, a patient develops palsy of the right superior oblique muscle. Which feature of the trochlear nerve explains why both superior oblique muscles may be affected together after a single midline head injury?

• A Its nucleus lies in the pons
• B It passes through the cavernous sinus below the oculomotor nerve
• C Its fibres decussate completely within the midbrain before emerging dorsally ✓
• D It supplies the superior rectus as well

Explanation

Trochlear fibres from each nucleus curve around the aqueduct and decussate completely in the superior medullary velum, emerging from the dorsal midbrain. This total decussation means a shearing injury at the tentorial edge or dorsal midbrain can strike both nerves where they lie close together, causing bilateral superior oblique palsies. The nucleus is in the caudal midbrain, not the pons, and the nerve supplies only one muscle.

Reference: Gray's Anatomy, 42nd ed.

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Written and medically reviewed by the StethoPrep medical team.