Trochlear nerve (Cranial Nerve IV)
Overview
The trochlear nerve supplies the superior oblique muscle. Its primary action is intorsion of the globe; secondarily it depresses the eye (especially in adduction) and assists abduction. Trochlear (CN IV) palsy produces characteristic binocular vertical diplopia that is worse on downgaze and can substantially affect reading and negotiating stairs.
Anatomy and key points
- Originates from the dorsal midbrain; it is the only cranial nerve to emerge dorsally and the only one whose fibres decussate before exiting.
- Intracranial course: subarachnoid space (between posterior cerebral and superior cerebellar arteries) → cavernous sinus (with CN III, VI and V1) → orbit via the superior orbital fissure.
- Motor to the superior oblique only; isolated palsy therefore produces a predictable motility deficit.
Clinical presentation
- Typical symptom: binocular vertical diplopia, worse on downgaze (reading, stairs) and when tilting the head.
- Typical posture: compensatory head tilt away from the affected eye (toward the contralateral shoulder) to minimise diplopia.
- Key signs:
- Hypertropia of the affected eye (it appears higher).
- Vertical deviation worsens on contralateral gaze and on downgaze.
- Bielschowsky head‑tilt test: measured hypertropia increases when the head is tilted toward the affected eye.
- No ptosis and usually no pupillary involvement (helps distinguish from CN III palsy).
- Associated symptoms depend on cause and may include periorbital pain, headache, other cranial nerve deficits, fever...