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Cranial nerves · Motor

IV. Trochlear nerve

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The trochlear nerve (CN IV) supplies only the superior oblique muscle. It is the thinnest cranial nerve, has the longest intracranial course, and is the only one to emerge from the dorsal surface of the brainstem and to decussate completely.

Terminologia Anatomica (Latin): Nervus trochlearis

Function

General somatic efferent to the superior oblique, which depresses and intorts the eye, most effectively when the eye is adducted. It is important for looking down and inward, for example when reading or walking downstairs.

Nuclei

Trochlear nucleus in the caudal midbrain at the level of the inferior colliculus, ventral to the cerebral aqueduct. Fibres pass dorsally, decussate in the superior medullary velum and emerge contralaterally, so the muscle is supplied by the opposite nucleus.

Exit from the skull

It runs around the cerebral peduncle, through the cavernous sinus, and enters the orbit through the superior orbital fissure above the common tendinous ring to reach the superior oblique.

Testing

Ask the patient to look down and in (adducted and depressed gaze) while observing for diplopia and failure of depression. Vertical diplopia worse on looking down, for example on stairs, and a head tilt are typical complaints.

Signs of a lesion

Vertical or oblique diplopia, maximal on downward gaze with the eye adducted. The affected eye is slightly elevated, and patients typically tilt the head away from the side of the lesion to minimise the diplopia. A nuclear lesion affects the contralateral eye.

Clinical correlations

Isolated fourth nerve palsy is commonly caused by head trauma (the long, thin dorsal course makes it vulnerable, even with minor injury), microvascular ischaemia in diabetes or hypertension, or may be congenital. Bilateral palsy can follow closed head injury.

Common exam points

  • Supplies only the superior oblique (SO4); thinnest nerve with the longest intracranial course.
  • Only cranial nerve to emerge dorsally and to decussate completely.
  • Vertical diplopia on looking down and in, such as reading or descending stairs.
  • Compensatory head tilt away from the side of the lesion.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)

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