CorpusHuman Anatomy
NervousNerve

Trochlear nerve (CN IV)

Nervus trochlearis

Also known as: CN IV, fourth cranial nerve, pathetic nerve

A purely motor nerve (GSE) to the superior oblique muscle. It is the thinnest cranial nerve, has the longest intracranial course, is the only one to leave the dorsal brainstem, and is the only one whose fibres fully decussate.

Show on body map

Overview

Nucleus levelMidbrain, inferior colliculus
Intracranial length≈7.5 cm (longest)
Axons≈2 000–3 000 (thinnest CN)
ExitSuperior orbital fissure (outside common tendinous ring)

The trochlear nucleus lies in the caudal midbrain at the level of the inferior colliculus, ventral to the cerebral aqueduct. Its axons run dorsally around the periaqueductal grey and decussate completely in the superior medullary velum. They emerge from the dorsal surface of the brainstem just below the inferior colliculi. Each superior oblique is therefore supplied by the contralateral nucleus.

The slender nerve curves around the cerebral peduncle, passes between the posterior cerebral and superior cerebellar arteries, and runs along the free edge of the tentorium. It then travels in the lateral wall of the cavernous sinus below CN III and enters the orbit through the superior orbital fissure, outside the common tendinous ring. It supplies the superior oblique, whose tendon passes through the trochlea (a fibrocartilaginous pulley) on the superomedial orbital rim. Because the tendon pulls from in front, the muscle depresses the eye (most effective in adduction), intorts it and abducts it. Its long, thin, exposed course makes the nerve vulnerable to head trauma, and it is the most commonly injured cranial nerve in closed head injury after CN I.

Location

Dorsal midbrain → around the cerebral peduncle → tentorial edge → lateral wall of the cavernous sinus → superior orbital fissure → superior oblique.

Parts & subdivisions

  1. Functional components
    Motor only (GSE), plus proprioceptive afferents.
  2. Clinical testing
    Ask the patient to look down and in, toward the nose, which isolates superior oblique depression. In a palsy, the eye fails to depress in adduction and vertical diplopia increases. The Parks–Bielschowsky three-step test confirms superior oblique weakness: the hypertropia increases on head tilt toward the affected side.
  3. Lesion signs
    Vertical (and torsional) diplopia, worse looking down, such as when reading or walking downstairs. The affected eye is elevated (hypertropia) and extorted. Patients adopt a compensatory head tilt away from the side of the lesion, with the chin tucked.

Anatomy

Innervation

Superior oblique muscle only (contralateral nucleus).

Course

Nucleus (caudal midbrain) → dorsal decussation in the superior medullary velum → emerges below the inferior colliculus → winds around the lateral midbrain → between the PCA and SCA → along the free tentorial edge → lateral wall of the cavernous sinus → superior orbital fissure (outside the tendinous ring, above the muscle cone) → superior oblique.

Function

  • Depression of the adducted eye (looking down and in)
  • Intorsion of the eye
  • Assists abduction

Clinical significance

Trochlear palsy and head tilt

The commonest causes are head trauma (the nerve is stretched against the tentorium), microvascular ischaemia and congenital palsy. Old childhood photographs showing a long-standing head tilt suggest a congenital, decompensated palsy. The tilt away from the lesion uses the intact superior oblique of the other eye to cancel the extorsion.

Mnemonics

Superior oblique action

"SO4 — Superior Oblique looks dOwn and in." Superior oblique depresses and intorts: "SIN" (Superiors INtort).

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.