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# Trochlear nerve (IV)

The trochlear nerve (cranial nerve IV) is the thinnest cranial nerve; it moves one eye muscle, and damage causes double vision when looking down.

Official name: Trochlear nerve

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## What it is

The thinnest cranial nerve, the only one leaving the back of the brainstem.

## What it does

Supplies the superior oblique; a palsy gives double vision walking downstairs.

## Key points

- The only cranial nerve that leaves the back of the brainstem, and its fibres cross: each nucleus supplies the opposite superior oblique
- Thinnest cranial nerve with the longest course inside the skull, so it is easily stretched in head injury, a common cause of an isolated palsy
- Palsy: vertical double vision worst looking down and in (reading, going downstairs); the affected eye sits higher (hypertropia)
- The head tilts away from the affected side, which makes up for the lost inward rotation (intorsion) of that eye

## Key facts

**Roots**
Trochlear nucleus (midbrain); only nerve exiting the dorsal brainstem; superior orbital fissure

**Moves**
Superior oblique

**Feeling from**
None

## Relations

**Course**
Exits the back of the [midbrain](https://durgical.com/en/anatomy/midbrain/), curves forward around the cerebral peduncle between the posterior cerebral and superior [cerebellar arteries](https://durgical.com/en/anatomy/superior-cerebellar-artery/), pierces the dura at the edge of the [tentorium](https://durgical.com/en/anatomy/tentorium-cerebelli/), runs in the lateral wall of the [cavernous sinus](https://durgical.com/en/anatomy/cavernous-sinus/) and enters the orbit through the [superior orbital fissure](https://durgical.com/en/anatomy/superior-orbital-fissure/), outside the [common tendinous ring](https://durgical.com/en/anatomy/common-tendinous-ring/), crossing above the [levator palpebrae](https://durgical.com/en/anatomy/levator-palpebrae-superioris/) to the [superior oblique](https://durgical.com/en/anatomy/superior-oblique-muscle/)

**Above**
In the lateral wall of the cavernous sinus, the [oculomotor nerve (III)](https://durgical.com/en/anatomy/oculomotor-nerve/)

**Below**
In the lateral wall of the cavernous sinus, the [V1 ophthalmic](https://durgical.com/en/anatomy/ophthalmic-nerve/) nerve

## How to find it

It cannot be felt; it is assessed by having the person look down while the eye is turned in. In dissection it is a very thin thread that appears just below the inferior colliculus and winds around the midbrain at the edge of the tentorium.

## Examination

- Eye movements: with the eye turned in, weak downward movement points to the superior oblique (CN IV)
- Three-step (Parks–Bielschowsky) test: a higher eye that gets worse on gaze to the opposite side and on head tilt toward the affected side points to a superior oblique palsy
- Head posture: a compensatory tilt away from the affected side; the same tilt in old photographs suggests a long-standing (congenital) palsy

## On imaging

Very thin: only high-resolution heavily T2-weighted MRI shows its cisternal part winding around the midbrain in the ambient cistern. In a palsy, MRI checks the back of the midbrain below the inferior colliculus, the cavernous sinus and the orbit; after head injury, CT looks for associated damage. A small superior oblique on coronal orbital MRI supports a long-standing palsy.

## Clinical notes

- Closed head injury is a classic cause: the nerve is stretched against the edge of the tentorium, and severe trauma can injure both sides
- A lesion of the nucleus weakens the opposite eye, because the fibres cross before they leave the brainstem
- Lesions of the cavernous sinus or the superior orbital fissure usually affect it together with III, V1 and VI

## Memory aid

"RAD SIN": the recti adduct (except the [lateral rectus](https://durgical.com/en/anatomy/lateral-rectus-muscle/)), and the superior muscles (superior oblique, [superior rectus](https://durgical.com/en/anatomy/superior-rectus-muscle/)) intort.

## Test yourself

Think of the answer, then tap the question to see it.

## What two features make the trochlear nerve (IV) unique among cranial nerves?

It is the only one leaving the back of the brainstem, and its fibres cross, so each nucleus supplies the opposite superior oblique.

## When is double vision worst in a trochlear nerve palsy, and which way does the head tilt?

Looking down and in (reading, going downstairs); the head tilts away from the affected side to make up for lost intorsion.

## Why is the trochlear nerve often injured in closed head trauma?

It is the thinnest cranial nerve with the longest course inside the skull, so it is stretched against the edge of the tentorium.

## Which named test confirms a superior oblique (trochlear nerve) palsy?

The three-step Parks–Bielschowsky test: a higher eye worse on gaze to the opposite side and on head tilt toward the affected side.

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Educational information, not medical advice. For any symptom, see a health professional.

## Its names

**Official Latin (TA2)**
nervus trochlearis TA2 6191

**Official Spanish (DeCS)**
Nervio Troclear DeCS D014321

**English (TA2)**
Trochlear nerve

**Also called**
cranial nerve IV

## Related

- [Superior oblique](https://durgical.com/en/anatomy/superior-oblique-muscle/)

Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=14724); [Neuroanatomy, Cranial Nerve 4 (Trochlear)](https://www.ncbi.nlm.nih.gov/books/NBK537244/).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/anatomy/trochlear-nerve/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/trochlear-nerve/