Third, fourth and sixth cranial nerve palsies and the three-step test

Updated 17 September 2026

Quick answer

A third nerve palsy causes ptosis and an eye that rests down and out, with limited elevation, depression and adduction, and sometimes a dilated pupil. A fourth nerve palsy weakens the superior oblique, causing a hypertropia of the affected eye that increases on gaze to the opposite side and on head tilt toward the same side, often with a compensatory head tilt away from the affected side. A sixth nerve palsy weakens the lateral rectus, causing an esotropia and limited abduction that increase on gaze toward the affected side and at distance.

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The three palsies at a glance

	Third nerve (oculomotor)	Fourth nerve (trochlear)	Sixth nerve (abducens)
Muscles affected	Medial, superior and inferior recti, inferior oblique, levator; pupil sphincter and ciliary muscle	Superior oblique	Lateral rectus
Eye position	Down and out, with ptosis	Affected eye higher (hypertropia)	Affected eye turned in (esotropia)
Deviation increases on	Adduction, elevation and depression of the affected eye	Gaze to the opposite side (especially down) and head tilt to the same side	Gaze toward the affected side and distance fixation
Double vision	Often masked by a complete ptosis	Vertical and torsional, worse reading or on stairs	Horizontal, worse at distance
Typical head posture	Variable	Head tilt to the opposite shoulder	Face turn toward the affected side
Common causes	Microvascular ischemia, posterior communicating artery aneurysm, trauma, cavernous sinus disease	Decompensated congenital palsy, head trauma, microvascular ischemia	Microvascular ischemia, raised intracranial pressure, trauma, cavernous sinus disease

The Parks–Bielschowsky three-step test

The three-step test identifies which of the eight vertically acting muscles is weak in an isolated vertical deviation.

Which eye is higher in primary position? A right hypertropia means a weak right depressor (superior oblique or inferior rectus) or a weak left elevator (superior rectus or inferior oblique).
Is the hypertropia worse on right or left gaze? Worse on left gaze leaves the muscles that act in left gaze: the right superior oblique or the left superior rectus.
Is it worse on right or left head tilt? Worse on right head tilt identifies the right superior oblique.

The classic result of a right fourth nerve palsy is therefore a right hypertropia that is worse on left gaze and worse on right head tilt. Measuring torsion (the affected eye is extorted) and checking for an increase in downgaze add confidence. Restrictive disease, skew deviation and myasthenia can mimic the pattern.

Red flags

A third nerve palsy with a dilated or poorly reactive pupil: urgent CT or MR angiography to exclude a posterior communicating artery aneurysm.
Headache, papilledema or bilateral sixth nerve palsies: think of raised intracranial pressure — a sixth nerve palsy can be a false localizing sign.
Several nerves together (III, IV, VI and the first or second division of V), with or without Horner syndrome: a cavernous sinus lesion.
Age over 50 with jaw claudication, scalp tenderness or visual symptoms: giant cell arteritis.
Fluctuating double vision and ptosis with normal pupils: consider myasthenia gravis.
A palsy in a child or a young adult without vascular risk factors, or one that worsens or has not recovered within about three months: neuroimaging is needed.

Aberrant regeneration of the third nerve

After a compressive or traumatic third nerve palsy, regrowing fibers can reach the wrong targets: the upper lid rises when the eye looks down or in, and the pupil constricts on adduction or downgaze. Aberrant regeneration is not expected after a microvascular palsy. When it appears without a preceding acute palsy (primary aberrant regeneration), suspect a slowly growing lesion in the cavernous sinus, such as a meningioma or aneurysm. The clinical repertoire entry covers it in detail.

Practice it on the virtual patient

Open a complete right third nerve palsy and check the lid, the eye position, the pupil and each direction of gaze.
Open a right fourth nerve palsy, use the head-tilt control and work through the three steps.
Open a right sixth nerve palsy and find where the esotropia is largest.
Finish with a cavernous sinus syndrome, where several nerves are affected together.

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Frequently asked questions

What does a third nerve palsy look like?

The upper lid droops, the eye rests down and out because the lateral rectus and superior oblique are unopposed, and the eye cannot move fully up, down or in. The pupil may be dilated and poorly reactive when the pupillomotor fibers are involved.

Does a pupil-sparing third nerve palsy rule out an aneurysm?

Not completely. A complete third nerve palsy with a normal pupil in an older patient with vascular risk factors is usually microvascular, but partial palsies, young patients and any later pupil involvement need urgent vascular imaging and close follow-up.

Why does a fourth nerve palsy cause a head tilt?

The superior oblique intorts and depresses the eye. Tilting the head toward the opposite shoulder reduces the demand on the weak muscle, which lessens the vertical and torsional double vision.

Why is a sixth nerve palsy called a false localizing sign?

The sixth nerve has a long course inside the skull and bends over the petrous apex, so raised intracranial pressure from a distant cause can stretch it. A sixth nerve palsy therefore does not necessarily mean the lesion is near the nerve.