Fourth Nerve Palsy: Three-Step Test

Master fourth nerve palsy localization with superior oblique actions, the Parks-Bielschowsky three-step test and board-focused clinical traps.

Reviewed for accuracyJuly 27, 20263 min read

Four-panel diagram of the Parks-Bielschowsky three-step test in right superior oblique fourth nerve palsy: primary position showing subtle right hypertropia, lateral gaze to the patient's left showing increased hypertropia, head tilt toward the patient's right shoulder showing greatest hypertropia, and compensatory head tilt toward the patient's left shoulder showing smallest hypertropia

Warm-up

Which cranial nerve innervates the superior oblique muscle?

Lesson

Fourth Nerve Palsy in 30 Seconds

Fourth nerve palsy causes vertical or torsional diplopia from superior oblique weakness. A trochlear nerve lesion produces ipsilateral superior oblique weakness and an ipsilateral hypertropia that typically increases in contralateral gaze and ipsilateral head tilt. Use the Parks-Bielschowsky three-step test to localize the weak cyclovertical muscle for NBEO and OKAP.

What the Superior Oblique Does

The trochlear nerve innervates the superior oblique, one of the extraocular muscles. The muscle has three actions: intorsion, depression that is greatest in adduction, and abduction. Weakness causes an ipsilateral hypertropia with excyclotorsion. The hypertropia is greatest when the affected eye is adducted, that is in contralateral gaze, and during ipsilateral head tilt. Patients often report worse diplopia when reading or descending stairs, since both require depression of the adducted eye. The trochlear nerve is the only cranial nerve that exits dorsally, and its fibers decussate before leaving the brainstem, so each trochlear nucleus supplies the contralateral superior oblique.

The Three-Step Test

Step one: Identify the hypertropic eye in primary gaze. A right hypertropia narrows the possibilities to depressors of the right eye or elevators of the left eye.
Step two: Determine which lateral gaze worsens the hypertropia. A right hypertropia that increases in left gaze implicates either the right superior oblique or the left superior rectus.
Step three: Determine which head tilt worsens the hypertropia. A right hypertropia that increases on right head tilt localizes to a right superior oblique palsy.

Worked example: right hypertropia in primary gaze, worse in left gaze and worse with right head tilt indicates a right fourth nerve palsy.

4 Facts Exams Always Ask

Patients typically adopt a compensatory head tilt away from the affected eye, toward the contralateral shoulder, often with slight chin depression and a face turn.
Congenital palsy often shows large vertical fusional amplitudes. Old photographs may reveal a longstanding compensatory head tilt.
Because the trochlear nerve has the longest intracranial course and exits dorsally, it is especially vulnerable to head trauma. Trauma is the most common cause of acquired fourth nerve palsy and traumatic palsies are frequently bilateral.
Bilateral involvement can produce V-pattern esotropia, large excyclotorsion and an alternating hypertropia that reverses in opposite gazes.

The Classic Trap

Do not assume a three-step pattern proves unilateral disease. Bilateral fourth nerve palsy may appear asymmetric and mimic a unilateral palsy, and it can satisfy the three-step test on only one side, producing false localization. Look for bilateral excyclotorsion, a V-pattern esotropia and evidence that both superior obliques are weak, especially after trauma.

Clinical Pearl

A patient may report little torsional diplopia despite clear excyclotorsion. Congenital cases often remain compensated for decades through unusually large vertical fusional amplitudes, then decompensate with age, illness or stress and present as new diplopia.

Check yourself

Checkpoint 1

The Parks-Bielschowsky three-step test is primarily used to localize:

Checkpoint 2

Which clinical feature favors congenital rather than acquired superior oblique palsy?

Clinical case

A patient develops vertical binocular diplopia after head trauma. Examination shows a right hypertropia in primary gaze that increases in left gaze and on right head tilt. The three-step test therefore localizes to the right superior oblique. Before labeling this an isolated right palsy, you consider whether the palsy is actually bilateral.

Which finding on examination is most suggestive of a bilateral, rather than unilateral, superior oblique (fourth nerve) palsy?

Explanation

Bilateral fourth nerve palsy typically presents with an alternating hypertropia that switches sides depending on gaze direction, a large objective excyclotorsion often exceeding 10 degrees, a V-pattern esotropia, and a chin-down head posture, distinguishing it from the more consistent, one-sided pattern seen in unilateral palsy. Masked bilateral cases can initially appear unilateral until double Maddox rod testing reveals significant torsion.

Case discussion

The three-step pattern localizes weakness to the right superior oblique, but trauma is the most common cause of acquired fourth nerve palsy and traumatic palsies are often bilateral. A bilateral palsy can present asymmetrically and mimic unilateral disease. The findings that point to bilateral involvement are an alternating hypertropia that reverses with lateral gaze, large excyclotorsion greater than 10 degrees and a V-pattern, so these should be sought before accepting an isolated unilateral diagnosis.

Sources & Reviewer Info

Reviewed by Vasilis Inglezis: Optometrist and Ocular Oncology Imaging Specialist, Ocular Oncology Center, Athens, Greece. Last updated July 27, 2026.

Diagram created with AI image generation, clinically reviewed for accuracy.

References

American Academy of Ophthalmology. Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology.
American Academy of Ophthalmology. Basic and Clinical Science Course, Section 6: Pediatric Ophthalmology and Strabismus.
American Academy of Ophthalmology EyeWiki. "Trochlear Nerve Palsy."

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