BASICS

DESCRIPTION

4th nerve palsy is caused by weakness or paralysis of the superior oblique muscle, and may result in diplopia.

EPIDEMIOLOGY

Prevalence

4th nerve palsy occurs frequently after closed head trauma due to its unique anatomic dorsal midbrain decussation.

ETIOLOGY

• Trauma and microvascular ischemia are the most common causes of acquired 4th nerve palsy. Congenital 4th nerve palsies are also common. Review of childhood photos showing head tilt to the side opposite the palsy can help determine the chronicity of the lesion.

• Tumor (e.g., pinealoma, tentorial meningioma), aneurysm, meningitis, and giant cell arteritis are uncommon causes of trochlear nerve palsy.

DIAGNOSIS

HISTORY

• Patients with 4th nerve palsy usually complain of binocular diplopia with vertical or diagonal separation that worsens on downgaze. They may complain that objects appear tilted in the affected eye.

• Patients often indicate that, when they tilt their head to the side opposite the palsy, they have less or no diplopia.

PHYSICAL EXAM

• On the palsied side, there may be deficient inferior movement of the eye, when the patient looks downward and inward. The antagonist inferior oblique muscle may appear to overact (over-elevation in adduction) on affected side.

• The vertical separation between the 2 eyes often increases when the head is tilted to side of the 4th nerve palsy, explaining why patients usually tilt their head to the side opposite the palsy.

• Alternate-cover tests in different positions of gaze, (3-step test) may help confirm the diagnosis of 4th nerve palsy.

• Patients with congenital 4th nerve palsy often have a high vertical fusional amplitude (>3 prism diopters).

• Patients with 4th nerve palsy should not have ptosis, but may volitionally close 1 eye to avert diplopia.

DIAGNOSTIC TESTS & INTERPRETATION

Lab

In adults, work-up for diabetes, hypertension and hypercholesterolemia should be considered.

Imaging

If a suspected 4th nerve palsy does not improve after a few months, neuroimaging should be performed.

DIFFERENTIAL DIAGNOSIS

• Bilateral 4th nerve palsies can occur, especially after trauma. Myasthenia gravis, Graves ophthalmopathy, and skew deviation may mimic a 4th nerve palsy. Patients with ocular myasthenia often have variable ptosis and variability in their eye misalignment with diurnal variation. Patients with Graves ophthalmopathy may show lid retraction, proptosis, conjunctival chemosis, and often have abnormal TSH. It is difficult for a non-specialist to differentiate 4th nerve palsy from skew deviation. Patients with skew deviation may have stroke history or brainstem findings, show incyclotorsion on Maddox rod testing, and often have less vertical strabismus in the supine, compared to upright, position.

• Rarely giant cell arteritis may mimic or cause 4th nerve palsy

– Patients with superior oblique myokymia (due to neurovascular compression of the 4th cranial nerve root exit zone, multiple sclerosis, or posterior fossa tumor) may complain of episodic tilting.

TREATMENT

ADDITIONAL TREATMENT

General Measures

Patients may find it helpful to tilt their head away from the side of the palsied nerve. Prism glasses are often helpful if there is vertical or diagonal diplopia. If the patient has intolerable torsional diplopia, patching or translucent tape over a glasses lens can be of help.

Issues for Referral

Any non-resolving 4th nerve palsy should be referred for ophthalmic assessment.

SURGERY/OTHER PROCEDURES

If new-onset diplopia does not improve after a year, and prism glasses do not help the patient, strabismus surgery can be considered.

ONGOING CARE

PROGNOSIS

Many patients with microvascular or traumatic 4th nerve palsy have some improvement 3–9 months after presentation.

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