4th Nerve Palsy

Written By Orthoptic Corner

The fourth cranial nerve (trochlear nerve) supplies the superior oblique (SO) muscle, which depresses and intorts the eye (moves it down and in). When impaired, the affected eye cannot move fullywards and inwards, leading to a hypertropia with vertical and torsional diplopia. It is the most commonly affected cranial nerve in vertical strabismus.


Aetiology

The trochlear nerve originates from the trochlear nucleus in the midbrain and exits from the posterior brainstem, with its fibres crossing to supply the contralateral superior oblique muscle. It passes through the subarachnoid space, pierces the dura, travels in the lateral wall of the cavernous sinus, and enters the orbit via the superior orbital fissure. The superior oblique muscle, anchored by the trochlea, primarily depresses and intorts the eye.

Causes

Unlike some cranial nerves, the trochlear nerve does not produce clear localisation patterns based on anatomy alone. In many cases, the precise lesion site cannot be determined clinically.

Most common known causes:

  • Head trauma

    • Can cause unilateral or bilateral palsy

  • Microvascular ischaemia (e.g. diabetes, hypertension)

  • Intracranial tumours

  • Myasthenia gravis

  • Congenital abnormalities

    • Often due to structural variation of the SO tendon

    • Tendon may be absent, malinserted, or excessively lax

Types

Congenital → unilateral, bilateral

Acquired → unilateral, bilateral


Clinical Presentation

Congenital - Most commonly present due to an abnormal head posture or facial asymmetry 

  • This develops due to chronic torticollis (Reduced distance between lateral canthus and mouth corner on the side of tilt)

Unilateral

  • Large hyperphoria with the AHP, large hypertropia without the AHP, larger at near 

  • May have associated eso or exo 

  • No subjective excyclotorsion, but objective excyclotorsion visible on fundus exam 

  • Saccadic speeds generally normal, though downgaze in adduction may be slightly reduced

  • Positive BHTT - Hypertropia increases on head tilt toward affected side

  • May or may not complain of diplopia (Large vertical fusion amplitudes may mask symptoms)

Bilateral

  • Constant V-esotropia with hypertropia of the non-fixing eye 

  • AHP likely chin down, may have slight tilt to less affected side  

  • IO overaction also seen - can be misdiagnosed as an infantile ET 

    • Differentially diagnose bilateral 4th vs ET with IO overaction vs infantile ET - with a positive BHTT to either side and look for features of infantile ET (DVD/ nystagmus)

  • Positive BHTT - reversal of HT seen on right and left tilt 

Note - adults can present with a decompensated congenital palsy or a congenital palsy may be picked up incidentally by an optometrist. 

Main differentials between acquired and congenital: 

Acquired cases report;

  • Recent onset of diplopia

  • Reduced/normal vertical fusion range (extended in congenital/longstanding cases)

  • Awareness of the AHP 

  • History of trauma 

Acquired 

Unilateral 

  • Positive BHTT tilting affected side 

  • May report vertical or torsional diplopia 

Bilateral 

  • Torsional diplopia 

  • AHP: chin down (marked) 

  • Reversal of HT seen on right and left gaze and on dextro and laevodepression 

    • If pt has IO overaction, may only see reversal on leavo and dextrodepression

  • BHTT positive to both sides

  • May have V-pattern 

  • Symptomatic torsion and recent-onset diplopia should prompt urgent neurological evaluation.

AHP

Unilateral - tilt unaffected, turn unaffected, chin down

Bilateral - marked chin down, tilt and turn to less a


Investigation

Clinical investigations

  • VA near and distance

  • CT near and distance, with and without AHP 

  • OM - versions, ductions and vergence movements

    • Watch eyelids and globe for any abnormalities - any MG signs?

  • PCT near and distance, and other gaze positions

  • Hess chart and field of BSV

Torsion Assessment

  • Subjective (Double Maddox rod)

  • Objective (fundus photography)

In acquired unilateral cases, objective torsion often measures larger than what the patient reports subjectively.

Neuroimaging is indicated in:

  • Acute onset

  • Bilateral involvement

  • Associated neurological signs

  • Any history of trauma 

Management

Non-surgical management 

Prisms can be helpful once the deviation becomes more comitant, as they help to realign images and reduce diplopia. They are particularly useful if an abnormal head posture (AHP) becomes uncomfortable or difficult for the patient to maintain. Only consider this if the prism does not induce double vision in primary position. 

Sector occlusion may also be used to eliminate diplopia in extreme positions of gaze, while preserving single vision in primary position.

Summary

  • Unilateral fourth nerve palsy is most common.

  • Congenital cases often compensate well and may lack torsional symptoms.

  • Facial asymmetry suggests longstanding AHP.

  • Positive BHTT is a hallmark sign.

  • Acquired bilateral cases frequently present with symptomatic torsion.

  • Always investigate acute or bilateral presentations.


We have created a quick reference page on how to differentiate between a unilateral, bilateral congenital and bilateral acquired 4th nerve palsy. You can access that here. 



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