Bitesize Orthoptics - 4th nerve palsy
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Today’s Bitesized Orthoptics is looking at how to differentiate between a unilateral, bilateral congenital and bilateral acquired 4th nerve palsy. It is important to know the difference because the presentation can be subtle, and knowing the clues can help you differentiate faster in clinic or exams.
Unilateral Fourth Nerve Palsy – Congenital vs Acquired
Unilateral fourth nerve palsy is the most common presentation and may be either congenital or acquired. A hallmark feature is an abnormal head posture (AHP), typically a head tilt and turn towards the unaffected side with a slight chin depression — this is to minimise diplopia and obtain binocular single vision. In longstanding congenital cases, this chronic torticollis can lead to facial asymmetry, what you will see here is a subtle reduction in the distance between the lateral canthus and the corner of the mouth on the side of the tilt. These patients often report intermittent diplopia or none at all, as they may have developed suppression or large vertical fusional amplitudes. A significant hyperphoria may be observed with the AHP in place, often larger than 20 prism dioptres, particularly at near fixation. Some also present with a horizontal deviation, either esotropia or exotropia, typically greater than 8 prism dioptres. An objective excyclodeviation may be visible on fundus examination, even if the patient is asymptomatic for torsion. Saccadic velocities are usually normal, though downward saccades in adduction may be slightly reduced. A key clinical sign is a positive Bielschowsky head tilt test (BHTT). This will show an increased hypertropia on head tilt toward the affected side, this helps differentiate it from other causes of vertical deviation.
Bilateral Fourth Nerve Palsy – Congenital
Bilateral congenital fourth nerve palsies are less common and may be subtle in presentation. Patients often adopt a compensatory chin-down posture to minimise the vertical misalignment and maintain fusion, though in milder cases there may be a slight head tilt and turn towards the less affected side. A classic feature is a V-pattern esotropia, often with hypertropia of the non-fixing eye, which can be mistaken for childhood primary esotropia. Many cases also demonstrate inferior oblique overaction. These patients do not report torsional symptoms, as the condition is longstanding and therefore the visual system adapts. The Bielschowsky head tilt test is typically positive bilaterally, and careful examination is needed to rule out other vertical strabismus patterns. Recognising the signs of bilateral congenital fourth nerve palsy helps avoid misdiagnosis and unnecessary intervention, particularly in children who present with complex strabismus patterns.
Bilateral Fourth Nerve Palsy – Acquired
Acquired bilateral fourth nerve palsies are often secondary to trauma or intracranial pathology and usually present with more striking clinical signs. Patients commonly experience torsional diplopia, often described as tilting or rotation of images. To compensate, they adopt a marked chin-down posture, sometimes combined with a head tilt. On examination, there is a reversal of hypertropia in right and left gaze, and similarly in dextro- and laevo-depression, due to alternating superior oblique underaction. A positive Bielschowsky head tilt test is typically observed bilaterally. As with congenital cases, a V-pattern esotropia may also be present, along with hypertropia of the non-fixing eye. The presence of symptomatic torsion, the pattern of deviation, and recent-onset diplopia are key distinguishing features that should prompt further neurological investigation. Timely identification is crucial to guide appropriate imaging and treatment planning.
