Bitesize Orthoptics - Red Flags in Third Nerve Palsy

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Today’s Bitesized Orthoptics is looking at red flags in patients with new onset 3rd nerve palsies. A 3rd nerve palsy can present in a variety of ways, but when the cause is not clearly established or there is no trauma, it must be treated as a potential emergency. Below are the key red flags you must consider when assessing a 3rd nerve palsy in the Orthoptic clinic.

1. Always Assume an Aneurysm Until Proven Otherwise

As an Orthoptist, you must assume that all non-traumatic or unexplained third nerve palsy are caused by an aneurysm until proven otherwise. You must refer these patients to eye casualty or neuro-ophthalmology to arrange urgent neuroimaging to rule out a potential aneurysm. Missing an aneurysm can have catastrophic consequences.

2. Assess the Pupil Carefully

A widely dilated, unreactive pupil in the context of 3rd nerve palsy is highly suspicious for compressive pathology (such as an aneurysm). However, if the pupil is not fully dilated, its reaction to light must be carefully assessed. A patient may have an associated Horner’s syndrome which means that they will not have the classic dilated pupil associated with a 3rd nerve palsy, as the Horner’s will cause relative pupillary constriction (atonic pupil). This may result in masking a compressive cause.

3. Check the Fourth Nerve (Trochlear Nerve)

A detailed assessment of the 4th cranial nerve should always be performed, as combined 3rd and 4th nerve involvement may suggest a lesion in the cavernous sinus or orbit. To properly test the 4th nerve, ask the patient to abduct the eye and then look down. The eye should show intorsion if the superior oblique muscle is functioning normally.

4. Evaluate the Fifth Nerve (Trigeminal Nerve)

The trigeminal nerve provides facial sensation and motor innervation to the muscles of mastication. Testing corneal sensitivity is especially important—if corneal reflex is reduced alongside a 3rd nerve palsy, this strongly suggests an orbital or cavernous sinus lesion rather than an isolated nerve palsy.

5. Look for Aberrant Regeneration

Aberrant regeneration (miswiring of regenerating third nerve fibres) can cause abnormal lid or pupillary movements, such as lid retraction on downgaze. If aberrant regeneration is present but there is no history of trauma, a compressive lesion should be strongly suspected and investigated further.

6. Test for Binocular Single Vision (BSV)

Loss of fusion mechanisms can occur in long-standing or complex third nerve palsy. Assessing the potential for binocular single vision is important for both diagnosis and future management, as disruption of fusion can make rehabilitation and prism management more challenging.

7. Consider Demyelinating Causes

In cases of pupil-sparing third nerve palsy with no clear vascular risk factors or trauma, demyelinating conditions such as multiple sclerosis should be investigated. This is particularly important in younger patients where microvascular causes are less likely.

Summary

When managing a third nerve palsy, the priority is to rule out life-threatening causes such as aneurysm. Careful examination of the pupil, neighbouring cranial nerves and corneal sensitivity provide crucial diagnostic clues to the potential underlying aetiology. Aberrant regeneration and atypical presentations should always raise suspicion of a compressive or demyelinating pathology.

In short: never dismiss an unexplained 3rd nerve palsy—investigate thoroughly and urgently.