Cranial Nerve Examination Viva

Short Case Viva Defence — Case A: Pupil-Involving Third Nerve Palsy

Candidate's opening statement (model answer)

"I examined Mrs Walsh, a 58-year-old woman with hypertension, who presents with a sudden severe right periorbital headache and double vision. On examination of the cranial nerves, the right eye is deviated down and out at rest with a complete ptosis. The right eye fails to adduct, elevate and depress, while abduction is preserved. Critically, the right pupil is 6 millimetres and unresponsive to light, while the left pupil is 3 millimetres and reactive. The swinging-flashlight test shows no relative afferent pupillary defect. The remaining cranial nerves, including the facial nerve and the tongue, are intact. ^[1]

In summary, this patient has a complete right third nerve palsy with the pupil involved. A pupil-involving complete palsy is compressive until proven otherwise; the classical cause is a posterior communicating artery aneurysm, and the sudden severe headache raises the possibility of a sentinel leak. This is a neurosurgical emergency, and my immediate step is urgent CT angiography of the intracranial circulation." ^[1]

Examiner probing questions and model answers

Q1: "Why does pupil involvement change your management?" ^[1]

"Because the parasympathetic fibres that constrict the pupil travel on the surface of the third nerve, while the motor fibres to the extraocular muscles travel in its centre. A compressive lesion, such as an expanding aneurysm at the posterior communicating artery to internal carotid junction, presses on the nerve from outside and picks off the surface pupillary fibres first, producing a dilated unresponsive pupil before or alongside the motor palsy. A microvascular, ischaemic palsy, by contrast, infarcts the centre of the nerve in a patient with diabetes or hypertension and spares the pupil. So a pupil-involving palsy points to a compressive, surgical lesion, and a pupil-sparing palsy points to a medical, microvascular one. The reason pupil involvement is so urgent is that an expanding aneurysm may rupture into a subarachnoid haemorrhage, which carries a high mortality, so I cannot afford to observe — I must image and, if confirmed, secure the aneurysm." ^[1]

Q2: "How sensitive is the rule of the pupil, and how do you apply it safely?" ^[1]

"It is a guide, not a law. The classic teaching — pupil-sparing and complete is microvascular, pupil-involving is compressive — holds in most cases, but there are two important exceptions. First, a pupil-involving palsy can rarely be microvascular, particularly in poorly controlled diabetes, where the ischaemia can involve the pupillary fibres. Second, and more dangerously, a compressive lesion can present with a pupil-sparing palsy, especially early, when it is partial or evolving; the aneurysm may not yet have compressed the surface fibres. So the safe application is this: a pupil-involving palsy is always imaged urgently. A pupil-sparing palsy that is complete and fits the microvascular pattern, in a patient with vascular risk factors, may be observed — but I re-examine the pupil every few hours for the first day or two, and any pupillary change, any incomplete palsy, or any severe pain prompts urgent imaging. In practice, many neuro-ophthalmologists image any new third nerve palsy that does not perfectly fit the microvascular pattern." ^[1]

Q3: "What would you do if the CT angiogram is negative?" ^[1]

"If the CT angiogram is negative but my clinical suspicion remains high — a painful, pupil-involving palsy — I proceed to digital subtraction angiography, which is the gold standard and can detect smaller aneurysms that CTA misses. I would also reconsider the differential: a cavernous sinus lesion such as a tumour, thrombosis or carotid-cavernous fistula can cause a painful third nerve palsy and may involve the neighbouring fourth, sixth and the first two divisions of the trigeminal nerve, so I would examine specifically for those. If the angiogram and imaging are negative and the picture is one of a painful pupil-involving palsy, I would involve neuro-ophthalmology and consider rare causes such as an inflammatory or infiltrative process. A microvascular cause becomes more likely if the vasculopathic risk factors are strong and the palsy resolves over the expected three months." ^[1]