Cranial Nerve Examination — Clinical Case

DCE Short Case — "Please examine this patient's cranial nerves"

Instruction

"Please examine this patient's cranial nerves. She has been complaining of a headache and double vision. You have 7 minutes for examination and 8 minutes for discussion." ^[1]

Patient brief (provided to examiner / simulated patient)

Patient: Mrs Eleanor Walsh, 58 years old, retired teacher, hypertensive. ^[1]

Findings the trainee elicits: The right eye is deviated down and out at rest with a complete ptosis. The right eye fails to adduct, elevate or depress; abduction is preserved. The right pupil is 6 millimetres and unresponsive to light; the left pupil is 3 millimetres and reactive. The swinging-flashlight test shows no relative afferent pupillary defect. The fundi, visual acuity, visual fields, facial nerve, hearing, palate, sternocleidomastoid, trapezius and tongue are all normal. Blood pressure 168/96. ^[1]

Systematic examination routine (model)

1. Set the stage: introduce, consent, position the patient sitting up at eye level, and state the plan to examine CN I to XII systematically.
2. Observe from the end of the bed: note the ptosis and the deviation of the right eye before touching the patient.
3. CN I: defer unless indicated.
4. CN II: acuity 6/6 both eyes; fields full by confrontation; pupils — note the anisocoria; swinging-flashlight test negative.
5. CN III, IV, VI: describe the ptosis, the down-and-out eye, the limitation of adduction, elevation and depression, and the fixed dilated pupil; abduction intact; no nystagmus; no diplopia complaint because the ptosis covers the eye.
6. CN V, VII, VIII, IX, X, XI, XII: all normal, stated and briefly tested.
7. Fundoscopy: normal discs. ^[1]

Presentation template (model)

"I examined Mrs Walsh, a 58-year-old woman with hypertension. On examination of the cranial nerves, the pupils are unequal: the right is 6 millimetres and unresponsive to light, the left is 3 millimetres and reactive, and there is no relative afferent pupillary defect. There is a complete ptosis on the right, and the right eye is deviated down and out. The right eye fails to adduct, elevate and depress, with abduction preserved. Visual acuity, fields and the fundi are normal, and the remaining cranial nerves 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 headache raises the possibility of a sentinel leak. This is a neurosurgical emergency. My immediate step is urgent CT angiography of the intracranial circulation, neurosurgical referral, and blood pressure control." ^[1]

Discussion questions

Q: "Why is the pupil involved, and why does that matter?" ^[1]

"The parasympathetic fibres to the pupil travel on the surface of the third nerve, so they are the first to be affected by a compressive lesion pressing from outside. An expanding aneurysm at the posterior communicating artery to internal carotid junction, which lies beside the third nerve, picks off these surface fibres and dilates the pupil. A microvascular, ischaemic palsy infarcts the centre of the nerve and spares the surface fibres, so the pupil is spared. Pupil involvement matters because it marks the palsy as compressive and therefore potentially fatal if the aneurysm ruptures. So this patient is imaged and referred urgently, never observed." ^[1]

Q: "How would a pupil-sparing palsy change your management?" ^[1]

"A pupil-sparing, complete palsy in a patient with strong vascular risk factors is managed conservatively as a microvascular palsy — I confirm the risk factors, control the blood pressure and diabetes, and expect recovery over three months, because the nerve infarcts and then regenerates. But the rule is a guide, not a law: I would still re-examine the pupil over the first 24 to 48 hours for any change, and any pupillary involvement, any partial or evolving palsy, or any severe pain would prompt urgent imaging. The threshold to image is lower than the textbook suggests, because a compressive lesion can occasionally present with a pupil-sparing palsy early in its course." ^[1]

Q: "What is the role of the swinging-flashlight test here?" ^[1]

"It excludes an afferent (optic nerve) contribution to the pupillary abnormality. A relative afferent pupillary defect is the most sensitive sign of an optic neuropathy, demonstrated by a paradoxical dilation of both pupils when the light swings into the affected eye ^[4]. In this patient the test is negative, which tells me the pupillary abnormality is efferent — a third nerve lesion — rather than afferent. The fixed dilated pupil here is from loss of the parasympathetic constrictor, not from an optic nerve problem, and that localises the lesion precisely to the third nerve."