Cranial Nerve Examination: A Practical Guide

Introduction

The cranial nerve examination is one of the most commonly assessed OSCE stations in medical finals, and one of the most clinically useful examinations in neurology. Its power lies in localisation: because the twelve cranial nerves have precisely known courses through the brainstem, skull base and orbit, the specific combination of nerves affected tells you where a lesion must be, often before any imaging is performed.¹

This article covers how to perform each component, what the classic abnormal findings mean, and - most importantly - how to reason from a pattern of findings to an anatomical site. It complements the articles on Bell's palsy, vestibular schwannoma and stroke, where these signs frequently appear.

Before you begin

• Introduce yourself, confirm the patient's identity, explain what you would like to do and obtain consent
• Position the patient sitting comfortably, roughly at your eye level and about an arm's length away
• Expose appropriately - the head and neck need to be visible
• General inspection first - look from the end of the bed for facial asymmetry, ptosis, obvious wasting, abnormal posture, hearing aids, walking aids, or a nasogastric tube (suggesting swallowing difficulty)
• Ask about pain before touching the patient

CN I - Olfactory

Test: ask whether the patient has noticed any change in their sense of smell or taste. Formal testing uses standardised odours presented to each nostril separately with the other occluded, though in most OSCE settings the question alone is accepted.

• Anosmia - causes include upper respiratory tract infection (much the commonest), head trauma shearing the olfactory fibres at the cribriform plate, nasal disease, a frontal lobe or olfactory groove meningioma, and Parkinson's disease, in which hyposmia can precede motor symptoms by years
• Kallmann syndrome - anosmia with hypogonadotropic hypogonadism, a classic association

CN II - Optic

The optic nerve is assessed in five parts, conventionally remembered as acuity, fields, reflexes, colour and fundoscopy.

1. Visual acuity - Snellen chart at the correct distance, testing each eye separately, with the patient's usual glasses on (correcting refractive error, which is not a neurological problem)
2. Visual fields - confrontation testing, comparing the patient's field with your own, quadrant by quadrant, each eye separately. Also test for visual inattention by presenting stimuli in both fields simultaneously
3. Pupillary reflexes - direct and consensual responses to light, the swinging light test for a relative afferent pupillary defect, and accommodation
4. Colour vision - Ishihara plates; red desaturation (asking whether a red object looks less vivid in one eye) is a sensitive early sign of optic nerve disease
5. Fundoscopy - looking for papilloedema, optic atrophy, and retinal pathology

CN III, IV and VI - Oculomotor, trochlear and abducens

These three are tested together because they act as a unit to move the eye. Test: inspect for ptosis and resting eye position, then ask the patient to follow your finger through an 'H' pattern, keeping their head still, and ask about diplopia at each position. Check for nystagmus at the extremes of gaze.

CN V - Trigeminal

Test: sensation in all three divisions (ophthalmic V1, maxillary V2, mandibular V3) bilaterally to light touch and, if indicated, pinprick and temperature; motor function by palpating masseter and temporalis while the patient clenches, and testing jaw opening against resistance; and the corneal reflex (afferent V1, efferent CN VII) with a wisp of cotton wool - though this is often only mentioned rather than performed in OSCEs.

• Sensory loss in a trigeminal distribution - consider a cerebellopontine angle lesion, cavernous sinus pathology, or a brainstem lesion
• Reduced or absent corneal reflex - often the earliest trigeminal sign, and an important finding in vestibular schwannoma
• Jaw deviation towards the weak side on opening, from unopposed action of the healthy pterygoid
• Trigeminal neuralgia - severe paroxysmal facial pain, but with a characteristically normal examination; sensory loss should prompt imaging for a secondary cause

CN VII - Facial

Test: inspect for facial asymmetry at rest, then ask the patient to raise their eyebrows, screw their eyes tightly shut (and try to open them against resistance), puff out their cheeks, show their teeth, and purse their lips. Ask about taste on the anterior two-thirds of the tongue and about hyperacusis.

• Lower motor neurone facial palsy - Bell's palsy, Ramsay Hunt syndrome (with a vesicular rash in the ear), parotid tumour, cerebellopontine angle lesion, or Lyme disease (which can cause bilateral palsy)
• Upper motor neurone facial weakness - stroke or other contralateral hemispheric lesion, with forehead sparing
• Inability to close the eye - risks exposure keratopathy and needs active eye protection

CN VIII - Vestibulocochlear

Test: gross hearing by whispering a number in each ear while masking the other, then perform Rinne's and Weber's tests with a 512 Hz tuning fork. Assess balance and, where vertigo is the complaint, consider positional testing.

CN IX and X - Glossopharyngeal and vagus

Test: listen to the patient's voice for hoarseness or a nasal quality, ask them to cough (a 'bovine' cough without the normal explosive onset suggests recurrent laryngeal nerve palsy), ask them to open their mouth and say 'aah' while observing palatal movement, and assess swallow if clinically indicated. The gag reflex (afferent IX, efferent X) is unpleasant and usually only mentioned rather than performed.

• Uvula deviates AWAY from the side of the lesion - the healthy side pulls it across, so the uvula points away from the weak side
• Hoarseness or bovine cough - recurrent laryngeal nerve palsy; consider lung cancer, thyroid surgery, or an aortic arch lesion given the left recurrent laryngeal nerve's course through the chest
• Dysphagia and nasal regurgitation - bulbar or pseudobulbar palsy
• Bulbar palsy (lower motor neurone) - a wasted, fasciculating tongue with a nasal voice, seen in motor neurone disease and Guillain-Barre syndrome; pseudobulbar palsy (upper motor neurone) - a spastic tongue, brisk jaw jerk and emotional lability

CN XI - Accessory

Test: ask the patient to shrug their shoulders against resistance (trapezius) and to turn their head to each side against resistance while you palpate the opposite sternocleidomastoid.

• Note the crossed innervation: the right sternocleidomastoid turns the head to the left, so weakness turning the head to the left indicates a right sternocleidomastoid (and therefore right accessory nerve) problem
• Causes of palsy: iatrogenic injury during posterior triangle neck surgery or lymph node biopsy is the commonest, plus trauma, and lesions at the jugular foramen or brainstem

CN XII - Hypoglossal

Test: inspect the tongue in the floor of the mouth at rest for wasting and fasciculation (fasciculations are unreliable to assess in a protruded tongue), then ask the patient to protrude it and move it side to side, and to push it into each cheek against your finger.

• Tongue deviates TOWARDS the side of the lesion in a lower motor neurone palsy - the healthy genioglossus pushes it across
• Wasting and fasciculation - a lower motor neurone sign; a wasted, fasciculating tongue is a classic and important finding in motor neurone disease
• A spastic, slow-moving tongue without wasting suggests an upper motor neurone (pseudobulbar) lesion

Localising from the pattern

The real diagnostic power comes from recognising combinations of affected nerves, because cranial nerves that travel together are affected together by a single lesion.

Completing the examination

• Thank the patient and ensure they are comfortable and appropriately covered
• State what you would do to complete the examination: a full peripheral neurological examination of the upper and lower limbs, cerebellar examination, fundoscopy if not already performed, otoscopy, blood pressure, and a swallow assessment where bulbar function is impaired
• Summarise your findings clearly, stating the abnormal nerves, whether the lesion is upper or lower motor neurone, and your proposed anatomical localisation
• Suggest relevant investigations - typically MRI brain for a structural cause, alongside bloods and any condition-specific tests