🩺 Cranial Nerve Examination

The cranial nerve examination is a comprehensive assessment of the twelve nerves arising from the brain and brainstem. It provides vital information regarding the integrity of the central nervous system and the base of the skull. The exam evaluates special senses (vision, hearing, smell), motor control of the face, eyes, and tongue, and autonomic functions (pupillary response). Systematic performance is crucial, as individual nerve deficits can pinpoint the exact anatomical location of intracranial pathology, from the cortex to the brainstem.

The cranial nerve exam is indicated for any patient presenting with headache, visual disturbances (diplopia, loss of vision), facial weakness, hearing loss, or speech and swallowing difficulties (dysarthria/dysphagia). It is a critical part of the assessment in suspected stroke, transient ischaemic attack (TIA), brain tumours, and multiple sclerosis. It also aids in the diagnosis of cranial neuropathies like Bell’s palsy or trigeminal neuralgia. Systematic evaluation is required for patients with unexplained dizziness or balance issues to differentiate central from peripheral vestibular causes.

There are no absolute contraindications to most components of the cranial nerve examination. However, specific tests such as the vestibular-ocular reflex (Head Impulse Test) should be avoided in patients with suspected cervical spine instability or severe neck pain. Evaluation of the corneal reflex should be performed sparingly and with caution to avoid corneal abrasion, particularly in patients with pre-existing ophthalmic conditions. Use of aromatic substances for olfactory testing should be avoided if the patient has known severe respiratory irritation to specific scents. Assessment of CN XI (Accessory) should be modified in patients with recent neck surgery or acute shoulder injury.

The examination requires a pen-torch for pupillary reflexes and inspection of the oropharynx. A Snellen chart (or near-vision card) is used for visual acuity, and Ishihara plates are needed for colour vision. A 128Hz tuning fork is used for Rinne’s and Weber’s tests (CN VIII). A cotton wool wisp and a 'Neuro-tip' are required for the trigeminal sensory assessment. A fundoscope is essential for inspecting the optic disc. For CN I, scent stimulants (e.g., coffee or peppermint) may be used, though this is often skipped in general clinical practice. A tongue depressor may be useful for visualising the palate and uvula.

Begin with CN I (Olfaction, often deferred). CN II involves visual acuity, fields, reflexes (direct/consensual), and fundoscopy. CN III, IV, and VI are tested via eye movements (H-test) and accommodation. CN V is assessed via facial sensation and muscles of mastication (jaw clench/opening). CN VII is evaluated through facial expressions (raise eyebrows, close eyes, smile, puff cheeks). CN VIII involves hearing (whisper test) and tuning fork tests. CN IX and X are checked via palatal movement ('Ah') and cough. CN XI involves shoulder shrug and head turn against resistance. CN XII is assessed by tongue inspection (wasting/fasciculation) and protrusion. Common practice includes checking the jaw jerk if UMN pathology is suspected.

Interpretation focuses on identifying focal deficits that localise pathology to the forebrain, midbrain, pons, or medulla. For example, a 3rd nerve palsy (down and out eye, ptosis, fixed dilated pupil) suggests a midbrain lesion or compression (e.g., PCom aneurysm). Facial nerve (VII) lesions are distinguished by forehead involvement: LMN lesions (Bell's) affect the entire half of the face, while UMN lesions (Stroke) spare the forehead due to bilateral cortical innervation. Bulbar vs. Pseudobulbar palsy can be differentiated by the appearance of the tongue (wasted/fasciculating vs. spastic) and the character of the gag reflex. Hearing loss interpretation requires combining Rinne’s (conductive) and Weber’s (sensorineural) findings.

A common error is failing to check for 'ptosis' before testing eye movements, which can obscure pupil assessment. Many students perform the 'H-test' too quickly, causing the patient to lose focus or missing subtle nystagmus at the extremes of gaze. Neglecting to offer a fundoscopy or a formal visual field assessment (Confrontation) is a frequent omission in OSCEs. During the facial nerve (CN VII) exam, many forget to ask about changes in taste or hearing (hyperacusis), focusing only on motor power. Finally, testing the gag reflex is often mentioned but rarely appropriate or necessary in a standard clinical exam; checking for uvula deviation is usually sufficient.

Ensure the room lighting is dimmed slightly for the pupillary reflex to be clearly visible. When testing the Trigeminial nerve (CN V), remember to test all three divisions (Ophthalmic, Maxillary, Mandibular) symmetrically. For the Extraocular muscles (CN III, IV, VI), ask the patient to report any double vision during the H-test. When testing the Accessory nerve (CN XI), remember it only innervates the Trapezius and Sternocleidomastoid. Always wash your hands before and after touching the patient's face. Make sure the patient's head is still during the H-test; only the eyes should move.

Critically linked to the 'Neurology' and 'Ophthalmology' sections of the MLA content map. Students must know the exit points (foramina) of the cranial nerves and the brainstem levels (2-2-4-4 rule: CN 1-2 above brainstem, 3-4 midbrain, 5-8 pons, 9-12 medulla). Integration of the pupillary light reflex pathway (Afferent CN II, Efferent CN III) is a common viva topic. Understanding the significance of papilloedema (raised ICP) versus optic atrophy is essential. Documentation must accurately describe the findings for each nerve (I-XII) clearly.