Cranial nerves - Overview and Testing

Overview of anatomy

• I - Olfactory cortex (base of brain)
• II - Visual cortex > Visual radiation > Optic tract > Optic chiasm > Optic nerve
• III, IV - Midbrain
• V, VI, VII, VIII - Pons
• iX, X, XII, XII - Medulla

• IV is the only nerve that innervates contralateral side
• XI ascends into cranium then exits
• Some nerves are multi-modality formed from several components e.g. CN VII - face motor (somato-motor), tongue taste (viscero-sense), saliva/lacrimation (viscero-motor)

I - Olfactory (smell)

• Ask to indentify clove or cinnamon via each nostril with eyes closed

II - Optic (vision, pupillary responses)

• Light response - direct and consensual (II > pretectal region of midbrain > Edinger-Westphal parasympathetic nucleus > CN III bilaterally > ciliary mm & iris sphincter)
• Accommodation response (as above)
• Swinging torch test (looking for Relative Afferent Pupillary Defect or Marcus Gunn pupil)
• Visual field testing - to confrontation and to each eye (use red hat pin)
• Things to note - pupillary irregularity, loss of consensual reflex, near-light diassociation

III, IV, VI - Oculomotor, Trochlear, Abducens nerve (eye movements)

• VI - eye abduction (lateral rectus)
• IV - eye turn in and down (superior oblique)
• III - all other movements
• Check gaze in H-pattern
• Things to note - loss of conjugate gaze, nystagmus

V - Trigeminal (facial sensation, jaw muscles)

• Divisions V1 (ophthalmic), V2 (maxillary), V3 (mandibular)
• Motor - clench jaw, open jaw
• Reflex - jaw jerk (stretch reflex), corneal reflex (cornea > V1 > VII > orbicularis oculi)
• Sensation - Touch / Pinprick to forehead, cheeks, chin (optional - anterior 2/3 tongue sensation)
• Things to note - jaw deviation

VII - Facial (facial movements)

• Motor - Squeeze eyes shut, Blow out cheeks, Show teeth
• Special sense (optional) - Identify salt or sugar on tongue
  + lacrimation, submandibular/sublingual salivation
• Things to note - eye sparing in facial palsy (= LMN lesion)

VIII - cochlear component (hearing)

• Whisper each ear (with contralateral distractor)
• 256 Hz tuning fork
• Weber Test: normal = midline, lateralises towards conduction defect and away from sensorineural defect
• Rinne test: normal, sensorineural defect = AC > BC, conduction defect = BC > AC

VIII - vestibular component (balance)

• Hallpikes manoeuver - examine for nystagmus

IX - Glossopharyngeal (pharyngeal sensation)

• Sensory - pharynx, posterior 1/3 tongue
• Special sense (optional) - indentify
  + parotid salivation

X - Vagus (pharyngeal movement)

• Motor - Pharyngeal muscle - Say 'ahhh' look for symmetrical palatal elevation
• Gag reflex (IX > X) - test each side

XI- Spinal accessory nerve (trapezius, sternocleidomastoid)

• Motor - shrug shoulders (sternocleidomastoid), turn head to side and turn back against resistance (sternocleidomastoid)
• NB left SCM turns head right and vice versa

XII - Hypoglossal nerve (tongue movement)

• Tongue movement - push out, push into each cheek
• Things to note - fasciculations, spasticity, tongue deviation (towards weak side)