Clinical Examination of Cranial Nerves I to VI - Physiology & Procedure

Overview

CN I - Olfactory Nerve

Physiology

• Olfactory fibres - bipolar neurons that project through the cribriform plate to the olfactory bulb, then to the primary olfactory cortex (piriform cortex) and limbic areas
• Trigeminal fibres - also present in the epithelium, responding to irritation and temperature (not true olfaction)

Examination Procedure

• Testing is often omitted unless inferior frontal lobe disease is suspected (e.g., meningioma, cribriform plate fracture)
• Ask the patient to close their eyes
• Occlude one nostril at a time
• Present a non-irritant odorant (toothpaste, coffee, vanilla) - avoid ammonia or alcohol as these stimulate trigeminal (not olfactory) fibres
• Ask the patient to identify the smell

Harrison's Principles of Internal Medicine 22E, p. 3424 | Scott-Brown's Otorhinolaryngology Head & Neck Surgery - Vol 2

CN II - Optic Nerve

Physiology

• Retinal ganglion cell axons form the optic nerve, which carries visual information to the optic chiasm
• At the chiasm: nasal fibers decussate (cross), temporal fibers remain ipsilateral - this arrangement means lesions at different points produce characteristic field defects
• Beyond the chiasm: optic tract → lateral geniculate nucleus (thalamus) → optic radiations → primary visual cortex (V1, occipital lobe)
• A separate retinomesencephalic pathway projects to the pretectal area to mediate the pupillary light reflex

Examination Procedure

• Use a Snellen chart at 6 m (20 ft) or a near-vision card at 30 cm
• Test each eye separately with optical correction (glasses/contacts) in place
• If no chart available, use counting fingers, hand motion, or light perception as fallback

• Face the patient at ~0.6-1.0 m (2-3 ft)
• Place your hands at the periphery of your visual field, in the plane equidistant between you and the patient
• Ask the patient to look directly at your face and indicate when they see finger movement
• Test all four quadrants (inferior then superior) by moving the index finger of each hand, or both simultaneously
• Compare patient's field to your own as a baseline

• Examine the optic disc: note color, size, margin sharpness, and elevation
• Look for papilloedema (blurred disc margins, venous engorgement, absence of venous pulsations)
• Examine the retinal vasculature for caliber changes, AV nicking, hemorrhages, and exudates

Harrison's Principles of Internal Medicine 22E, p. 3424

CN III, IV, VI - Oculomotor, Trochlear, Abducens

Physiology

• Afferent: retinal ganglion cells → optic nerve → optic tract → pretectal nucleus (midbrain)
• Efferent: Edinger-Westphal nucleus → preganglionic parasympathetic fibers via CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae
• Both direct (ipsilateral) and consensual (contralateral) responses occur because pretectal fibers project bilaterally

1. Convergence (bilateral medial recti via CN III)
2. Accommodation (ciliary muscle contracts via CN III parasympathetics - lens thickens)
3. Miosis (sphincter pupillae contracts via CN III)

Examination Procedure

• Inspect in ambient light: note size, shape, equality (anisocoria)
• Shine a bright light into each eye:
  • Direct reflex: same eye constricts
  • Consensual reflex: opposite eye constricts simultaneously
• Relative Afferent Pupillary Defect (RAPD) / Swinging flashlight test: alternate light rapidly between eyes; if the pupil dilates when light moves to it, RAPD is present (CN II lesion on that side)

• Ask patient to look at a distant target, then follow your finger as it moves toward the bridge of their nose
• Normal response: pupils constrict (miosis) and eyes converge

• Check for ptosis (drooping - suggests CN III palsy or Horner syndrome)
• CN III palsy: complete ptosis + "down and out" eye position + fixed dilated pupil

• Ask patient to hold their head still
• Move your finger slowly in an "H" pattern covering all cardinal positions of gaze
• Observe for:
  • Paresis (limited movement in one direction)
  • Nystagmus (involuntary oscillation - assess at 45° lateral gaze, not extreme gaze)
  • Diplopia (true diplopia resolves on closing one eye)
  • Smooth pursuit vs. saccadic breakdown
• Corneal light reflex test: compare reflections of bright light off both pupils to detect misalignment

Harrison's Principles of Internal Medicine 22E, p. 3424 | Goldman-Cecil Medicine | Bradley and Daroff's Neurology in Clinical Practice

CN V - Trigeminal Nerve

Physiology

• V1 (Ophthalmic): forehead, scalp, upper eyelid, cornea, nose (above nostril)
• V2 (Maxillary): cheek, lower eyelid, upper lip, upper teeth/gum, hard palate, nose (below nostril)
• V3 (Mandibular): lower lip, lower teeth/gum, chin, jaw; also carries the motor root to muscles of mastication (masseter, temporalis, pterygoids)

• Main (principal) sensory nucleus (pons): fine touch
• Spinal nucleus (descends into cervical cord): pain and temperature
• Mesencephalic nucleus: proprioception from jaw

• Afferent: V1 (ophthalmic branch) → trigeminal sensory nucleus
• Efferent: facial nerve (CN VII) → orbicularis oculi → blink

Examination Procedure

• Test light touch with a wisp of cotton
• Test pain/temperature with a pin or cold object
• Compare symmetry V1 (forehead), V2 (cheek), V3 (jaw/chin) on both sides
• Two modalities from different anatomical pathways (touch + temperature) suffice for screening

• Ask patient to look up and away
• Lightly touch the cornea (not the sclera) with a fine wisp of cotton from the lateral side
• Normal response: bilateral blink (direct + consensual)
• Absent blink with intact touch sensation suggests CN VII lesion; absent blink with absent corneal sensation suggests CN V (V1) lesion

• Ask patient to clench the jaw - palpate masseter and temporalis for bulk and symmetry
• Ask patient to open the mouth against resistance - jaw deviates toward the side of weakness (pterygoids)
• Test the jaw jerk reflex: tap the chin downward while mouth is slightly open - normally minimal or absent; exaggerated = upper motor neuron lesion above the pons

Harrison's Principles of Internal Medicine 22E, p. 3424 | Gray's Anatomy for Students | Bradley and Daroff's Neurology in Clinical Practice

Summary Table: CN I-VI Examination

• CN I has no thalamic relay - unique among sensory nerves
• CN IV is the only CN to exit dorsally and decussates entirely
• The parasympathetic fibers of CN III run on the outer surface of the nerve - making them vulnerable to compressive lesions (aneurysm) before motor fibers are affected, causing a "surgical" (dilated) pupil
• The abducens nucleus is a conjugate gaze center, not just an abductor - a nuclear CN VI lesion causes an ipsilateral gaze palsy (both eyes), not just isolated abduction failure
• The corneal reflex tests both CN V (afferent) and CN VII (efferent) simultaneously