CRANIAL NERVES (I–XII)

This chapter provides a detailed, nerve-by-nerve approach to the cranial nerves. It integrates relevant anatomy with bedside examination techniques, patterns of UMN and LMN lesions, and clinical localization. Common exam scenarios and frequently tested questions are highlighted to strengthen diagnostic accuracy.

The cranial nerves arise from the brain and brainstem and are responsible for motor, sensory, and autonomic functions of the head and neck. Proper cranial nerve examination allows precise localization, often even before imaging.

Functional Classification

Pure sensory: CN I, II, VIII
Pure motor: CN III, IV, VI, XI, XII
Mixed: CN V, VII, IX, X

Anatomical Localization

Forebrain: CN I
Diencephalon: CN II
Midbrain: CN III, IV
Pons: CN V–VIII
Medulla: CN IX–XII

Olfactory Nerve (CN I)

Function

Sense of smell

Examination

Each nostril is tested separately using non-irritant odors (coffee, soap). Irritant substances (ammonia) should be avoided as they stimulate trigeminal nerve endings.

Abnormalities

Anosmia – complete loss of smell
Hyposmia – reduced smell
Parosmia – distorted smell perception

Causes of Anosmia

Upper respiratory infections
Nasal obstruction
Head injury (cribriform plate fracture)
Frontal lobe tumors
Neurodegenerative disorders (Parkinson disease)

Clinical Correlation

Loss of smell after head injury strongly suggests shearing of olfactory filaments.

Optic Nerve (CN II)

Functions

Vision
Afferent limb of pupillary light reflex

Examination

1. Visual Acuity

Assessed using Snellen chart. Reduced acuity suggests optic nerve, retinal, or refractive pathology.

2. Visual Fields

Tested by confrontation method.

Visual Field DefectLocalization

Monocular blindnessOptic nerve

Bitemporal hemianopiaOptic chiasm

Homonymous hemianopiaOptic tract/radiations

QuadrantanopiaTemporal/parietal lobe

3. Fundus Examination

Look for:

Papilledema
Optic atrophy
Retinal hemorrhages
Exudates

Papilledema indicates raised intracranial pressure, not optic nerve disease.

Optic Atrophy

Primary – demyelination, compression
Secondary – papilledema resolution

Oculomotor (III), Trochlear (IV), Abducens (VI) Nerves

These nerves control extraocular movements.

Oculomotor Nerve (CN III)

Functions

Eye movements (most muscles)
Pupillary constriction
Eyelid elevation

Examination

Ptosis
Eye position
Pupillary reaction
Extraocular movements

CN III Palsy

Eye deviated down and out
Ptosis
Dilated pupil (if parasympathetic fibers involved)

Pupil-involving CN III palsy suggests compressive lesion (aneurysm).

Trochlear Nerve (CN IV)

Function

Innervates superior oblique muscle

Lesion

Vertical diplopia
Worse on looking down and in
Patient tilts head away from lesion

Abducens Nerve (CN VI)

Function

Lateral rectus muscle

Lesion

Inability to abduct eye
Commonly affected in raised ICP due to long intracranial course

Facial Nerve (CN VII)

Functions

Facial expression
Taste (anterior 2/3 tongue)
Lacrimation and salivation

Bell’s Palsy

Acute LMN facial palsy
Often idiopathic
Good prognosis

Vestibulocochlear Nerve (CN VIII)

Functions

Hearing (cochlear)
Balance (vestibular)

Hearing Tests

Rinne’s test
Weber’s test

Vestibular Dysfunction

Vertigo
Nystagmus
Gait imbalance

Central causes produce direction-changing nystagmus.

Glossopharyngeal (IX) & Vagus (X) Nerves

Functions

Swallowing
Palatal movement
Gag reflex
Voice

Examination

Say “ah”
Observe palatal movement
Assess voice quality

Palate deviates away from the lesion in LMN vagal palsy.

Accessory Nerve (CN XI)

Functions

Sternocleidomastoid
Trapezius

Examination

Shoulder shrug
Head rotation

Hypoglossal Nerve (CN XII)

Function

Tongue movements

Examination

Protrude tongue
Look for wasting, fasciculations

Tongue deviates towards lesion in LMN palsy.