Auditory-vestibular nerve - anatomy, physiology, examination, syndromes
Anatomy
The vestibulocochlear nerve (CN VIII) is a purely sensory nerve consisting of two parts:
1. Cochlear Division (Hearing)
- Originates from the cochlea
- Carries auditory impulses to the auditory cortex (temporal lobe)
2. Vestibular Division (Balance)
- Originates from the semicircular canals, utricle, and saccule
- Carries information about head position and movement to vestibular nuclei and cerebellum
Physiology
Cochlear Function
- Responsible for: Hearing. Sound perception and discrimination
Vestibular Function
- Responsible for: Balance and posture. Spatial orientation. Coordination of eye and head movements. Maintenance of equilibrium
Examination
A. Hearing Examination
Whisper Test: Patient repeats whispered words from a distance.
Rinne Test (512 Hz tuning fork)
- Compares: Air conduction (AC). Bone conduction (BC)
- Normal: AC > BC
Weber Test: Tuning fork placed on forehead.
- Normal: Sound heard equally in both ears.
B. Vestibular Examination
Observe for: Vertigo. Nystagmus. Balance disturbances
Romberg Test: Patient stands with feet together and eyes closed.
Gait Examination: Normal gait. Tandem (heel-to-toe) walking
Nystagmus Examination: Observe eye movements in different directions of gaze.
Syndromes
Cochlear Syndrome
1.Sensorineural Hearing Loss
Causes: Acoustic trauma.Aging (presbycusis). Infections. Vestibular schwannoma
Clinical Features: Reduced hearing. Difficulty understanding speech. Tinnitus
Tuning Fork Findings
- Rinne: AC > BC (positive) Weber: Lateralizes to healthy ear
2.Conductive Hearing Loss
Causes: Cerumen impaction. Otitis media. Otosclerosis
Tuning Fork Findings
- Rinne: BC > AC (negative) Weber: Lateralizes to affected ear
Vestibular Syndrome
1.Peripheral Vestibular Syndrome: Lesion of the labyrinth or vestibular nerve.
Features: Severe vertigo. Nausea and vomiting. Horizontal or rotatory nystagmus. Tendency to fall toward affected side. Hearing symptoms may be present
Causes: Vestibular neuritis. Ménière disease. Labyrinthitis
2.Central Vestibular Syndrome: Lesion of vestibular nuclei, cerebellum, or brainstem.
Features: Mild vertigo. Prominent gait instability. Vertical or multidirectional nystagmus. Other neurological deficits often present. Hearing usually preserved
3.Vestibular Schwannoma (Acoustic Neuroma): Benign tumor of CN VIII.
Clinical Features: Progressive unilateral sensorineural hearing loss. Tinnitus. Vertigo or imbalance. Large tumors may compress CN V and CN VII