Lesions of the vestibulocochlear nerve (CN VIII)

CN VIII is routinely tested in the unconscious ICU patient as one of the the sensory components of the vestibulo-ocular and oculocephalic reflexes.  In the conscious patient, one may also test hearing directly. It is a well-known phenomenon among ICU staff to mistake coma for deafness, and to raise their voices to a frighteningly high volume when addressing a semi-conscious patient.

Lesions of the vestibulocochlear nerve

Expected features

  • Hearing loss
  • Absent oculocephalic reflex on the affected side
  • Absent caloric reflex on the affected side

Causes of a unilateral vestibulocochlear nerve lesion

Unilateral hearing loss:

  • Tumour (cerebellopontine angle, acoustic neuroma)
  • Fracture of the petrous temporal bone
  • Vascular disease of the internal auditory artery

Unilateral vestibular reflex loss:

  • Tumour (cerebellopontine angle, acoustic neuroma)
  • Fracture of the petrous temporal bone
  • Vascular disease of the internal auditory artery
  • Vestibular neuritis

Causes of a bilateral vestibulocochlear nerve lesion

Bilateral hearing loss:

  • Industrial deafness (not really a CN VIII lesion)
  • Presbyacousis (age-related hearing loss; not strictly speaking a CN VIII lesion)
  • Drug toxicity (gentamicin, salicylate etc)
  • Meniere's disease (also pathologically not a CN VIII lesion)
  • Brainstem lesion (eg. stroke)- freakishly rare

Bilateral vestibular reflex loss:

  • Demyelinating illness, eg. MS
  • Migraine

Important localising feature:

  • Lateral gaze deviation from a pontine lesion cannot be overcome by stimulating oculocephalic or ocularvestibular reflexes, whereas supranuclear (e.g. frontal lesions) can. (Thank you, Plum and Posner via LITFL )

The "Doll's Eye" oculocephalic reflex

  • Tests the vestibulocochlear nerve, the brainstem nuclei of the vestibulocochlear nerve, the fibers to the cerebellum, the fibers from the cerebellum, the medial longitudinal fasciculus (MLF) and the 3rd and 6th cranial nerves.
  • The cause of the unconsciousness in a patient with a negative oculocephalic reflex is some sort of destructive brainstem pathology or brain death. Conversely, an intact oculocephalic reflex suggests that the coma is of a non-structural cause, because much of the brainstem must be intact.

References

The LITFL summary of cranial nerve lesions is without peer in terms of useful information density.

Walker, H. Kenneth, W. Dallas Hall, and J. Willis Hurst. "Clinical methods." 3rd edition.(1990).Chapter 62. Cranial Nerve VII: The Facial Nerve and Taste - by Kenneth Walker

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