Cranial nerve 8 (CN 8) contains two components: auditory (cochlear) and vestibular. Both begin in the inner ear and travel to the brainstem: the auditory component projects to the cochlear nuclei (at the pontomedullary junction) and the vestibular component projects to the vestibular nuclei (in the medulla).

Auditory information travels from the inner ear through the auditory (cochlear) portion of CN 8 to arrive at the cochlear nuclei at the pontomedullary junction (Fig. 12–1). The cochlear nuclei project to the inferior colliculi of the lower midbrain via the lateral lemniscus, and also project to the superior olives. Each inferior colliculus projects to the ipsilateral medial geniculate nucleus (MGN) of the thalamus, and each MGN projects to the ipsilateral auditory cortex in the superior temporal gyrus (Heschel’s gyrus).

Auditory information crosses to become bilateral early in its connections within the brainstem, so unilateral hearing loss can only occur due to pathology of the inner ear or CN 8 (or rarely the entry zone of CN 8 or cochlear nuclei at the pontomedullary junction). Central lesions (in the brainstem or temporal lobe) only rarely cause deafness, and must be extensive and bilateral to do so. Therefore, central etiologies of deafness are usually associated with other signs due to involvement of neighboring structures. Left temporal lobe lesions can lead to deficits in word processing (pure word deafness) and right temporal lobe lesions can cause deficits in music processing (amusia).

Hearing loss due to a peripheral lesion is called conductive hearing loss if it caused by problems in the outer or middle ear, and called sensorineural hearing loss if it is due to problems in the cochlea or auditory component of CN 8. Both conductive and sensorineural etiologies of hearing loss may be acquired or may have a congenital/genetic basis. Acquired causes of hearing loss are listed below.

Acquired causes of conductive hearing loss include:

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  Outer ear: cerumen impaction

  
  
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  Middle ear: infection (otitis media), otosclerosis

Acquired causes of sensorineural hearing loss include:

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  Unilateral

  
  
  
  
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    Internal auditory artery infarct (the internal auditory artery [also called the labyrinthine artery] is usually a branch of the anterior inferior cerebellar artery [AICA])

    
    
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    Sudden sensorineural hearing loss (often idiopathic; may respond to steroids)

    
    
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    Ménière’s disease (see “Ménière’s Disease” below)

    
    
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    Vestibular schwannoma (also called acoustic neuroma; see “Vestibular Schwannoma” in Ch. 24)

  
  
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  Bilateral

  
  
  
  
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    Aging (presbyacusis)

    
    
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    Ototoxic medications (e.g., aminoglycosides)

    
    
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    Sequela of meningitis (especially in children)

    
    
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    Neurofibromatosis type II with bilateral vestibular schwannomas (see “Neurocutaneous Syndromes” in Ch. 24)

    
    
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    Susac’s syndrome (a syndrome causing branch retinal artery occlusion, sensorineural hearing loss, and encephalopathy; see “Susac Syndrome” in Ch. 19)

    
    
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    Superficial siderosis (which causes hearing loss accompanied by cerebellar dysfunction and/or upper motor neuron signs; see “Superficial Siderosis” in Ch. 19)

    
    
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    Mitochondrial disorders (see “Mitochondrial Diseases” in Ch. 31)

Sudden-onset unilateral hearing loss can be caused by infarction of the inner ear structures due to ischemia in the territory of the internal auditory artery (also called the labyrinthine artery), which is usually a branch of the AICA. This diagnosis should be strongly considered in patients whose acute-onset unilateral hearing loss is accompanied by vertigo or other brainstem or cerebellar symptoms/signs. If hearing loss is episodic with a sense of ear fullness and vertigo, Ménière’s disease should be considered.

Distinguishing Conductive Hearing Loss From Sensorineural Hearing Loss

The function of the eardrum and middle ear bones is to amplify sound waves for transmission into neural impulses. In conductive hearing loss, CN 8 works properly, but sound is not able to be transmitted to it by the outer/middle ear. In sensorineural hearing loss, the pathway for auditory conduction is not functioning properly. The Weber and Rinne tests can help to distinguish between conductive and sensorineural hearing loss (Table 12–1).

Weber’s Test

In Weber’s test, a vibrating tuning fork is placed on the top of the patient’s head. Normally, the pitch should be heard equally in both ears. If the pitch is louder on one side, this means that either the opposite (softer) side is affected by sensorineural hearing loss or the ipsilateral (louder) side is affected by conductive hearing loss (outer/middle ear blockage masks outside noise, making conduction by way of the skull louder in that ear).

Rinne’s Test

In Rinne’s test, a vibrating tuning fork is placed on the patient’s mastoid and then next to the ear, and the patient is asked which is louder. Normally, the sound next to the ear should be louder because it takes advantage of the amplifier function of the middle ear bones. In conductive hearing loss, the middle ear bones are not working normally (e.g., otosclerosis, otitis media), or sound cannot adequately access the middle ear (e.g., cerumen impaction, otitis externa). Therefore, the tuning fork will sound softer next to the ear (since the amplifying mechanism of the middle ear is not functioning), but louder on the mastoid because it uses skull vibration to arrive directly at CN 8, taking a “detour” around the problem in the outer or middle ear. In sensorineural hearing loss, the tuning fork will be heard as diminished both on the mastoid and next to the ear since the final pathway (inner ear/CN 8) is not functioning. Air conduction is still usually louder than bone conduction in sensorineural hearing loss, as is the case normally.

The inner ear vestibular structures (saccule, utricle, and semicircular canals) detect movements of the head (Fig. 12–2). This information is transmitted by the vestibular portion of each CN 8 to the vestibular nuclei in the dorsolateral medulla, which then communicate with the cranial nerve nuclei for eye movements (CNs 3, 4, 6) via the medial longitudinal fasciculus (MLF). Vestibular information is also communicated to the vestibulocerebellum (the flocculus/nodulus of the cerebellum) via the inferior cerebellar peduncles (see Ch. 8). This coordination of eye movements with head movements allows for the vestibulo-ocular reflex.

The Vestibulo-Ocular Reflex (VOR)

When attempting to fixate gaze on a point while moving the head, the eyes must move in the opposite direction from the head (Fig. 12–3). For example, if you ask a patient to continue looking at your nose while turning the head to the right, the patient’s eyes will have to move to the patient’s left to maintain fixation. This vestibulo-ocular reflex (VOR) is accomplished by communication between the vestibular nuclei and the abducens nucleus on the left so as to coordinate left eye abduction (via CN 6) and right eye adduction (via MLF to CN 3).

The brain determines which way the head is turning by comparing information from the left and right vestibular systems. The side to which the head turns stimulates greater vestibular system excitation on that side, and this is communicated to the ocular motor nuclei by way of the MLF to turn the eyes to the opposite side. Turning the head toward the left causes the left vestibular system to become more activated than the right side, and this activation pattern is communicated through the MLF to the right abducens nucleus to send the eyes to the right (via the right CN 6 and left CN 3). Turning the head toward the right causes the right vestibular system to become more excited than the left side, and this excitement pattern is communicated through the MLF to the left abducens nucleus to send the eyes to the left (via the left CN 6 and right CN 3).

The VOR can be suppressed when needed. For example, if you want to turn both your head and your eyes to the left to look at something on the left side, you would not want your eyes being dragged to the right against your will as you try to look left. Although the decision to suppress the VOR likely comes from the hemispheres, suppression of the VOR is mediated by the vestibular portion of the cerebellum (flocculus and nodulus). The ability to suppress the VOR can be tested by having the patient sit in a swivel chair with the thumb held out at arms’ length, and then rotating the chair from side to side while the patient maintains fixation on the thumb. If the VOR cannot be suppressed, the eyes will move in the direction opposite chair/head turning rather than following the thumb. Failure to suppress the VOR can be seen in neurodegenerative disorders such as progressive supranuclear palsy (see “Progressive Supranuclear Palsy” in Ch. 23).

Oculocephalic Reflex

The oculocephalic reflex can be used to assess the VORs in a comatose patient to determine the integrity of brainstem pathways (although this should not be attempted if the cervical spine is injured). To test the oculocephalic reflex, the head is turned passively to look for the presence of conjugate eye movements in the opposite direction. Although the term “doll’s eyes” is often used for this test, it can be unclear whether “having doll’s eyes” means having an intact oculocephalic reflex or an abnormal one, so it is more precise to state whether the oculocephalic reflex is present or absent in each direction. Presence of oculocephalic reflexes in a comatose patient indicates that the brainstem pathways are intact, suggesting that the comatose state is due to pathology affecting the hemispheres. Oculocephalic reflexes may be impaired with brainstem pathology, but can also be diminished or absent with sedating medications, complicating assessment in comatose patients who are sedated.

Cold Caloric Testing

If a patient has neck trauma prohibiting oculocephalic reflexes from being safely tested by passive head turning, cold caloric testing can be pursued (as long as the tympanic membrane is intact). In this test, cold water is placed into one ear. Cold water “turns off” the vestibular apparatus. If one side is turned off, the other side’s normal baseline activity is “greater than off,” so the brain thinks the head is moving toward the not-cold side. This causes the eyes to move away from the not-cold side and toward the cold side.

For example, cold water in the left ear causes the left ear to be “off,” so the brain detects more activity from the right vestibular system compared to the “off” left ear. This simulates the head turning toward the right, and so the eyes move to the left (if the VOR pathway is intact). If the frontal lobes are intact, there will be nystagmus with the fast phase in the direction opposite the slow phase (i.e., the fast phase of the nystagmus is away from the cold side).

Cold caloric testing assesses both brainstem and hemispheric function. If both the brainstem and hemispheres are functioning, there will be both a slow phase toward the cold water side and nystagmus with fast phase away from the cold water side. If the brainstem is functioning but there is hemispheric dysfunction, there will only be a slow phase (toward the cold water side) and no fast phase. If there is no slow phase at all, this suggests that there is either disruption of brainstem vestibulo-ocular pathways or that the patient has taken or received sedating medications.

Nystagmus

Nystagmus is abnormal spontaneous rhythmic movement of the eyes. The type of nystagmus most commonly seen has a slow-phase in one direction and a fast phase in the opposite direction (jerk nystagmus). When nystagmus is present, the goal is to determine if the cause is peripheral (inner ear or CN 8) or central (brainstem/cerebellum).