Chapter 6 - Auditory & Vestibular Function

In this chapter, the functions and clinical examination of the vestibulocochlear nerve (CN VIII) and its central connections will be discussed. The two distinct functions are in hearing and in control of balance. In the case of the auditory part of CN VIII, the symptoms are deafness or tinnitus (ringing in the ears). In the case of the vestibular part of CN VIII, the symptoms are vertigo or imbalance, although visual disturbance when moving may also be a complaint.

Auditory function

Testing

The vast majority of hearing problems result from peripheral disease, i.e., involvement of the eighth nerve or inner ear. Testing of the peripheral system at the bedside is simple and rewarding. For screening of persons who do not complain of hearing loss, asking them to compare the sound of rustling fingers or a ticking watch in the two ears is a useful test of acuity. This, combined with the Weber test (see below), is adequate. To this might be added the use of a whispered voice, which represents midrange frequencies that frequently are involved in neural deafness.

Two basic instruments can aid in testing the auditory system: a C512 tuning fork (C256 is adequate but not as sensitive; C128 is inadequate except for testing for hyperacusis and cutaneous and bony vibratory perception), and a mechanical watch (watch-ticking is in the 1,500 cps range). The watch is placed next to the patient's ear and gradually moved away. The distance at which the patient ceases to hear the tick is noted and compared with the distance from the opposite side. If the examiner has normal hearing, a useful comparison can be made. High-tone deafness is measured by this test. The C512 (or C256) fork is then used to test for lower tone falloff and, more importantly, to determine whether hearing loss is caused by defects in the conduction system (conductive deafness) or by damage to the inner ear-auditory nerve system (sensorineural deafness). This distinction is important since different types of conditions produce these types of deafness and the Weber test and Rinne test permit bedside differentiation of these conditions.

More detailed clinical evaluation, including special audiometric testing, is carried out in otolaryngological laboratories and can be very useful in differentiating cochlear (inner ear) disease from direct eighth-nerve involvement.

Weber test

Both the Weber and Rinne tests are most valuable in the patient with a documented hearing loss (see above). These tests are particularly focused on determining whether the loss is sensorineural or conductive. In the Weber test, the stem of a tuning fork is placed gently against a midline structure of the skull (i.e., the maxillary incisor teeth or vertex of the cranium or forehead) and the patient is asked where s/he hears the sound. Sound is transmitted to both ears through the air but particularly through the vibrations of the bones of the skull. If sound is transmitted to both sides equally, the sound is heard in the midline and it can be presumed that the conduction and neural apparatus is intact. With neural deafness, the sound transmits best to the normal side and the patient lateralizes the sound to that side. With conduction deafness, sound transmits best to the side of the deafness. This is thought to occur because ambient sound is prevented from getting to the cochlea on the blocked side. This causes the nervous system to amplify sounds on that side by sensitizing cochlear transduction. You can demonstrate this yourself by plugging an ear with your finger, causing conduction deafness, and then humming. The sound will be heard better on the occluded side.

By the way, you notice the effects of ambient sound on hearing acuity when you must talk to a friend at the top of your voice in a noisy, crowded room and then continue talking and walk into a silent room where you find yourselves shouting at each other.

Rinne test

In some cases the Rinne test can provide some additional information. This tests both bone and air conduction. The examiner places the butt of a vibrating tuning fork on the mastoid region, and when the patient ceases to hear the vibration, the examiner places the tines close to the external auditory meatus to check air conduction. Vibrations perceived through air are heard twice as long as those perceived through bone, so the normal individual reports, for example, hearing the bone vibration for 30 seconds and then continues to hear the vibration through air for another 30-60 seconds altogether. If there is conductive deafness, bony conduction is either normal or slightly enhanced, whereas air conduction is decreased. If there is neural deafness, both bone conduction and air conduction are equally suppressed. As with the watch tick, the examiner should compare the ability of both sides to perceive the fork. A comparison of the patient's ability to perceive the fork, as well as the watch tick, with the examiner's ability is also useful (Schwabach test).

Pathology

Conductive deafness results from processes that occlude the sound conduction pathways (the external auditory canal, tympanic membrane, middle ear, or the ossicles). These may be easily detected when they result from conditions that are visible with the otoscope (blockage of the external canal or rupture or scarring of the tympanic membrane). Some conditions of the middle ear, such as suppurative otitis media (where there is pressure in the middle ear due to infection), or serous otitis media (where there is obstruction of the auditory tube with a vacuum in the middle ear and retraction of the ear drum and accumulation of some serous fluid), may be visible, as well. Other conditions, such as otosclerosis, which results in progressive fusion of the ossicles, may not be detectible by observation and may require more testing.

There are many conditions that can damage the delicate hair cells of the organ of Corti or the auditory component of CN VIII. These conditions produce "sensorineural" hearing loss. By far, the most common cause of this is exposure to loud noises, which typically affects high-tone hearing. Other conditions should be considered in acute sensorineural hearing loss, including: infectious (usually viral) or inflammatory attack on the inner ear; ischemia (the labyrinthian artery usually arises from the anterior inferior cerebellar artery); or trauma (especially with fracture of the skull base). A more insidious loss of hearing can occur with Meniere syndrome. This condition occurs due to buildup of pressure in the inner ear due to obstructed resorption of endolymph. This pressure can cause "blowouts" of the membranes, with attacks of sudden vertigo that improves over hours (see the vestibular system below). It also affects hearing, with tinnitus (usually a buzz or hum) and hearing loss (usually of low tones). The hearing loss can be detected even between attacks of vertigo.

While damage to the cochlear nuclei, located at the lateral aspect of the pontomedullary junction (where CN VIII enters the brain), can cause unilateral hearing loss, damage to other regions of the central nervous system is unlikely to cause recognizable hearing loss. Beyond the cochlear nuclei the auditory system makes multiple decussations in the brainstem up to the level of the medial geniculate, and therefore auditory signals are bilaterally distributed in the brainstem, thalamus and primary auditory cortex (Heschl's gyri of the posterior-superior temporal lobes). Significant loss of hearing therefore, does not occur following unilateral lesions of the auditory system above the cochlear nuclei. Bilateral lesions may affect hearing but are usually so devastating as to preclude clinical testing of hearing (there are laboratory tests of hearing, described in Chapter 11, that may help localize brain stem lesions and do not require patient cooperation).

Damage to the central nervous system occasionally impairs localization of sound, even without affecting acuity. For example, localization difficulty can occur with large unilateral cerebral cortical lesions. Deficits in sound localization are most likely to occur when the primary auditory cortex is involved but is less frequently observed with large lesions of the frontal and/or parietal cortex. Such patients are unable, with eyes closed, to localize an auditory stimulus with their eyes closed in the auditory field opposite the damaged hemisphere. This can be tested at the bedside by asking the patient to reach for a sound (such as snapping fingers) with their eyes closed. Double simultaneous stimulation (DSS) can also be useful; the patient may extinguish (ignore) the stimulus opposite the lesioned hemisphere (some of this may be a neglect phenomenon, especially when the frontal and parietal lobes are involved).

Tinnitus

Tinnitus (ringing or buzzing in the ears) is a common complaint. It is usually due to some damage of cochlear hair cells, with spontaneous nerve activity being produced by the damaged cells. High-pitched tinnitus is most commonly due to damage to cells at the base of the cochlea due to excessive sound exposure. However, it can also result from virtually any cause of inner ear or CN VIII damage. Low pitch tinnitus (buzzing or humming) is less common in general, but may result from excessive pressure in the inner ear (conditions such as Meniere syndrome). Pulsatile tinnitus is most often due to turbulence in the carotid blood flow. This can be normal but it can also occur with carotid bruits (turbulence due to arterial narrowing). Tinnitus that is not accompanied by hearing loss can result from medications (notably high doses of aspirin), but often defies diagnosis. There is no cure for tinnitus (unless a curable cause of inner ear damage is identified), although it can occasionally be masked with other sounds.

Vestibular function

The vestibular apparatus of the inner ear is specialized to detect movement of the head and, to a lesser extent, position in space. The vestibular portion of CN VIII conveys these signals to the vestibular complex of the dorsolateral brainstem at the pontomedullary junction and also to the vestibulocerebellum (Figure 6-1). The chief symptom of damage to the system is vertigo, i.e., the illusion of movement. Stabilization of the eyes is one of the most important functions of the vestibular system. It is largely through this effect on eye movement that we can objectively evaluate the vestibular system. The fundamental reflex that we will discuss is the vestibulo-ocular reflex (VOR), which moves the eyes opposite to head movement in order to stabilize vision. Without a vestibular system, we would be unable to see anything clearly while the head is moving. There is another reflex, the fixation reflex, that we will discuss as well. This reflex attempts to fix the image of an object on the retina. Between these two reflexes our vision is actually quite good even when the head is moving (try reading a sign while nodding your head or shaking your head "no" and you are using these reflexes, predominantly the VOR).

We are particularly good at detecting angular acceleration (i.e., spinning, pitching or tumbling). If you think about it, these are the movements that would be most important to compensate for, since they would tend to move your eyes off of a visual target. Smooth, linear motion would not tend to blur vision as much, nor would acceleration in a straight line (linear acceleration). The organs of the inner ears that detect angular acceleration are the cristae within the semicircular canals. Since the canals are at right angles to one another in three major planes, angular acceleration in any direction will move fluid in the particular canals that are in the plane of movement because of the flow of endolymph (actually, in normal movements, the endolymph stays behind and the inner ear moves with the skull). At rest, the hair cells are tonically active, releasing transmitter that activates the peripheral end of the vestibular nerve fibers. The tonic activity on one side is balanced by the tonic activity in the other ear and the patient perceives that they are stable. Differential movement of fluid in one direction in the semicircular canal increases this activity in one ear and decreases it in the corresponding canal of the other ear, leading to a perception of movement and reflex movement of the eyes. Most diseases of the inner ear or vestibular nerve are destructive in nature, decreasing input from that ear. Therefore, the tonic firing level of the opposite canal system is no longer opposed and the patient perceives motion. Additionally, there will be VOR-induced eye movements that result from the brain attempting to move the eyes opposite to the direction of perceived motion. This will trigger competing reflexes (as part of the visual fixation reflex) that will result in eye movement in an attempt to maintain a stable image. This to-and-fro motion is termed nystagmus.

Actually, there are two major types of nystagmus, "jerk nystagmus" and "pendular nystagmus". The first type (and the type generated by the vestibular system) is" jerk nystagmus." In this type of nystagmus, there is relatively slow eye drift to one side produced by the VOR, with a fast compensatory "jerk" of the eyes to reacquire the visual target. Jerk nystagmus is named according to the direction of the fast movement, since this is the easiest to see. This type of nystagmus can normally be seen when an individual spins around. Here the nystagmus is initiated by the VOR produced by head movement. Spontaneous nystagmus is produced by vestibular damage because of the imbalance of inputs from the ears. Jerk nystagmus can also be elicited by the visual input of objects passing-by rapidly (for example, if one stares out the side window of a moving vehicle with posts or trees flashing past). In this case, the nystagmus is elicited by the fixation reflex that is attempting to lock onto and track objects visually. This type of jerk nystagmus has been termed "optokinetic" nystagmus or "railway" nystagmus.

The other major type of nystagmus is termed "pendular" nystagmus. This type of oscillation of the eyes does not have a fast and slow direction of movement but rather consists of an even motion from side to side. This is most often due to poor visual acuity (especially when young), and the fixation reflexes that stabilize the eyes are, therefore, poorly formed. This is more of a tremor of the eyes and does not reflect problems with the vestibular system.

If the two inner ears are damaged symmetrically, there is little in the way of vertigo or nystagmus. If the horizontal canals are damaged, there is predominantly horizontal nystagmus (see Figure 6-1), while damage to the anterior and posterior canals will tend to produce rotary nystagmus (clockwise or counterclockwise) due to the addition of a vertical component to the horizontal nystagmus. Damage to the inner ear does not produce vertical nystagmus. Rather, this suggests damage to the brain stem vestibular apparatus.

As an illustration of what happens with unilateral vestibular damage, let's consider the effects of sudden loss of the right labyrinthine system, for example, as in vestibular neuronitis (a common affliction, presumably viral in origin). In this example (Figure 6-2), the normal response of the now unopposed opposite (left) horizontal canal system is to tonically drive the eyes conjugately to the right. In an alert individual, there is a reflex attempt to contain the abnormal tonic drive. This checking attempt is called the fast component and in combination with the tonic or slow component, forms the rhythmic to-and-fro movement - nystagmus. The tonic component encompasses the vestibular-oculomotor brain stem systems, whereas the fast component depends on the integrity of the cerebral hemispheres. The right hemisphere, including cortex, basal ganglia, and diencephalon, is responsible for the fast component to the left and the left hemisphere for the fast component to the right, just as for voluntary and visual tracking horizontal gaze (see Figures 4-5 and 4-6). With loss of hemispheric function and preservation of basic brain stem functions (e.g., in a coma from sedative overdose), the fast component becomes weak, irregular, and finally disappears, leaving only tonic deviation of the eyes following vestibular-oculomotor activation. With acute unilateral hemispheric depression, such as caused by a middle cerebral artery occlusion, the fast component to the opposite side is depressed. The tonic component is predominant during vestibular-oculomotor activation and drives the eyes toward the side of the abnormal hemisphere, which is capable of little, if any, checking.

It is important to note that the brain has compensatory mechanisms for damage to the vestibular system. Therefore, with chronic, slowly progressive disease such as an acoustic neuroma (a tumor arising from the neurolemmal sheaths of the eighth nerve at the internal auditory meatus), a person is much less likely to complain of vertigo or to have significant nystagmus. This is true also in persons following recovery from an acute destructive process despite the lack of effective function in the destroyed system. Compensation for even massive damage to an inner ear is quite effective. The lack of symptoms and signs is due to central compensation and in large part, though not entirely, depends on visual fixation. If a patient with chronic disease or compensated acute disease closes their eyes, the examiner may be able to detect the reappearance of the nystagmus by using an electrical test of eye position (the electronystagmogram) or simply by feeling the elevated corneas move through the closed lids. Vertigo usually does not reappear, which suggests that there are means other than visual for suppressing the illusion of movement. An excellent example of the suppression of nystagmus and vertigo is seen in the figure skater who is subjected to marked acceleration and deceleration of the horizontal endolymph-cristae systems during every spin. Using visual fixation (a fix on one object as long as possible while spinning), skaters learn to suppress after-spin nystagmus and vertigo almost entirely. Imagine what figure skating would be like if this were not possible!

Examination

On examining a patient with suspected vestibular dysfunction, observation for nystagmus is of primary importance prior to formal testing. A person with, for example, acute right vestibular apparatus destructive disease has horizontal nystagmus with the tonic component toward the diseased right side (release of the normal left) and the fast component toward the left (see Figure 6-2). Usually s/he complains of vertigo (illusion of movement of self or environment), saying that the room is spinning in the direction of the fast component, to the left -- an illusion caused by the forced tonic movement of the eyes and retinae. This should be called object vertigo as opposed to subject vertigo, which is the sensation that the subject is spinning and which occurs almost exclusively with the eyes closed. Subject vertigo is the true vestibular illusion, unsuppressed by the retinal image. The patient usually complains that s/he feels s/he is rotating in the direction of the clinically observed fast component of the nystagmus.

Testing of the inner ear is not a simple matter. Asking the patient to focus on your nose while using your hands to rapidly move the head to either side ("head thrust") can provide some information. Usually, individuals are able to maintain good focus if the inner ears are intact since the head thrust activates the VOR. Inability to maintain fixation when doing this indicates damage to the inner ear.

Another test of vestibular function employs rotation of the patient in a spinning chair (a Barany chair). It is very awkward to do this in the clinic and creates considerable discomfort for the subject, particularly nausea. Additionally, it cannot test each ear individually (since the whole head is spinning and since both ears are moved simultaneously). Because of these limitations, the use of a spinning chair has largely been replaced by caloric testing, which is easier to carry out and tends to be less noxious. In addition, only one horizontal canal is involved and therefore evaluated in routine caloric testing, whereas both are involved in Barany rotation. Barany rotation is more useful for testing the vertical canals; this can also be done with bilateral simultaneous caloric testing with, however, some inconsistency in the results. Vertical canal testing is rarely necessary, so a rotating barber's chair need not be part of a physician's clinical armamentarium.

Caloric testing

Caloric testing is an elegant method for evaluating the integrity of the vestibular apparatus of each ear, independently. Caloric testing is carried out most simply by irrigating the external auditory canal (observed by otoscope to be unobstructed by wax, not infected, and with no tympanic perforation) with water warmer or colder than body temperature, the presumed resting temperature of the labyrinths. The differential warming or cooling of the horizontal semicircular canal where it lies closest to the external auditory canal causes a decrease or increase, respectively, in the specific gravity of the endolymph at that point. If the head is positioned so that the horizontal canal is vertical (see the position of the canal in Figure 6-3), significant convection currents are caused in the canal by the induced changes in specific gravity (Figures 6-4 and 6-5). Vertical upward currents are caused by warming because of the decreased specific gravity and, with the patient supine, the current in the horizontal canal is toward the ampulla and crista (see Figure 6-5). This direction of flow is excitatory to the crista, causing increased firing over the pathways diagrammed in Figures 6-1, 6-2, and 6-5. This results in vertigo (in which the patient feels that they are spinning toward the ear being irrigated with warm water), and there will be VOR-induced reflex movement of the eyes away from that ear. If the patient is awake and alert, the drift of vision that is produced by the VOR will result in a rapid corrective "jerk" of the eyes to try to keep them focused on a target. Nystagmus is the result. Remember, nystagmus is named by the direction of the fast phase, so that nystagmus "to the right" means nystagmus with the fast component to the right. In order to maintain clarity, many examiners use the term "right-beating" to clarify that they are referring to the direction of the fast phase. Cold-water irrigation will have opposite effects. With the horizontal canal in the vertical position (i.e., patient supine with their head on a slight pillow), cold-water irrigation increases the specific gravity of the endolymph closest to the external auditory canal. Therefore the fluid sinks and a current is created away from the crista/ampulla (see Figure 6-5). This decreases the spontaneous firing of the ipsilateral horizontal canal vestibular system and causes an imbalance with the resting tone of the opposite horizontal canal system becoming dominant. The eyes are thus driven tonically toward the irrigated side and the checking or fast component is opposite in direction. This same nystagmus (and concomitant vertigo) is seen in persons with destructive lesions of the vestibular apparatus (see Figure 6-2).

In performing caloric tests with warm water, 20 cc of approximately 48 degrees C water (higher temperature is painful) is irrigated into the external auditory canal, which should be clear of wax, uninfected, and with no tympanic membrane perforation (Figure 6-6). Each auditory canal should be irrigated separately for the same duration (30 seconds is convenient), and the time of onset of nystagmus from the beginning of irrigation, as well as its duration and direction should be recorded. The findings from the two sides should be compared; a difference of approximately 20% is considered significantly abnormal. At least five minutes should elapse between irrigations to allow the stimulated canal to return to body temperature. The patient should be asked whether s/he is experiencing spinning sensations or nausea and whether there is a difference between the two sides. If there is less vertigo on one side, you must consider that there is hypofunction of the inner ear on that side.

You may have already surmised that vestibular-oculomotor testing has considerable diagnostic usefulness in the unconscious patient since it is objective and not dependent on patient cooperation. The vestibular-oculomotor reflex pathway encompasses an expanse of the brain stem (upper medulla through mesencephalon) that contains much of the reticular formation necessary for the maintenance of consciousness. Caloric testing is good at assessing the integrity of the brain stem (see Chapter 17). There are two basic causes of depression of consciousness: diffuse bilateral hemispheric dysfunction; or dysfunction of the brain stem reticular formation (patients can also have both). Caloric testing provides a method for rapid screening to determine which of these causes is producing the depressed consciousness.

From our discussion of the mechanisms of the VOR it can be surmised that the patient with an intact reflex has an intact brain stem. Also, since the fast phase of nystagmus is mediated by activity in the cerebral cortex, a vestibulo-ocular reflex with tonic eye deviation but no fast, corrective movement, indicates that the brain stem is intact and that the cause of depressed consciousness is diffuse cortical depression. This is most often related to toxic, metabolic or drug-related effects. This occurs because the brain stem response is more resistant to these effects than is cerebral cortical function (of course, if brain activity is sufficiently depressed by toxic or metabolic upsets, even the brain stem can be ultimately affected). In cases of encephalopathy (i.e., depressed consciousness due to diffuse cerebral cortical suppression), caloric irrigation thus elicits only tonic deviation of the eyes. Warm caloric irrigation causes tonic conjugate deviation of the eyes to the side opposite the irrigation, and cold irrigation elicits deviation of the eyes toward the irrigated ear (Figure 6-7A).

An interesting and important observation is the finding of normal oculocephalic test results in the patient who is apparently in "coma". The normal slow component of nystagmus indicates the integrity of the brain stem and the normal rapid phase indicates that the cerebral cortex is awake, alert and functional. Therefore this "coma" is actually fictitious and the patient is more appropriately labeled as "catatonic".

Brain stem damage produces variable effects on the reflex depending on the location of the reflex. For example, a destructive process (e.g., infarction, hemorrhage or tumor) at the midbrain level involves the oculomotor complex with subsequent loss of the medial rectus portion of conjugate horizontal deviation, with preserved lateral rectus deviation during irrigation (Figure 6-7B). A bilateral lesion of the pons, involving the abducens nuclei and the proximate medial longitudinal fasciculi, destroys the vestibular-oculomotor reflexes entirely (Figure 6-7C). What effect would be seen after complete transection of the basis pontis sparing the tegmentum (see Figures 4-5 and 6-1)?

"Doll's eyes"

The poorly named "doll's eye" maneuver is a simple mechanical test that is particularly useful in the patient with depressed consciousness. More appropriately called the oculocephalic maneuver, it is composed of a rapid passive rotation of the head laterally, which causes an inertial flow of the horizontal canal endolymph in the opposite direction of the head rotation. As seen in Figure 6-8, the eyes are driven in a direction opposite the head rotation.

If the patient is awake, the hemispheric checking component (this has the same substrate as the fast component of the nystagmus) keeps the eyes from deviating from midposition and actually may drive the eyes beyond the midposition toward the direction of turning. If the patient is in a coma due to bilateral hemispheric suppression, such as with toxic or metabolic disease (e.g., sedative overdose or uremia), the checking component (also the fast component of nystagmus) is lost. In this case, the eyes deviate away from the direction of head rotation in an unchecked manner (the reflex response is not inhibited by cerebral cortical input). Of course, if dysconjugate gaze is produced during the maneuver, damage to the brain stem in areas that control brain stem extraocular function must be assumed.

Conditions affecting vestibular function

There are a large number of conditions that can affect the vestibular apparatus. Broadly, these can be divided into peripheral causes and central causes. These two types of causes can often be distinguished on clinical grounds (see below). Peripheral causes include conditions damaging the inner ear or the vestibulocochlear nerve while central causes affect the brain stem, vestibulocerebellum or, in rare cases, the cortex.

The most common cause of peripheral vertigo has been termed acute labyrinthitis or vestibular neuronitis. While there may be subtle distinctions between these conditions, the presumed etiology is inflammation. In this condition the vertigo comes on quickly and patients often have severe nausea and can't walk. They are at their worst in a matter of hours and then there is slow improvement over days to weeks. There is usually no hearing loss. If it comes on very rapidly (and particularly if there is hearing loss), you should consider that the condition might result from infarction due to occlusion of the labyrinthian artery. Meniere syndrome is not uncommon. It is believed to result from obstructed drainage of endolymph, resulting in increased pressure due to continued production. The pressure damages the delicate hair cells (both vestibular and auditory) with loss of sensitivity. The clinical course is punctuated by paroxysms of sudden vertigo (often with worsened tinnitus), lasting hours with spontaneous resolution. This is believed to occur due to sudden puncture of the membranes, with resolution of symptoms dependent on sealing the puncture and reestablishment of the normal equilibrium between the fluid compartments of the inner ear. These attacks of vertigo (which can occasionally be triggered by loud noises) can be violent enough to throw the patient to the ground, though, in between attacks, there may be little residual other than some low-tone hearing loss.

Perilymph fistula is another cause of peripheral vertigo that is due to leakage of fluid. This condition is often precipitated by barotrauma (abrupt pressure changes) and individual attacks can occasionally be precipitated by pressure changes (including Valsalva maneuver, coughing, sneezing, airplanes, scuba diving, etc). Fluid usually leaks around the round window into the middle ear (and can occasionally be seen there).

Acoustic neuroma (actually a neurolemmoma) is a common tumor that grows on the vestibular nerve. Ironically, despite the fact that it damages vestibular nerve fibers, it is a rare cause of vertigo. This is because it progresses slowly, with ample time for compensation of deficits.

Positional vertigo will be discussed below.

Central causes of vertigo include damage to the brain stem or vestibulocerebellum. Stroke, usually involving the posterior inferior cerebellar artery (which supplies the lateral brain stem and part of the cerebellum) often produces severe vertigo (along with diminished pain and temperature sensations in the face). Isolated infarction or hemorrhage in the cerebellum can produce vertigo. These are particularly important to recognize because they can produce swelling and mass effect that can occasionally be fatal due to brain stem damage. Both of infarction and hemorrhage often produce occipital headache (particularly common with hemorrhage). It is important to consider this before attributing vertigo to vestibular neuronitis, which shouldn't produce headache.

Neoplasms of the cerebellum and brain stem usually don't produce much vertigo (for the same reason of slow growth with compensation that we invoked with acoustic neuroma). Inflammatory disease (such as MS or rare conditions such as neurosarcoid) can produce vertigo although this is usually not severe. Paroxysmal vertigo can result from the aura of migraine or seizure. This is presumed to result from activation of the part of the sensory cortex that perceives motion. If vertigo is the only symptom, it is difficult to diagnose seizure or migraine until or unless more characteristic features arise.

Peripheral versus central vertigo

As can be seen in the preceding section, there are quite different conditions producing central and peripheral vertigo. Fortunately, it is usually possible to distinguish central from peripheral vertigo on clinical grounds (Table 6-1). First of all, acute central vestibular involvement (as contrasted with acute peripheral disease) is associated with less severe vertiginous symptoms and less nausea. Additionally, central disease often produces more severe nystagmus than peripheral conditions. As distinct from central conditions, where the nystagmus is out of proportion to the vertiginous sensations, with peripheral conditions it is usually possible to predict how vertiginous the patient is by examining the nystagmus. Furthermore, central vertigo often is quite bizarre, changing directions depending on the direction that the patient is looking. That does not occur with peripheral vertigo, which is unidirectional and most evident when the patient endeavors to look in the direction of the fast phase of the nystagmus. Vertical (upward or downward) nystagmus does not occur with any normal lesion of the peripheral vestibular apparatus. Therefore, vertical nystagmus must be presumed to be central. If not present on forward gaze, vertical nystagmus is best observed by having the patient look directly up or down (Figure 6-9). Horizontal and rotary nystagmus can occur with either peripheral or central disease and are therefore not of value in differentiation.

Positional vertigo

Positional nystagmus and vertigo are relatively common disorders which have several potential causes (both peripheral and central). The patient complains of vertigo only when the head is in certain positions, commonly looking up. The vertigo may persist if the head is kept in the same position (this is particularly true with central disease) or it may rapidly fade (typical of the more common peripheral disease). The most common single cause of positional vertigo is so-called "benign paroxysmal positional vertigo" (BPPV). The characteristic complaint is of vertigo, which is severe and relatively brief, after turning in bed. This can also be triggered by looking up, lying back, getting up quickly or bending to tie the shoes (either when bending forward or, more commonly, when bringing the head back up). This condition results from loose otoliths in the inner ear. When these are in the semicircular canals, position-induced movement of the stones can produce severe vertigo that resolves in under a minute (often leaving the patient quite shaky and nauseated). This can occur after head trauma but is increasingly common with age where the otoliths are less securely anchored to the macula. Testing for positional nystagmus and vertigo is done by rapidly dropping the patient backward as in Figure 6-10 (the Hall Pike or Barany maneuver). The patient's head is held right side down, left side down, and in the midline on each of three trials. Vertigo and attendant rotatory nystagmus are seen usually beginning after a few seconds and terminating in less than a minute. The condition usually is self-limiting (in months, with dissolution of the stones). However, the "canalith repositioning maneuver" can often move these stones to a less sensitive part of the inner ear, terminating the attacks. Some examples of etiologic significance are head trauma, frequently of only minor severity, vertebrobasilar distribution ischemia, and acoustic neurolemmoma, the latter involving both the nerve directly and the brain stem by compression. The most commonly affected individual is the elderly patient with no predisposing factors and no threatening pathology. Presumably the dysfunction, which is called benign positional vertigo, is caused by aging and minor asymmetrical degenerative changes in the macula-otolith apparatus.

Cervical problems can produce positional vertigo by either of two mechanisms: by impairing blood flow through the vertebral artery system; or by activating sensory nerves from cervical muscles. With aging, cervical osteoarthritis becomes common. Occasionally, the bony overgrowth impinges on the transverse foramina through which the vertebral arteries course. Turning the head may increase the foraminal narrowing and compress the vertebral arteries to such a degree that brain stem ischemia occurs. Vertigo on head turning may be the presenting symptom, but usually other evidences of brain stem involvement clarify the picture. The vertigo and other symptoms and signs should be reproducible by turning the head; it is usually not necessary to go through the whole Barany maneuver (see Figure 6-10). In the case of vascular insufficiency, the vertigo and nystagmus usually take significantly longer to develop than with other causes of positional vertigo, up to 20-30 seconds. However, the nystagmus is variable in type, persists for longer periods, and is associated with only mild vertigo.

It has been shown that sensory nerve fibers coming from the cervical musculature have connections with the vestibular nuclei. These connections probably mediate head-neck-trunk axis orientation information. Disorders of the neck that are associated with abnormal muscle tightness or spasm can produce vertigo with head movement even without impairing the circulation.

References

Questions

Define the following terms:

conductive hearing loss, sensorineural hearing loss, tinnitus, vertigo, nystagmus.
Conductive hearing loss is the loss of the ability to transmit sound waves to the inner ear. Obstruction of the external ear, problems with the tympanic membrane or problems with the middle ear (or the ossicular chain) are the cause.
Sensorineural hearing loss is hearing loss due to damage to the hair cells of the organ of Corti or to the auditory part of the vestibulocochlear nerve.
Tinnitus is ringing or buzzing in the ears.
Vertigo is the illusion of movement.
Nystagmus is a to-and-fro movement of the eyes. It can be pendular (even swings in both directions, often due to congenital vision problems) or can be "jerk." In jerk nystagmus, there is a fast and a slow component, typically due to vestibular problems (the direction is named by the fast component).

6-1. What effect do lesions of the CNS have on hearing in one ear?

Answer 6-1. It is nearly impossible to cause loss of hearing in one ear by damage to the brain beyond the point where the vestibulocochlear nerve enters the brain stem. This is because the distribution of hearing is bilateral at all levels of the central nervous system auditory pathway. Localization of sound may be slightly affected by auditory cortex lesions. In fact it is nearly impossible for central nervous system lesions to cause clinically detectable hearing loss and if these is hearing loss you must look at the conductive system, the inner ear and the vestibulocochlear nerve.

6-2. What kind of hearing loss will be produced by damage to the inner ear?

Answer 6-2. Inner ear and CN VIII lesions will produce senorineural deafness.

6-3. What do you call problems in which the sound wave can not reach the inner ear?

Answer 6-3. Blockage of the outer ear or damage to the tympanic membrane or middle ear is called conductive deafness.

6-4. What would it mean if Weber's test lateralized to the left?

Answer 6-4. Weber's test lateralized toward the side of conductive deafness and away from sensorineural.

6-5. What is the most common symptom of damage to the vestibular system?

Answer 6-5. Vertigo is common with vestibular damage.

6-6. What does it mean when someone says that nystagmus was in a particular direction?

Answer 6-6. Nystagmus is named according to the direction of fast movement.

6-7. How can you distinguish vertigo from inner ear damage from that caused by damage to the central nervous system?

Answer 6-7. With peripheral causes of vertigo (the illusion of movement), nystagmus is proportional to the amount of vertigo and nystagmus is always in the same direction regardless of the direction that the patient looks. Also, peripheral nystagmus is almost never in a vertical (up or down) direction. With central vertigo (cerebellum, vestibular nuclei and brain stem) nystagmus is usually greater than vertigo and may shift direction depending on gaze direction.

6-8. What is the only way to examine the integrity of the vestibular system on one side?

Answer 6-8. Caloric testing is the only way to check each inner ear vestibular function independently.

6-9. What would you anticipate finding during cold-water caloric testing in the intact patient who is awake?

Answer 6-9. When the patient is awake, the eyes will drift slowly toward the side of cold water with rapid correction to the opposite side (the opposite for warm water). There is tonic balance in vestibular input from each ear; cold water caloric testing decreases tonic input from the inner ear (warm water increases it).

6-10. What would you expect to find in the comatose patient whose brain stem was still working if ice-water was infused into the right ear?

Answer 6-10. The eyes would drift toward the side of the ice-water infusion and remain there for several minutes. There would be no nystagmus (which is a response generated by the conscious cerebral cortex when the visual image slips across the retina).

6-11. Damage to the inner ear produces responses that look like (cold/warm) water caloric testing (choose one)?

Answer 6-11. Damage to the inner ear or vestibular nerve decreases vestibular input from that ear (acts like cold water caloric).
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