8 Cranial Nerve VIII: Vestibular Disorders
Abstract
Dizziness is a common patient complaint and may be challenging to diagnose a definitive cause. Only a fraction of vertigo or imbalance cases are attributable to peripheral (i.e., vestibular nerve and inner ear) pathology. It is critical to consider and evaluate for central, cardiogenic, ophthalmologic, and proprioceptive causes of dizziness. A history focusing on nature of dizziness, its duration, and associated symptoms is essential. Examination includes complete head and neck examination, neurologic examination, and neurotologic maneuvers such as head-impulse test, Dix–Hallpike test, and the head shake test. There are many inner ear function tests available for evaluation of the dizzy patient and can be used to evaluate for specific vestibular disorders. Imaging including CT of the temporal bones and MRI of the internal auditory canals and brain may be useful for ruling out pathology.
8.1 Vestibular Anatomy and Physiology
The vestibular system is comprised of three semicircular canals, the utricle, the saccule, and the superior and inferior vestibular nerves, which join together with the cochlear nerve to form the eighth vestibulocochlear cranial nerve. The superior, posterior, and lateral semicircular canals are three orthogonally positioned canals that sense rotational acceleration. The utricle senses horizontal acceleration, while the saccule senses vertical acceleration. Each of the vestibular organs contains a gelatinous material that deflects hair cells during acceleration or deceleration, resulting in transmission of an electrical signal through the vestibular nerve. Impulses from the superior semicircular canal (SCC), lateral SCC, and utricle travel through the superior vestibular nerve, while impulses from the posterior SCC and saccule travel through the inferior vestibular nerve.
8.2 Overview of Dizziness
The body’s sense of position and balance relies on the accurate integration of multiple sensory signal inputs within the brain. Information on body position and movement is conveyed through the vestibular system as well as through vision and proprioception. In addition, the sensation of dizziness can be induced as a self-preservation technique—a brain that is not sufficiently perfused will trigger a sense of faintness in order to induce sitting down or lying down in order to increase brain perfusion.
Dizziness is a common patient complaint without a specific definition. As common as dizziness is, it is a fairly imprecise symptom. The differential diagnosis is broad with even the most common etiologies accounting for less than 10% of cases. 1 Dizziness can encapsulate vertigo, disequilibrium, vision changes (e.g., nystagmus), and lightheadedness. Vertigo refers to a sensation of rotational motion. Disequilibrium is a feeling of instability or imbalance. Lightheadedness is the sense of impending faintness.
There are many etiologies for dizziness, and dizziness can be multifactorial as well. Therefore, it is crucial to consider causes of dizziness beyond the vestibular system including neurologic, ophthalmologic, cardiogenic, psychogenic, and proprioceptive (such as peripheral neuropathy from diabetes mellitus).
Vestibular causes of dizziness are usually the result of dysfunction of the semicircular canals or the vestibular nerve.
8.3 Epidemiology and Economic Impact of Dizziness
Dizziness and balance disorders are common conditions that result in a substantial number of visits to health care providers. Dizziness accounts for an estimated 5% of all primary care clinic visits and 3% of all emergency room visits. 2 , 3 This is particularly true in older adults, where up 30% of patients age 60 years and up to 50% of community-dwelling adults older than 80 years have some degree of dizziness. 4 The total cost of evaluation and management of these patients is enormous. In 2008, Saber Terhani and colleagues reported the estimated national cost of patients presenting with dizziness to the emergency department alone exceeded $4 billion. 5 These costs have almost certainly increased since that publication. Individuals with symptomatic balance dysfunction have a 12-fold increase in the odds of falling, with one in three community-dwelling adults older than 65 years falling each year. Ten years of falls result in major injuries with a cost of over $20 billion annually. 6 In general, dizziness and vertigo have a huge negative impact not only on health care resource use but also on work productivity. 7
8.4 History
A general approach should focus on obtaining a thorough history. Timing (onset, duration, evolution, etc.), severity, frequency, triggers, and associated symptoms are likely to clue the clinician as to whether the problem area is central (brain, vestibular nuclei in brainstem, cerebellum, etc.), peripheral (inner ear, cochleovestibular nerve), or nonvestibular (cardiovascular insufficiency, peripheral neuropathy, etc.).
Patients with peripheral inner ear sources of their dizziness tend to have vertigo, or a subjective illusion of movement. Lightheadedness, imbalance and unsteadiness, nausea, and blackouts are generally not seen with peripheral vestibulopathies. Vertigo may be accompanied by hearing loss and is more commonly episodic (rather than constant) when compared to central nervous system causes. For an excellent review of how to differentiate the source (central, peripheral, or nonvestibular), the reader is encouraged to see Muncie et al. 8 (For the purpose of this discussion, disorders of the vestibular branch of vestibulocochlear nerve [CN VIII] are considered peripheral [or inner ear] causes of dizziness.)
A complete history of a patient presenting with dizziness must focus on the duration of the dizziness as well as a detailed description of the dizziness. Does the dizziness last for seconds, minutes, hours, or days? Is it intermittent or constant? What is the frequency of each dizziness episode and when did they first begin? Is the dizziness vertigolike or is it a sense of imbalance or of feeling faint? Is the dizziness associated with headaches (e.g., vestibular migraine), hearing loss, aural fullness, or tinnitus? Is the dizziness associated with changes in vision? Is the dizziness positional? Has there been recent barotrauma? A detailed past medical history, medication history, and family and social history should be obtained.
For the smaller percentage of patients with suspected inner ear disorders, their workup and management can be both incredibly challenging and rewarding. Fortunately, duration of symptoms can point toward the likely diagnosis (▶Table 8.1). In addition to history, some simple, targeted “bedside” testing such as the Dix–Hallpike maneuver and head shake test can aid in establishing the correct diagnosis. When available, pure-tone audiometry with speech comprehension testing can be helpful, and may even identify the suspected affected side as the inner ear functions of hearing and balance can be affected in disorders such as Meniere’s disease and labyrinthitis. Taken in aggregate, history, limited physical examination, and audiometry can frequently result in the correct diagnosis or at least suggest if the dizziness is central, peripheral, or nonvestibular.
Duration | Disorder |
s–min | Benign paroxysmal positional vertigo |
min–h | Meniere’s disease Vestibular migraine |
d–mo | Vestibular neuronitis Labyrinthitis Perilymph fistula |
mo–y | Superior semicircular canal dehiscence Vestibular schwannoma |
8.5 Physical Examination
In addition to the physical examination described earlier for the patient with hearing loss, specific tests help elucidate the diagnosis in dizzy patients.
Testing can be performed in the office for orthostatic hypotension using commonly available sphygmomanometers. During examination of the tympanic membranes (TMs), a fistula test should be performed to examine for pressure-induced vertigo or nystagmus.
Nystagmus should be evaluated with a neutral gaze with and without fixation. Evaluation for gaze-evoked nystagmus to the right, left, up, and down should also be performed. Ocular alignment, saccades, and smooth pursuit should be tested.
The vestibulo-ocular reflex (VOR) moves the eyes contrary to the head in order to maintain steady vision during head motion. Bilateral vestibular weakness or poorly compensated unilateral vestibular weakness can result in loss of the VOR and lead to symptoms of oscillopsia. Several physical examination maneuvers can test the integrity of the VOR. In the head impulse test (HIT), the examiner rapidly turns the patient’s head 30 degrees while the patient attempts to maintain their gaze on a target. In a patient with bilateral vestibular weakness, the patient’s eyes do not remain on the target with the test done in either direction. In a patient with a unilateral vestibular weakness, the eyes will lag in returning to the target when head motion is rotated toward the side with vestibular weakness. The post-head-shake nystagmus test can be used to assess for unilateral vestibular weakness. The patient’s head is shaken from side to side for 20 seconds at 2 Hz. The head shake is then stopped. In a patient with symmetric vestibular function, there should be no nystagmus. If there is unilateral weakness, there will be fast-phase beating nystagmus away from the weaker side.
The Dix–Hallpike maneuver test is used for posterior canal benign paroxysmal positional vertigo (BPPV). Starting in a sitting position, the patient’s head is rotated 45 degrees to one side. The patient is then laid supine with the neck slightly extended. The Dix–Hallpike test is considered positive when it elicits up-beating and torsional nystagmus when the head is tilted toward the affected ear. Nystagmus typically starts at least 1 second after positioning and reaches a maximum at about 10 seconds before fatiguing in less than 60 seconds.
The Romberg test assesses proprioception. The patient first stands in a neutral position with feet slightly apart and arms at the side or folded across the chest. The patient starts with eyes open for 30 seconds and then with eyes closed for 30 seconds. The Romberg test is considered positive if the patient is stable with eyes open and then loses balance with eyes closed. A positive Romberg test occurs in patients with proprioceptive deficits.
The Fukuda stepping test assesses for unilateral peripheral vestibular hypofunction. The patient marches in place with their eyes closed. A rotation to one side may signify an ipsilateral vestibular weakness.
When available, pure-tone audiometry with speech comprehension testing can be helpful, and may even identify the suspected affected side as the inner ear functions of hearing and balance can be affected in disorders such as Meniere’s disease and labyrinthitis. Taken in aggregate, history, limited physical examination, and audiometry can frequently result in the correct diagnosis or at least suggest if the dizziness is central, peripheral, or nonvestibular.
If doubt as to the correct diagnosis persists, a variety of tests (behavioral, electrophysiologic, and radiographic) are available to the clinician. Yet judicious use of these tests should be employed. The “shotgun” approach of ordering every test for every patient may only prove to obfuscate the actual diagnosis while only increasing cost.
8.6 Laboratory Testing
8.6.1 Cost-Effectiveness of Laboratory Testing for Peripheral Vestibular Disorders
There is no clear evidence in the literature pertaining to the cost-effectiveness of the laboratory tests for vestibular disorders described below. The below tests rely on expensive specialized equipment and highly trained operators. Many diagnoses can be made without laboratory testing. Clinicians typically do obtain laboratory testing such as videonystagmography (VNG) prior to performing an ablative procedure on the inner ear (e.g., labyrinthectomy or transtympanic gentamycin injection) in order to confirm the side of vestibulopathy.
8.6.2 Videonystagmography/Electronystagmography
VNG and electronystagmography (ENG) are techniques of tracking eye movement in order to assess for nystagmus. VNG and ENG can be used interchangeably. VNG relies on video to capture eye movement, whereas ENG assesses changes in the corneoretinal potentials, which can be translated into eye movements.
VNG generally refers to a series of vestibular tests that rely on quantified measurement of nystagmus: caloric testing, gaze test, Dix–Hallpike maneuver, saccades, positional testing, and smooth pursuit.
8.6.3 Caloric Testing
Caloric testing, first described by Barany in 1906, has long been considered the standard method of evaluating the function of an individual’s inner ear. The caloric test allows for the assessment of a unilateral vestibulopathy as peripheral or central and for identification of the side of the vestibulopathy. In the caloric test, heated or cooled air or water into the external auditory canal is used to generate endolymphatic flow in the lateral semicircular canal. Each ear is irrigated while the patient lies supine with the head tilted up 30 degrees, placing the lateral SCC in the vertical plane. Cold irrigation causes slow-phase nystagmus toward the irrigated ear, while warm irrigation causes nystagmus away from the irrigated ear. The velocities of irrigation-induced nystagmus with hot and cold stimulus are compared between ears in order to assess for asymmetry between sides. The canal paresis (CP) index is used to quantify asymmetry and is calculated according to the following formula:
where slow-phase velocities are measured for RW = right warm, RC = right cool, LW = left warm, and LC = left cool. Unilateral weakness is considered significant when CP is greater than 20%.
Caloric testing is advantageous as it can localize a lesion to the right or left side. However, as it relies on stimulating the lateral SCC, it is a test of the superior vestibular nerve (innervates the lateral SCC) and not the inferior vestibular nerve. As it relies on comparing the right to the left side, it is not a good test for symmetric bilateral vestibular weakness.
VNG/ENG testing only assesses the lateral semicircular canal and only at low-frequency stimulation. Some have even asserted that caloric testing and ENG/VNG add little value to diagnosis other than to identify central versus peripheral etiology, and laterality when peripheral. 9 , 10 Such testing can be useful prior to ablative therapy in which the entire malfunctioning vestibular labyrinth is deafferented (chemically or surgically). In their study of cost-effectiveness of workup of unilateral vestibular weakness found on ENG, Gandolfi and colleagues concluded that further workup (MRI) is high cost and low yielding in patients with one isolated abnormality and no other risk factors. 11
8.6.4 Gaze Test
The eyes are assessed for nystagmus as the patient gazes 30 degrees to the right, to the left, up, and down with and without fixation suppression. Nystagmus on gaze test is an abnormal finding that may be of vestibular or central origin. Generally, in an acute unilateral vestibular weakness, there will be nystagmus with the fast phase directed toward the unaffected ear, which will be accentuated with gaze toward the healthy ear and suppressed with gaze toward the affected ear. With any other type of nystagmus on gaze test, a central cause must be considered. 12
8.6.5 Dix–Hallpike Maneuver
The Dix–Hallpike maneuver (described earlier in the Physical Examination section) can be performed with VNG in order to quantify any associated nystagmus.
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