Anatomy and Pathophysiology:
The vestibular system consists of the semicircular canals and otolithic organs (saccule and utricle), which detect information regarding acceleration and orientation of the body. This information is then transmitted via the vestibular nerve (a division of cranial nerve VIII) to the brainstem and cortex, effecting compensatory positional adjustments of the eyes, head, and muscles of the trunk and limbs to maintain balance. Additional details regarding anatomy of the vestibular system can be found in Anatomy of the Ear.
Disorders of the vestibular system may be central or peripheral in etiology and can produce a variety of symptoms. Although patients may use the term dizziness to describe a number of different sensations involving abnormal sense of orientation, a distinction should be made between vertigo and disequilibrium or lightheadedness. Vertigo refers specifically to the illusion of movement, such as when the patient feels that the room is spinning . Pathologic nystagmus is another sign of vestibular dysfunction; in order to interpret this finding, an understanding of the underlying physiology is required. Physiologic nystagmus is a normal vestibular response that allows for maintenance of visual focus despite movement of the head. Stimulation of the semicircular canal due to rotational acceleration of the head causes a compensatory movement of the eyes in the direction opposite to the rotation (vestibulo-ocular reflex). In cases of aberrant or hyperstimulation of the semicircular canal on one side, the eyes move toward the opposing side despite lack of movement of the head. This response represents the slow phase or beat of pathologic nystagmus. In order to correct this aberrant movement, there is a rapid movement of the eyes back toward center. This compensatory response is rapid and constitutes the fast phase or beat of pathologic nystagmus. By convention, pathologic nystagmus is described according to the direction of this fast phase (i.e., in right-beating nystagmus, the pathologic response is leftward movement of eyes, which is then corrected by a fast movement of the eyes to the right). In the case of ablative lesions or other disruption of semicircular canal function on one side, the unaffected (normal) semicircular canal is then interpreted as being relatively hyperstimulated, such that the slow phase of the pathologic nystagmus is in the direction of the lesion.
This chapter will cover the most common causes of peripheral vestibular dysfunction, including benign paroxysmal positional vertigo (BPPV), Meniere’s disease, and vestibular neuritis. A more complete list of vestibular disorders, including central causes, is given in the subsequent section on differential diagnosis.
Benign paroxysmal positional vertigo is a condition caused by debris in a semicircular canal (usually the posterior canal). In most cases, the debris likely consists of stray otoconia that have migrated from the utricle . Movement of the head in a direction that places the canal in a dependent position causes the debris to shift with gravity and produces movement of the cupula. This produces the abnormal sensation of movement (vertigo).
Meniere’s disease (also known as endolymphatic hydrops) is a disorder of unknown etiology that causes episodic hearing loss, tinnitus, and vertigo. Various causes have been suggested, including anatomic abnormalities, infection, autoimmune response, and allergic reaction, but no definitive explanation of its pathogenesis has been found thus far. As such, it remains a clinical diagnosis based on the constellation of symptoms.
Vestibular neuritis, the third most common peripheral vestibular disorder, is an isolated disorder of the vestibular system in the absence of additional neurologic symptoms (such as hearing loss). Its etiology is also unclear, with proposed mechanisms including viral infection, ischemic injury, and inflammatory immune response.
BPPV has an estimated incidence of 10-100 cases per 100,000 individuals. There is no gender predilection, and it may affect individuals of all ages, although the mean age at presentation is 50 years. BPPV may be more common in individuals with history of prior head trauma or episodes of vestibular neuritis, as well as those with a family history of BPPV.
Meniere’s disease has an estimated incidence of 10-150 cases per 100,000 individuals. There is no gender predilection, and the disorder is most common in middle-aged patients (average age of presentation is 40-50 years). Meniere’s disease is rare in children.
Vestibular neuritis is diagnosed in approximately 150,000 patients per year in the United States. There is no gender predilection, and the disorder is most common in middle-aged patients. Some cases may be associated with a preceding viral infection, but, in the majority of patients, no inciting event is identified.
BPPV is typically characterized by sudden onset. The majority of patients experience spontaneous recovery over the course of months. Approximately 30% of patients have symptoms persisting for longer than 12 months.
Patients with Meniere’s disease exhibit a variable disease course, often with periods of remission between episodes. Studies have suggested that the progression of hearing loss and frequency of attacks gradually decrease over the course of 10-20 years. While the disease is almost always unilateral at the time of diagnosis, up to 45% of patients eventually develop the condition in the contralateral ear, with the risk of contralateral involvement increasing over time.
The natural history of vestibular neuritis is characterized by sudden onset and spontaneous resolution in the vast majority of cases. Patients typically experience acute symptoms lasting several days, followed by recovery over weeks to months. An estimated 15% of patients have persistent symptoms lasting for more than 12 months.
The characteristic presentation of BPPV involves episodic vertigo triggered by particular head positions, such as when sitting up from a supine position, rotating the head in a certain direction, or bending forward at the waist. The vertigo attacks are sudden in onset and typically last from a few seconds to a minute. There are usually few associated symptoms aside from nausea related to vertigo; patients do not experience hearing loss or pain during the episodes and are asymptomatic between attacks.
Patients with Meniere’s disease also experience episodic attacks; however, the episodes typically last for hours. There are four main symptoms associated with Meniere’s disease: fluctuating unilateral hearing loss, tinnitus, sensation of aural fullness, and vertigo. During the attack, patients may also experience nausea and vomiting.
Vestibular neuritis presents with sudden onset vertigo in the absence of other symptoms, such as hearing loss or headache. Unlike BPPV, the vertigo is constant in vestibular neuritis, and may last several days. Patients may experience nausea and vomiting associated with the vertigo, as well as some postural instability.
Differential Diagnosis of Sensorineural Hearing Loss:
- Central vestibular dysfunction
- Vascular disorders (e.g., vertebrobasilar insufficiency)
- Cerebrovascular accident (e.g., cerebellar or brainstem infarction)
- Neoplasm (e.g., vestibular schwannoma, brain stem tumor)
- Other neurologic disorders (e.g., multiple sclerosis, cerebellar ataxia, normal pressure hydrocephalus, pseudo tumor cerebri)
- Motion sickness
- Peripheral vestibular dysfunction
- Benign paroxysmal positional vertigo
- Meniere’s disease
- Vestibular neuritis
- Labyrinthine fistula
- Trauma (e.g., barotrauma, blunt head trauma)
- Acute or Chronic Otitis Media
- Acute or Chronic Labyrinthitis
- Non-vestibular causes
- Ophthalmic dysfunction (e.g., poor visual acuity, disconjugate gaze)
- Cardiovascular dysfunction
- Autonomic dysfunction (e.g., vasovagal response, orthostatic hypotension)
- Metabolic disorders (e.g., diabetes mellitus)
- Drug effect
When taking the patient history, it is important to determine whether the patient is experiencing vertigo, lightheadedness, imbalance (i.e., inability to maintain center of gravity), or some other sensation of disequilibrium. Information regarding symptom onset (sudden or gradual), disease course (continuous or episodic), associated symptoms (particularly other neurologic complaints), family history, and antecedent or triggering events can also be very useful in determining diagnosis.
The physical exam should include evaluation of cranial nerve function and a comprehensive assessment of vestibular function. Ocular movement should be observed during smooth pursuit and with changes in gaze direction. The presence of spontaneous (pathologic) nystagmus should be noted, as well as the direction of nystagmus, whether the nystagmus is fatigable, and whether the nystagmus changes with gaze fixation or eye movement. Table 15.1 outlines the characteristic pattern of nystagmus in central versus peripheral vestibular disorders. Frenzel glasses are specialized goggles worn during assessment of nystagmus; they prevent gaze fixation by the patient, as well as illuminate and magnify the patient’s eyes so the examiner can clearly evaluate ocular movement.
|Origin of Disorder||Direction of Nystagmus||Effect of Gaze Fixation||Effect of Gaze Direction||Fatigability|
|Peripheral||Mixed linear and torsional components (BPPV exhibits geotropic nystagmus, which is down beating and rotatory)||Nystagmus suppressed||Does not impact direction of nystagmus; nystagmus intensifies when looking toward the fast phase||Yes|
|Central||Purely vertical, purely horizontal, or purely torsional||No change||Direction of nystagmus may change to match gaze direction||No|
The Dix-Hallpike test is a positional maneuver that is used to assess for BPPV. The test places the posterior semicircular canal in a dependent position. If the patient’s symptoms are a result of debris in the posterior canal, the maneuver should trigger a vertiginous episode. To perform the test, the patient begins in a seated position with the head rotated 45° toward the right. The examiner then rapidly moves the patient into the supine position, maintaining the rotation of the head toward the right. In the supine position, the head should hang slightly over the edge of the exam table, so that the neck is extended approximately 20°. In this position, the patient is asked whether vertigo is present, and the eyes are observed for characteristic geotropic (downward, rotatory) nystagmus. There may be a latency period of a few seconds before symptoms appear; if no symptoms are elicited after 20 seconds, the test is recorded as negative. The procedure can then be repeated for the left ear.
Additional physical examination components that evaluate vestibular function include assessment of gait (heel-toe), posture (Romberg and Fukuda tests ), and cerebellar function (finger-to-nose, rapid alternating movements).
Specialized Vestibular Testing
Further characterization of vestibular function can be achieved through specialized testing. Electronystagmography (ENG) and the newer videonystagmography (VNG) can localize the site of a vestibular lesion via oculomotor, positional, and caloric testing. The oculomotor test battery evaluates the speed and accuracy of eye movements in response to various stimuli, such as moving and fixed targets. The positional test battery involves placing the head in various positions and observing the effect on nystagmus. Finally, caloric testing allows comparison between the left and right sides via direct stimulation of one semicircular canal at a time. The physiologic basis of caloric testing involves the movement of the cupula within the semicircular canal in response to a temperature change. Cold stimuli cause the cupula to move away from the utricle and result in nystagmus in the opposite direction of the stimulus. Warm stimuli cause nystagmus toward the direction of the stimulus.
Other types of vestibular testing include the rotary chair test, in which the patient’s eye movements are recorded while undergoing rotational acceleration, and vestibular evoked myogenic potentials (VEMP). VEMP are responses recorded from the sternocleidomastoid muscle in response to auditory stimuli. The pathway for generation of VEMP involves the saccule and vestibular nerve; thus the test can be used to assess their function.
The use of radiographic imaging, mainly magnetic resonance imaging (MRI), in cases of vertigo or dizziness is indicated if a CNS lesion, neoplasm or anatomic abnormality is suspected. Atypical nystagmus pattern, unresponsiveness to treatment, and unexpected neurologic symptoms or examination findings generally warrant further investigation. MRI is useful in identifying intracranial neoplasms (such as cerebellopontine angle tumors) or infarction that may account for unusual presentation.
Laboratory tests may be useful in distinguishing Meniere’s disease from otosyphilis or autoimmune ear disease.
The Epley maneuver, also known as the canalith repositioning procedure (CRP), is a repositioning technique used to treat BPPV. It attempts to move debris in the posterior or lateral semicircular canal back into the utricle. The maneuver involves a series of four steps in which the head is rotated as the patient moves from seated to supine and back to seated position. A vibratory device may optionally be placed over the ipsilateral mastoid during the Epley maneuver to promote dislodgement of the debris. For rare patients who experience continued and intractable episodes of BPPV despite multiple positioning maneuvers, surgical options include occluding or denervating the posterior semicircular canal.
Because the pathogenesis of Meniere’s disease is not fully understood, current treatment approaches are primarily directed at symptomatic reduction. Conservative management includes dietary sodium restriction and diuretic therapy, typically with hydrochlorothiazide/triamterene. These measures are thought to improve symptoms by reducing fluid pressure within the inner ear. Some patients with specific triggers may benefit from avoiding certain foods or substances, such as caffeine, chocolate or alcohol. Symptoms during acute attacks may be partially alleviated with antiemetic or vestibular suppressant drugs (such as prochlorperazine or meclizine). A short course of oral or intratympanic corticosteroid therapy may also be tried in the setting of acute exacerbation. Gentamicin is an ototoxic aminoglycoside antibiotic that can be given intratympanically in an attempt to selectively ablate vestibular function in the inner ear. Gentamicin injection effectively induces a chemical labyrinthectomy; it is associated with a relatively high risk of hearing loss in the affected ear.
In patients with severe, persistent symptoms of Meniere’s disease despite medical management, surgical intervention may be considered. Endolymphatic sac surgery involves decompression of the endolymphatic sac (the blind pouch where the endolymphatic duct terminates). In some cases, a shunt may be placed to allow communication between the endolymphatic duct and the subarachnoid or mastoid space. Alternatively, vestibular nerve section or surgical labyrinthectomy may be performed to completely eliminate vestibular input on the affected side. Transection of the vestibular nerve may be performed via either a retrosigmoidal or a middle fossa approach (see Chapter 18 for details). It preserves auditory function, unlike labyrinthectomy, which is typically reserved for patients with poor hearing on the affected side.
Because vestibular neuritis spontaneously resolves in the majority of cases, treatment is typically limited to supportive care. Antiemetic or vestibular suppressant drugs may be administered, but should be used conservatively to prevent interference with normal compensatory adaptation of the central vestibular system. Surgical therapy or invasive medical therapy is best avoided.
Complications, Prognosis and Follow-Up:
The success rate of the Epley maneuver for BPPV is high, with an 80% cure rate after one repositioning. The rate of recurrence has been estimated to be 10-15% per year. Recurrent episodes of BPPV can usually be retreated with additional repositioning. Patients who fail repeated Epley maneuvers and go on to posterior canal occlusion may experience transient hearing loss postoperatively, but generally have good long term outcomes without permanent complications.
The use of intratympanic gentamicin has been shown to be very effective in controlling symptoms of vertigo in patients with Meniere’s disease. Up to 90% of patients achieve satisfactory relief of vertigo with monthly gentamicin administration. Although intratympanic administration promotes selective destruction of vestibular function, risk of hearing loss due to ototoxicity remains. Treatment is generally discontinued in patients experiencing persistent or progressive hearing loss with gentamicin. Most patients experience only mild (< 10 dB) impairment in auditory function.
The efficacy of endolymphatic sac surgery for treatment of Meniere’s disease remains questionable. Studies have generated conflicting data as to whether the procedure produces substantial symptomatic improvement. However, endolymphatic sac surgery remains a relatively low risk alternative for intractable Meniere’s disease. The data regarding vestibular nerve section is more definitive, with over 90% of patients achieving vertigo control. Vestibular nerve section produces temporary disequilibrium immediately postoperatively, but the decrease in vestibular input on the affected side is usually centrally compensated for within a few days to a few weeks. Vestibular nerve section is a major intracranial procedure with potential, but rare, serious complications that include injury to the auditory nerve and hearing loss, as well as facial nerve injury. Although vestibular nerve section is effective for treatment of vertigo, it does not impact the natural course of Meniere’s disease with relation to hearing loss.
Vestibular rehabilitation therapy may be a useful adjunct for patients with residual symptoms after treatment. It is usually administered by a physical or occupational therapist and consists of a series of exercises designed to promote central compensation and acclimation to altered vestibular function.
- Peripheral and central causes of vestibular dysfunction can be differentiated on the basis of nystagmus characteristics; electronystagmography and rotational chair testing are methods of assessing vestibular function based upon eye movements.
- Benign paroxysmal positional vertigo is a peripheral vestibular disorder caused by debris in the semicircular canal; it can be diagnosed with the Dix-Hallpike test and is often successfully treated with repositioning maneuvers to remove the debris from the canal.
- Meniere’s disease is a condition of unknown etiology that produces episodic vertigo, fluctuating unilateral hearing loss, tinnitus, and aural fullness; treatment is primarily aimed at reducing symptoms of vertigo and can include diuretics and sodium restriction, intratympanic gentamicin, endolymphatic sac surgery, vestibular nerve section, or labyrinthectomy.
- Vestibular neuritis is a self-limited peripheral vestibular disorder that produces sudden onset of vertigo in the absence of other neurologic symptoms; the vertigo typically spontaneously resolves after several days without treatment.