Chapter 56. Vestibular Disorders

The value and function of the vestibular system may often be underestimated when considering the various special senses that we possess. However, of all the special senses, unilateral loss of the vestibular system may cause the most significant determent for our daily function and survival. Millions of people present annually to their physician with the complaint of dizziness. The goal of this chapter is to discuss the common disorders that affect the vestibular system and provide a framework for the evaluation, diagnosis, and treatment of patients with vestibular disorders.

Injury to the peripheral or central vestibular system causes asymmetry in the baseline input into the vestibular centers and this causes vertigo, nystagmus, vomiting, and a sense of falling toward the side of the injury. Vertigo is defined as the illusion of movement. However, the chief complaint of patients with injury to the vestibular system is usually not vertigo but dizziness. If the complaint is clarified to be vertigo, the duration, periodicity, and circumstance of the vertigo and the presence of other neurological signs or symptoms allow for categorization of the vertigo.

The proximity of the vestibular system to the auditory system often causes vertigo to be coupled with hearing loss. The role of the otolaryngologist includes clarifying the subset of patients who have vertigo due to injury to the vestibular system and differentiating central from peripheral vestibular disorders. The evaluation includes a complete head and neck and vestibular examination (Table 56–1). The diagnostic evaluation includes audiology, vestibular testing, and imaging. Knowing the duration of the vertigo or disequilibrium and the presence or absence of hearing loss allows for a narrowing of the differential diagnosis (Table 56–2). The vertigo may be due to injury of the peripheral or central vestibular system. Often, the presence of other neurological abnormalities leads to an investigation for a central cause of the vertigo. However, central vestibular injury due to a lesion or stroke may mimic a peripheral vestibular disorder.

Most patients with peripheral vestibular disorders have benign paroxysmal positional vertigo (BPPV), Meniere disease, or vestibular neuronitis. These patients generally improve with supportive or conservative care (medical or physical therapy). The small percentage of medically recalcitrant patients can then be helped with surgical intervention. The surgical interventions, in general, ablate the vestibular system and rely on central compensation and vestibular rehabilitation to improve the patient's condition.

The central compensation for vestibular injury occurs via the cerebellum. The cerebellum provides a “clamping” response to the injured vestibular system to reduce the effects of the abnormal vestibular signal. In an acute injury such as vestibular neuronitis, the vertiginous response lasts 3–5 days, and then the central compensation is able to modulate the signal from the injured vestibular system. In episodic injuries, such as Meniere disease, the central compensation is not able to be as effective; therefore, with each new episode, there are acute vertiginous symptoms. In a slowly evolving process such as a vestibular schwannoma, the central compensation occurs in step with the vestibular dysfunction, and the patient may have minimal to no vestibular symptoms. The central compensation is enhanced by vestibular activity and delayed by the prolonged use of medical vestibular suppression. This observation has led to the development of vestibular rehabilitation programs.

Vestibular rehabilitation programs use three strategies: (1) habituation exercises, which facilitate central compensation by extinguishing pathologic responses to head motion; (2) postural control exercises; and (3) general conditioning exercises. Vestibular rehabilitation is critically important in the elderly because their ability to have optimal central compensation is diminished.

Essentials of Diagnosis

• Sudden vertigo lasting seconds to minutes with head movement
• No associated hearing loss
• Characteristic nystagmus (latent, geotropic, fatigable) with Dix-Hallpike test.

General Considerations

BPPV is one of the most common types of peripheral vertigo, arising as a result of debris in the posterior semicircular canal. Patients complain of vertigo lasting seconds, with no associated hearing loss when in certain positions. The average age of presentation is in the fifth decade and there is no gender bias. The incidence may range from 10 to 100 cases per 100,000 individuals per year. Nearly 20% of patients seen at vertigo clinics are given the diagnosis of BPPV. Ten to fifteen percent of patients have an antecedent history of vestibular neuronitis and another 20% have a history of head trauma.

Pathogenesis

BPPV occurs because a semicircular canal has debris either attached to the cupula or free floating in the endolymph. The semicircular canal becomes stimulated by the movement of these particles in response to gravity. The study of temporal bones from patients with BPPV showed basophilic deposits adherent to the cupula; this finding was termed cupulolithiasis. Intraoperative findings during posterior canal occlusion in patients with resistant BPPV have shown free-floating debris in the endolymph. Electron microscopy of these particles shows that they are likely otoconia originating from the macula of the gravity-sensitive utricle. This process has been termed canalolithiasis.

The cupula of the semicircular canal has the same specific gravity as the endolymph and so is not sensitive to gravity. However, the debris in the semicircular canal moves in response to gravity and when the patient places the semicircular canal in a dependent position, the particles move and entrain endolymph with them and cause deflection of the cupula. The unexpected gravity-sensitive response from the semicircular canal causes vertigo. The majority of BPPV is due to debris in the posterior canal, but debris may also enter the horizontal and superior semicircular canals.

Clinical Findings

Symptoms and Signs

Patients usually complain of a sudden onset of vertigo that lasts 10–20 seconds with certain head positions. The triggering positions include rolling over in bed into a lateral position, getting out of bed, looking up and back (top-shelf vertigo), and bending over. The vertigo may be associated with nausea. Patients have normal hearing (no new loss), no spontaneous nystagmus, and a normal neurological evaluation.

Imaging Studies

Imaging is reserved for patients who do not have the characteristic nystagmus, have associated neurological findings, or do not respond to treatment. The imaging choice is a magnetic resonance imaging (MRI) scan with gadolinium contrast to evaluate the brainstem, the cerebellopontine angle (CPA), and the internal carotid artery (IAC). The MRI is the most sensitive and specific test to identify posterior fossa tumors.

Special Tests

Patients should have no new hearing loss. The audiogram should show symmetric hearing with appropriate speech discrimination scores. The tympanogram should be normal. An asymmetric hearing loss calls into question the diagnosis of BPPV and further evaluation is required.

Special Examinations

BPPV is diagnosed by observing a characteristic nystagmus when performing the Dix-Hallpike test (Figure 56–1). There is a latency of 1–2 seconds before the onset of the nystagmus and vertigo. The nystagmus is mixed with a torsional and vertical component and is geotropic (down-beating, rotatory nystagmus). The nystagmus follows the Ewald law for excitation of the dependent posterior semicircular canal. The nystagmus is in the plane of the canal, and the fast phase is toward the stimulated canal. The vertigo and nystagmus increase and then decrease within 20 seconds; they are reduced with repeated Dix-Hallpike tests and so the nystagmus is fatigable. All these criteria need to be present to diagnose a patient with BPPV due to debris in the posterior semicircular canal.

Treatment

Nonsurgical Measures

The primary management for BPPV includes maneuvers, Epley (particle or canalith repositioning procedure), and Semont (liberatory maneuver) to reposition the debris into the utricle. The most widely used Epley repositioning maneuvers is depicted in Figure 56–2. The maneuver may be repeated if the patient is still symptomatic. Eighty percent of patients are cured by a single repositioning maneuver. If the symptoms persist after a single maneuver or if patients have recurrent symptoms, the repositioning maneuver may be repeated or patient can be given exercises to perform at home.

Surgical Measures

Surgical treatment is available for a very small number of patients with intractable BPPV. These patients have failed repositioning maneuvers and have no intracranial pathology on imaging studies. The primary surgical option is posterior semicircular canal occlusion. A standard mastoidectomy is performed and the posterior semicircular canal is fenestrated. The membranous canal is occluded with muscle, fascia, or bone pate, or collapsed with a laser. The occlusion prevents debris and subsequent endolymph movement to deflect the cupula. There may be a temporary mixed hearing loss that usually recovers. The success rate for an occlusion of the posterior semicircular canal is high. A more technically challenging surgical option with an increased risk to hearing involves ablating the nerve supply of the posterior semicircular canal via a singular neurectomy.

Prognosis

The natural history of BPPV includes an acute onset and remission over a few months. However, up to 30% of patients may have symptoms for longer than 1 year. Most patients improve with a repositioning maneuver. Patients may have unpredictable recurrences and remissions, and the rate of recurrence may be 10–15% per year. These patients may be retreated with a repositioning maneuver. A subset of patients who have adapted by not using certain positions in order to avoid the vertigo or who have other balance disorders can benefit from balance rehabilitation therapy.

Essentials of Diagnosis

• Episodic vertigo lasting hours
• Fluctuating hearing loss
• Tinnitus
• Aural pressure.

General Considerations

Meniere disease or endolymphatic hydrops is an idiopathic inner ear disorder characterized by attacks of vertigo, fluctuating hearing loss, tinnitus, and aural fullness. The incidence of Meniere disease ranges from 10 to 150 cases per 100,000 persons each year. There is no gender bias and patients typically present in the fifth decade of life. A new diagnosis of Meniere disease in someone younger than age 20 or older than age 70 is unusual. There is no right or left ear predilection for the disease.

Pathogenesis

The cause of Meniere disease remains elusive and has been attributed to anatomic, infectious, immunologic, and allergic factors. The focus of most studies has been the endolymphatic duct and sac based on the basic premise that there is increased endolymphatic fluid owing to impaired reabsorption of endolymphatic fluid in the endolymphatic duct and sac. Histopathological studies have shown blockage in the longitudinal flow of endolymph in the endolymphatic duct, the endolymphatic sinuses, the utricular ducts, the saccular ducts, and the ductus reuniens. Studies have reported that the endolymphatic sacs in patients with Meniere disease are smaller, have less absorptive tubular epithelium, and have increased perisaccular fibrosis. Results of a blinded control study, however, did not show any difference in the connective tissue or fibrosis surrounding the endolymphatic sac in patients with Meniere disease. The vestibular duct has also been shown to be smaller in patients with Meniere disease. Recent studies have shown a decrease in Type II vestibular hair cells in cases of Meniere disease. The role and significance of the decrease in these Type II hair cells are currently not known. The endolymphatic sac has been shown to be important in inner ear metabolic homeostasis. The endolymphatic sac secretes glycoprotein conjugates in response to osmotic challenges, and preliminary studies have shown an alteration in glycoprotein metabolism in Meniere disease. There has been no conclusive proof of an infectious agent related to this disease.

The roles of allergy and immunology in Meniere disease are under active investigation. The “seat” of immunity in the inner ear may be the endolymphatic sac, which is able to process antigens and mount a local antibody response. The endolymphatic sac may be vulnerable to immunologic injury because of the hyperosmolarity of its contents and the fenestrations in its vasculature. These two properties increase the risk of immune complex deposition and injury. IgG deposition is seen in the endolymphatic sacs of patients undergoing shunt procedures of the endolymphatic sac. Patients with Meniere disease also have elevated IgM complexes and C1q component of complement, and low levels of IgA complexes in their serum. These patients have also shown vulnerability to autoimmune (cytotoxic) reactions. Thirty percent of patients with Meniere disease had autoantibodies to an inner ear antigen by Western blot analysis. The response of some patients to steroid therapy and the increased rate of expression of certain HLA antigens (eg, A3, Cw7, B7, and DR2) in patients with Meniere disease support the presence of an underlying immune mechanism.

A similar argument may be made regarding Meniere disease and allergy. A significant percentage (50%) of affected patients have concomitant inhalant or food allergies (or both), and treating these allergies with immunotherapy and diet modification has improved the manifestations of their allergies and Meniere disease.

The role of genetic influences in the pathogenesis in Meniere disease is also being elucidated. Mutation in the COCH gene is associated with Meniere disease. The family of water channels (AQPs) and ion channels have also been implicated.

Clinical Findings

Symptoms and Signs

Meniere disease occurs as episodic attacks lasting for hours. The four symptoms and signs include (1) a unilateral, fluctuating sensorineural hearing loss (often involving low frequencies); (2) vertigo that lasts minutes to hours; (3) a constant or intermittent tinnitus typically increasing in intensity before or during the vertiginous attack; and (4) aural fullness. The acute attack is also associated with nausea and vomiting and, following the acute attack, patients feel exhausted for a few days. Table 56–3 shows the diagnostic scale for Meniere disease created by the Committee on Hearing and Equilibrium of the American Academy of Otolaryngology–Head and Neck Surgery. As emphasized in the diagnostic scale, the diagnosis of Meniere disease is based on the longitudinal course of the disease rather than on a single attack.

Laboratory Findings

Meniere disease is a clinical diagnosis. The diagnostic evaluation primarily includes audiometry and a fluorescent treponemal antibody absorption (FTA-ABS) test to rule out syphilis. FTA-ABS testing is mandatory in any patient given the diagnosis of an idiopathic disease, because syphilis may perfectly imitate Meniere disease. Electrophysiologic studies, other serologic studies, and imaging are obtained as needed. The role of allergy testing continues to be defined. Initially, autoimmune ear disease may be clinically indistinguishable from Meniere disease.

The distinguishing characteristics of an autoimmune ear disease include a more aggressive course and early bilateral involvement. Autoimmune serologic tests may also be helpful. There is no diagnostic test for Meniere disease.

Imaging Studies

MRI with gadolinium contrast allows the exclusion of retrocochlear pathology, such as a vestibular neuroma, and should be considered in all patients with asymmetric hearing loss.

Special Tests

Audiology

Audiologic assessment initially shows a low-frequency or a low- and high-frequency (inverted V) sensorineural hearing loss. As the disease progresses, there is a flat sensorineural hearing loss. A glycerol dehydration test involves measuring serial pure-tone thresholds and discrimination scores during diuresis. The diagnosis of Meniere disease is supported if there is improvement in the patient's hearing. The test is positive in only 50% of patients suspected to have the disease and is not routinely performed in the US.

Electrocochleography

Electrocochleography (ECOG) measures the sound-evoked electrical potentials from the inner ear. The three phenomena measured from the external canal (tympanic membrane) or on the promontory in response to clicks include (1) the cochlear microphonic, (2) the summating potential, and (3) the action potential. The endolymphatic hydrops of Meniere disease causes a larger summating potential and so the ratio of the summating potential to the action potential (SP/AP) is elevated. ECOG lacks the specificity or sensitivity to reliably use the SP/AP ratio to consistently diagnose Meniere disease or predict the clinical course.

Electronystagmography (ENG)

ENG with caloric testing shows peripheral vestibular dysfunction. The caloric response decreases during the first decade of the disease and usually stabilizes at 50% of normal function.

Vestibular-Evoked Myogenic Potential (VEMP) Testing

VEMP is a vestibulo-collic reflex whose afferent limb arises from acoustically sensitive cells in the saccule, with signals conducted via the inferior vestibular nerve. VEMP is a biphasic, short-latency response recorded from the tonically contracted sternocleidomastoid muscle in response to loud auditory clicks or tones. VEMPs may be diminished or absent in patients with early and late Meniere disease, vestibular neuritis, BPPV, and vestibular schwannoma. On the other hand, the threshold for VEMPs may be lower in cases of superior canal dehiscence and perilymphatic fistula.

Differential Diagnosis

In addition to the vestibular system, dizziness may be caused by poor vision, decreased proprioception (diabetes mellitus), cardiovascular insufficiency, cerebellar or brainstem strokes, neurological conditions (eg, migraines, multiple sclerosis), metabolic disorders, and the side effects of medications (see Table 56–2).

Treatment

Nonsurgical Measures

Since the introduction of aminoglycoside therapy in 1948, no significant conceptual advances have been made in the treatment of Meniere disease. The current treatments focus on relieving vertigo without further injuring the patient's hearing. Hearing may be temporarily improved or stabilized by the current treatments, but the hearing does not have long-term stability.

Dietary Modifications and Vestibular Suppressants

The primary management of Meniere disease involves a sodium-restricted diet (≤2000 mg/d) and diuretics (eg, diazide). In a crossover placebo study of diazide, it was shown that diuretics seem to improve vestibular complaints but have no effect on hearing or tinnitus. Some patients benefit from dietary restrictions on caffeine, nicotine, alcohol, and foods containing theophylline (eg, chocolate). Acute attacks are managed with vestibular suppressants (eg, meclizine and diazepam [Valium]) and antiemetic medications (eg, prochlorperazine [Compazine] suppository). Most patients are controlled with conservative management.

Aminoglycoside Therapy

Medically refractory patients with or without serviceable hearing may benefit from intratympanic gentamicin therapy. Intratympanic gentamicin is absorbed into the inner ear primarily via the round window and selectively damages the vestibular hair cells relative to the cochlear hair cells. Gentamicin may also decrease endolymph production by affecting dark cells in the stria vascularis. Intratympanic gentamicin has nearly a 90% vertigo control rate with a follow-up of at least 2 years; the extent of hearing loss depends on the protocol for gentamicin delivery. A variety of treatment protocols (daily, biweekly, weekly, or monthly injections) using fixed-dose or titration end-point regimens exist but a few trends are present. Treatments are stopped if there is persistent hearing loss. Vertigo control is nearly always obtained if vestibular function is ablated. However, the risk of hearing loss increases as the total dose and frequency of gentamicin injections are increased. Current protocols are reducing the dose and frequency of injections to decrease hearing loss and still obtain vertigo control. Vertigo control can be obtained with some residual vestibular function, and this residual function may be useful if patients develop bilateral Meniere disease. Recent studies with monthly injections have shown a nearly 90% vertigo control with a 17% (<10 dB) hearing loss.

Steroid Therapy

Acute exacerbation of Meniere disease may respond to a short burst of oral steroids. Intratympanic steroids have also been used to treat active disease and avoid the systemic complications associated with oral steroids.

Surgical Measures

Occasionally, patients who fail medical and gentamicin treatment may require surgical intervention. Endolymphatic sac surgery and vestibular nerve sections preserve hearing while labyrinthectomy ablates hearing.

Endolymphatic Sac Surgery

The role of endolymphatic sac surgery in the management of Meniere disease remains controversial. One double-blinded, placebo-controlled comparison of the endolymphatic mastoid shunt versus a simple cortical mastoidectomy showed no benefit of the sac surgery. A 9-year follow-up showed a 70% control of vertigo in both surgical groups. Re-analysis of the study suggested a greater benefit in the group who had endolymphatic sac surgery, and a recent study with a 5-year follow-up showed an 88% functional level 1 or 2 response after an endolymphatic mastoid shunt operation. Endolymphatic sac surgery involves a mastoidectomy and locating the endolymphatic sac on the posterior fossa dura (Figure 56–3A). The sac is medial to the sigmoid sinus and inferior to the posterior semicircular canal. The endolymphatic sac is also located along an imaginary line (Donaldson line) in the plane of the horizontal semicircular canal. The endolymphatic sac may be decompressed or have a shunt placed that communicates into the subarachnoid space or mastoid cavity. Endolymphatic shunt surgery provides a nondestructive option for patients who fail medical or aminoglycoside therapy and have good hearing.

Vestibular Nerve Section

A vestibular nerve section provides a definitive treatment of unilateral Meniere disease in patients with serviceable hearing. Ninety-five percent of patients achieve vertigo control, and hearing is preserved in more than 95% of patients. The vestibular neurectomy may be approached via a retrosigmoidal or middle fossa approach (Figure 56–3B). The risk to the facial nerve is <1% in the retrosigmoidal approach and <5% in the middle fossa approach. The patients are acutely vertiginous and have nystagmus (fast phase away from the operated ear) for a few days until central compensation takes effect.

Labyrinthectomy

A transmastoid labyrinthectomy with fenestration of the bony semicircular canals and vestibule and removal of the membranous neuroepithelium provides control of vertigo in nearly all patients with unilateral Meniere disease and poor hearing. The rate of control may decline in 10 years owing to the development of vertebral-basilar insufficiency (aging), poorer vision, and the development of Meniere disease in the contralateral ear. The complete loss of unilateral vestibular function due to the labyrinthectomy leads to unsteadiness in up to 30% of patients.

Prognosis

Meniere disease is characterized by remissions and exacerbations, making it difficult to predict the future behavior of the disease in any individual patient based on the patient's own history, diagnostic evaluations, or epidemiologic profiles. The initial manifestation may be vertigo or hearing loss, but within 1 year of onset, the typical syndrome–attacks of vertigo, tinnitus, fluctuating hearing loss, and aural fullness–is present. Longitudinal studies have shown that after 10–20 years, the vertigo attacks subside in most patients and the hearing loss stabilizes to a moderate to severe level (50 dB). Meniere disease is usually a unilateral disease, and the risk of developing this disease in the contralateral ear appears to be linear with time. Of the patients, 25–45% may develop disease in the contralateral ear.

Essentials of Diagnosis

• Vertigo lasting days after an upper respiratory infection
• No hearing loss
• No other neurological signs or symptoms.

General Consideration

Vestibular neuronitis is the third most common cause of peripheral vestibular vertigo after BPPV and Meniere disease. Vestibular neuronitis has no gender bias and typically affects middle-aged people. Less than half the patients have an antecedent or concurrent viral illness. The presentation includes acute vertigo. Like Meniere disease, the pathogenesis is not known but most patients recover with no sequelae. The primary role of the physician is to rule out a central cause of the acute vertigo. The treatment is primarily supportive care.

Pathogenesis

The proposed etiologies for vestibular neuronitis include viral infection, vascular occlusion, and immunologic mechanisms. The most likely cause is the reactivation of a latent herpes simplex virus type 1 (HSV-1) infection. The study of the available temporal bones of patients with vestibular neuronitis shows a spectrum of injury: from normal to significant degenerative changes in the vestibular nerve, Scarpa ganglion, and vestibular neuroepithelium without evidence of vascular occlusion. The injury is often seen in the superior vestibular nerve.

Clinical Findings

Symptoms and Signs

The presentation of vestibular neuronitis includes the sudden onset of vertigo with nausea and vomiting. The patient has normal hearing and a normal neurological examination. The patient may have postural instability toward the injured ear but is still able to walk without falling. He or she usually does not have a headache and has spontaneous nystagmus characteristic of an acute peripheral vestibular injury. The nystagmus is usually horizontal with a torsional component and is suppressed by visual fixation. The reduction in the vestibular signal in the injured ear leads to relative vestibular excitation in the opposite ear. The result is that the slow phase of nystagmus is toward the injured ear and the fast phase is away from the injured ear. The nystagmus is intensified by looking toward the fast phase and decreased by looking toward the slow phase or toward the injured ear. This principle is Alexander's law. The direction of the nystagmus does not change with changes in the direction of gaze.

Imaging Studies

MRI, with emphasis on the identification of both infarction and hemorrhage in the brainstem and cerebellum, is obtained in patients with risk factors for stroke, with additional neurological abnormalities, and who do not show improvement within 48 hours. Alternately, computed tomography (CT) scanning with thin cuts to evaluate the brainstem, cerebellum, and fourth ventricle may be obtained.

Special Tests

In most patients, vestibular testing shows a complete or reduced caloric response in the injured ear. The caloric response eventually normalizes in 42% of patients. VEMP responses are attenuated or absent.

Differential Diagnosis

The diagnosis of vestibular neuronitis is based on the constellation of symptoms and signs described above; however, they may be mimicked by other disorders as well (see Table 57–2). The need for further evaluation is necessary only if there is a concern for a central cause of the acute vertigo or if the acute vertigo does not substantially improve in 48 hours. The primary central cause for acute vertigo lasting days is a brainstem or cerebellar stroke. In most cases, there are other neurological findings: diplopia, dysmetria, dysarthria, motor and sensory deficits, abnormal reflexes, the inability to walk without falling, and a central nystagmus. Central nystagmus is not affected by visual fixation and may change directions with changes in gaze. A purely vertical or purely torsional nystagmus is highly suggestive of a central disorder. In the event of an isolated inferior cerebellar stroke, the presentation may be indistinguishable with vestibular neuronitis. Also, 25% of patients with risk factors for stroke who present with vertigo, nystagmus, and postural instability have had an inferior cerebellar stroke. Therefore, patients with significant risk factors for stroke should have an imaging study if they present with these symptoms.

Treatment

The primary management includes symptomatic and supportive care during the acute phase of the illness. The patients are given vestibular suppressants and antiemetics to control the vertigo, nausea, and vomiting. These medications are withdrawn as soon as possible to avoid interfering with the central vestibular compensation.

Prognosis

The natural history of vestibular neuronitis includes an acute attack of vertigo that lasts a few days with complete or at least partial recovery within a few weeks to months. Some patients (15% in one study) may have significant vestibular symptoms even after 1 year. Recurrent attacks in the same or contralateral ear have been reported but are unusual. Some patients may later develop BPPV. Vestibular rehabilitation is of benefit in patients with residual symptoms.

Essentials of Diagnosis

• Vertigo or oscillopsia induced by loud sounds or pressure changes in the middle ear
• Conductive or mixed hearing loss with presence of acoustic reflexes
• Nystagmus align with the plane of the dehiscent superior semicircular canal.

General Considerations

In 1998, Lloyd Minor and colleagues described sound- and/or pressure-induced vertigo associated with the bony dehiscence of the superior semicircular canal. Patients complain of vertigo when exposed to loud noises (Tullio phenomenon), with Valsalva maneuvers, with pressure changes in the ear (Hennebert sign), or with factors that raise intracranial pressures. Auditory symptoms include sensitivity to bone-conducted sounds and autophony. Similar symptoms have also been noted with dehiscence of other semicircular canals.

Pathogenesis

The auditory and vestibular symptoms are due to exposure to external pressure along the dehiscent superior canal that is transmitted to the inner ear. Histologic and radiologic studies suggest that superior semicircular canal dehiscence is either congenital or developmental. Approximately 1% of temporal bone CT scans demonstrated significant thinning (≤0.1 mm) or dehiscence of the superior canal in the floor of the middle cranial fossa; this finding is usually bilateral. Over time, this thin bone may be further eroded by pressure transmitted by dura-encased temporal lobe.

Clinical Findings

Symptoms and Signs

Although dehiscence of the superior canal may be congenital, symptoms and signs usually do not present early in life; the youngest patients have been in their teens. Patients may complain of vestibular symptoms only, auditory and vestibular symptoms, or, less commonly, isolated auditory symptoms. Typically, loud noises, pressure on the external auditory canal, and factors that change intracranial pressure (Valsalva maneuver, jogging, jugular venous compression) lead to vertigo or oscillopsia. Many patients complain of chronic disequilibrium.

Patients report increased sensitivity to bone-conducted sounds, hearing their pulse sound, hearing their eye movements, and autophony. “Inner ear conductive hearing loss” is also common. The hearing loss is artifactual and mimics otosclerosis (low-frequency conductive hearing loss); in contrast to otosclerosis, the stapedius reflexes are present. The dehiscent portion of the superior canal acts as a third mobile window allowing acoustic energy to be dissipated there. The presence of stapedius reflex with low-frequency conductive hearing loss should prompt radiological imaging of the inner ear to exclude the possibility of dehiscence of the inner ear.

Imaging Studies

The presence of vestibular symptoms with loud noises or pressure changes, abnormally enhanced bone conduction hearing, or conductive hearing loss with normal stapedius reflexes should prompt radiological imaging with high-resolution CT of the temporal bone. Superior semicircular canal dehiscence is best seen with 0.5-mm collimated helical CT scans with reformation of the images in the plane of the superior canal (instead of the tradition 1.0-mm collimated images in the axial and coronal plane) (Figure 56–4).

Audiologic Testing

Audiologic testing demonstrates low-frequency conductive hearing loss with the presence of stapedius reflex. In otosclerosis, with fixation of the stapes footplate, the reflexes are absent. Presentation of loud auditory signal may elicit typical symptoms of vertigo and eye movements.

Special Examinations

Eye movements, examined with the use of Frenzel glasses (to prevent visual fixation and abolition of the nystagmus) typically align with the affected superior canal and follow Ewald law. The nystagmus is in the plane of the canal, and the fast phase is toward the stimulated canal. Loud noises, positive pressure in the ear canal, and the Valsalva maneuver against pinched nostril lead to excitation of the superior canal. The eye movements associated with the ampullifugal deflection of the cupula have the slow phase that is directed upward and torsion of the superior pole of the eye away from the affected ear. With inhibition of the superior canal due to negative pressure in the ear canal, Valsalva against closed glottis, and jugular vein compression, the eye moves downward and torsions toward the affected ear.

VEMP testing is also useful in the evaluation of patients with SSCCD. Patients with superior semicircular canal dehiscence have lower than normal threshold for eliciting the VEMP response (81 dB NHL vs 99 dB NHL).

Treatment

Nonsurgical Measures

Avoidance of symptom-provoking stimuli such as loud noises, jogging, or singing may be sufficient therapy for patients with mild symptoms. Correct diagnosis of superior semicircular canal dehiscence as the cause of low-frequency conductive hearing loss prevents unnecessary otosclerosis surgery.

Surgical Measures

Patients in whom the symptoms are associated with pressure in the ear canal can be treated with a tympanostomy tube. Patients with debilitating symptoms may require surgical repair of the dehiscence of the superior canal either through the middle cranial fossa approach or the transmastoid approach. The dehiscent area of the canal may be repaired with canal plugging or resurfacing procedure. Hearing loss following surgical repair is more common in revision surgery. The symptoms can recur following surgical repair.

Prognosis

Correct diagnosis of superior semicircular canal dehiscence is a critical first step in the management of patients with this clinical syndrome. Most patients can be helped by avoiding provoking stimuli. Surgery is reserved for the debilitated patient and is often curative.