Clinical and Health Affairs
The First New Otologic Disorder in a Century
Superior Canal Dehiscence Syndrome
By Meredith E. Adams, M.D., and Samuel C. Levine, M.D.
■ Superior canal dehiscence syndrome is a recently described vestibular and hearing condition caused by an absence of bone over the arc of the superior semicircular canal. Patients with the condition present with a variety of perplexing symptoms including conductive hearing loss, hyperacusis, vertigo, autophony, and eye movement in response to sound. This article presents the case of a patient with the condition, discusses how it is distinct from other vestibular and hearing disorders, and describes how patients with this disorder can be diagnosed and treated.
A 58-year-old man first presented to his primary care physician after being in a car accident. He complained that his hearing had changed “in a very odd way.” The patient described the accident in which his car had been struck by an oncoming ambulance and explained that he suffered only minor bumps and bruises. A few weeks later, he started to notice changes in his hearing as well as his sense of balance. He began to abnormally hear himself talk, chew, and breathe. If it were quiet enough, he could even hear his heart beating. These symptoms increased if he held his breath.
The man also noticed increased fatigue throughout the day and found that if he held the phone to his right ear and heard a loud sound through the receiver, or if he just pressed on the opening of his right ear canal (tragal compression), he experienced brief dizziness and a peculiar sensation of his eyes moving. He also described a feeling of imbalance at times when walking. After the sound of falling tools almost caused him to lose his balance while on a ladder, he stopped working as an electrician.
A head CT and a brain MRI performed after the accident looked normal, and his primary care physician and a neurologist diagnosed him as having post-concussive syndrome. The patient was enrolled in intensive vestibular rehabilitation therapy for several months. But when his symptoms did not improve and he was unable to safely return to work after a year, his insurance company indicated that they thought he was malingering. He was then referred for a neurotology evaluation at the University of Minnesota.
When the patient presented to us, we learned that he had no other otologic history—no ear surgery or significant ear infections as a child or adult. He denied having any other symptoms such as otorrhea, ear pain, aural fullness, vertigo, or hearing loss.
His examination was normal except for a few unusual findings. One was his response to the Weber tuning fork test. He heard sound only in his right ear. And although he appeared somewhat confused during the Rinne test, which compares the perception of sound transmitted by bone conduction to that transmitted by air conduction, it appeared he had negative results on the right side (bone conduction louder than air conduction) and positive results on the left side, suggesting a conductive hearing loss in his right ear. Surprisingly, no matter where the tuning fork was placed—even on his ankle—he heard it better in his right ear. His audiogram confirmed a mild right conductive loss; but he actually had better-than-normal bone conduction thresholds. He also had some high-frequency sensorineural hearing loss that was symmetric bilaterally.
A new set of fine-cut temporal bone CT images was obtained and reformatted in the plane of the superior semicircular canal, revealing an absence of bone over the arc of the superior semicircular canal. Further specialized electrodiagnostic testing confirmed that the patient had superior semicircular canal dehiscence syndrome (SCDS).
The patient was shocked to learn that there was such a condition and that his doctors thought he needed a craniotomy to resolve it. Had he walked through our clinic doors a dozen years earlier, that wouldn’t have been the case. That’s because the disorder, which may arise spontaneously or after head trauma, was only recently discovered.
A New Condition
In 1998, Lloyd Minor at Johns Hopkins University published an article on a series of eight patients with a distinct syndrome of sound- and pressure-induced vertigo (an illusory sense of motion) and better-than-normal bone-conduction thresholds.1 When one of those patients heard a loud sound in the affected ear, his or her eyes were observed to deviate. He or she developed nystagmus (spontaneous compensatory eye movement) with a rotatory component. This finding of eye movement in response to sound is known as Tullio phenomenon. Similarly, when negative pressure was applied to the ear canal (a fistula test), the patients showed Hennebert sign, which features symptoms of nystagmus and vertigo.
What was causing these signs, which once were strongly associated with advanced syphilitic bone disease in the skull base but had rarely been seen since the advent of penicillin?2 Meniere’s disease could cause similar signs; but Minor’s patients lacked the characteristic fluctuating hearing loss, tinnitus, ear fullness, and hours of spontaneous vertigo commonly experienced by those with Meniere’s.3 Tullio phenomenon and Hennebert sign had been observed in patients with fistulas in the inner ear—whether from injury, chronic inflammation, or eroding cholesteatomas—suggesting that an abnormal hole in the labyrinth allowed sound and pressure energy to stimulate the vestibular system instead of the cochlea alone.4-6 But Minor’s patients did not have evidence of any of these conditions.
The key to understanding this perplexing syndrome was Minor’s careful examination of the eye movements using testing known as electronystagmography or ENG, which assesses the function of the semicircular canals. The three fluid-filled canals (superior, posterior, and lateral) are the inner ear’s rotation sensors. Normally, in response to angular acceleration such as head turning, fluid movement inside the canals deflects specialized sensory cells at the dilated end of each arc (the ampulla). The vestibular nerve signal is relayed to nuclei controlling eye movements. As the head turns, our eyes reflexively turn almost equally in the opposite direction, so our visual field remains stationary and we maintain balance. The canals are at right angles to one another, arranged in X, Y, and Z planes, so that they can detect head movement. When one set of canals is stimulated, the corresponding canals in the opposite ear are inhibited. Vertigo and nystagmus occur when asymmetric stimulation of one inner ear occurs.
When Minor’s patients heard certain sounds or felt pressure, they exhibited nystagmus, with both vertical and rotary eye movements. The pattern was what one would expect if the superior canal alone was activated. Minor postulated that there was a fistula in the superior canal, and that the hole in the bone allowing the fluid movement was inside the skull rather than in the mastoid. No one had been looking for holes in the superior canal because its location high and deep in the temporal bone, abutting the dura of the temporal lobe, makes it more immune to erosion from middle-ear and mastoid disease. But careful inspection of fine-cut temporal bone CT scans revealed that all of Minor’s patients lacked the bony covering over the superior semicircular canal in the affected ear. Surgical occlusion of the affected canal led to complete symptom resolution, further bolstering the idea that the superior canal was causing the symptoms. Superior semicircular canal dehiscence syndrome, the first new otologic disorder in a century or more, had been discovered.
Although the incidence rate for SCDS has not been estimated systematically, a study of 1,000 postmortem temporal bone specimens demonstrated a 0.5% prevalence of complete dehiscence of the bone overlying the superior semicircular canal. In an additional 1.4% of specimens, the thickness of overlying bone was 0.1 mm or less.7 Although less common, dehiscences in the absence of inflammatory disease have been identified in the posterior semicircular canal and are associated with similar symptoms and signs.8 Patients may become symptomatic at any time in their life; but most diagnoses have been made in adults.9 The cause of these dehiscences and the time between their development and symptom onset remain unknown; but predisposing conditions that have been proposed include developmental abnormalities, head trauma, and increased intracranial pressure.9,10
The Approach to the Patient
As with all cases in which patients complain of dizziness and balance problems, taking a complete clinical history is the most important part of the diagnostic evaluation. Patients with SCDS may report vestibular symptoms, auditory symptoms, or both. But because the symptoms of SCDS can be quite unusual, patients do not always volunteer them without prompting. Thus, it is important to ask about a number of symptoms specifically. Vestibular symptoms include vertigo, disequilibrium, and oscillopsia (the illusion that stationary objects are moving in the visual field). In 90% of patients with SCDS, oscillopsia occurs in response to loud sound; in 73% of patients, it occurs as the result of pressure changes in the middle ear.9 Middle-ear and intracranial pressure changes can be induced by sneezing, coughing, straining, or squatting. Other patients may complain of unsteadiness, generalized fatigue, and difficulty multitasking, likely because they are expending considerable mental energy to maintain balance.
Physicians should ask about several characteristic auditory symptoms. One is autophony or the bothersome perception of internal noises such as the sound of one’s own voice, breathing, and chewing. It should be noted that autophony can result from Eustachian tube dysfunction, middle-ear effusion, or an abnormally patent Eustachian tube as well as SCDS. More than half of patients with SCDS (52%) also have an unusual sensitivity to bone-conducted noises (hyperacusis).9 Patients may report hearing their eyes move, feet striking on a hard surface, and pulse in the affected ear.1,11 The reasons for conductive hyperacusis in SCDS patients are not fully known; but the phenomenon can be triggered by placing a 512 Hz tuning fork on the medial malleolus. Patients with SCDS will often hear the tone in the affected ear.
An audiogram should then be obtained. Patients with SCDS typically have a low-frequency conductive hearing loss (Figure 1). The audiogram will look similar to those seen with middle-ear pathologies such as otosclerosis and chronic ear disease. In fact, it is now recognized that many patients who have “failed” stapes operations for otosclerosis actually had SCDS. Unlike hearing loss caused by otosclerosis, however, the hearing loss associated with SCDS is not the result of middle-ear pathology. Instead, SCDS hearing loss is the result of sound energy dissipating away from the cochlea toward the dehiscence. In other words, the sound wave reaches the inner ear because the ear drum and ossicles are working normally, but then some of the energy takes the path of least resistance created by the dehiscence. So what looks like hearing loss related to a middle-ear source is, in fact, a disorder of the inner ear. Additionally, hyperacusis results in abnormally low bone-conduction thresholds, resulting in an air-bone gap. It is not uncommon to see thresholds of 0 or even (-)10 dB at 250 and 500 Hz,12 which show that bone-conducted hearing at the lowest-tested frequencies is far better than the established averages for persons with normal hearing. Unlike other causes of conductive hearing loss, an inner-ear conductive loss is associated with an intact acoustic stapedial reflex.9
High-resolution CT imaging of the temporal bone is ordered for patients suspected of having SCDS. The key finding is an absence of bone anywhere over the arc of the superior canal (Figure 2). During the last decade, we have learned that traditional coronal CT images significantly overestimate the prevalence of canal dehiscence because of the effects of volume averaging.13 To compensate for this, 0.5 mm collimated CT images are obtained and reformatted in the planes parallel and orthogonal to the superior canal (known as Stenver and Poschl views).14 The possibility of false-positive findings is much lower with this approach, but some risk of over-calling the disease remains.
Given that risk, as well as the fact that dehiscences apparent on radiography may not result in the patient having symptoms, it also is important to obtain physiologic confirmation that the superior canal dehiscence is affecting inner ear function before rendering a diagnosis of SCDS. Clinical findings that have been used for this purpose include: 1) conductive hearing loss with preserved acoustic reflexes,12 2) nystagmus in the plane of the affected canal in response to sound and pressure stimuli,9 3) reduced thresholds for vestibular-evoked myogenic potentials (VEMPs), which represent neck muscle responses to loud sounds.15 Evoked eye movements may be observed under infrared video goggles or lenses designed to eliminate visual fixation (Frenzel lenses). A calibrated audiometer can be used to deliver high-intensity sound to one ear, and a pneumatic otoscope may deliver pressure to the external auditory canal, eliciting nystagmus in the plane of the dehiscent canal. Testing for VEMPs may be performed by specially trained audiologists. Recently, electrocochleography, a test more commonly used for Meniere’s disease, has been found to yield abnormal results for patients with SCDS. Unlike the other tests, this one can be performed in the operating room, and can help a surgeon identify the dehiscence and confirm that it is satisfactorily occluded.16
For those meeting the criteria for symptomatic, electrophysiologically active SCDS, management is geared toward the patient’s degree of disability. The benefits of symptom resolution must be weighed against the risk of surgical intervention. Often after years of uncertainty or previous diagnoses of psychosomatic or psychiatric illness, many patients are relieved just to receive a diagnosis. Those who are mildly affected may choose to adapt to the condition or avoid situations that exacerbate it. A sound-reducing ear plug in the affected ear can help manage sound-induced symptoms.11 For some patients, pressure-induced symptoms can be improved by placing a pressure equalization tube in the tympanic membrane.11
Surgical therapy is offered to patients who find their vestibular symptoms or autophony disabling and who are otherwise in good health. Early attempts to resurface the canal wall, leaving fluid flow intact, showed short-lived resolution of symptoms.9,17 Therefore, superior canal occlusion has become the preferred surgical treatment. Occlusion may be performed from above, through a craniotomy of the middle cranial fossa, or from behind, through the mastoid. The middle fossa craniotomy approach is currently favored, as it allows for direct visualization of the dehiscence. Through an approximately 3 x 4 cm opening in the bone of the skull above the auricle, the dura of the temporal lobe is elevated from the middle fossa floor until the superior semicircular canal dehiscence is identified. It is then plugged. The transmastoid approach involves making separate openings in front of and behind the dehiscence from below, and then occluding the canal in between.18 A variety of substances have been used to plug the canal, ranging from fascia and moistened bone dust to wax.
Successful canal occlusion results in resolution or satisfactory improvement in vestibular symptoms, autophony, and hyperacusis. A recent study found that patients who had SCDS surgery saw twice as much improvement on the Dizziness Handicap Inventory as patients who underwent surgical treatment for Meniere’s disease.19,20 The fact that this condition responds better to surgery than Meniere’s disease is important because SCDS has often been mistaken for Meniere’s disease in the past. Conductive hearing loss is less consistently improved by surgery, and traditional amplification is recommended if this is the main symptom.21
Summary
The index patient had a superior canal plugging procedure and is doing much better. He is back at work and can enjoy sound again without feeling sick to his stomach. Just knowing that the odd collection of symptoms were from an anatomical disorder seems to give him some comfort.
We now know that SCDS results from the absence of bone overlying the arc of the superior semicircular canal. Patients may present with unusual sound and pressure-evoked vestibular symptoms, autophony, conductive hyperacusis, and hearing loss. High-resolution CT reformatted in the plane of the superior canal is used to diagnose the anatomic abnormality. The diagnosis of SCDS is reserved for symptomatic patients with radiographically apparent dehiscences who also have evidence of physiologically active disease, as demonstrated by audiometry, evoked nystagmus, or specialized electrophysiologic testing. Surgical occlusion of the canal can alleviate symptoms and allow disabled patients to return to work and have a good quality of life. MM
Meredith Adams is an assistant professor of otolaryngology and Samuel Levine is a professor of otolaryngology at the University of Minnesota.
References
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