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Superior Semicircular Canal Dehiscence (SSCD)

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Superior Semicircular Canal Dehiscence (SSCD) TH 5018 NE 15 AVE · PORTLAND, OR 97211 · FAX: (503) 229-8064 · (800) 837-8428 · [email protected] · VESTIBULAR.ORG Superior Semicircular Canal Dehiscence A Cause of Balance and Hearing Problems By Lloyd B. Minor, MD, Andelot Professor and Director, Dept. of Otolaryngology—Head and Neck Surgery; and John P. Carey, MD, Associate Professor, Dept. of Otolaryngology—Head and Neck Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland with the Vestibular Disorders Association What is superior semicircular canal patients have exclusively auditory dehiscence? complaints. Cause Vestibular and auditory symptoms and With a dehiscence in the bone that is signs can result from a dehiscence supposed to cover the superior (opening) in the bone overlying the semicircular canal (see diagram on page superior semicircular canal of the inner 2), the fluid in the membranous superior ear. This clinical syndrome—superior canal (which is located within the lumen— semicircular canal dehiscence syndrome tubular cavity—of the bony canal) can be (SSCD)—was first described by Minor and displaced by sound and pressure stimuli. colleagues There are normally only two points of in 1998. increased compliance (yielding to pressure) in the inner ear: the oval Patients with SSCD can experience vertigo window, through which sound energy is and oscillopsia (the apparent motion of transmitted into the inner ear via the objects that are known to be stationary) stapes bone; and the round window, evoked by loud noises and/or by through which sound energy is dissipated maneuvers that change middle-ear or from the inner ear after traveling around intracranial pressure (such as coughing, the cochlea. SSCD creates a third mobile sneezing, or straining). Auditory window into the inner ear. The signs and manifestations of the syndrome include symptoms in this syndrome are due to the autophony (increased resonance of one’s physiological consequences of this third own voice), hypersensitivity to bone- window. conducted sounds, and an apparent conductive hearing loss revealed on The mean age at the time of diagnosis is audiometry. Some patients have around 45 years. Unilateral SSCD occurs exclusively vestibular symptoms and relatively equally in the right and left ears. signs; some have both auditory and About one-third of patients have evidence vestibular manifestations; and still other © Vestibular Disorders Association ◦ vestibular.org ◦ Page 1 of 6 of bilateral SSCD at the time of diagnosis. that loud noises cause them to see things In patients with bilateral dehiscence, there moving or that they experience a similar is typically one ear from which the sensation when they cough, sneeze, or symptoms and signs are greater. In strain to lift something heavy. They may patients with unilateral dehiscence, the perceive that objects are moving in time bone overlying the contralateral superior with their pulse (pulsatile oscillopsia). canal (in the opposite ear) is often Some individuals can bring on the abnormally thin. sensation of motion—and cause their eyes to move—by pressing on their tragus (the These findings support the notion that area of skin and cartilage located just SSCD is due to a developmental outside the ear canal). Patients may abnormality. Temporal-bone experience a feeling of constant histopathological studies suggest that 1– disequilibrium and imbalance. 2% of the population have abnormally thin bone overlying the superior canal (Carey et The signs of vestibular abnormalities in SSCD al. 2000). Disruption of this thin layer (as relate directly to the effect of the dehiscence may perhaps occur with trauma or over in creating a third mobile window into the time due to the pressure of the overlying inner ear. One of the most important temporal lobe of the brain) leads to the functions of the vestibular system is to keep onset of symptoms and signs. the eyes focused on objects of interest during head movements. A principle underlying the Diagnosis organization of these vestibulo-ocular reflexes is that the eyes move in the plane of the Vestibular symptoms and signs: The semicircular canal that is being activated. vestibular symptoms in SSCD can be debilitating and often provoke patients to seek medical attention. Patients may note © Vestibular Disorders Association ◦ vestibular.org ◦ Page 2 of 6 in outward motion of the stapes footplate Analysis of the eye movements evoked by (such as negative pressure applied to the sound and pressure stimuli in patients with external ear canal) or that increase SSCD led Minor and colleagues (1998) to intracranial pressure (such as taking a deep the identification of this syndrome. These breath and bearing down, or compressing evoked eye movements often align with the the jugular vein with pressure on the neck) plane of the superior canal. Furthermore, typically result in eye movements that are in the direction of the eye movements the same plane but opposite in direction. provides support for the theory of a third- The eyes move down, and the superior pole mobile-window mechanism. Stimuli that of each eye moves toward the SSCD ear. result in inward motion of the stapes foot- plate (such as loud sounds, applying Auditory symptoms and signs: The pressure to the external ear canal, or auditory symptoms and signs in SSCD blowing pressure through the nose while may mimic those in other ear disorders pinching the nostrils) produce an excitation and may at times seem bizarre. Some of the superior canal. Evoked eye patients have a conductive hearing loss movements in these situations are typically for low-frequency sounds that can vertical-torsional, with the eyes moving up resemble the pattern in otosclerosis. and the superior pole of each eye moving These diagnostic entities can be away from the SSCD ear. Stimuli that result distinguished by acoustic reflex testing: © Vestibular Disorders Association Ear Anatomy and Superior semicircular canal dehiscence An opening in abnormally thin bone overlying the superior semicircular canal creates a third mobile window into the inner ear (in addition to the oval and round windows). © Vestibular Disorders Association ◦ vestibular.org ◦ Page 3 of 6 patients with otosclerosis lose the SSCD. It must be emphasized that even acoustic reflex response in the affected with these scans, the diagnosis depends ear early in the course of the disorder, upon characteristic clinical findings and whereas the response remains intact in other physiologic tests. SSCD. This distinction is important because patients who have conductive Vestibular evoked myogenic potentials hearing loss resulting from SSCD will not (VEMP): In SSCD, loud tones evoke a benefit from stapedectomy surgery. short-latency relaxation potential in the ipsilateral sternocleidomastoid muscle. Patients with SSCD may also complain of Patients with SSCD typically have a lower- symptoms such as hearing their eye move- than-normal threshold for the VEMP ments, hearing their own voice too loudly response, and the amplitude of the VEMP in the affected ear (autophony), or having waveform in an SSCD ear is greater for a distorted sensation of sound in the comparable stimulus intensities than in an affected ear during activities such as ear without dehiscence. The VEMP test running. These auditory symptoms and plays an important role in the evaluation of signs are also manifestations of the third patients with suspected SSCD (Zhou et al. mobile window created by the dehiscence. 2007). Bone-conducted sounds are amplified by the effects of the dehiscence, whereas the Treatment energy from air-conducted sounds is Many patients with SSCD are able to partially shunted away from the cochlea tolerate their symptoms and reduce the and through the dehiscence. more severe effects by avoiding the stimuli that make the symptoms worse, CT imaging: High-resolution CT scans of such as loud noises. For other patients, the temporal bones are very useful in the symptoms are much more debilitating. making the diagnosis of SSCD. These Pulsatile oscillopsia, chronic disequilibrium, scans demonstrate the opening in the and autophony are some of the symptoms bone that should cover the superior canal. for which avoidance of stimuli is unlikely to Care must be exercised, however, because be helpful. such scans may miss a thin layer of intact bone overlying the canal. Applying specific Surgical correction: For patients whose parameters for the CT imaging can well-being is severely affected by SSCD, improve the specificity of the scans surgical repair of the dehiscence can be (Belden et al. 2003), but false positives very beneficial. The middle cranial fossa can still occur, even with the highest- approach has been used most commonly. resolution scans. Individuals who are Plugging of the canal with fascia (fibrous suspected of having SSCD are strongly tissue), using small bone chips to secure urged to have their CT scans performed at the fascia in place, has been shown to be a center experienced in the diagnosis of more effective than canal resurfacing in © Vestibular Disorders Association ◦ vestibular.org ◦ Page 4 of 6 achieving long-term control of symptoms. are debilitated by their symptoms, surgical The main risk of this procedure is hearing plugging of the superior canal can be very loss in the affected ear, although this risk beneficial in alleviating or substantially is low in patients who have not undergone reducing the symptoms and signs. prior SSCD surgery or prior stapedectomy. References The procedure is very effective in 1. Minor LB, Solomon D, Zinreich J, Zee DS. relieving both the vestibular symptoms Sound- and/or pressure-induced vertigo and the autophony associated with SSCD. due to bone dehiscence of the superior Plugging of the superior canal typically semicircular canal. Archives of Otolaryngology—Head & Neck Surgery results in decreased function in this canal 1998;124:249–258. alone, while preserving function in the 2. Carey JP, Minor LB, Nager GT. other semicircular canals (Carey et al. Dehiscence or thinning of bone overlying 2007). The reduction of function in the the superior semicircular canal in a superior canal has minimal negative temporal bone survey.
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