Superior Semicircular Canal Dehiscence




Abstract


It has been 20 years since the initial description of superior semicircular canal dehiscence (SSCD) first appeared in the literature, and information regarding the diagnosis and treatment of this protean condition continues to emerge. SSCD can have multiple unique presentations, with varying degrees of symptoms. Treatment options have expanded; however, with variations in techniques and the small patient numbers reported, comparison of different surgical techniques is challenging. Since 1998, our group has performed more than 400 SSCD surgical procedures of varying types, and we have evaluated many more patients who were managed nonsurgically. Much of this chapter is supported by this experience. First, the incidence, pathophysiology, presentation, evaluation, and treatments for SSCD will be reviewed. Treatment options can range from nonsurgical to minimally invasive to a variety of more invasive procedures. Knowing the range of symptoms, pathophysiology, and outcomes from various interventions will aid the physician in discussing evidence-based management options with patients most appropriately.




Keywords

Autophony, Superior semicircular canal dehiscence, Surgery, Tullio phenomenon, Vertigo, Vestibulopathy

 




Introduction


Superior semicircular canal dehiscence (SSCD) can be defined as an anatomic anomaly found (occasionally incidentally) on high-resolution temporal bone computed tomography (CT) scan or at surgical exploration ( Fig. 12.1 ) or as a syndrome, a constellation of symptoms, that frequently accompanies this anatomic deviation. An anatomic dehiscence of bone overlying the superior semicircular canal (SSC) at its interface with the middle cranial fossa dura has been proposed to be a developmental abnormality. Bone overlying the middle ear cavity and mastoid air cell system (tegmen tympani and tegmen mastoideum) separating the ear from the middle cranial fossa (a.k.a. the floor of the middle cranial fossa) thickens progressively during the first 3 years of life, such that CT findings of SSCD in a 1-year-old patient may “disappear” by the time the patient is 3 or 4 years old. At this age, the incidence of an anatomic finding of SSCD is believed to be relatively stable, rendering 1%–2% of the general population at risk for developing SSCD syndrome.




FIG. 12.1


Intraoperative, middle fossa craniotomy view of the middle fossa floor demonstrating superior semicircular canal dehiscence (SSCD) ( multiple arrows ). MFR , middle fossa retractor; TLD , temporal lobe dura.


The calvarium initially thickens in early childhood, but then, later in life, the thickness of the calvarium and the area of the middle fossa floor in question, very slowly thins. This has also been proposed as a possible cause for development of SSCD in patients who had very thin bone covering the SSC earlier in life. Additional factors that may make an individual susceptible to SSCD are erosive processes such as arachnoid granulations, cholesteatoma, other neoplastic processes, and trauma/fractures. More recently, the CDH23 gene (associated with Usher Syndrome and nonsyndromic hearing loss) has been found to be a genetic risk factor for the development of SSCD.


SSCD has been called the great otologic mimicker because of the myriad of clinical presentations that are similar or identical to other major otologic disorders, such as patulous Eustachian tube, otosclerosis, Meniere’s disease, benign paroxysmal positional vertigo (BPPV), perilymph fistula, sudden sensorineural hearing loss, and acute vestibular neuritis. This variety of presentations gives rise to a disorder that can go undiagnosed or misdiagnosed for quite some time.


Consider the SSCD patient who presents initially with conductive hearing loss without vestibular symptoms and is diagnosed with otosclerosis. Unless the patient undergoes some investigation beyond basic audiometry, the patient can go for quite some time labeled as having “otosclerosis” before a different diagnosis is considered. Even after the patient develops vestibular symptoms, many clinicians may preserve the otosclerosis label while adding other diagnoses, such as BPPV or “otosclerosis with Meniere’s disease.” Clinicians must maintain a level of open-mindedness and not “pigeonhole” patients into a single diagnosis when dealing with patients with symptoms of SSCD.


To make the condition even more interesting, a patient with the absence of bone overlying the SSC may be completely asymptomatic. It has been recognized since the initial description of the problem in 1998 that a “second event” is suspected to be the root cause of the onset of the symptoms of SSCD. The leading suspected cause is head trauma or conditions that cause increased intracranial pressure. This is the presumed reason that SSCD symptoms are rarely seen in children.




Incidence and Etiology


The prevalence of SSCD has been found to be much higher in a series of analyzed CT scans than on temporal bone histology. Carey et al. identified a complete absence of bone over the superior canal histologically in 0.5% of 1000 vertically sectioned adult temporal bones. There was an additional 1.4% with very thin (<0.1 mm) bone covering the SSC. Added together, the histologic prevalence of thin or dehiscent SSCs approached 2%. This study also reported that 50% of patients with SSCD had bilateral involvement. Carey and colleagues also analyzed 36 infant temporal bones and concluded that the thickness of the bone overlying the superior canal was consistently thin. The thickness of the bone covering the SSC gradually increased with age, reaching adult thickness by 3 years of age.


Roberto et al. used tetracycline staining to investigate the deposition of bone in a dog model at 10, 25, and 50 days of age. This study demonstrated progressive deposition of endochondral and endosteal bone over the SSC postnatally. The bone deposition decreased with age. These findings are in agreement with observations in the study by Carey et al.


In a related study, Hirvonen and colleagues reported a CT temporal bone study of bone thickness over the SSC in patients with SSCD and those without SSCD. Among those with SSCD, the contralateral SSC bone was thinner (or dehiscent), compared with those patients without SSCD. This finding of thin bone overlying the SSC bilaterally supports the notion of SSCD as a developmental anomaly related to bony deposition in early life.


Several observations point to SSCD as a developmental anomaly requiring a “second event” to produce symptoms:



  • 1.

    Studies demonstrating development of bone over the SSC occurring later (postnatally) than other parts of the inner ear.


  • 2.

    Clinical observations of asymptomatic but anatomic SSCD noted during intraoperative exploration of the middle cranial fossa for encephalocele repair.


  • 3.

    Symptoms of SSCD rarely present in the pediatric population.



Thus, a second event may be required in addition to the congenital anomaly of thin or absent superior canal bone to produce clinical symptoms. Roughly half of patients with SSCD report an event they attribute to symptom onset. This “second event” is typically noted to be head trauma, a Valsalva-type episode, barotrauma, or some other type of intracranial pressure-altering event.




Clinical Presentation


SSCD was first reported by Minor et al. in eight patients who exhibited symptoms of short-lived episodic vertigo in response to certain sounds or activities that would cause transient increases in intracranial or middle ear pressure (e.g., Valsalva, coughing, sneezing, nose-blowing, auto-insufflation). These activities produce torsional nystagmus, which directly implicates SSC stimulation. Activities causing increased middle ear pressure (e.g., loud sound, positive pressure in the ear canal, autoinsufflation) induce nystagmus with the slow phase upward and the superior pole of the eye directed away from the affected ear. Activities causing transient elevation in intracranial pressure (e.g., Valsalva against a closed glottis, jugular venous compression) or negative pressure in the ear canal result in the slow phase of nystagmus directed downward and the superior pole of the eye directed toward the affected ear. The clinical findings of Tullio’s phenomenon and pressure-induced nystagmus associated with SSCD has been termed “Minor’s syndrome.” Although the vertigo caused by SSCD is most characteristically reported as short-lived, other descriptions of vestibular symptoms have been reported as well, including more prolonged vertigo spells, chronic disequilibrium, and drop attacks.


Since the first identification of SSCD as a cause for Minor’s syndrome, other symptoms and clinical presentations have been identified.


Although the presentation of patients with SSCD is highly variable, the most recognizable presentation will include Tullio’s phenomenon, pressure-induced vertigo with transient increases in intracranial or middle ear pressure, and autophony. While these symptoms are characteristic of SSCD, they are certainly not present in all SSCD patients, and their absence cannot be used as a means to exclude the diagnosis of SSCD. The more nonspecific symptoms of vestibulopathy such as head movement–induced disequilibrium is frequently described by patients but not particularly helpful in confirming the diagnosis of SSCD. Aural pressure and aural fullness are commonly reported, as are complaints of hearing loss, distorted hearing, pulsatile tinnitus, aural fullness, and hyperacute hearing.


There are also vague cognitive and neurobehavioral symptoms frequently reported by SSCD patients that are not specific to SSCD but should be included in the discussion during patient education. These symptoms include depression, “brain fog,” short-term memory problems, and difficulty with concentration. These symptoms often improve after surgical repair of SSCD.


Patients with SSCD may have been given other diagnoses prior to presentation. The biggest indicator of a misdiagnosis is nonresponse to treatment. This should always prompt the clinician to reassess the prior diagnosis.




Physical Examination


Routine head and neck examinations are typically normal in the SSCD patient. Microscopic otoscopy is normal as well, unless there has been prior surgical intervention. The vestibular component of the physical examination should include evaluation with infrared video goggles. This is typically unremarkable but may reveal spontaneous nystagmus and head thrust or head-shake abnormalities if there has been any vestibular loss. In some extreme cases, one can identify spontaneous torsional nystagmus that is synchronous with the pulse.


Tuning fork testing can also be helpful. Patients may “hear” the tuning fork in the affected (SSCD) ear when the tuning fork is placed on the ankle or some other remote bone location.


Pneumatic otoscopy can be an effective screening tool for SSCD. Symptoms of vertigo, or a feeling that the world shifts or moves, will often be reported during pneumatic otoscopy. Infrared video-oculography (VOG) cameras can be used to observe and/or record the resulting pressure-induced nystagmus. If VOG cameras are not available, a second examiner, an assistant or family member can also watch for pressure-induced nystagmus. The direction of pressure-induced nystagmus is dependent on the location of dehiscence and whether positive or negative pressure is applied.




Testing


Much of the published literature on SSCD discusses CT imaging and vestibular evoked myogenic potential (VEMP), an electrical potential measured from the sternocleidomastoid muscle elicited by a sound stimulus to the ipsilateral ear. Relatively high-intensity sound stimulates the hair cells in the ampulla of the saccule, which in turn send afferent impulses along the inferior vestibular nerve to vestibular nuclei in the brainstem. The vestibular nuclei send efferent projections along the vestibulospinal (vestibulocollic) tract to the sternocleidomastoid muscle to facilitate head stabilization. Athough the CT scan is imperative (the “gold standard”), and VEMP testing is often helpful, we feel this limited testing is inadequate for patients presenting with symptoms of SSCD. Because SSCD is a disorder that can mimic many other otologic disorders, can cause a number of secondary pathologies, and may require invasive surgery to resolve, we feel a full audiovestibular test battery is warranted.


CT scan sections should be performed at the submillimeter level, preferably 0.24 mm thickness, but no thicker than 0.6 mm. The thinner section scan gives a more accurate portrayal of the defect. Thicker scans can be prone to both false-positive and false-negative findings. Both coronal and Pöschl views will demonstrate the dehiscence, but the Pöschl view is a parasagittal view perpendicular to the petrous ridge/long axis of the temporal bone displaying the entirety of the SSC ( Fig. 12.2 ). Because of this, Poschl views tend to only identify large dehiscences. Slices perpendicular to the SSC, such as coronal or Stenvers views, are more sensitive in detecting a dehiscence. MRI should be performed to evaluate for concomitant intracranial abnormalities. One of the more frequent findings in SSCD patients is Chiari malformation. Additionally, MRI findings suggestive of elevated intracranial pressure, such as empty sella, vertical tortuosity of the optic nerves, prominent arachnoid spaces around the optic nerves, flattening of the globe, slit-like ventricles, venous sinus abnormalities, and Chiari/cerebellar ectopia should also be sought.


Jul 4, 2019 | Posted by in OTOLARYNGOLOGY | Comments Off on Superior Semicircular Canal Dehiscence
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