NORD gratefully acknowledges Gerard Gianoli, MD, The Ear and Balance Institute, Covington, Louisiana, for the preparation of this report.
Superior semicircular canal dehiscence (SSCD) has been defined as the absence of bone overlying the superior semicircular canal facing toward the dura of the middle cranial fossa. SSCD has been implicated as the cause of a variety of inner ear symptoms including Tullio’s phenomenon, pressure induced vertigo, aural fullness, autophony, conductive hearing loss, and fluctuating or progressive sensorineural hearing loss. Additionally, SSCD has also been reported to be asymptomatic. In the past, many patients with SSCD had been misdiagnosed as having otosclerosis, patulous eustachian tubes, middle ear perilymphatic fistulas, or Ménière’s disease. Identification of this entity requires a high degree of suspicion, appropriate findings on physical exam, lab testing, and confirmation on high-resolution CT scan. Surgical repair of the SSCD or occlusion of the superior canal has been reported with a high degree of symptom resolution.
SSCD has been labelled the “great otologic mimicker” because it can simulate the symptoms of so many other ear disorders. However, the most common symptoms are vertigo/dizziness elicited by pressure altering activity, Tullio’s phenomenon (sound-induced vertigo), fullness/pressure in the ear and autophony. Autophony is hearing internal noises louder than would be expected, such as hearing your eyes move/blink, heart beat or joint movements. Hearing loss and fluctuating hearing loss can occur, mimicking otosclerosis or Meniere’s disease. Fullness in the ear and hearing your breathing loudly in the ear are symptoms of patulous eustachian tube, but can also be found with SSCD. Finally, most patients with the anatomic defect of superior semicircular canal dehiscence have no symptoms at all for some time prior to developing symptoms.
The anatomic defect of SSCD is believed to be a developmental anomaly of the temporal bone in the vast majority of cases. At birth, the bone over the top of the inner ear, which is the floor of the middle cranial fossa, is very thin or absent. Over the first three years of life, this bone thickens considerably. However, this is variable and approximately 20% of the population is left with holes in the middle fossa floor that extend into the middle ear or mastoid. These are typically free of any symptoms unless pressure from intracranially causes the brain to prolapse through one of these holes – an event typically not seen until the latter decades of life. Approximately 1-2% of the population will be left with a defect of the bone over the superior semicircular canal resulting in SSCD. However, this is almost always free of any symptoms until later in life.
A second event occurs in the vast majority of patients prior to the onset of symptoms. This second event is most commonly head trauma or a major pressure-altering event such as scuba diving, air travel, or straining. There are a still a significant number of patients where it is uncertain why the symptoms started.
There is also evidence of gradual thinning of the temporal bone over the superior semicircular canal throughout the rest of one’s life. This thinning is an extremely slow process, but it does present another possibility for SSCD development. Theoretically, if someone has thin bone over the SSC, over time the bone could dissolve away. This could set up the environment for SSCD.
Much less common are erosive processes that can cause the development of SSCD. This can be seen with benign or malignant tumors and with arachnoid granulations. Lastly, trauma can also result in fractures across the SSC that can produce SSCD.
SSCD can affect all age groups. There are no studies looking at the demographics of SSCD, however, the vast majority of patients diagnosed with SSCD are adults. Although the incidence of finding SSCD on a scan is the same for adults and children, the incidence of symptomatic SSCD in children is much less.
There are three essential elements to make the diagnosis of SSCD:
As mentioned earlier, there are many patients who have the anatomic defect of SSCD but have no symptoms at all. These patients do not have SSCD syndrome and should not be treated, but observed. They may or may not develop symptoms later in life. However, if someone has the symptoms of SSCD, the next step is physiologic testing. Several findings on physiologic tests that can be suggestive of SSCD include:
If the above two criteria are met, then a high resolution thin-sliced CT scan of the temporal bone is ordered.The slice thickness should be as thin as possible (<0.6mm, but preferably 0.125 mm). The orientation of the slices should be done in the axial, coronal, Stenvers and Poschl planes. Identification of a dehiscence on a CT scan with thicker slices could easily not be a true dehiscence, but the result of averaging artifact. Once these three elements are met, the diagnosis of SSCD can be confirmed. However, it should be noted that patients with SSCD can also have other dehiscences of the otic capsule. The otic capsule in SSCD patients is of less volume than the norm which may explain this phenomenon. There is also a higher incidence of Arnold Chiari malformation among SSCD patients.
Because the condition of SSCD is still fairly new (identified in 1998), it could be argued that all treatments are considered experimental or investigational. However, the most frequently employed are surgical. Surgery for SSCD has included resurfacing the defect with a variety of materials, plugging the superior semicircular canal, and a combination of resurfacing and plugging. These techniques can be done through a transmastoid or a middle fossa craniotomy approach. These techniques have found great success in reducing or eliminating the vestibular symptoms of SSCD. These techniques are also effective in reducing or eliminating autophony but they have not been effective in improving hearing. A lesser approach that seems to give similar benefits is reinforcement of the middle ear windows. This surgery has the advantage of being a more minimally invasive procedure but the success rate seems to be much lower with a higher recurrence rate in the long term.
There are a number of treatments that have been employed with limited or no success in SSCD. Among these are placement of pressure equalizing tubes (PE tubes) and endolymphatic sac surgery. Alternatives to surgical plugging/repair that have merit are 1) reassurance and observation if the symptoms are relatively minor, and 2) avoidance of provocative stimuli (e.g. sound stimuli, straining, etc.). SSCD is not a life-threatening disorder as long as it does not result in a fall, motor vehicle accident or some other trauma from poor balance. Consequently, the decision to do any treatment is elective and not mandatory.
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The SCDS Society https://www.scdssociety.com/ Accessed August 27, 2019.
Facebook SCDS Support Group https://www.facebook.com/SuperiorSemicircularCanalDehiscence/ Accessed August 27, 2019.
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