• Disease Overview
  • Synonyms
  • Signs & Symptoms
  • Causes
  • Affected Populations
  • Disorders with Similar Symptoms
  • Diagnosis
  • Standard Therapies
  • Clinical Trials and Studies
  • References
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Superior Semicircular Canal Dehiscence

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Last updated: 03/23/2023
Years published: 2016, 2019, 2023


Acknowledgment

NORD gratefully acknowledges Gerard Gianoli, MD, The Ear and Balance Institute, Covington, Louisiana, for the preparation of this report.


Disease Overview

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. SSCD has also been reported to be asymptomatic in some people. In the past, many patients with SSCD were misdiagnosed as having otosclerosis, patulous eustachian tubes, middle ear perilymphatic fistulas or Ménière’s disease. Identification of this condition 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.

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Synonyms

  • superior semicircular canal dehiscence syndrome (SCDS)
  • canal dehiscence syndrome
  • superior canal syndrome
  • third mobile window syndrome
  • Minor’s syndrome
  • SSCD
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Signs & Symptoms

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, heartbeat 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.

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Causes

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 intracranial pressure 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 usually does not cause 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. For many patients, it is uncertain why the symptoms started.

There is also evidence of gradual thinning of the temporal bone over the superior semicircular canal throughout 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.

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Affected populations

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 lower.

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Diagnosis

There are three essential elements to make the diagnosis of SSCD:

  • History of symptoms compatible with SSCD
  • Physiologic testing consistent with SSCD
  • CT scan confirmation of an anatomic deficiency of the SSC

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:

  • Supranormal hearing thresholds on audiometry
  • Abnormal fistula/pressure testing
  • Abnormal Valsalva testing
  • Abnormal Tullio’s testing
  • Reduced threshold or increased magnitude of response on VEMP testing
  • Vibration induced torsional nystagmus
  • Abnormal results on electrocochleography

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.

Third mobile window disorders (TMWD) are defined as a group of disorders, among which superior semicircular canal dehiscence syndrome is the most recognized. They have a common clinical presentation including a combination of Tullio’s phenomenon, pressure/strain-induced vertigo and/or autophony, but there are certainly other associated symptoms. The “third window” effect results from the altered inner ear mechanics due to a defect in the inner ear or an aberration of its structural integrity. Initially thought of as rare when the first paper was published in 1998, TMWD are in fact very much more common than realized and it is important to be aware of this. If doctors elicit a full medical history from their patients, including any head trauma, otic barotrauma resulting from air travel etc, the chance of a wrong diagnosis will be greatly reduced. The superior canal may have a complete bony coverage but one of the lesser recognised TMWD must not be overlooked.

The John Hopkins temporal bone histology study of more than 1,000 temporal bones demonstrated 0.5% incidence of SSCD, and about 1.5% incidence of extreme bony thinning. Cochlear-facial dehiscence is actually more common than SSCD, although not recognized or diagnosed nearly as frequently, and their combined incidence is 1.1% in the general population.

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Standard Therapies

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.

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Clinical Trials and Studies

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.

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov

Some current clinical trials also are posted on the following page on the NORD website:
https://rarediseases.org/living-with-a-rare-disease/find-clinical-trials/

For information about clinical trials sponsored by private sources, contact:
www.centerwatch.com

For information about clinical trials conducted in Europe, contact:
https://www.clinicaltrialsregister.eu/

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References

BOOKS
Gianoli G & Thomson P (Eds.). Third Mobile Window Syndrome of the Inner Ear. Superior Semicircular Canal Dehiscence and Associated Disorders. Springer, 2023.

Thomson P. A Hole in My Life. Battling Chronic Dizziness. Createspace, 2016.

JOURNAL ARTICLES
Minor LB. Superior canal dehiscence syndrome. Am J Otol. 2000;21:9–19. Merchant SN, Rosowski JJ. Conductive hearing loss caused by third window lesions of the inner ear. Otol Neurotol. 2008;29:282–289.

White JA, Hughes GB, Ruggieri PN. Vibration-induced nystagmus as an office procedure for the diagnosis of superior semicircular canal dehiscence. Otol Neurotol. 2007;28(7):911–916.

Zhou G, Gopen Q, Poe DS. Clinical and diagnostic characterization of canal dehiscence syndrome: a great otologic mimicker. Otol Neurotol. 2007;28(7):920–926.

Belden CJ, Weg N, Minor LB, Zinreich SJ. CT evaluation of bone dehiscence of the superior semicircular canal as a cause of sound- and/or pressure-induced vertigo. Radiology. 2003;226(2):337¬–343.

Hirvonen TP, Weg N, Zinreich SJ, Minor LB. High-resolution CT findings suggest a developmental abnormality underlying superior canal dehiscence syndrome. Acta Otolaryngol. 2003;123(4):477-481.

Williamson RA, Vrabec JT, Coker NJ, Sandlin M. Coronal computed tomography prevalence of superior semicircular cnaal dehiscence. Otolaryngol Head Neck Surg. 2003;129(5):481–485.

Kartush JK. Superior canal dehiscence syndrome symptoms resolved by reinforcement of the oval and round windows. Unpublished data. 2002.

Gianoli GJ. Deficiency of the superior semicircular canal. Curr Opin Otolaryngol Head Neck Surg. 2001;9:336–341.

Carey JP, Minor LB, Nager GT. Dehiscence or thinning of the bone overlying the superior semicircular canal in a temporal bone survey. Arch Otolaryngol Head Neck Surg. 2000; 126 (2):137-47.

Brantburg K, Bergenius J, Tribukait A. Vestibular-evoked myogenic potentials in patients with dehiscence of the superior semicircular canal. Acta Otolaryngol (Stockh). 1999;119:633–640.

Smullen JL, Andrist EC, Gianoli GJ. Superior semicircular canal dehiscence: a new cause of vertigo. Journal LA State Med Soc. 1999;151:397–400.

Minor LB, Solomon D, Zinreich JS, Zee DS. Sound- and/or pressure-induced vertigo due to bone dehiscence of the superior semicircular canal. Arch Otolaryngol Head Neck Surg. 1998;124:249–258.

INTERNET
The SCDS Society https://www.scdssociety.com/ Accessed March 21, 2023.

Facebook SCDS Support Group https://www.facebook.com/SuperiorSemicircularCanalDehiscence/ Accessed March 21, 2023.

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