What Audiologists Need to Know About Superior Canal Dehiscence Syndrome



Author: Samuel N. Bittel, Au.D.

Superior canal dehiscence syndrome (SCDS) was first described by Lloyd Minor, M.D., at Johns Hopkins in 1998. It is important for audiologists to be well versed in SCDS, as it can present with a number of unique symptoms and diagnostic findings. This condition is often initially suspected through a specific audiometric profile, which may affect hearing aid fitting, influence patient counseling, and substantiate the need for advanced vestibular testing.
Pathophysiology
SCDS is caused by a thinning or absence of the bone covering the superior semicircular canal. With this absence, a false inner ear third window (in addition to the normally-occurring oval and round windows) is created. SCDS is thought to be congenital, arise from head trauma, or be the product of increased intracranial pressure. However, it is important to note that, oftentimes, the cause may be unknown.

With a typically structured inner ear, sound will travel from the oval window, via stapes vibration, in the direction of least resistance. This direction is towards the round window, which is essentially the cochlea’s pressure outlet valve. With SCDS, sound may travel in the direction of the dehiscence and the superior semicircular canal. This will allow sound to directly stimulate the vestibular system, which it should not. Additionally, with an absence of bone over the canal, sound can more easily stimulate the ear via bone conduction. This may lead to a number of interesting symptoms.
Symptoms
Patients may present with any degree or combination of:
  • Autophony (sensitivity to one’s own voice)
  • Dizziness/lightheadedness
  • Disequilibrium
  • Tullio’s phenomenon (dizziness provoked from sound)
  • Hennebert’s sign (dizziness with a change in intracranial pressure)
  • Hyperacusis
  • Low-frequency pseudo conductive hearing loss
  • Aural distortion with loud and/or specific sound
Audiologists should be sensitive to this condition when taking case histories and during diagnostic testing. For those audiologists not completing vestibular work, this condition is still important to watch for, as hyperacusis, aural distortion, and Tullio’s phenomenon may directly impact hearing aid fitting and counseling.
Clinical Findings
There are a number of clinical findings that can be observed in patients with SCDS:
  • Often, these individuals will demonstrate what appears to be a low-frequency conductive hearing loss. Low frequency bone conduction scores are often better than expected.
  • There will also not be a clinical correlate with other middle ear tests. That is, the patient will demonstrate what appears to be a low frequency conductive hearing loss, but may have normal tympanograms, present stapedial reflexes, and present otoacoustic emissions.
  • The vestibular evoked myogenic potential (VEMP) test may show abnormally low thresholds and/or atypically large amplitudes.
  • If a patient demonstrates what appears to be a low frequency conductive hearing loss, but has a present VEMP response, SCDS should be included with the differential. The VEMP response is highly influenced by conductive hearing loss, so the response should be obliterated by true middle ear pathology.
  • Patients with SCDS will often demonstrate nystagmus or dizziness with vocalization in a vision denied condition. This finding would be consistent with Tullio’s phenomenon.
  • SCDS may also cause dizziness or nystagmus during tympanometric or perilymphatic testing, which is consistent with Hennebert’s sign.
Diagnosis
If SCDS is suspected through audiometric testing and/or the patient’s case history, the next logical step would be an advanced vestibular evaluation. This evaluation can look for the presence of sound and/or pressure-induced nystagmus (consistent with the involved canal), VEMP amplitude and thresholds, and postural stability. If the preliminary diagnosis of SCDS continues to be substantiated, the patient should be referred for a medical otological evaluation.

The gold standard testing for identifying SCDS is a fine detail CT-Scan of the temporal bone. It will be important for the reading radiologist to know that SCDS is suspected, as this condition can be missed during reconstruction or with an incorrect plane. Figure 1 shows SCDS on CT-Scan.
Figure 1. CT-scan of SCDS. Photo courtesy of James Lin, M.D., University of Kansas Medical Center.


Management
If SCDS is confirmed through imaging, this condition can be surgically repaired through plugging of the dehiscence by a neurotologist. The surgeon will often take either a middle fossa craniotomy or transmastoid approach. However, in this author’s experience, many individuals do not elect to have this somewhat invasive surgery, so must be counseled to avoid provoking factors. Please refer to Figure 2 for a surgical picture of the middle fossa approach.
Figure 2. Surgical photo of middle fossa approach. Photo courtesy of James Lin, M.D., University of Kansas Medical Center.


Conclusion
Although SCDS is relatively rare, it presents with a number of interesting symptoms and test findings that should not be ignored. The author has personally found this condition is a number of patients whose only abnormal test findings were similar to the audiogram in Figure 3 (SCDS was ultimately diagnosed with radiographic study). In fact, this patient’s comprehensive vestibular test battery (including cVEMP) was entirely normal.

Figure 3. Audiologic assessment results from patient diagnosed with SCDS. This audiogram is from a patient with a rather large left dehiscence. Please note the atypically good left bone conduction scores, and the low-frequency air-bone gaps with present stapedial reflexes.


Please notice the large air-bone gap at 250 Hz, which may substantiate the prudence of bone conduction testing at this frequency. Many of the patients evaluated in the author’s clinic have had excellent surgical results, which have ultimately resulted in a dramatic improvement in quality of life.    
Samuel N. Bittel, Au.D., FAAA is Director of Vestibular Services at Associated Audiologists Inc. in Shawnee Mission, KS. He can be contacted at (913) 403-0018. www.HearingYourBest.com