Writer: Samuel N. Bittel, Au.D.

Superior canal dehiscence syndrome (SCDS) was starting time described past Lloyd Minor, M.D., at Johns Hopkins in 1998. It is of import for audiologists to exist well versed in SCDS, as it tin nowadays with a number of unique symptoms and diagnostic findings. This condition is often initially suspected through a specific audiometric contour, which may affect hearing aid plumbing equipment, influence patient counseling, and substantiate the need for avant-garde vestibular testing.

Pathophysiology

SCDS is caused by a thinning or absence of the bone covering the superior semicircular canal. With this absenteeism, a false inner ear third window (in add-on to the commonly-occurring oval and circular windows) is created. SCDS is thought to exist congenital, ascend from head trauma, or be the product of increased intracranial pressure. However, it is important to notation 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 management of to the lowest degree resistance. This direction is towards the round window, which is substantially the cochlea's pressure outlet valve. With SCDS, sound may travel in the direction of the dehiscence and the superior semicircular canal. This will let audio to directly stimulate the vestibular system, which it should non. 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'due south phenomenon (dizziness provoked from audio)
  • 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 exist sensitive to this condition when taking case histories and during diagnostic testing. For those audiologists not completing vestibular work, this status is still important to picket for, as hyperacusis, audible distortion, and Tullio'southward phenomenon may straight bear on hearing assistance fitting and counseling.

Clinical Findings

There are a number of clinical findings that tin be observed in patients with SCDS:

  • Oftentimes, these individuals will demonstrate what appears to be a low-frequency conductive hearing loss. Depression frequency bone conduction scores are often better than expected.
  • In that location will also not be a clinical correlate with other middle ear tests. That is, the patient will demonstrate what appears to exist a depression frequency conductive hearing loss, but may accept normal tympanograms, present stapedial reflexes, and nowadays otoacoustic emissions.
  • The vestibular evoked myogenic potential (VEMP) examination may show abnormally depression thresholds and/or atypically large amplitudes.
  • If a patient demonstrates what appears to be a depression 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, and so the response should be obliterated by true middle ear pathology.
  • Patients with SCDS will ofttimes demonstrate nystagmus or dizziness with vox in a vision denied status. This finding would be consistent with Tullio'due south miracle.
  • SCDS may also crusade dizziness or nystagmus during tympanometric or perilymphatic testing, which is consistent with Hennebert'south sign.

Diagnosis

If SCDS is suspected through audiometric testing and/or the patient'due south 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 particular CT-Scan of the temporal bone. It will exist of import 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-Browse.

Effigy 1. CT-scan of SCDS. Photograph 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 heart fossa craniotomy or transmastoid approach. However, in this writer's feel, many individuals do not elect to have this somewhat invasive surgery, so must be counseled to avoid provoking factors. Please refer to Effigy 2 for a surgical flick of the middle fossa approach.

Effigy 2. Surgical photograph of centre fossa approach. Photo courtesy of James Lin, Yard.D., University of Kansas Medical Center.

Conclusion

Although SCDS is relatively rare, information technology presents with a number of interesting symptoms and test findings that should not be ignored. The writer has personally plant this condition is a number of patients whose only aberrant test findings were similar to the audiogram in Figure iii (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 Managing director of Vestibular Services at Associated Audiologists Inc. in Shawnee Mission, KS. He can be contacted at (913) 403-0018. world wide web.HearingYourBest.com