When performing an exam for Superior Canal Dehiscence (SCDS), you may have two different goals. In a patient with many of the symptoms described in last week’s post, but no diagnosis, you are trying to establish the likely source of those symptoms. This might lead to the referring physician ordering a high resolution CT scan to investigate the possibility of a canal dehiscence.
Another goal would be establish if a patient, already identified with Superior Canal Dehiscence by CT scan, was suffering symptoms related to this finding. Not all patients identified with SCDS require surgical repair, so it is important to connect symptoms and signs to the specific disorder if possible.
Clinical Signs associated with SCDS:
Air/Bone Gap
The finding of a low frequency air/bone gap on audiometric evaluation is common in SCDS. It is important that the Audiologist performing the test search for true bone conduction thresholds, and not stop seeking lower thresholds if the patient responds at 0dB.
In a series of patients with confirmed SCDS, Minor (2005) reports frequency specific air/bone gap findings. They consider a 10dB or more air/bone gap to be significant, and list the percentage of patients with said gap.
250Hz (70%)
500 Hz (64%),
1KHz (64%)
2KHZ (21%).
It is also important that these patients undergo tympanometry and acoustic reflex testing. Many patients with SCDS have been misdiagnosed with Otosclerosis due to the air/bone gap and normal tympanogram. In Otosclerosis, you would expect acoustic reflexes to be absent. In SCDS, they should be normal.
Tuning fork Weber –Just like any conductive hearing loss, the tuning fork Weber should lateralize to the affected ear. An interesting take on this routine test is that patients with SCDS may lateralize to the affected ear when the tuning fork is placed on the ankle.
Nystagmus induced by sound or pressure:
Valsalva with pinched nostrils (eye movements noted in 75%)
Introduction of brief loud sound (250 and 1KHz at 105dBNHL). Subjective increased symptoms noted in 90% – Nystagmus or other sound evoked eye movements noted in 82%
Insufflation of the external ear canal with pnuematic otoscopy –Nystagmus or evoked eye movements noted in 45%
C VEMP (Cervical Myogenic Evoked Potential)
– average CVEMP threshold for the affected ear in subjects with SCDS (81dB)
– average CVEMP threshold in the healthy ear of these same subjects (98 dB)
-average CVEMP threshold for control group (98 dB)
Data obtained from Minor, L (2005) Clinical Manifestations of Superior Semicircular Canal Dehiscence
