These peculiar symptoms can be associated with an unusual condition of the inner ear called superior canal dehiscence (SCD) syndrome. Lloyd Minor, MD, in 1998 described a series of patients with vestibular and auditory symptoms caused by a defect or dehiscence of the bony covering of the superior semicircular canal. The superior canal is part of the inner ear, and is one of three pairs of balance organs that detect angular head acceleration.
Patients with SCD syndrome can present with a wide range of clinical signs and symptoms that include ear (aural) fullness, sensitivity to self-generated sounds such as swallowing or speaking (autophony), hearing loss, pulsatile tinnitus, dizziness, and vertigo. Patients with SCD syndrome are often misdiagnosed with more common otologic conditions such as Eustachian tube dysfunction, otosclerosis, Ménière's disease, and benign paroxysmal positional vertigo. Some patients with intractable symptoms seek psychiatric care.
The diagnosis is based on presenting signs and symptoms, the absence of external or middle ear pathology on otoscopic examination, 512-Hz tuning forks lateralizing to the more severely affected ear, nystagmus triggered by loud, low-frequency sound stimuli or a Valsalva maneuver, and computed tomography imaging confirming a bony defect of the semicircular canal. Additional testing used to confirm SCD syndrome includes audiometric threshold testing demonstrating a low-frequency air-bone gap (conductive hearing loss), bone-conduction thresholds better than 0 dB (in the range of -5 to -10 dB, called supranormal bone conduction).
Tympanometry and stapedial reflex testing are generally normal, excluding a middle ear or ossicular abnormality as a possible cause of the patient's symptoms. A few patients, however, do have co-existing SCD and either ossicular fixation or Eustachian tube dysfunction. A sensitive measure of an active SCD is the cervical vestibular evoked myogenic potential (cVEMP) test that measures inhibitory responses of the sternocleidomastoid, a neck muscle. The cVEMP tests the integrity of the saccule (one of two balance organs that detect linear acceleration) and inferior vestibular nerve. Most patients with symptoms associated with SCD will have low-threshold cVEMPs compared with the better ear.
Two types of SCD are seen based on CT imaging: dehiscence of arcuate eminence (top of the superior semicircular canal) and dehiscence associated with the superior petrosal sinus, a vein that runs in a bony groove along the ridge of the temporal bone. (Otol Neurotol 2011;32:1312; figure.) The pathologic process, or first event, by which SCD may occur includes congenital bone-thinning over the superior semicircular canal with low-lying skull base (arcuate eminence defect) or a close anatomic relationship between the superior petrosal sinus and the medial limb of the superior semicircular canal. Twenty to 30 percent of SCD patients report a second event that triggered symptoms, resulting from a loud sound, heavy lifting, or childbirth.
Surgical repair, either by plugging or resurfacing the defect, is successful in the majority of patients who have intolerable symptoms associated with SCD in one ear. The approach can be done using a middle fossa craniotomy to visualize and repair the SCD directly (arcuate eminence defect). Mastoidectomy can also be used to repair the SCD indirectly (superior petrosal sinus associated defect). Surgery is generally safe and effective. Complications are not common and include hearing loss, dizziness, facial nerve injury, and brain fluid leakage. As vestibular dysfunction in the operated ear is a known risk (or even expected outcome) after SCD repair, patient selection and choice of which ear to operate on is important, especially in patients with bilateral SCD. Most patients with bilateral SCD seen on CT have symptoms in only one ear, but managing patients with symptoms of SCD from both ears is more challenging. Findings from our institution suggest that a larger conductive loss and lower cVEMP thresholds correlate with the worse ear in patients with bilateral SCD. (Otol Neurotol 2012;33:824.) A recent study provides new insight on outcomes in patients who underwent surgical repair of bilateral SCD syndrome.
Second-Side Surgery in Superior Canal Dehiscence Syndrome
Agrawal Y, Minor LB, et al Otol Neurotol 2012;33:72
The symptom profile of adult patients with bilateral SCD syndrome who underwent second-side surgery was examined, and the effects of surgery on dizziness and quality of life were assessed. Five patients underwent second-side SCD repair, with reported results on four patients. One patient did not have postoperative follow-up at the time of publication. Follow-up ranged from three to 34 months. All patients received a middle fossa craniotomy with plugging of the bony defect; none experienced major surgical complications.
Symptoms were present after the first SCD repair in the contralateral ear immediately after surgery in three of five patients. The remaining two patients developed progressive symptoms of the other ear following repair of the first ear. The symptoms that prompted patients to pursue second-side surgery were autophony and sound- and pressure-induced dizziness. All five patients reported severe symptoms caused by the contralateral SCD ear; their daily activities were impaired, and they were on disability. Symptoms resulted in falls that caused a fracture in one case.
Questionnaire scores on dizziness and quality of life were measured in two patients before and after second-side SCD surgery. Patients were also asked to rate their satisfaction on a scale ranging from zero (very happy) to 10 (very unhappy). One patient said his dizziness lessened, but he reported little improvement in quality of life (score 2/10) and was happy that he underwent surgery. The second patient described increased dizziness and worse quality of life (0/10) following her second surgery, and was also very happy. All four patients with postoperative outcome data reported an improvement of symptoms following surgery.
Research in patients who have had SCD surgery has shown that the function of superior canal is reduced, but what happens to the vestibular system following bilateral SCD surgery? Vestibular testing was performed after second-side surgery in one patient. He underwent dynamic visual acuity testing, which assesses the ability to see an object precisely during head movements; it was within normal limits for both of the surgically repaired superior semicircular canals as well as for the other semicircular canals.
These findings suggest that compensation for the loss in peripheral function of both superior canals occurs following surgery. This patient, however, reported a sensation that objects were moving back and forth in his visual field (oscillopsia). The other three patients with postoperative outcome data reported oscillopsia as well, which resolved in two of the patients, and all patients reported that they were happy to have undergone second-side SCD repair.
What do these findings mean for patients with bilateral SCD syndrome? Should they all undergo bilateral surgical repair? This study showed that second-side SCD repair is safe and effective but that the risk of dizziness following surgery mandates careful patient selection. Only those with intractable symptoms in both ears should be considered for surgery, and a more conservative approach should be taken for bilateral SCD patients with mild symptoms in the unrepaired ear. This study also suggested that the ear and brain can compensate for reduced function of the semicircular canals even when the function was decreased in both canals. Second-side surgery, however, can cause temporary or permanent oscillopsia, and patients must be counseled to expect a prolonged recovery period. The results of this small patient cohort are promising, but the risks and benefits should be considered carefully for each patient before deciding to pursue a second-side SCD repair.
© 2012 Lippincott Williams & Wilkins, Inc.
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