Histopathology of the temporal bone in a case of superior canal dehiscence syndrome

Abstract

Objectives: We describe the histopathologic findings in the temporal bones of a patient who had, during life, received a diagnosis of superior canal dehiscence (SCD) syndrome.

Methods: The patient was found to have SCD syndrome at 59 years of age. She became a temporal bone donor, and died of unrelated causes at 62 years of age. Both temporal bones were prepared in celloidin and examined by light microscopy.

Results: The patient developed bilateral aural fullness, pulsatile tinnitus, and difficulty tolerating loud noises after minor head trauma at 53 years of age. The symptoms were worse on the right. She also had Valsalva-induced dizziness and eye movements, as well as sound-induced dizziness (more prominent on the right). Audiometry showed a small air-bone gap of 10 dB in the right ear. Vestibular evoked myogenic potential testing showed an abnormally low threshold of 66 dB on the right, and a computed tomography scan showed dehiscence of the superior canal on the right. Histopathologic examination of the right ear showed a 1.4 x 0.6-mm dehiscence of bone covering the superior canal. Dura was in direct contact with the endosteum and the membranous duct at the level of the dehiscence. No osteoclastic process was evident within the otic capsule bone surrounding the dehiscence. The left ear showed thin but intact bone over the superior canal. Both ears showed focal microdehiscences of the tegmen tympani and tegmen mastoideum. The auditory and vestibular sense organs on both sides appeared normal. No endolymphatic hydrops was observed.

Conclusions: The findings were consistent with the hypothesis put forth by Carey and colleagues that SCD may arise from a failure of postnatal bone development, and that minor trauma may disrupt thin bone or stable dura over the superior canal.

Figure 1

Audiometric evaluation at age 59 at the time of the diagnosis of SCD. There was a relatively flat, mild, sensorineural hearing loss bilaterally with a small air bone gap noted on the right. Note that the bone thresholds were unmasked. Speech discrimination was 100% in both ears.

Figure 2

A. Low power view of vertical section from right temporal bone at the level of the superior semicircular canal. The external auditory, middle ear, ossicles and otic capsule were normal. There were multiple focal bony dehiscences of the tegmen (arrowheads), probably as a result of developmental anatomical variation. There were no instances of arachnoid granulation or meningocele at any of the dehiscences. There was dehiscence of the bone covering the superior canal, which is shown in more detail in B. TM, tympanic membrane. B. Section through the superior semicircular canal on the right showing bony dehiscence. The dura showed artifactual separation of its layers. There were no abnormalities within the endolymphatic or perilymphatic fluid spaces of the canal. The bone of the otic capsule surrounding the dehiscence appeared normal without any evidence of an osteoclastic process. Inset shows higher power view of dehiscence. Dura was apposed to the endosteum and wall of the membranous duct within the dehiscence. The morphologic appearance of the dura and the semicircular duct was normal. The folded appearance of the dura was artifactual. C. Vertical section through the left temporal bone at the level of superior canal showing thin bone covering the entire canal. The external auditory canal, middle ear, ossicles, mastoid air spaces and otic capsule were normal. There were multiple asymptomatic focal dehiscences of the tegmen (arrowheads). TM, tympanic membrane. D. A higher power view of the thin bone over the superior canal on the left side. The dura had been artifactually separated into layers.

Figure 3

Mid-modiolar section from right temporal bone showing absence of endolymphatic hydrops. Reissner’s membrane (arrows) is in normal position in all turns.