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Neuroradiology Case of the Week

Case 113

Clinical Presentation:The patient is a 73-year-old female who presented with chronic dizziness and imbalance. Evaluation was requested for semi-circular canal or labyrinthial abnormality.

Radiological Findings: Non-contrast CT of the temporal bones reveals an opacified area of the right mastoid region likely due to chronic mastoiditis changes. The superior semicircular canal on the right side of the head is noted to lack a superior border.

Diagnosis: Tullio’s phenomenon, also known as superior semicircular canal dehiscence syndrome

Discussion: In 1929, Tullio found that if a pigeon has a small hole drilled into its superior semicircular canal (SCC), then the pigeon will develop nystagmus after being subjected to a loud noise. This discovery was the first piece of evidence for what is now referred to as Tullio’s Phenomenon, or superior semicircular canal dehiscence syndrome. This syndrome is one in which sound and/or pressure induces vertigo (an illusion of motion) and/or oscillopsia (apparent motion of objects that are known to be stationary). The pathology of this condition is attributed to a dehiscence, or opening, of the superior semicircular canal.
      Under normal circumstances, sound is transmitted from the outer to the inner ear in the following sequence. Sound vibrations travel through the external acoustic meatus, cause the tympanic membrane to vibrate, are transmitted to the ossicles, at which point the stapes pushes against the oval window and sends a pressure wave through the perilymph of the cochlea. To counteract this inward movement of the oval window into the cochlea, there is a corresponding outward movement of the round window. This compensatory movement occurs because fluids are not compressible. Normally, the oval and round windows are the only two openings in the inner ear. The semicircular canals are a hydraulically closed system, and so inward movement of the oval window by the stapes does not cause any movement of the endolymph in the canals.
     In Tullio’s Phenomenon, there is a dehiscence, or a “third window,” in the superior semicircular canal. Therefore, when the oval window moves inward, there is movement of the endolymph within the canal, and this third opening responds by moving outward. The brain interprets the motion of the endolymph as angular movement of the body, causing the patient to experience vertigo and/or oscillopsia.
     Diagnosis of this syndrome is dependent on certain signs and symptoms as well as CT demonstration of a bony defect of the superior semicircular canal. Upon exposure to a loud sound or to stimuli that cause changes in the middle ear or intracranial pressure, such as pressure on the tragus or Valsalva maneuvers, the patient will have vertigo and nystagmus. The nystagmus is specific in that it is a vertical-torsional eye movement that is in the plane of the SSC. Finally, for diagnosis, CT imaging must demonstrate that there is dehiscence of the SSC. Belden et al. [1] has shown that CT at 0.5-mm collimation with reformation in the plane of the SCC has superior positive predictive value than CT at 1.0-mm collimation. With use of smaller voxels, partial volume averaging becomes less problematic, and there are fewer false positives.
     Tullio’s Phenomenon may be present in a variety of circumstances. Otosclerosis that has been treated by the creation of a fenestration in the lateral semicircular canal, cholesteatoma that has lead to erosion of one of the canals, and syphilis with osteitis of the otic capsule of one of the canals, may all lead to Tullio’s Phenomenon. Trauma, Ménière disease, perilymphatic fistula, and Lyme disease have also been implicated as etiologies.
     Many patients do not need treatment for this syndrome because they are able to minimize any aggravating symptoms by merely avoiding provocative stimuli. However, for those patients who do require treatment, a craniotomy with plugging of the bony defect in the SSC may be performed. It is important to have reliable radiographic evidence of dehiscence before a patient is to undergo treatment because a craniotomy is surely not a minor procedure.

References:

1. Belden CJ, Weg N, Minor LB, Zinreich SJ. CT Evaluation of bone dehiscence of the superior semicircular canal as a cause of sound- and/or pressure-induced vertigo. Radiology Feb. 2003; 226(2):337-343. [Medline]
2. Curtin HD. Superior semicircular canal dehiscence syndrome and multi-detector row CT. Radiology Feb 2003; 226(2):312-314. [Medline]
3. Som and Curtin: Head and Neck Imaging, 4th ed., Volume One. 2003 Mosby: 1127-1129, 1209.
4. Russolo M. Sound-Evoked Postural Responses in Normal Subjects. Acta Otolaryngology 2002; 122:21-37. [Medline]
5. Minor LB, Solomon D, Zinreich JS, Zee DS. Sound- and/or pressure-induced vertigo due to bone dehiscence of the superior semicircular canal. Arch Otolaryngology Head and Neck Surgery Mar 1998; 124:249-258. [Medline]