Disclaimer: The information contained within the Grand Rounds Archive is intended for use by doctors and other health care professionals. These documents were prepared by resident physicians for presentation and discussion at a conference held at Baylor College of Medicine in Houston, Texas. No guarantees are made with respect to accuracy or timeliness of this material. This material should not be used as a basis for treatment decisions, and is not a substitute for professional consultation and/or peer-reviewed medical literature. Superior Canal Dehiscence Syndrome The objectives of this talk are to go over the etiology and epidemiology of superior canal dehiscence syndrome (SCDS). I would also like to review the histology and the pathogenesis as well as a brief review of vestibular physiology. I will discuss the unique signs and symptoms of superior canal dehiscence syndrome. In addition, I would like to go over the objective evaluation of patients with SCDS including vestibulocolic and vestibulo-ocular reflexes and some of these tests that have been used for further study of this syndrome. I would also like to review imaging and some of the unique audiologic findings. Finally, treatment will be outlined. Tullio first described sound-induced nystagmus in pigeons in 1929. He noted head and eye movements in response to loud noises and subsequently this has been referred to as Tullio’s phenomenon. Similar findings may be seen in fistulas of the lateral semicircular canal, perilymph fistula, Ménière’s disease, congenital deafness, hypermobile stapes, and syphilis. In 1911 Hennebert noted eye movements evoked with pressure in the external auditory canals in patients with congenital syphilis. Nadol further speculated this is most likely due to adhesions between the footplate and sacculus. This histology slide demonstrates an adhesion between the footplate and the saccule. This can also be seen in Ménière’s disease and perilymph fistula as well. In 1998 Dr. Minor described superior canal dehiscence syndrome. He description originally included eight patients who had sound and/or pressure-induced vertigo. This is all based on a third mobile window effect that I will speak about later. Here is a diagram showing increased pressure in the external auditory canal displacing the tympanic membrane medially. The conduction of the fluid will be dissipated because of a third window created by the dehiscence in the superior semicircular canal. Minor further described 28 patients in 2001, the age ranged from 20 to 70 years of age with the mean age of 41. Nine patients had this syndrome in the right ear, 13 in the left, and 6 bilateral. Twenty five percent had undergone previous middle ear explorations for presumed perilymph fistula, 36% reported prior barotrauma or head trauma, and one patient developed symptoms after stapedectomy. Kerry and others in 2000 looked at 1000 temporal bones, and they noted a 0.7% incidence of complete dehiscence per temporal bone. In addition, they described that 1.3% had less than 0.1 mm thickness. This is important because the finest temporal bone scans currently available will not detect a bone thickness of 0.1 mm. A dehiscence occurred at two sites most commonly, that being the superior most canal or between the canal and the superior petrosal sinus. They noted that abnormalities were typically bilateral. This is a histologic cut showing the superior petrosal sinus, the superior canal, and a very thin layer of bone between these. Kerry measure the thickness of bone between the superior canal and the middle fossa and the superior petrosal fossa. The red shows the thickness between the superior canal and the middle fossa with an average of 0.96 mm. The yellow shows the thickness between the superior canal and the superior petrosal sinus at 1.79 mm. The following slides show the variation in thickness of the bone overlying the superior semicicrcular canal. This shows a patient with a lot of pneumatization of the bone and it measures 4.3 mm thick. This demonstrates a more standard thickness of bone at 1.38 mm. This shows is a very thin covering of bone at 0.12. Finally, this slide demonstrates a very thin layer of bone measuring only 0.02 mm. Senoto looked at 244 temporal bones in cadaveric specimens and only noted one patient with findings consistent with a superior canal dehiscence. Unfortunately, this finding did not have correlating clinical data. This slide shows the temporal bone with the dura covering and you can see an indentation where the superior canal lies. The dura is reflected and you can see the dehiscence of the canal. This is a scanning electron micrograph of this defect. You can notice the very smooth surface which argues against this being just artifact. In this patient, this is a 5.2 mm dehiscence and the contralateral side had a 3.5 mm dehiscence. The pathogenesis is not totally clear. The most common theory is that this begins as a developmental abnormality. This is the temporal bone of a 24-week-old which shows the very thin bone between the superior canal and the middle fossa This bone actually develops in three separate bones, being first an innermost endosteal layer which is here and then a mid and an outer layer will develop. It is suggested that perhaps the bone fails to mature and the patient continues to have a very thin overlying bone. Then perhaps the disruption of bone happens later with trauma, barotrauma, or with gradual erosion with aging. Notably the bone seems reach maturity at three years of age. There are three semicircular canals and these are responsible for sensing angular acceleration. The two lateral canals are paired and then the superior is paired with the contralateral posterior canal. Ampullopetal flow is defined as endolymph flow toward the utricle. Ampullofugal flow is endolymph flow away from the utricle. The lateral semicircular canals have increased electrical output with ampullopetal flow and the vertical canals, being the superior and posterior canals, have increased electrical output with ampullofugal. This will be important when we are talking about some of the effects of endolymph flow. This is a diagram showing the third mobile window effect. Increased intracranial pressure will create a down force on the superior canal that will displace the endolymph towards the utricle resulting in ampuloopetal flow. This will decrease firing rate and inhibition and result in a vertical rotation of nystagmus towards the affected ear. On the other hand, a Valsalva maneuver against closed nostrils or increased external auditory pressure will create an ampullofugal flow displacing the cupula away from the utricle. This will result in increased firing rate and excitation and a vertical rotation nystagmus in the opposite direction or away from the affected ear. The typical signs of SCDS are vertigo and oscillopsia evoked by sounds. Sound induced vertigo occurs in about 97% of patients and activities that change middle ear intracranial pressure cause vertigo in approximately 82% of patients. Most patients also complain of chronic dysequilibrium. Hyperacusis was found at 39% of patients and gaze-evoked nystagmus is also a common finding. In addition, patients will have eye movements in the plane of the superior canal evoked by sounds in 89%, evoked by Valsalva maneuvers in 82% or tragal pressure in 54%. In addition, 21% of patients showed head movement evoked by sounds. Another unique finding is sensitivity to bone conduction. Patients may describe very unusual findings such as hearing their joints moving, hearing their eye movements, hearing their heart beat, hearing their heels strike during walking, or be able to hear a tuning fork placed at a distal extremity. Often the bone conduction threshold is negative. This is important because many times audiologists will not check negative bone thresholds, so if you are suspecting superior canal dehiscence, it is important to ask your audiologist to check even negative bone conduction thresholds. On physical examination, spontaneous nystagmus is generally not appreciated. It is important to use Frenzel goggles because a subtle nystagmus is common. In addition, pneumatic otoscopy and Valsalva maneuvers may produce the nystagmus. Weber is usually to the affected side and usually with a 256 Hz tuning fork. Often bone conduction is greater than air conduction on Rinne tuning fork test. As I mentioned earlier, patients are often able to hear a tuning fork placed on the extremity. Objective evaluations include vestibular-evoked myogenic potential testing, vestibulo-ocular reflexes, computed tomography, and audiology. Vestibular-evoked myogenic potential testing (VEMP) basically measures short latency relaxation potentials in the sternocleidomastoid muscle. This is based on the fact that the saccule is actually sensitive to sound. This technique is useful for detecting abnormal sensitivity to otoliths to sound. EMG electrodes are placed to the sternocleidomastoid muscle and the patient holds their head up to tonically contract the sternocleidomastoid muscle. Loud clicks at 200 millisecond intervals are presented to the ear and short latency relaxation potentials are recorded average. There is a characteristic P12 N23 wave that is measured. Normal potentials will begin to occur around 96 decibels. With superior canal dehiscence syndrome, decreased thresholds are present with an average of 72 decibels. This demonstrates a typical tracing of a patient with right-sided superior canal dehiscence syndrome. You can see they begin to have the characteristic waveform hereat approximately 80 decibels compared to the normal ear where it is starting around 100 to 105 dB. In addition, the amplitude is about two to three times higher, so this is a typical finding in a patient with superior canal dehiscence syndrome in one ear and then the contralateral ear with normal depth thresholds. Vestibular-evoked myogenic potentials also have been described for other conditions. Notably low thresholds can be seen, as I mentioned in superior canal dehiscence syndrome, as well as perilymph fistula. Elevated thresholds with saccule disorders occur since the saccule is the source of the phenomenon. Prolonged latency can be seen in vestibular nerve disorders or retrocochlear lesions. Absent potentials are seen in vestibular neuritis, conductive hearing disorders and also after vestibular nerve sections. Other uses have been described for Ménière’s disease, multiple sclerosis, and brain stem stroke. The vestibulo-ocular reflex uses electro-oculography to measure eye movements in response to rapid clicks. Normal subjects will display little movement, 0.25 degrees or less of the VOR. Subjects with superior canal dehiscence syndrome will display lower thresholds, increased amplitude, and an onset of latency that is consistent with head thrust maneuvers. This shows two patients, both of these patients have superior canal dehiscence syndrome and you can see they have a fairly large response. This diagram is showing the eye movements in the ear that is affected. Radiographic evaluation is very important for confirmation when you are suspicious of the patient having superior canal dehiscence syndrome. Perhaps the best view is the plane of Poschl that is basically a 45 degree oblique view perpendicular to the petrous bone. This radiograph in the plane of Poschl presents the superior canal as a ring. You can seappreciate the bony covering over the superior canal. You can see the malleolus and tympanic membrane here. Here is a patient showing superior canal dehiscence where you can see that there is an obvious dehiscent here. These radiographs are at 0.5 mm collimation allowing the fine detail neede to appreciate dehiscence. Drs. Williamson, Vrabec, and Coker looked at 442 temporal bone CT scans. This is a retrospective review on any patient that had a coronal CT scan. The scans were at 1 mm collimation. The maximum resolution showed to be 0.324 mm. They noted a 9% possible dehiscence of the superior semicircular canal and 46% of the time this was bilateral. No patients had retrospective symptoms consistent with superior canal dehiscence. They concluded the importance of clinical correlation to CT findings to avoid false positives. Belvin looked at 50 patients, or 100 temporal bones, in patients that complained of either sound and/or pressure-induced vertigo. These basically show that the patients that had 0.5 mm collimated CT scan had a higher specificity and positive predicted value compared to 1 mm scans. If there is an equivocal case, it is important to get the finest cuts available at your institution. There are also interesting audiologic findings with superior canal dehiscence. As I mentioned earlier, there is a decreased bone conduction threshold. In addition, there can be an apparent conductive hearing loss that is most likely due to dissipation of sound energy due to the third window effect. Minor described four patients that had a significant air-bone gap of 24 +/- 7 decibels. Three of these patients underwent previous stapedectomy at another institution. They did not have improvement in their air-bone gap. It is important to realize that patients with superior canal dehiscence will inevitably have intact acoustic reflexes that could help differentiate otosclerosis from superior canal dehiscence syndrome. Treatment varies in corresponding to the severity of symptoms. Many times patient can be educated and they can identify and avoid aggravating factors and not have to undergo surgery. Occasionally placement of a pressure equalization tube may relieve symptoms of disequilibrium and unsteadiness, especially in patients with concurrent eustachian tube dysfunction. Surgical treatment was been described in two techniques. Canal pluggind has been described in the past for intractable benign positional vertigo. This does require a middle cranial fossa approach. It may result in loss of vestibular function and sensorineural hearing loss has been reported after this procedure. This cartoon just shows a dehiscence here. A plug composed of fascia and bone dust is used to plug the superior canal and then fascia is overlaid over this. Alternatively, superior canal resurfacing can be undertaken. This may theoretically preserve the physiologic function of the semicircular canal that is not possible with canal plugging procedures. In this technique, a piece of fascia is placed over the dehiscence and tucked under the edge of the bone A bone graft is then placed and fascia is placed over this and held into place with fibrin glue. This technique may also result in vestibular hypofunction. There have been speculation that the bone graft may resorb with time. In addition, the fascia or the bone graft could slip out of place and you can have return of symptoms. In conclusion, superior canal dehiscence syndrome is characterized by pressure and/or sound-induced vertigo. Physical exam findings include pressure-induced nystagmus, localization of Weber tuning fork test and vertigo with sound. This entity is diagnosed with clinical findings and confirmed by CT scan or other objective testing. High resolution CT scan should be used to confirm diagnosis and ultra high resolution may be needed for equivocal presentations. Vestibular-evoked myogenic potentials, VOR, and audiology can be useful for a diagnosis and post treatment evaluation. Audiology may reveal decreased bone conduction thresholds, an apparent air-bone gap, and it is important to realize they will have intact acoustic reflexes. Treatment options now include avoidance of aggravating factors, superior canal plugging, or resurfacing. Long-term postoperative follow up will be needed. Case Presentation: LGR is a 33 year-old white female who presented with disequilibrium, unsteadiness, and vertigo with head movements. She identified the onset of symptoms after sneezing during an upper respiratory infection a month prior to presentation. Blowing her nose, coughing, and loud voices in her left ear produced similar symptoms. She also complained of left aural fullness, autophony, and mild hyperacusis. She denied a history of ear surgery, recurrent ear infections, or head trauma. Her past medical history was noncontributory. Physical examination revealed normal extraocular eye movements and normal vestibule-ocular reflexes. Examination with Frenzel goggles revealed no spontaneous nystagmus or nystagmus with head thrusts. Valsalva maneuver against pinched nostrils revealed a downward, counter-clockwise nystagmus that reversed direction after release. Valsalva maneuver against a closed glottis produced nystagmus in an upward, clockwise direction. No eye movements were evoked when tones of 250 to 4000 Hz were presented at intensities of 100-110 dB in each ear. Positive pressure in the left ear produced symptoms of vertigo. Weber localized to the left ear with air conduction greater than bone conduction bilateral. Otherwise the rest of her head and neck exam was normal. Vestibular evoked myogenic potentials (VEMP) testing revealed normal thresholds in the right ear at 106 dB with abnormally low thresholds at 75 dB in the left ear. The VEMP amplitude was 2-3 times larger in the left ear than the right. CT scan of the temporal bones performed at 0.5 mm collimation revealed dehiscence of the left superior semicircular canal. The patient underwent resurfacing of the dehiscent bone covering the superior semicircular canal through a middle craniotomy approach. The area of dehiscence measured 2.5 to 3.0 mm. Temporalis fascia was placed over the area of dehiscence and secured by placing under the bony overhang of the dehiscence. Next a calvarial bone graft followed by another piece of temporalis fascia was placed over the area of dehiscence. Fibrin glue was used to secure the graft. Following surgery, the patient’s symptoms are much improved, although she still complains of occasional unsteadiness. 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