Literature Review
Superior Semicircular Canal Dehisnce Syndrome (SSCD) was first described in 1998 by Minor (Minor, 1998)
For SSCD, 83% patients presented with vertigo, 10% with hearing loss or tinnitus, 7% with symptoms unrelated to the inner ear.
For Posterior Semicircular Canal Dehiscence (PSCD), 86% persented with vertigo, 9% with hearing loss or tinnitus, and 5% with symptoms unrelated to the inner ear. (Krombach, DiMartino, et al, 2003)
The symptoms of SSCD:
Recurrent attacks of vertigo following stimuli which lead to change in intracranial or inner ear pressure, such as abrupt noise, coughing or Valsalca manoeuver;
Involuntary verticaltorsional eye movement, the direntions of which can be explained by excitation or inhibition of the SCD have been observed. (Kromabach, DiMartino, et al, 2003)
More than 50 cases of SSCD have been presented in literature (Kromabach, DiMartino, et al, 2003)
PSCD was seen in combination with SSCD in 8 patients and as an isolated finding in 15 cases of the total 507 images patients. (Kromabach, DiMartino, et al, 2003)
Tullio phenomenon: Vestibular activation by sound (Kromabach, DiMartino, et al, 2003)
Carey et al, found SSCD in 0.7% of subjects, incidence of SSCD was 1% in patients with semicircular canal dehiscence. (Kromabach, DiMartino, et al, 2003)
Aetiology:
The most probable aetiology for the dehiscence syndrome is incomplete development of the bony cover during early childhood.
Carey et al found consistently only a monolayer built from the inner layer covering the surface of the superior semicircular canal dehiscence.
Symptoms related to dehiscence are not congenital, but evolve in adulthood. (Kromabach, DiMartino, et al, 2003)
European radiographic survey reported an incidence of PSCD of 4.5%.(Anthony, 2006)
138 paediatric patients over the age of 3 years underwent a high resolution CT scan of the temporal bone. Based upon review of CT scans from the remaining 131 patients and using the histogram method to confirm and objectively quantify dehiscence, 18 patients were found to have at least 1 semicircular canal dehiscence. No dehiscence was found in the horizontal semicircular canal (Chen et al, 2009).
Lateral Semicircular Canal Dehiscence (LSCD):
LSCD was associated with CHL in 14%, SNHL in 71%, normal hearing in 11%. (Johnson and Lalwani, 2000)
In large vestibular aqueduct syndrome, there is a conductive conponent to the hearing loss in 30% of patients. (Johnson and Lalwani, 2000)
In general, in patients with partial LSCD, the cochlea tends to be normal.
A strong relation is seen between the posterior limb of the LSC and the vestibular aqueduct.
In patients with complete LSCD, the cochlea tends to be abnormal. (Johnson and Lalwani, 2000)
In general, 13% of patients had vestibular symptoms related to inner ear malformations at least involving the LSCD.
In general, patient history dis not suggest a cause of the inner ear malformation.
None of the patients had a family history of hearing loss or a disgnosis of any syndromeic hearing loss or syndromes related to inner ear malformation. (Johnson and Lalwani, 2000).
Correlation between PTA and the associated inner ear malformations involving the cochlea and vestibular aqueduct shows that increasely severe inner ear malformations are associated with increasingly impaired PTA.
Only LSCD, the PTA is most often in the lowest range of 0 to 40 dB (Johnson and Lalwani, 2000).
The patients with isolated LSCD had no progressive hearing loss.
Half the parients had greater than 20 dB asymmetrical hearing loss,
Hearing level is not linearly related to the extent of LSCD.
Isolated LSCD does not lead to progression of hearing loss (Johnson and Lalwani, 2000)
The SSCD is caused by failure of normal postnatal bone development in the middle cranial fossa leading to absence of bone at the most superior part of the superior semicircular canal.
The typical features for SSCD are:
sound- and pressure-induced vertigo with torsional eye movement
pulse synchronous tinnitus
apparent conductive hearing loss in spite of normal middle ear function. (Brantberg et al, 2006)
stapedius reflexes are normal
the Vestibular-evoked myogenic potentials (VEMPs) have a reduced thresholds. (Brantberg et al, 2006) , (Milulec et al, 2004)
umbo velocity measured by laser Doppler vibrometry was above mean normal (Mikulec, et al, 2004)
Patients with semicircular canal dehiscence have common auditory-vestibular features regardless of which of the two vertical senicircular canals is affected.The only obvious difference between the two is the vertical component of the sound and pressure-induced eye movements (which beats in opposite direction). (Brantberg et al, 2006)A postnatal developmental failure of the middle and outer bone layers to cause SSCD. A strong argument for this explanation is the fact that the syndrome is most often found bilaterally. Furthermore, the syndrome has been seen in brothers, which suggests a family predisposition. (Krombach and DiMartino, 2003)Audiometric testing with attention to absolute bone conduction thresholds, acoustic reflex testing, VEMP testing, laser vibrometry of the umbo, and computer tomography scanning can help to identify patients with SSCD presenting with conductive hearing loss without vertigo. (Mikulec, et al, 2004)Patients with SSCD and typical vestibular symptoms sometimes have a small, low frequency air-bone gap of 5 to 10 dB. This small conductive loss resolves after surgical repair of the SSCD. (Mikulec, et al, 2004)Our observations and those of Minor er al (2003) support the concept that SSCD can result in significant conductive hearing loss in the absence of vestibular symptoms. (Mikulec et al, 2004)The hallmark features of conductive hearing loss in SSCD are a low frequency air-bone gap with cone conduction thresholds that are better than 9 dB in the lower frequencies.The latter finding underscores the importance of accurate assessment of audiometric bone-conduction thresholds to levels below 0 dB HL. (Mikulec et al, 2004)Experience accrued with occlusion of the posterior semicircular canal for benign paroxysmal vertigo has revealed an incidence of significant sensorineural hearing loss of approximately 2%. (Mikulec et al, 2004).Specimens from infants demonstrated uniformly thin bone over the superior canal in the middle fossa at birth, with gradual thickening until 3 years of age.The incidence of bone overlying the superior canal occured in approximately 0.5% of temporal bone specimens, often bilaterally. (Carey et al 2000)
Limited clinical histories suggested that most of these individuals were not symptomatic. (Carey et al, 2000).
The bilateral nature of this abnormality and its similarity to the appearance of the infant temporal bone suggest that this is a developmental anomaly.
Our clinical experience had been that symptoms and signs of superior canal dehiscence first develop in adulthood. (Carey et al, 2000)
All 20 patients presented with typical vestibular symptoms of SSCD. Seventeen (85%) patients also had auditory symptoms, including autophony (40%), hyperacusis to bodily sounds (65%), hearing loss (40%), aural pressure (45%), and tinnitus (35%). Of the 17 patients, 14(82%) patients had an ABG on audiometry, but only 7(41%) patients demonstrated negatrive bone conductive thresholds.
Auditory symptoms are common in SSCD patients.
No firm conclusion could be drawn regarding the association between symptoms and ABG,
Different pathways or mechamisms may exist in SSCD for bone-conducted sounds arising form different sources,
Surgery repair of the dehiscence results in resolution of auditory symptoms in most patients (Yuen, Eikelboom, and Atlas, 2009).
The larger the dehiscence, the lower the frequency of stimuli required to provoke a vestibular response (Gunesh et al, 2008).
The combination effects of a congenital aberration in the thickness of the bony converting of the superior semicircular canal, the influence of constant pulsating cerebrospinal fluid / intracranial pressure, or a precipitating event resulting in a breach of the abnormally thin bone is thought to cause SSCD (Gunesh et al, 2008).
Animal model studies showed the size of the dihiscence influenced the magnitude of the recorded cochlear potential (Gunesh et al, 2008).
