|Year : 2020 | Volume
| Issue : 1 | Page : 22-25
Superior semicircular canal dehiscence syndrome: Often a missing clinical entity in vertigo management
Santosh Kumar Swain1, Sidharth Mohanty2, Mahesh Chandra Sahu3
1 Department of Otorhinolaryngology, IMS and SUM Hospital, Siksha “O” Anusandhan Deemed to be University, Bhubaneswar, Odisha, India
2 Department of Anesthesiology, Apollo Hospital, Bhubaneswar, Odisha, India
3 Department of Directorate of Medical Research, IMS and SUM Hospital, Siksha “O” Anusandhan Deemed to be University, Bhubaneswar, Odisha, India
|Date of Submission||04-May-2018|
|Date of Acceptance||30-Dec-2019|
|Date of Web Publication||17-Mar-2020|
Santosh Kumar Swain
Department of Otorhinolaryngology, IMS and SUM Hospital, Siksha “O” Anusandhan Deemed to be University, K8, Kalinga Nagar, Bhubaneswar - 751 003, Odisha
Source of Support: None, Conflict of Interest: None
Superior semicircular canal dehiscence syndrome (SSCDS) is a rare and recently described inner ear lesion presenting with disequilibrium and associated with dehiscence of the bony covering of SSC. SSCDS patients may present with a variety of vestibular or auditory symptoms or both. Patients present with vertigo induced by sound or pressure. The diagnosis of SSCDS depends on the demonstration of a defect in the bony wall of the roof of the SSC. A high-resolution computed tomography is helpful for diagnosis. It is usually treated by plugging of dehiscence. The aim of this review article is to discuss the etiopathology, clinical presentation, investigations, and recent treatment of SSCDS.
Keywords: Superior semicircular canal, superior semicircular canal dehiscence syndrome, vertigo
|How to cite this article:|
Swain SK, Mohanty S, Sahu MC. Superior semicircular canal dehiscence syndrome: Often a missing clinical entity in vertigo management. Apollo Med 2020;17:22-5
| Introduction|| |
The superior semicircular canal dehiscence syndrome (SSCDS) is rare clinical entity characterized by vestibular symptoms in response to loud sound or change in intracranial or middle ear pressure due to the dehiscence of bony plate covering the SSC. SSCDS was first described by Minor et al. in 1998. It is an uncommon clinical entity with prevalence of 0.5% which represents a set of clinical, vestibular, and cochlear symptoms associated with dehiscence of bony covering at the roof of SSC. Due to formation of third window of the inner ear, there will be dissipation of the acoustic energy of sound waves, abnormal mobility of endolymph and reduced threshold of bone conduction. This phenomenon leads to pseudoconductive hearing loss, vertigo/imbalance, and autophony. The exact etiology is still unknown. Few patients of SSCDS present with only hearing loss without any vestibular symptoms. SSCD is the discontinuity of the bony overlying of the SSC making a third window of the inner ear. Here, we are discussing the etiopathology, clinical presentations, investigations, and recent treatment of SSCDS.
| Etiopathogenesis|| |
SSCD is caused by the absence of bone over SSC in one or both sides which lead to the dysfunction of the vestibular end organs due to the formation of the third window other than round window and oval window resulting in altered fluid dynamics. SSCDS is an uncommon clinical entity. Although congenital etiology is often proposed for SSCDS, some authors believe it as due to acquired cause. Pressure from the overlying dura or temporal lobe of the brain or trauma may cause erosion of the congenitally thin bone leading to dehiscence over a period of time. This may explain why majority of cases of SSCDS seen in adult or middle age of life. The acquired causes for SSCDS are chronic otitis media with cholesteatoma, high jugular bulb, and fibrous dysplasia. The dehiscence in the SSC acts as a third window, causing pressure transmission into the labyrinth and accounting for vestibular symptoms of the patient. This third window causes dissipation of sound and increases the sensitivity of bone conduction sounds. The thickness between the SSC and the middle cranial fossa is classified into Grade 1 – >2.5 mm; Grade 2 – <2 mm; Grade 3 – SSC in contact with middle cranial fossa, and Grade 4 – SSC opens into middle cranial fossa. Grades 1 and 2 are considered as normal, whereas Grade 4 is considered as dehiscence and Grade 3 as predehiscence. The bony defect causes the SSC to make pressure changes in the membranous labyrinth on exposure to sound and may be responsible for inner ear conductive hearing loss. An air-bone gap is present constantly at low frequencies when the bony dehiscence is >3 mm and increases with the size of dehiscence. Sound flow occurs through SSCD by sound-induced velocity within the perilymph or endolymph of a SSCD. The increased bone-conducted sounds have been demonstrated on cochlear potential measurement. The dissipation of sound energy may give rise to false interpretation of the air-bone gap in case of SSCDS and give a false diagnosis of middle ear abnormality.
| Clinical Presentations|| |
SSCDS is a new and exciting topic in the field of clinical medicine. SSCDS has been seen in adult population and very rarely reported in pediatric age group. Patients usually present with vertigo which is aggravated with intense sound, Valsalva maneuvers, and other maneuvers which change the intracranial pressure. Patients often present with autophony and high sensitivity to bone-conducted sounds such as jaw movement, eye movement, and even the movement of joints in extremities. The vertigo is also induced by coughing, straining, and sneezing. The symptoms of SSCDS also include autophony (echoing of one's own voice, hearing one's own heartbeat, pulse, or eye movements – similar to symptoms of patulous Eustachian tube More Details) and oscillopsia (shift of visual field induced by sound or pressure). Patients often present with conductive hearing loss in the affected ear due to decreased bone conduction thresholds, as result of increased sensitivity to bone-conducted sounds. In case of tuning fork test, Weber's test may be lateralized to the affected ear. SSCDS is often seen in middle-aged group and has also seen in pediatric ages. Only a few cases have been documented in the literature among adults with bilateral SSCD and very few cases of pediatric patients with this findings. It may be unilateral (two-third of cases) or bilateral (one-third of cases). In case of SSCD, exposure to loud sound or positive pressure in the external auditory canal or Valsalva maneuver with closed mouth and nostril leads to ampullofugal (excitatory) deflection of the cupula in the SSC. This causes conjugated eye movements with an upward vertical slow phase component and a torsional component where the upper ocular pole moves toward the normal side. The application of negative pressure in the external auditory canal causes ampullopetal (inhibitory) deflection in the cupula in the SSC where the eye movement is downward vertical with a torsional component toward the affected side. A vertical-torsional nystagmus with the slow phase rotating up and away from the affected side may be induced by exposure of loud sound (Tullio phenomenon) or by giving pressure in the external auditory canal (Hennebert sign). SSCDS can present with conductive hearing loss which mimics to otosclerosis and explains some cases of persistent conductive hearing loss surgery such as stapedectomy. A bony dehiscence of the SSC leads to hyperacusis for bone-conducted sounds and autophony. This abnormal third window creates abnormal sound pathways, allows abnormal pressure inside the inner ear, and causes vestibular symptoms such as vertigo.
| Investigations|| |
There are several cochlear and vestibular tests utilized to diagnose SSCDS. In SSCDS, the pure-tone audiometry often reveals a low-frequency hearing loss. Increased sensitivity of the bone-conducted sounds may lead to lower bone conduction thresholds and so air-bone gap is seen in the affected ear and shows conductive type of audiogram. Impedance audiometry usually shows normal tympanogram. Vestibular evoked myogenic potential (VEMP) thresholds may be decreased for both air and bone conduction in affected ear. VEMP amplitudes may be high and the responses are useful to distinguish between a conductive loss due to SSCD and ossicular fixation as VEMP response is absent or slow elevated thresholds in ossicular fixation. Cervical VEMP increases the specificity of diagnostic test for physiologic dehiscence in comparison to high-resolution computed tomography (HRCT) scan used to demonstrate the anatomical dehiscence. HRCT is considered as imaging modality for confirmation of SSCD. HRCT with thin slices centered on the temporal bones with oblique coronal multiplanar reconstruction in the plane of SSC, called as the plane of Poschi, will give view of the dehiscence on the roof of the SSC [Figure 1]. Preoperative CT scan is not only confirm the diagnosis of SSCDS but also assess the pneumatization of the mastoid air cells for allowing adequate surgical exposure through the transmastoid route. Preoperative CT scan is often essential for knowing low dura or sclerotic mastoid preoperatively which prevent adequate surgical exposure and preventing complications. The presence of both vestibular and auditory symptoms has been often described with a dehiscence ≥2.5 mm on the CT scan. Sometimes, asymptomatic false-positive findings and radiologically apparent dehiscence limit the diagnostic usefulness of the HRCT. HRCT does not differentiate bone thickness <0.1 mm from true dehiscence. Normally, bone thickness over SSC is approximately 0.67 mm and standard deviation of 0.38 mm, whereas proportion of normal individuals with bony overlying of semicircular canal thickness <0.1 mm is more than the incidence of SSCD among general population (0.05%). Accordingly, the positive predictive value for HRCT in SSCD is only 57%. Along with HRCT, SSCD is confirmed by pure-tone audiometry and calorimetry. Pure-tone audiometry findings in SSCDS reveal air-bone gaps and negative bone conduction thresholds at the lower frequencies. A wrong diagnosis of ossicular fixation may be done due to air-bone gap in audiogram, which often misleads the clinician. Caloric test is usually done routinely among vertigo patients. However, the usefulness of caloric test as diagnostic test for SSCDS is questioned because the SSC is not reached by caloric stimulation. The differential diagnosis of SSCDS is always considered while evaluating a patient with a conductive hearing loss with normal reflex on a tympanogram, particularly in the presence of vestibular symptoms. A HRCT of the temporal bone is helpful for the diagnosis SSCDS in doubtful cases; thus, it will avoid unnecessary middle ear/mastoid exploration and help appropriate treatment of the patient. Video head impulse test helps to assess the individual semicircular canal function. To evaluate each canal plane, the person's head is positioned in a way so that the delivery of the head impulse is coplanar to the paired semicircular canals. In case of left and right anterior/posterior semicircular canal planes, the head are tilted 45° to the right and left respectively for getting the delivery of head shakes in the pitch plane.
|Figure 1: High-resolution computed tomography (Poschi plane) showing the dehiscence of the right superior semicircular canal|
Click here to view
| Treatment|| |
Patients of SSCDS with mild symptoms can be managed by conservative means. Medical treatments such as salt restriction, vestibular suppressants, diuretics, vestibular rehabilitation, and avoiding stimulus are useful. Grommet placement may reduce pressure-induced symptoms but shows variable results. This condition is treated with surgery when the symptoms are severe and affecting greatly to the daily life. In meta-analysis by Watters et al., 53% of patients of SSCD required surgery on the left, 23% on the right, and 23% on both sides. The principle of the surgery is to block the extra window with bone wax and the operative area with muscular fascia, bone chips, or conchal cartilage. The transmastoid approach for this disease has less complication and avoids craniotomy and temporal lobe retraction. In this case, the transmastoid approach was done for plugging the SCC. This surgery is usually done under local anesthesia which identifies accurately with certainty as a patient will complain vertigo when fistula area handled. A cortical mastoidectomy is done, and the lateral semicircular canal is identified. The subarcuate cell tracts exenterated for skeletonizing the SSC. The tegmen bone is drilled away with diamond burr for exposing the middle cranial fossa dura. The dura over the area of dehiscence is elevated from the area of the arcuate eminence with a blunt probe, taking care for avoiding the suction cannula on the fistula. The dehiscence over the semicircular canal is identified and is blocked with bone wax or connective tissue or conchal cartilage. The middle cranial fossa is gently released to drape over the tissue seal at the dehiscence area. The fistula is closed in layers. The surgical procedure should be done with caution in view of possible sensorineural hearing loss. Transmastoid surgery is easy and can be done easily by practicing otologists. This approach allows excellent exposure of the SSC. It obviates the need for craniotomy and temporal lobe retraction. Transmastoid approach for SSC occlusion has also financial advantages over craniotomy as in middle fossa approach. Other than transmastoid approach, middle fossa approach has been also used to repair the superior semicircular dehiscence by plugging the canal. It is often associated with morbidity on account of the craniotomy and temporal lobe elevation. Initially, access to the SSC was done through craniotomy, i.e., middle cranial fossa approach. This approach allows surgeon for showing the dehiscence of SSC. Morbidities associated with middle cranial fossa approach include transient temporal lobe dysfunction (seizures) by temporal lobe retraction. The cranial nerves such as the 3rd, 4th, 6th, 7th, and 8th nerve dysfunctions, hemiparesis, and cerebrospinal fluid leak may occur in middle cranial fossa approach. Temporal lobe gliosis is another new complication which can also occur after middle cranial fossa approach. Transcanal round window reinforcement technique is also done in SSCDS where occlusion of the round window is done through transcanal approach. It reduces the sensitivity of the inner ear to sudden fluctuations in sound or pressure.
| Conclusion|| |
SSCDS is an uncommon diagnosis for causing vertigo in clinical practice. It may present with vestibular or auditory symptoms or both in clinical presentations. The diagnosis should be done on the basis of clinical findings and investigations. Asymptomatic or mild symptoms of SSCDS can be managed conservatively. Surgery like plugging of the dehiscence of SSC is usually done in case of debilitating symptoms. SSCDS should be kept in mind of clinicians during evaluating vertigo patients for accurate diagnosis and treatment.
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