CT and MRI Evaluation of Bilateral Enlarged Vestibular Aqueduct Syndrome with Curved Reconstructions – A Case Report and Short Review of Literature

Enlarged Vestibular Aqueduct Syndrome (EVAS) is a known, but rare entity. It represents a common cause of congenital sensorineural hearing loss, diagnosed more often in children with a slight female predominance. Herein, we report a case of bilateral Enlarged Vestibular Aqueduct Syndrome [EVAS] in a 26-year-old male. As it is a subtle finding on imaging, the interpreting radiologist must be aware of this entity to make the diagnosis. The diagnostic CT and MRI images of this patient are given along with curved CT and MRI reconstructions along the plane of the Vestibular Aqueduct [VA] extending to the endolymphatic sac [ES]. The CT reconstructions demonstrate the bony anatomy in great detail and the bilateral dilated VA. These help in excluding diseases like otospongiosis and inner ear anomalies. The curved MRI reconstruction CISS images through the VA demonstrate the entire endolymph channel. The reconstructed MRI Images clearly demonstrate a patent endolymph channel without any focal abnormality such as stricture, or other associated congenital anomalies.


Introduction
Enlarged Vestibular Aqueduct Syndrome (EVAS) is a known but rare entity. It is an important cause of sensorineural hearing loss. EVAS is diagnosed more frequently in children. The median age at diagnosis is 13 years. EVAS can be unilateral or bilateral. Bilateral involvement is more common than unilateral. EVAS ranges from complete deafness in infancy to reasonably well retained hearing in adulthood. Interestingly, the degree of aqueduct enlargement has not been correlated with the rate of attenuation of hearing in bilateral enlarged vestibular aqueduct syndrome (BEVAS). BEVAS is associated with mutations of SLC26A4, and these mutations correlate with the rate of hearing attenuation. Unilateral EVAS is not associated with SLC26A4, nor with any genetic syndromes. This condition can be congenital or acquired but is more commonly congenital.

Case report
26-year-old male was referred to the ENT clinic by his primary care physician for progressively worsening bilateral severe hearing loss. The patient had a childhood history significant for attention deficit disorder. Review of other systems was unremarkable. The patient was evaluated with audiogram, CT temporal bone, MRI of brain and IAC. Audiogram demonstrated bilateral down sloping which is associated with severe bilateral sensorineural hearing loss. Gene mapping for SLC26A4 mutations were not performed in this patient. Based on the findings, patient was scheduled to undergo cochlear implantation.

Methods
CT temporal bone without contrast was performed in a Somatom Definition Edge Siemens scanner [Siemens Medical Systems, USA]. Standard 0.6 mm axial CT bone algorithm images were obtained followed by 0.6 mm reconstructed images in coronal and oblique [Stenver's and Poschl's] planes. For the purpose of this case report, curved CT reformats were obtained from the vestibule through the vestibular aqueduct towards the endolymphatic sac.
MRI of the brain was performed on a 3T Siemens Spectra scanner [Siemens Medical Systems, Malvern, Pennsylvania, USA] using a standard head coil. Standard axial T1, axial T2, fat suppressed axial T2 FLAIR and GRE sequences without contrast were obtained at 22 FOV, 5 mm thickness and 1 mm spacing. Axial DWI and ADC brain pulse sequences were acquired without contrast using 28 FOV, 5mm thickness and 0 spacing. After intravenous administration of 20 ml of Multihance [gadobenate dimeglumine, Bracco Diagnostics Inc., Monroe Township, NJ, USA], axial and coronal post contrast sequences were acquired using 5mm thick slices with 1 mm spacing and 22 FOV.
MRI of the internal auditory canals was performed on a 3T Siemens Spectra scanner [Siemens Medical Systems, Malvern, Pennsylvania, USA] using a standard head coil. 3mm Axial T1 and 1mm axial CISS sequence were obtained pre contrast followed by post contrast 3 mm axial T1 and coronal T1 sequences. For the purpose of this case report, using CISS sequence, curved MRI reformats were obtained from the vestibule through the vestibular aqueduct to the endolymphatic sac.

Imaging findings
Nonenhanced temporal bone CT images show enlarged bilateral VA [ Fig  1a].     [5]. There is some controversy regarding EVA's origin, and two prevalent, competing hypotheses exist. One claims that EVA arises in early gestation due to arrested development, while the other states that EVA arises during later fetal/postnatal life from aberrant development [6,7].

Clinical presentation
The vast majority of patients experience progressive and/or fluctuating bilateral onset of sensorineural hearing loss [2,9]. It is believed that the SLC26A4 mutations causing EVA causes hearing loss, rather than EVA directly resulting in hearing loss [8]. Vertigo and dizziness are typically present to some degree. EVA's clinical picture often mimics otosclerosis and other common etiologies of hearing loss when presenting in the adult population [9]. Further, about 60% of EVA patients also possess additional anatomic deficits, such as enlarged semicircular canals or hypoplastic cochlea, among others [10].

Co-existing anomalies:
The pathogenesis of EVA has been associated with inner ear anomalies and genetic syndromes in about 85% of patients, including Pendred syndrome, cochlear anomalies (i.e., cochlear hypoplasia, Mondini malformation, etc.), otofaciocervical syndrome, CHARGE syndrome, vestibular and semicircular canal anomalies, etc. The majority of these anomalies are linked to mutations in SLC26A4, which codes for pendrin, a cellular transporter of various negatively charged ions. Yet, polygenic influences from still-undefined genes may still play a role [5]. Pendred syndrome refers to EVA that is coupled with genetic thyroid dysfunction [1,10].

Pathophysiology
The pathophysiological mechanism of hearing loss in EVA has not yet been fully elucidated. Back pressure of endolymph onto the cochlea, leak of hyperosmolar endolymph into the cochlea, and ion pump saturation leading to inner ear electrolyte imbalances are widely proposed models [10].

Imaging findings
EVAS displays enlarged VA on CT. The endolymphatic duct and sac usually are also enlarged and are better seen on MRI. Abnormalities involving the cochlea, vestibule and semicircular canals can be associated. Middle and external ear structures are usually within normal limits.

Non-enhanced CT:
In literature, multiple thresholds regarding the size of enlarged VA exist with some studies cautioning against the use of any specific threshold whatsoever. Statistically, at least 120 reference values are required to calculate any valid 95% mid-interval (2.5 th -97.5 th percentile). Using 146 reference values, Vijayasekaran et al. calculated that VA measured <0.9 mm at midpoint and <1.9 mm at the operculum, ideally measured halfway between the crus and the aperture on an axial view (i.e., AP dimension) [11]. As such, the vestibular aqueduct is considered enlarged if measuring ≥ 1 mm at the midpoint of the aqueduct and ≥ 1.9 mm at operculum, perpendicular to the long axis of the VA. In Poschl's plane (images obtained parallel to the superior semicircular canal) measurements ≥ 1.2 mm at midpoint and ≥ 1.3 mm at operculum have been suggested [12]. Some studies suggest mid vestibular aqueduct measurement of ≥ 1.5 mm as enlarged [5]. The posterior margin of the petrous bone can be scalloped due to the enlarged ES..

MRI:
T1WI shows low to intermediate signal endolymphatic sac along the posterior petrous bone. 3D CISS [Siemens] or equivalent FIESTA [GE], is the most specific and sensitive sequence and it shows enlarged ES and duct with variable signal likely based on its proteinaceous content. Often, the typical high-signal fluid within the posterior duct is supplanted by low-signal fluid, owing to its proximity to loose or fibrous connective tissue. In over 80% of cases, additional inner ear structural abnormalities are also visualized, ranging from conspicuous dysplasia of inner ear structures to more subtle asymmetry [13,14].

Prognosis
The degree of VA enlargement (ranging from 1.5mm to 8mm) has not been correlated with the rate of attenuation of hearing in BEVAS. However, mutations of SLC26A4 have been shown to do so. [10] This rate ranges from complete deafness in infancy to decently retained hearing into adulthood. Prognosis is considered better for unilateral hearing loss and/or late onset hearing loss. While unilateral EVAS still holds a significant risk for hearing loss in the affected ear, this condition's etiology may be different altogether as it is not associated with SLC26A4, nor with the aforementioned genetic syndromes seen in BEVAS [8,15].
Overall prognosis of EVAS is difficult to predict reliably.

Treatment
Therapeutic planning is often guided by accompanying inner ear abnormalities. First-line treatment of EVAS involves temporary hearing aids and rehabilitation followed by cochlear implantation. Cochlear implantation consistently yields profoundly improved outcomes in audiometric function, comparable to that of healthy patients [13]. The presence of incomplete partition of the cochlea or other concurrent inner ear anomalies may temper efficacy of implantation by up to a third, though function is still greatly improved [16]. Accordingly, this procedure represents the curative mainstay of EVAS. Intratympanic corticosteroid use has been studied, but the results remain controversial. Some studies report that 80-85% of their patients demonstrated hearing improvement with steroid therapy, but meta-analyses on the topic show no significant benefit. This may be due to the tendency for patients with mild EVAS cases to receive steroids, while those with severe progression generally require cochlear implantation [17]. Other previously attempted surgeries, such as endolymphatic sac obliteration or decompression shunting, are ineffective for EVAS and may even hasten its progressive hearing loss [18].

Conclusion
EVAS is a relatively rare entity, which can lead to sensorineural deafness. Though it's most commonly seen in childhood, with female preponderance, this is a case report in a 26-year-old male, underscoring the need to be alert to this diagnosis in adults. The CT and MRI images clearly demonstrate previously undiagnosed bilateral EVAS in this patient. Awareness of this subtle entity is essential for the interpreting radiologist in order to make a prompt and correct diagnosis. In this case, curved reconstructions through the entire VA extending to the ES were created using both CT images and thin volumetric axial CISS sequence. The reconstructed images clearly demonstrated the entire dilated channel of the VA, ED and ES. Focal lesions like distal widening, stricture or associated congenital anomalies can be more easily identified using these reconstructed images.