Diagnosis: Otosclerosis and an Enlarged Vestibular Aqueduct
At first glance, the patient's audiogram shows mixed hearing loss. All things being equal, with a normal ear exam, conductive hearing loss in an adult with no history of ear disease is most likely caused by an ossicular fixation, which is usually due to otosclerosis. After stapes fixation, malleus fixation due to calcification of the anterior ligament of the malleus is the most common cause of conductive hearing loss. Incus fixation is uncommon, but can certainly occur.
The next step in evaluating a patient with this condition is to take an in-depth look at other aspects of the audiologic evaluation. In the case of this patient, he had absent acoustic reflexes, which supported the possibility of otosclerosis. However, he was found to have low-frequency sensorineural hearing loss upon bone conduction testing. In addition, a Carhart notch, which is commonly seen in patients with otosclerosis, did not appear in the patient's audiogram. A Carhart notch is a drop in the bone conduction threshold usually at 2,000 Hz. The bone conduction threshold at 2,000 Hz is usually worse than that at 1,000 and 4,000 Hz. In this patient, the bone conduction threshold at 1,000 Hz on the right was worse than that at 2,000 Hz. This made the audiogram suspicious. Therefore, further workup was warranted prior to proceeding with surgery.
The patient's CT of the temporal bone showed bilateral otosclerosis foci in the region of the fistula ante fenestrum (Figs. 2 and 4). This area, which is located anterior to the stapes footplate, is the most common site of otosclerosis involvement. When evaluating the CT, the clinician should look at the entire scan for other causes of hearing loss, such as canal dehiscence, among others. Upon in-depth examination of the patient's CT scan, we found that he also had an enlarged vestibular aqueduct.
An enlarged vestibular aqueduct is the most common congenital anomaly of the bony inner ear. This condition usually manifests itself in childhood with sensorineural or mixed hearing loss. The hearing loss tends to fluctuate and can get worse after head trauma.
Enlarged vestibular aqueduct is caused by a mutation of the SLC26A4 gene, which regulates fluid absorption in the inner ear. During embryonic development, fluid is produced and re-absorbed in many parts of the body. An imbalance between fluid production and reabsorption leads to an increase in fluid pressure, which likely causes enlargement of the endolymphatic duct and sac. This in turn leads to bony enlargement.
An enlarged vestibular aqueduct can present with conductive or mixed hearing loss. The conductive component of this condition is related to the third window effect, like superior canal dehiscence. Some of the mechanical energy of sound is lost when some of the energy is translated into movement that goes into the vestibular system and does not reach the cochlea, thus causing conductive hearing loss. The sensorineural component is due to involvement of the inner ear, possibly from increased pressure within the inner ear as a whole. Endolymphatic sac surgery has been done to reduce hearing fluctuations in patients with this condition, but it has caused significant hearing loss in some patients.
Different definitions of enlarged vestibular aqueduct have been proposed. One commonly used definition is comparing the width of the posterior semicircular canal with the vestibular aqueduct at the half-way point between the posterior fossa and the vestibule. Others have proposed 1.3 mm as the cut-off point for a normal vestibular aqueduct. This patient's vestibular aqueducts measured 1.0 mm on the right and 1.5 mm on the left, though both were larger than the width of the posterior canal (Figs. 3 and 5).
This patient showed evidence of otosclerosis on CT imaging bilaterally (Figs. 2 and 4). The imaging showed otosclerosis in the fistula ante fenestrum, which is the most common site of involvement. We would consider performing a stapedectomy to treat this patient, but his slightly enlarged vestibular aqueduct was a cause for concern. The enlarged vestibular aqueduct and secondary enlarged endolymphatic duct can lead to an increase in endolymphatic fluid pressure and consequently to an enlarged saccule or utricle. Normally, the saccule and utricle are approximately 0.6 to 1 mm away from the footplate. During stapedotomy, a laser or drill is used to perforate the oval window. The proximity of the utricle and saccule during surgery can make them susceptible to possible damage and increase the chance of deafness.
In this case, we elected to treat the patient conservatively with hearing aids rather than risking hearing loss from a stapedotomy procedure.
BONUS ONLINE VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial (horizontal) CT of the right temporal bone showing the otosclerosis and enlarged vestibular aqueduct.
- Video 2. Axial (horizontal) CT of the left temporal bone showing the otosclerosis and enlarged vestibular aqueduct.
- Video 3. Coronal (parallel to the face superiorly-inferiorly) CT of the right temporal bone showing the enlarged vestibular aqueduct in the coronal direction.
- Video 4. Coronal (parallel to the face superiorly-inferiorly) CT of the left temporal bone showing the enlarged vestibular aqueduct in the coronal direction.
- Video 5. Axial (horizontal) CISS-sequence MRI does not show significant hyperintensity in the vestibular aqueduct.
- Video 6. Coronal (parallel to the face superiorly-inferiorly) T2 MRI showing the inner ear with no hyperintensity in the vestibular aqueduct.
WATCH the patient videos on our website at thehearingjournal.com https://journals.lww.com/thehearingjournal/Pages/collectiondetails.aspx?TopicalCollectionId=23.Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.