Diagnosis: Acoustic Neuroma
At first glance, the patient's audiogram showed conductive hearing loss in addition to sensorineural loss on the same side, suggesting mixed hearing loss. When making a differential diagnosis for mixed hearing loss like this patient's, otosclerosis is usually at the top of the diagnostic list. Cochlear involvement of otosclerosis can lead to sensorineural hearing loss in a similar pattern, as seen on this patient's audiogram. Other possible pathologies that could lead to mixed hearing loss include the third-window effect (in addition to the round and oval windows), which occurs when there is an abnormal opening into the inner ear and an enlarged vestibular aqueduct. However, it is uncommon that someone with an enlarged vestibular aqueduct would reach adulthood and not have severe sensorineural hearing loss, which usually occurs after minor head trauma. This pattern of mixed loss is usually seen in childhood wherein an enlarged vestibular aqueduct results in sensorineural hearing loss as well as conductive hearing loss from the third-window effect. An enlarged cochlear aqueduct could also create the third-window effect and resultant mixed hearing loss. Rarely, an anomaly in the basal turn of the cochlea, where there is an abnormal connection between the internal auditory canal (IAC) cerebrospinal fluid (CSF) space and the basal turn, results in stapes gusher syndrome, a condition that looks like conductive hearing loss. This happens because the abnormal connection of the basal turn of the cochlea with the CSF space causes a loss of energy on the air conduction testing (as in all third-window disorders), which appears like conductive hearing loss on the audiogram. Finally, superior canal dehiscence can cause mixed hearing loss, though it would rarely be this severe.
When looking at asymmetric sensorineural hearing loss, the clinician should always have a high index of suspicion and consider all possibilities before recommending an intervention. The patient brought his CT of temporal bones from five years ago. A cursory evaluation of the temporal bone CT did not reveal otosclerotic plaques. The report of the scan was normal. However, a more thorough evaluation of the scan revealed a significant asymmetry of the IACs. While some asymmetry in the width of the IACs is acceptable, the level of asymmetry in this patient was unusual and raised the suspicion of a possible tumor in the IAC on the right. A malignant tumor in the IAC will usually cause destruction and a moth-eaten appearance of the bone around it. A benign tumor in the IAC will slowly grow and gradually widen the IAC, as in the case of this patient.
When faced with an asymmetric IAC, the next step in the evaluation is to obtain an MRI of the IACs, which can be performed with or without contrast enhancement. While contrast enhancement provides visualization of much smaller tumors (less than 2 mm) for screening, an MRI without contrast provides a good visualization of the IAC and the inner ear anatomy. In this case wherein we are looking for a possible tumor of about 6 mm in size that has widened the IAC, a non-contrast MRI will provide good information. It also costs less, requires less time for a patient to be in the MRI device, and eliminates the need for an IV placement for contrast infusion.
The MRI of the patient showed a mass in the IAC on the right. Based on the location and shape of the mass (well-circumscribed and smooth), it is most likely a vestibular schwannoma (acoustic neuroma). In general, a newly diagnosed patient with a small tumor such as this has three options for management. The first option is expectant observation. The patient will need to get an MRI six months after the initial MRI. If the second MRI doesn't show any changes, then a subsequent MRI will be obtained usually six to 12 months afterwards. If no further changes are seen, yearly MRIs are obtained for a few years, then spaced longer to every other year. After five years, the intervals can be increased to every three years and then to every five years. Lifelong MRIs are needed because vestibular schwannomas may grow many years later. If the tumor grows, the patient will need to undergo treatment. The primary disadvantage of observation is that the patient's hearing can decline during the period of observation.
Treatment of vestibular schwannomas (acoustic neuromas) involves either stereotactic radiation (e.g., CyberKnife or GammaKnife) or surgical resection. Radiosurgery is usually performed for tumors that are smaller than 3 cm in the cerebellopontine angle and in older patients. Younger patients and larger tumors are generally considered for surgical treatment.
Given the benefit of having the CT scan from 2014 that showed an enlarged IAC of about the same size as the current one, we elected to observe this patient. No significant change had occurred over the past five years, so we will check the patient's MRI in one year rather than the traditional six months after the diagnosis.
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 widened IAC.
- Video 2. Axial (horizontal) CT of the left temporal bone demonstrating the normal IAC.
- Video 3. Coronal (parallel to face) CT showing the normal vs. an abnormally widened IAC in the coronal plane.
- Video 4. Axial (horizontal) CISS-sequence MRI showing the tumor on the right (left side of the images) IAC.
- Video 4. Axial (horizontal) T1 non-contrast MRI demonstrating the mass on the right IAC.
- Video 6. Coronal (parallel to face) T1 MRI showing the mass on the coronal plane.
Watch the patient videos online at thehearingjournal.com https://journals.lww.com/thehearingjournal/Pages/collectiondetails.aspx?TopicalCollectionId=23.Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.