Diagnosis: Acoustic Neuroma
The patient's audiogram shows an asymmetric high-frequency sensorineural hearing loss. The thresholds in the right ear were 15 dB worse than the left ear at 4,000 and 8,000 Hz. The rest of the thresholds were essentially symmetric. His word discrimination was technically not asymmetric, though there was a slight difference between the two sides.
Patients often come in with an old audiogram that shows a slight asymmetry. The question is whether the asymmetry is real and whether the patient needs further work-up. There have been many attempts to find criteria, then have low false-positive and low false-negative rates on further work-up when looking for retrocochlear lesions. The goal is to not order too many additional tests when it's not necessary. Before the era of MRIs, the work-up of asymmetric hearing loss primarily depended on the use of auditory brainstem response (ABR) testing. Over time, ABR was found to have a higher false-negative rate in the presence of a small (< 1cm) acoustic neuroma. Today, MRI is the standard in screening for acoustic normal when asymmetric hearing loss is present. The primary downside of MRIs is the cost to the patient and to the health care system as a whole. While shrinking reimbursement rates have reduced the overall cost of MRIs, the current high-deductible insurances compel some patients to bear the entire cost of MRIs on their own. In the case of asymmetric hearing loss, physicians request MRIs to avoid a potential malpractice suit even if the suspicion for a tumor is low.
In previous years, MRI with and without intravenous gadolinium were performed to evaluate for the presence of an acoustic neuroma. However, with the development of the ultra high-resolution, non-enhanced fast spin echo (also known as CISS, FIESTA, or 3D MPR), a non-contrast (gadolinium) MRI has been shown to be accurate down to a 2 mm-sized tumor (Otolaryngol Head Neck Surg. 1998;119:364). The use of a non-contrast enhanced MRI is accurate most of the time when screening for an acoustic neuroma. Non-enhanced MRIs are also advantageous, as these reduce the cost, cut the time a patient is in the MRI machine by nearly half, and prevent exposure to gadolinium. In recent years, gadolinium has been found to be associated with potential kidney dysfunction. Sometimes the patient may have claustrophobia and requests for an open MRI. MRIs obtain images of soft tissues using a combination of high-strength magnetic and radio waves. The magnet aligns the protons in the hydrogen atom that are stimulated by the radio waves. Relaxation time and return of the hydrogen atom to baseline are measured, creating a cross-section image. In general, the strength of the magnet in the MRI is mostly responsible for the visibility of details in the resulting image. Most closed (the magnet is in one piece) MRI devices have magnet strengths between 1.5T and 3.0T, while most open (the magnets are in a few pieces with gaps between them) MRIs have between 0.2T and 0.3T. Open MRI images are notoriously fuzzy and lacking in detail, especially on the CISS sequence.
This patient previously obtained a non-contrast open MRI (Fig. 2) before coming to our clinic. A new audiogram was obtained that showed a significant hearing decline in the patient's right ear, with profound loss in the high frequencies. A post-gadolinium MRI of the internal auditory canal (IAC) was also obtained in an open MRI, which showed a small mass in the IAC on the right side (Fig. 3). Due to the small size of the tumor and the age of the patient, a follow-up MRI was obtained six months later using a closed 1.5T MRI. CISS sequence showed the tumor at a significantly better resolution (Fig. 5).
There are no set rules or guidelines on when an MRI should be repeated. However, if the suspicion for the tumor is high and the previously obtained MRI was inadequate, a repeat MRI is warranted. This is usually the case when the previous MRI was a brain MRI and high-resolution images of the IAC were not obtained. While a brain MRI would likely demonstrate a tumor extending beyond the IAC, it could easily miss a small tumor. It is our practice to repeat the MRI if there is further decline of hearing unilaterally or if new symptoms develop such as dizziness or imbalance. Obtaining a non-contrast MRI of the IAC is generally adequate for screening for an acoustic neuroma. However, if a patient prefers an open MRI, gadolinium-enhanced (with and without contrast) images need to be obtained because a small tumor will be missed in the IAC.
Looking back at the patient's old audiogram from 2015 (Fig. 1), the patient had an asymmetric hearing loss on the right. While this small degree of hearing loss is not alarming on its own, it becomes significant when noting the patient's history of right-handed rifle use. His asymmetric hearing loss would be expected to be more on the left as he was a right-handed shooter. A higher degree of hearing loss on the right may indicate a possible retrocochlear abnormality.
BONUS 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 CISS sequence MRI of the IACs obtained in an open 0.2T MRI. The tumor cannot be seen.
- Video 2. Axial T1-weighted pre-gadolinium (non-enhanced) open 0.2T MRI of the IAC which does not show the intracanalicular tumor.
- Video 3. Axial T1-weighted post-gadolinium open 0.2T MRI of the IAC showing the tumor. The borders of the tumor cannot be distinguished well due to the low-resolution images.
- Video 4. Axial CISS sequence MRI obtained in a closed 1.5T MRI showing the higher resolution images compared with Video 1.
- Video 5. Axial T1-weighted non-enhanced closed 1.5T MRI of the IAC which does not show the intracanalicular tumor due to the lack of enhancement.
- Video 6. Axial T1-weighted post-gadolinium enhanced closed 1.5T MRI of the IAC that shows the sharp margins of the tumor.
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