Hearing Tests Can Help Diagnose, Monitor MG, Study Suggests

The inner ear function in myasthenia gravis (MG) patients is significantly impaired, and hearing tests can serve as a diagnostic and monitoring tool, scientists from South Korea say.

Their study, “Abnormalities of Otoacoustic Emissions in Myasthenia Gravis: Association With Serological and Electrophysiological Features” was published in the journal Frontiers in Neurology.

MG is an autoimmune condition in which communication between the nerves and muscles is impaired. It causes severe muscle weakness, and patients can tire easily. In the most common form of MG, the body generates antibodies that attack  acetylcholine receptors (AChRs) and impede that communication. Acetylcholine receptors are present in the muscle cell surface and are crucial in neuromuscular interaction.

AChRs are also present on the outer hair cells (OHC) that line the inner ear. The OHCs serve as sound pre-amplifiers and help in hearing, allowing people to hear very soft sounds.

In this study, researchers assessed whether otoacoustic emissions (OAE) — the low-level signals generated by the inner ear — in response to sound stimuli are affected in people with MG.

A total of 15 MG patients (mean age 35.5 years) and 10 healthy individuals (mean age 33.3 years) with no history of hearing problems were included.

OAE testing was performed in both ears using two methods, the Transient Evoked Otoacoustic Emissions (TEOAE) test and Distortion Product Otoacoustic Emissions (DPOAE) testing. The two methods vary based on the stimuli used to assess the OAE response.

In TEOAE, clicks within the frequency ranges of 2,000-4,000 Hz are used to stimulate a response. For DPOAE testing, two tones of moderate pitch within a frequency range from low to high are used as triggers. A small probe or catheter placed in the ear canal is used to send the stimuli. The OAE response from the inner ear is then measured in decibels (dB).

The team used the ILO-v6 clinical OAE system to perform the two tests. To avoid the masking effects of treatment, patients were asked to stop taking medication before the test. Twelve hours after stopping therapy, OAE was assessed.

Both the amplitudes of TEOAE and DPOAE in MG patients were significantly reduced compared to the healthy control group, the researchers reported. The mean amplitude of TEOAE was 3.41 dB in MG patients compared to 8.69 dB in the healthy controls. The DPOAE results were similar, and it was highly significant especially at higher frequency stimuli between 2,026 and 4,053 Hz, the authors found.

Researchers also found that OAE test results were significantly comparable to the blood test for AChR antibody detection, the standard blood test for MG diagnosis. Lower OAEs were detected in AChR antibody-positive MG patients. A similar correlation was also reported in those MG patients who were positive for repetitive nerve stimulation, another MG diagnostic tool.

No marked difference in OAE was reported among patients with different types of MG and different disease durations, the authors wrote.

“This study, therefore, found strong evidence for an association between OAEs and electrophysiological and serological characteristics of MG,” the authors wrote. “Our findings imply that the measurement of OAEs might increase the diagnostic accuracy and help to monitor the severity of MG.”