A new study finds that Charcot-Marie-tooth (CMT) can cause hearing difficulties even if a person has no hearing loss. The study suggests that the disorder of the neuron cells which transmit signals carried from the brain to ears causes CMT.
What is CMT
CMT is a disorder of the peripheral nervous system. It is often inherited from parents to off-springs.
‘Hair cells’ are the cells that absorb the sound waves as they approach the ear. The vibrations cause chemical differences in the cells due to which neurons send signals to the brain. The process is called the ‘primary process of hearing’.
Individuals with CMT have more common hearing problems. A previous study recommended that these individuals may have difficulty hearing sounds amid background noises. However, they can hear normal sounds easily.
CMT is linked with nerve damage. The reason for hearing problems might be the abnormality in neurons that carry signals from ear to brain. However, there is little research data on the topic.
Research on CMT and Hearing problems
The research included 43 CMT1A and 15 CMT2A patients. None of the CMT patients had any sound‐detection problem using pure‐tone audiometry.
The study included three groups of people; CMT1A, CMT2A, and the control group. The researchers compared spectral‐ripple discrimination, auditory frequency‐following response, and temporal modulation detection.
The audiograms were normal for all the participants. However, CMT patients could not understand speech in noisy backgrounds.
They also measured auditory frequency-following responses (FFRs). Moreover, they also measured electric signals, sent along the neurons during the process of hearing, using electrodes which are located on the scalp, and are called FFR.
The author explained the importance of recognition while clinically managing complaints of CMT patients, with no audiometric proof of listening loss, the encoded sign could be deficient in the accuracy required for communication understanding in a clamorous background.
The electric signs had a weaker amplitude in CMT subjects as compared to the control group. This means that neurons fire less actively in these individuals. However, neuronal firing in the patients had equal time to that of the control group.
According to the researchers, the deficiency in auditory temporal processing has a link with the flat firing rate of the average neuron response over time.
The reason could be that the neurons get lesser signals or and could not send signals to the brain. There were some differences in the FFRs of CMT1A and CMT2A, because of a different kind of neuron loss between the two types.
Statistical models analyzed whether the advancement of the disease affects the hearing abnormalities or not. However, most of the hearing features did not correlate with the age of the patients.
The ability to discriminate sounds upon hearing close together is audio temporal resolution. Researchers found that the ability was dependent upon age at disease duration, poorer motor neuron function, and the onset of the patients.
According to the authors, even if CMT1A and CMT2A patients had normal audiograms, still they had irregular auditory processing in comparison with the control group, healthy people.
Although both the types of patients had some differences in the pathophysiology in specifically the auditory dysfunction, both had decreased the firing frequency of the average nerve response.
The researcher added that the study would make help the care of patients with CMT.