Patients with Charcot-Marie-Tooth type 1A (CMT1A) can have difficulty understanding speech in a noisy background, despite having a normal hearing test, according to a study.
The study, “Hidden hearing loss in patients with Charcot-Marie-Tooth disease type 1A,” which appeared in the journal Scientific Reports, also indicates that certain hearing assessments could be used as a marker for nerve damage in this disease subtype.
CMT1A, the most common subtype of CMT1, is a demyelinating peripheral neuropathy, which are disorders characterized by the loss of myelin, or the protective layer, in peripheral nerves. One of these nerves is the cochlear, or auditory, nerve, which connects the inner ear to the brain.
Hearing loss is a well-known symptom of CMT1A. Because selective loss of neurons does not affect sensitivity to weak sounds, it cannot be predicted with common hearing assessments using audiograms. As a result, CMT1A patients could have normal audiograms despite having difficulty hearing.
Hidden hearing loss, or reduced speech perception in a noisy background, has recently been described as an auditory neuropathy believed to contribute to impaired speech discrimination — which refers to how well an individual perceives speech — and intelligibility (understanding) in people with normal audiograms.
Since the correlation between hearing deficits and CMT1A progression is still unclear, a research team in Korea assessed hidden hearing loss and determined the diagnostic value of new auditory tests in this patient population.
A total of 43 CMT1A patients, including 20 males, 23 females, and 60 healthy controls with normal hearing ability were included. They were subjected to evaluations of speech perception in quiet and noisy backgrounds as well as to analyses of spectral ripple discrimination (SRD) and temporal modulation detection (TMD), which have both been shown to correlate with speech and music perception.
Results revealed that CMT1A patients and controls had normal audiogram and speech perception scores in a quiet background. However, the CMT1A patients had significantly impaired speech perception in a noisy background, as well as worse SRD and TMD.
Data also revealed that, in CMT1A patients, auditory temporal processing — the processing of sound over time — correlated with speech perception in a noisy background, disease duration, age at onset, and motor nerve conduction velocity.
“Therefore, hearing measurements can be used to track neural damage as the disease progresses,” the researchers wrote.
Overall, “auditory nerve demyelination in CMT1A might have resulted in decreased speech perception in a noisy background,” the scientists hypothesized. They considered this defective cochlear nerve transmission in CMT1A patients a different type of hidden hearing loss.
Additionally, because the temporal resolution results were associated with the extent of demyelination in both cochlear and peripheral motor nerves, the investigators suggested that “temporal resolution testing could be performed as an additional marker for CMT1A.”