"Doctor, I'm puzzled. Why does my child react to sounds but can't understand our speech?" This is a common question from parents of children with auditory neuropathy in clinics. What causes a child to be able to "hear" but not "understand"? How is auditory neuropathy different from the more common sensorineural hearing loss?
Concept and Causes of Auditory Neuropathy
The term "Auditory Neuropathy (AN)" was first introduced by the foreign scholar Starr in 1996. This condition is characterized by the ability to hear sounds but an inability to comprehend language, hence the phrase "hear but can't understand." Auditory neuropathy is a challenging and incurable disorder affecting speech and communication in infants and young people, accounting for 10% of permanent childhood hearing loss. It can occur in the neonatal period or during adolescence.
The causes of auditory neuropathy may include environmental and genetic factors:
- Environmental Factors: Neonatal hyperbilirubinemia, hypoxia or asphyxia, infections, medications, noise, etc., are major environmental factors leading to auditory neuropathy.
- Genetic Factors: Over 40% of cases are believed to be related to genetic factors. Currently, more than 20 pathogenic genes associated with auditory neuropathy have been identified, with the OTOF gene being significant in infantile auditory neuropathy.
Common sensorineural hearing loss, such as noise-induced deafness, is caused by damage to the outer hair cells inside the cochlea. The main function of outer hair cells is to enhance the perception of low to moderate intensity sounds, acting like an "amplifier" for sound perception. In auditory neuropathy patients, while the outer hair cell function is normal and can effectively "amplify sound," there is a pathology in the inner hair cells, auditory nerve, or higher auditory pathways, leading to errors in the transmission of neurotransmitters and impulses. This impairs the processing and discrimination ability of auditory information, resulting in a situation where sounds can be heard but not understood.
Audiological manifestations in auditory neuropathy patients include otoacoustic emissions (OAE) and/or cochlear microphonic (CM) responses that reflect outer hair cell function. However, auditory brainstem responses (ABR), which reflect the entire auditory transmission pathway, may be absent or abnormal. Difficulty in speech recognition in noise and disproportionate decreases in speech recognition rates compared to pure tone thresholds are common.
Auditory neuropathy should be distinguished from sensorineural hearing loss, cochlear nerve dysplasia, and lesions behind the cochlea or central lesions. Comprehensive audiological and imaging examinations should be conducted to rule out structural abnormalities in the cochlear nerve and lesions behind and within the central nervous system.
- Hearing Aid Intervention: Children with elevated behavioral hearing thresholds in auditory neuropathy can benefit from hearing aids. However, the use of hearing aids in children with behaviorally normal hearing thresholds is still controversial. Follow-up monitoring after hearing aid fitting is crucial due to the different pathogenesis from sensorineural hearing loss.
- Cochlear Implantation: The effectiveness of cochlear implants in auditory neuropathy patients varies, with some benefiting significantly, while others show large differences in postoperative outcomes. The age of cochlear implantation is recommended not to be too early, generally around 2 years old.
- Assistive Listening Technologies: Optimizing the signal-to-noise ratio through methods such as reducing environmental noise, using amplifiers to increase speaker volume, employing frequency modulation (FM) systems, and using sign language can help improve communication skills in infants and children with auditory neuropathy.
- Drug Therapy: The effectiveness of drug therapy for auditory neuropathy is not well-documented. Consideration may be given to drugs such as demyelinating agents, nutritional agents for the auditory nerve, antioxidants, agents for synaptic function reshaping, and mitochondrial energy metabolism agents.
Genetic counseling for auditory neuropathy patients and their families involves two main parts:
- Pre-genetic Testing Counseling: This includes gathering patient information, preliminary clinical diagnosis, accurate collection and organization of phenotype information, and providing helpful insights for choosing precise and efficient genetic testing methods to assist in developing a personalized testing strategy.
- Post-genetic Testing Counseling: This covers the etiology, genetic modes, lesion locations, prognosis, intervention and prevention, and reassessment of recurrence risk based on different test results. Counseling for hereditary auditory neuropathy, which includes various genetic patterns and lesion locations, should give appropriate guidance based on the purpose of counseling and the relationship between the counselee and the proband. It should also address the psychological issues arising from genetic testing results for patients and their families.
Auditory neuropathy is relatively rare, and our understanding of the disease is not comprehensive. Further research is needed to explore the lesion sites and mechanisms of auditory neuropathy. Early diagnosis and intervention with hearing aids, cochlear implants, assistive listening technologies, and drugs are essential for patients with auditory neuropathy. Patients and their families should establish realistic expectations for the treatment outcomes of this condition.
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