Auditory Neuropathy Spectrum Disorder (ANSD) is a hearing condition where the ear detects sound, but the transmission of that sound to the brain is disrupted. Unlike more common forms of hearing loss, the problem in ANSD is not with the initial capture of sound waves. Instead, it lies in the pathway that carries sound information from the inner ear to the brain, causing the messages to become jumbled or lost.
This can result in fluctuating hearing, where a person seems to hear well one day and not the next. Individuals with ANSD have difficulty distinguishing between different sounds and may struggle to understand speech, especially in noisy environments.
The Mechanics of Auditory Neuropathy
In a functioning ear, cells in the cochlea receive sound vibrations and convert them into electrical signals. These signals are sent along the auditory nerve to the brain for interpretation. With ANSD, this system experiences a breakdown in transmission. The issue in ANSD is not with the first step, as the cochlea’s outer hair cells, which act as amplifiers for sound, are working correctly.
The disruption occurs after this point, either at the level of the inner hair cells, which are responsible for sending information to the brain, or within the auditory nerve itself. This is analogous to a faulty cable connecting a microphone to a speaker; the microphone picks up the sound, but the corrupted signal results in distorted or unintelligible output.
This mechanism is different from sensorineural hearing loss, in which the outer hair cells are damaged and cannot properly detect or amplify sound. In ANSD, the sound is detected, but the timing and organization of the nerve signals are compromised. This neural dyssynchrony is why making sounds louder often fails to make them clearer for someone with ANSD.
Potential Causes and Risk Factors
The causes of ANSD are varied, and in many instances, a specific cause cannot be pinpointed. A number of cases are identified in newborns, particularly those who have experienced certain health challenges. Factors such as premature birth, low birth weight, and a lack of oxygen before or during birth are recognized risk factors. Severe jaundice in the newborn period (hyperbilirubinemia) has also been linked to the development of ANSD.
Genetic factors play a role, with some estimates suggesting that genetics underlie about 40% of cases. The condition can be inherited and may exist as an isolated issue or as part of a broader syndrome that affects other body systems. For example, certain neurological disorders that affect peripheral nerves, such as Charcot-Marie-Tooth disease and Friedreich’s ataxia, can also be associated with ANSD that may develop later in childhood.
Exposure to ototoxic (ear-damaging) medications can contribute to the condition. Infectious diseases, like mumps, and certain immune disorders have also been identified as potential causes.
The Diagnostic Process
Diagnosing ANSD involves a set of audiological tests that measure how different parts of the auditory system are functioning. ANSD is defined by a pattern of results from two main tests: Otoacoustic Emissions (OAEs) and the Auditory Brainstem Response (ABR) test.
The OAE test assesses the function of the outer hair cells within the cochlea. A small probe placed in the ear canal presents sounds and measures the faint echo-like responses, or emissions, produced by healthy outer hair cells. In a person with ANSD, these emissions are present, indicating that the cochlea is receiving sound. This test is used in newborn hearing screenings and can be passed by a baby with ANSD, which can delay diagnosis.
The ABR test measures the hearing nerve’s and the brain’s response to sound. Electrodes on the head record brainwave activity as clicking sounds or tones are played through earphones. For an individual with ANSD, the ABR test will be absent or show highly abnormal results, demonstrating the sound signal is not being transmitted effectively to the brain. This combination of present OAEs with an absent or abnormal ABR confirms a diagnosis of ANSD.
Management and Communication Approaches
Managing ANSD requires an individualized approach, as the effects of the condition are unique for each person. A team, including an audiologist, an ear, nose, and throat (ENT) doctor, and a speech-language pathologist, works with the family to determine the most suitable path forward. The goal is to find the right combination of technology and communication strategies to support language development.
Hearing aids may be recommended, but their benefit can be limited because they amplify a sound that is already disorganized. Pairing a hearing aid with a frequency modulation (FM) or digital modulation (DM) system can be helpful, especially in noisy settings like a classroom. These systems use a microphone worn by the speaker to transmit sound directly to a receiver worn by the child, improving the clarity of the speaker’s voice over background noise.
When hearing aids provide insufficient benefit, cochlear implants may be an option. A cochlear implant bypasses the damaged parts of the auditory system and directly stimulates the hearing nerve, which can help synchronize the neural signals reaching the brain. Alongside technological support, communication methods that incorporate visual information are encouraged. Approaches like cued speech, which uses handshapes near the mouth to clarify lip movements, and sign language can be used to supplement or replace spoken language, ensuring the child develops robust communication skills.