Sensorineural Hearing Loss (SNHL)

Sensorineural hearing loss (SNHL) or nerve deafness can be defined as a type of hearing impairment that is caused due to dysfunction of the sensory receptors present in the inner ear. This condition causes a defect in the cochlea and/or the auditory section of the vestibulocochlear nerve, thus creating difficulty in the perception of sound.

SNHL can be either unilateral or bilateral. Although it is an irreversible condition, medical science has proposed certain beneficial hearing aids/implant techniques to support such patients.

Causes and Classification

There are numerous causes for this condition. Depending upon these causes, SNHL can be grouped into three categories:

• Congenital hearing loss
• Acquired hearing loss
• Auditory neuropathy

Congenital Hearing Loss

This is the type of nerve deafness that is present from birth, and can be genetic or non-genetic. Genetic causes may be delayed in action and affect only the hearing. This is also called “Familial Progressive Sensorineural Hearing Loss.”

Non-genetic causes develop during the prenatal and perinatal stages of the baby. Maternal rubella, German measles are examples which were known to cause congenital hearing loss before the development of a proper vaccine.

Acquired Hearing Loss

This can be categorized as the hearing loss that is caused after birth due to several reasons such as infections, inflammations, toxic drugs and threatening medical conditions, traumas, exposure to loud noise, age-related factors, tumors, injuries, and sudden accidents.

Presbycusis is a type of SNHL that is caused due to age-related factors. It commonly affects both ears and it takes a long time for the disease to affect a person. More than one-third of the elderly is affected by this hearing loss.

SNHL that is caused by exposure to loud noises can be divided into two types—Acoustic Trauma and Noise-Induced Hearing Loss (NIHL). Acoustic trauma is the hearing loss caused by the sudden brief exposure to loud noise, while NIHL is the hearing loss that is caused by chronic exposure of noisy occupations. The former leads to permanent damage to hearing while the latter is preventable.

Acquired factors can also cause “Sudden Sensorineural Hearing Loss” that develops over a period of hours or a few days. This may be either partial or complete.

“Immune-Mediated Sensorineural Hearing Loss” that is caused due to autoimmune disorders is medically treatable.

Auditory Neuropathy

This is a type of SNHL that can be congenital or acquired. This can be differentiated from other types of hearing loss by the combination of normal OAE (Otoacoustic Emissions) results and abnormal ABR (Auditory Brainstem Responses) responses. Abnormal auditory functions are considered as the reasons for this condition.

Signs and Symptoms

The symptoms exhibited by people who are affected by SNHL are as follows:

• Difficulty in understanding speech, even though it is loud enough.
• Difficulty in locating sounds if only one ear is affected with SNHL.
• Difficulty in hearing when there is background noise.
• Some sounds seem too loud.
• Difficulty in following the conversation within a group.
• Men’s voices seem more audible than the female voice.
• Difficulty in distinguishing high-pitched sounds (such as "s" or "th") from each another.
• The sounds of other people seem mumbled or slurred.
• Feeling of being off-balance or dizzy (more common with Meniere disease and acoustic neuromas).
• Ringing or buzzing sound in the ears (tinnitus).

Diagnosis

The first step in the examination of a patient involves the gathering of case histories related to the patient. This step involves collecting of details such as follows:

• Major concern
• Pregnancy and childbirth information
• Medical history
• Development history
• Family history.

ENT specialists use many techniques for the diagnosis of this condition. There are two main techniques used for this purpose:

• Otoacoustic Emissions: Low-intensity sounds that are produced by the movement of outer hairs cells in the cochlea are picked up by a miniature microphone for testing.
• Auditory Brainstem Response: it is a noninvasive technique that finds the integrity of central auditory pathway.

Other methods such as differential testing, audiometry, tympanometry are also used for the diagnosis of SNHL.

Rehabilitation Methods

Different rehabilitation methods help the patient to overcome the difficulties they face due to hearing loss. To support them, various hearing aids are provided in the initial stages. These hearing aids can help them in mild to moderate SHLS for both the ears. Hearing aids amplify the sound, but cannot make it clearer. Apart from this, telephone amplifiers, as well as other assistive devices, can be useful.

People with intense hearing loss, who do not benefit from using hearing aids, can opt for implant devices. The cochlear implant device is one of such implant devices. It bypasses the damaged structure of the cochlea and stimulates the function of the auditory nerve.

For patients with neurofibromatosis type 2, auditory brainstem implants are mostly used.

Patients are also trained in sign language and speech reading techniques to support their understanding level.

Reviewed by Afsaneh Khetrapal BSc (Hons)