Question

Outer hair cells contribute not only to amplification but also to the frequency tuning in the cochlea. What are some of the difficulties that might arise for people with cochlear hearing loss, due to these two factors? Why do hearing aids not solve all these problems?

Answer 1

The cochlea is a tiny, snail-shaped structure. It is the main organ of hearing and is part of your inner ear. Cochlear Damage means that all or part of your inner ear has been hurt. Damage to the cochlea typically causes permanent hearing loss.This is called sensorineural hearing loss (SNHL).

What are the causes ?

The causes of sensorineural hearing loss are varied but can be generally put into two categories: congenital and acquired.

Congenital hearing loss is present at birth and is the most common problem seen in newborn babies. It can be either inherited or caused by abnormal development in the fetal stages of life. Before the development of a vaccine, maternal rubella or German Measles was a common cause of congenital hearing loss.

Acquired hearing loss, which happens after birth, can be caused by a wide range of factors. These include trauma, presbycusis (age-related hearing loss), noise exposure from machinery or firearms, Meniere’s disease and meningitis. Ototoxic drugs - which damage the ear but may be necessary to treat some life-threatening medical conditions – can also trigger hearing loss. A tumour on the hearing nerve can cause the rarer neural hearing loss.

What happens when cochlear damage occurs?

Thousands and thousands of tiny nerves called ‘hair cells’ line the inside of the snail-shaped structure, the cochlea. Each hair cell is responsible for picking up a different sound, sort of like keys on a piano, but on a much more detailed scale. All of the hair cells work in concert to code the incoming sound and send it on to the brain, where sound is heard and understood.

magine the tiny hair cells, standing tall and waving back and forth as sound comes into the ear. All of a sudden, an extremely loud sound enters the ear and the cochlea. The hair cells are hit with sound so hard that the hair cells are bent, broken, and in some cases, totally sheared off. Once this cochlear damage occurs, the damage is done. Hair cells in the cochlea are not able to regenerate themselves. Unlike your skin, hair, and many other cells in the body, once cochlear damage occurs, there’s no ‘growing’ back.

Difficulties in cochlear hearing loss

Since the inner ear is not directly accessible to instruments, identification is by patient report of the symptoms and audiometric testing. Of those who present to their doctor with sensorineural hearing loss, 90% report having diminished hearing, 57% report having a plugged feeling in ear, and 49% report having ringing in ear (tinnitus). About half report vestibular (vertigo) problems.

Most cases of SNHL present with a gradual deterioration of hearing thresholds occurring over years to decades. In some, the loss may eventually affect large portions of the frequency range. It may be accompanied by other symptoms such as ringing in the ears (tinnitus) and dizziness or lightheadedness (vertigo).

Frequent symptoms of SNHL are loss of acuity in distinguishing foreground voices against noisy backgrounds, difficulty understanding on the telephone, some kinds of sounds seeming excessively loud or shrill, difficulty understanding some parts of speech (fricatives and sibilants), loss of directionality of sound (especially with high frequency sounds), perception that people mumble when speaking, and difficulty understanding speech.

Why do hearing aids not solve all these problems?

Hearing aids can help most people with mild to moderate sensorineural hearing loss in both ears. For more severe levels of hearing loss, sometimes hearing aids just do not help enough. This is because sensorineural hearing loss causes sounds to become distorted. Amplifying sounds through hearing aids makes them sound louder, but not necessarily clearer. Even good quality hearing aids can sound distorted if the inner ear is severely damaged.

Hearing aids amplify sounds, if the cochlea is severely damaged and many hair cells are missing, amplified sound won’t get past the ‘relay station’ (i.e. the hair cells in the cochlea), to be sent on to the brain.