Types of Hearing Loss

CONDUCTIVE HEARING LOSS: Sound waves are conducted into and through the hearing system in a variety of ways. The term conductive hearing loss refers to a disruption or mechanical blockage of the movement of sound waves (vibrations) at some point in the hearing system before they reach the inner ear. A conductive loss occurs in the outer and/or middle ear.

Sound is first conducted through the air into the ear canal and then transmitted further via vibration of the middle ear bones. This conduction of sound can be disrupted by something as simple as a build-up of wax in the ear canal. It can also be disrupted by a hole in the ear drum, or by the presence of fluid in the middle ear.

Otitis Media is a common cause of conductive hearing loss associated with children who have frequent ear infections. Normally, the air pressure in the middle ear should be equal to the air pressure on the outer side of the eardrum. The Eustation Tube, which connects the middle ear and the back of the throat, helps to maintain this balance of pressure. Allergies, a cold, or a nose and throat infection might close the Eustation Tube and will block the exchange of pressure. Nature will complain, and can cause a painful earache or a plugged ear. Fluid can also build up in the middle ear. If this condition is not treated, infection can spread to the inner ear (through the mastoid bone) and/or cause the eardrum to burst. Children are more susceptible to middle ear infections because their Eustation Tubes are straighter and shorter than those in an adult. Often a tiny cut, called a myringotomy, is made in the eardrum and a small ventilating tube is inserted. The tube usually remains within the ear for 2 to 8 months. Normally, it works it way out naturally during the healing process. Hearing loss can fluctuate anywhere from a 20 dB to 40 dB loss when a child is suffering from an infection.

Many conductive losses can be treated and eliminated with medication or surgery. But conductive loss can occur due to malformations of the outer or inner ear. If the conductive hearing loss occurs because the bony or soft tissues of the outer ear have not fully developed, it is known as microtia. If there is no visible canal opening, there is atresia of the ear canal. These types of problems do not allow enough sound to the ear drum for the middle and inner ear to detect sound. The three small bones in the middle ear may be fused, or stuck together so that they cannot carry the sound from the eardrum to the cochlea. This is most common with the last ossicle, or the stapes. If the rest of the middle and outer ear are working well, a surgeon may be able to go into the middle ear cavity and loosen the stuck bones. These procedures are usually done after a child is at least four years old, and the procedures may not completely correct the hearing loss

SENSORI-NEURAL HEARING LOSS: This type of hearing loss is caused by a problem with the reception of sound in the inner ear or in the transmission of electrical impulses along the auditory nerve to the brain.
Possible causes of sensorineural hearing loss are: heredity; drugs; permanent damage due to excessive noise; prenatal exposure to rubella, cytomegelovirus; RH incompatibility at birth; low birth weight caused by prematurity; elevated biliruben levels (jaundice); meningitis; and some infectious diseases such as mumps.
A sensor-neural loss implies damage to the sensors or nerve fibers which connect the inner ear to the hearing center in the brain. Since damaged nerve fibers do not regenerate or repair themselves like some other parts of the body, this damage is permanent.

MIXED HEARING LOSS: This is a combination of conductive and sensor-neural hearing losses. The conductive loss often masks the sensorineural loss. It is important to have a child with a conductive loss tested for the presence of a sensorineural loss. There is often some improvement in the hearing of a child with a mixed hearing loss when the conductive loss is corrected or improved upon.

UNILATERAL HEARING LOSS: This type of hearing loss only affects one ear and may range from mild to profound. Most children with unilateral loss have problems localizing or locating the source of sound. Others may have difficulty with paying attention in noisy situations and language-based academic subjects.

AUDITORY NEUROPATHY (AN): Auditory neuropathy is a type of hearing loss where the cochlea, or organ of hearing, appears to be functioning normally. However, sound cannot travel to the hearing centers of the brain because the auditory nerve is not working properly. Children with AN sometimes can hear sounds but cannot understand what those sound mean or hear better on different days, with no obvious reason for the difference. And, most children, even those that can discriminate words in a quiet environment, cannot use their residual hearing in noise. Doctors don’t know the exact cause of this type of hearing loss, but there seems to be an increased risk for children who have a brother or sister with AN or babies who were born premature or had severe jaundice.

Illustration Source: MED-EL Corporation