- What is a conductive hearing loss?
- What is a sensory hearing loss?
- What is a neural hearing loss?
- What is cortical deafness?
What is a conductive hearing loss?
A conductive hearing loss is one in which the inner ear and hearing nerves are healthy but sound cannot travel to the inner ear as a result of mechanical blockage of sound transmission.
Conductive hearing loss can be caused by a variety of factors, including the malformation of the ear canal (congenital atresia of the ear canal) or a blockage of the external canal caused by ear wax (in children) or vernix (in newborns). However, the most common cause of conductive hearing loss is the presence of fluid in the middle ear (behind the eardrum).
A conductive loss can also occur if there is a hole in the eardrum which decreases its surface area and ability to collect sound to pass on to the fluids of the inner ear. Additionally, recurrent infections can cause the hearing bones to stiffen, preventing sound vibrations from passing through the chain of hearing bones as easily. Recurrent infections can also result in erosion of the chain of hearing bones, causing the chain to become disconnected.
The treatment of conductive hearing loss varies depending on the cause. Because causes of conductive loss are mechanical, surgery is often possible. Although tubes can be placed in newborns, other ear surgeries must wait until a child is older. Treatment, however, cannot wait. Infants may be fitted for hearing aids and, in some cases, a bone conduction hearing aid may be the best option. A bone conduction aid delivers the sound vibrations directly to the bone around the ear. The sound travels through the bone to the inner ear very efficiently. A bone conduction aid is worn with a headband to keep the vibrator against the skull. There are surgically implantable forms of bone conduction hearing aids as well, which eliminate the need for a head band. This category of devices is called bone anchored hearing aids (BAHA).
What is a sensory hearing loss?.
A sensory hearing loss can also develop as a result of significant fluid pressure changes in the inner ear. This can happen in an inner ear concussion in individuals who have an extra opening in the inner ear, such as an enlarged vestibular aqueduct. It is not unusual for a sudden total or partial loss of hearing to be associated with head trauma in these patients. When sensorineural hearing loss is present it may be partial and putting sound in more loudly with a hearing aid may be helpful, as long as the information that is decoded by the hair cells and sent to the brain is clear enough to understand speech. In some cases the clarity is more important than the actual amount of sound sensitivity. A child who can hear only distorted or unclear sounds will not learn speech and language. Rather, they will give others the impression that he is hearing but simply not paying attention to instructions. This “unruly child” behavior is commonly seen in children with hearing loss when hearing aids are not helping.
What is a neural hearing loss?.
A neural hearing loss may occur in patients who possess a normal auditory system, with the exception of the auditory nerve, which carries nerve impulses from the hair cells of the inner ear to the brain.
The cochlear nerve is a complex structure with 30,000 nerve fibers which fire in carefully synchronized volleys in response to incoming sound. This nerve does not sit idle when it is not stimulated -it has a constant resting firing rate. Therefore, if physiological changes in the electrical conduction properties are changed enough, the synchronization of the nerve volleys fails to adequately carry clear information to the brain. We call this condition auditory neuropathy. A child with auditory neuropathy will not have detectable electric nerve responses when sounds are delivered to the ear because each nerve impulse in response to a sound is so spread out in time that it cannot be measured.
Children with a neural hearing loss will not learn meaningful speech and language with hearing aids, which simply make sounds louder. Fortunately, this desynchrony can be overdriven with the electrical stimulation of a cochlear implant.
What is cortical deafness?
Cortical deafness is a condition is which the ear canal, the middle ear, the inner ear, and the auditory nerve are all normal, but incoming sound vibrations will not be registered as sounds as a result of an injury to the cortex of the brain, which is specialized in receiving auditory signals. Cortical deafness can occur as a result of a stroke, infection, malformation or serious injury. There is little that can be done to rehabilitate cortical deafness. Communication with sign language is necessary.