Causes of Hearing Loss in Children
There are two main categories of hearing loss, conductive and sensorineural. Conductive hearing losses involve the outer and/or the middle ear, the latter being the problem most often. Sensorineural hearing losses may involve the inner ear (the snail-shaped cochlea) and/or the auditory nerve. Let's examine these classifications and their causes.
Conductive hearing losses may be caused by malformation of the outer ear or ear canal. Wax may clog up the ear canal; this is called impacted cerumen. Most frequently, children develop infections in the middle ear (otitis media). Fluid accumulates in the middle ear causing reduced hearing thresholds. (Fluid may also accumulate with colds.) If it persists, doctors may choose to treat this more aggressively with insertion of tubes in the tympanic membrane (ear drum) to allow the fluid to drain. This procedure is called a myringotomy. Generally, after the fluid is gone, hearing thresholds returns to normal levels. Thus, children with recurrent ear infections may have hearing levels that fluctuate. In the young child who is just developing language skills, this can have a devastating effect on both the developing sound system and the grammatical system. We have also noticed that some children develop a hypersensitivity to sound which is often a feature of an auditory processing disorder.
One disorder (which may be genetic) that causes a conductive hearing loss is called otosclerosis. This condition causes excess bony cell growth which fixates the three little bones in the middle ear. If these bones cannot vibrate, the sound is not conducted from the middle ear to the inner ear. This condition can be treated surgically if necessary with a procedure called a stapedectomy.
Sensorineural hearing losses tend to be more severe than conductive losses. The cochlea, which is the snail-shaped structure in the inner ear and consists of three fluid filled chambers and besides being an organ of hearing, also is responsible for our sense of balance. The cochlea also contains sensory hair cells. In the healthy cochlea, the fluid and the hair cells amplify sound and ultimately convert the sound to an electrical signal. Ultimately, this signal travels through the auditory nerve and then to the auditory cortex in the brain. Most sensorineural hearing losses are the result of a problem in the cochlea, rather than the auditory nerve. Those delicate hair cells, in particular, are easily damaged by long term exposure to loud sounds (think of the teens on the subway listening to their MP3 players) or even to one extremely loud event. According to the American Speech-Language & Hearing Association (ASHA), more than 50% of congenital hearing losses have heredity as a factor. After birth, there are a number of viruses that can cause hearing losses including meningitis, influenza and, in fact, anything that can cause a high fever. Certain drugs are ototoxic; that is, they can damage the inner ear.
We all probably know someone who uses a hearing aid in one or both ears. Another one of our articles discusses cochlear implants, which can be used as long as the auditory nerve is not damaged. Cochlear implants in young children, when accompanied by auditory training and assistance from a speech-language pathologist, can result in fully functional speech recognition and verbal expression.