Etiology of Hearing Loss
Causes of permanent hearing loss in children
Causes of conductive hearing loss in children
Can Middle Ear Dysfunction Cause Hearing Loss?
Can Middle Ear Dysfunction Affect Speech and Language Development or School Performance?
Most professionals working with hearing loss are in general agreement that the cause of hearing loss in about one-quarter of cases is Unknown, one-quarter Non-genetic, and about one-half Genetic.
Non-genetic hearing loss
In about 25% of cases of hearing loss there is a non-genetic cause that can be identified. Non-genetic hearing loss is most often caused by illness or trauma before birth or during the birth process. Older infants and young children can also develop non-genetic hearing loss due to illness or trauma.
- Congenital infections
Some viral infections are known to be associated with hearing loss. These infections carry the highest risk of causing hearing loss if the mother has the illness during pregnancy or passes the infection to her baby during the birth process. The primary infections are toxoplasmosis, syphilis, rubella (german measles), cytomegalovirus (also known by the initials CMV) and herpes. The amount of hearing loss that can result varies widely and some babies show no hearing loss at all, even if they have one of these infections. These infections can affect other systems in the body as well and medical professionals will need extensive birth history and test information to identify these infections as a cause for hearing loss. Congenital CMV infection is the leading cause of acquired hearing loss in infants.
- Complications associated with prematurity
Low birth weight has also been identified as a risk factor for hearing loss. Newborn specialists identify 1500 grams (approx. 3.3 lbs.) as a cut-off point, with children weighing less than 1500 grams having an increased likelihood of hearing loss. For infants that are born premature, illnesses associated with prematurity can cause hearing loss.
Hyperbilirubinemia (jaundice) that is severe enough to require a blood transfusion can also result in hearing loss. This is related to the potential damage that high levels of bilirubin can cause to the nerves of hearing.
Sometimes medications that are known to be ototoxic (damaging to hearing) are prescribed to babies, usually to treat serious infections or birth complications. The most common ototoxic medications used at this time include a family of antibiotics called aminoglycosides with names such as gentamycin, tobramycin, kanamycin, and streptomycin. They present more of a risk to hearing when they are used multiple times or in combination with other medications, such as diuretics.
All babies are evaluated at birth on a 10-point scale, called an APGAR score, given at 1 minute and 5 minutes after birth. The higher the score, the healthier the baby is. When babies have scores of 0-4 at one minute or 0-6 at five minutes, their risk for having hearing loss increases.
Also, prolonged mechanical ventilation for a duration of five days or longer due to persistent pulmonary hypertension increases the risk for hearing loss. These conditions of breathing problems and other distress at birth do not mean that a baby will always have a hearing loss, but do indicate the need to monitor hearing closely.
One illness that carries a high risk of causing hearing loss and/or balance problems is bacterial or viral meningitis. Because meningitis is an infection of the lining of the brain and spinal cord, the sense organs of hearing and balance are especially sensitive to this infection. Meningitis can cause hearing loss in infants and children.
Genetic hearing loss
With genetic forms of hearing loss, an estimated 70% are due to recessive causes, about 15% have a dominant cause; and the remaining 15% include all the other forms of inheritance. Genetic scientists subdivide genetic hearing loss into two general categories: non-syndromic, meaning hearing loss and nothing else, and syndromic, meaning hearing loss with other clinical findings. By far, the more common is non-syndromic hearing loss which includes 2/3 of all genetic hearing losses. There are over 400 known genetic causes involving hearing loss. One gene, known as Connexin 26 (abbreviated CX26) alone is responsible for about 1/3 of all the cases of genetic hearing loss. Since CX26 accounts for about 1/3 of all cases of genetic hearing loss, that leaves about 1/3 of all cases as non-syndromic (this includes all types of inheritance) with the remaining 1/3 as syndromic. Among the remaining 1/3 of non-syndromic cases of genetic hearing loss, 13 dominant and 8 other recessive genes have been described.
There are a number of types of conductive hearing loss in children that can be permanent due to congenital malformations of the outer and middle ear.
Otitis Media and Hearing Loss
Anything which interferes with the proper working of all parts of the middle ear is called a dysfunction. The most common dysfunction seen in children is an improperly working eustachian tube. This tube opens and closes many times during the day, providing an exchange of air between the middle ear and the air around us. If the eustachian tube does not open often enough, the middle ear pressure changes and fluid may fill the middle ear space. The fluid can become infected resulting in an ear infection, or otitis media.
Middle ear dysfunction is a health problem that requires medical attention. If left untreated, it may result in hearing loss and communication problems. Hearing loss caused by middle ear dysfunction is called conductive hearing loss. This type of hearing loss is usually temporary, and hearing can be restored with appropriate medical treatment. The amount of hearing loss varies from child to child and may even change from day to day. A complete hearing evaluation is needed to determine the extent of the hearing loss. The hearing test, ear examination, and a history of ear problems are used by the doctor to create a medical treatment plan which is right for the individual child.
Children learn speech and language by listening to the people around them. If middle ear dysfunction results in long-standing or repeated episodes of conductive hearing loss, speech and language development may be delayed. It also can cause listening problems in daycare or school, as well as at home. In some cases, the hearing problems may be mistaken for behavioral problems such as poor attention or distractibility.
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