Deafness and genetics
Deafness, also referred to as hearing loss or hearing impairment, encompasses a spectrum of auditory deficits ranging from partial to complete loss of hearing, affecting one or both ears. Hearing loss can be categorized into three main types: conductive, sensorineural, and mixed. Conductive loss arises from issues in the outer or middle ear, often responsive to medical intervention, while sensorineural loss is associated with inner ear damage or auditory nerve disorders and is typically permanent. Genetic factors play a significant role in deafness, with over half of children born with profound or partial hearing loss having a genetic component. Specific genes, such as GJB2 and GJB6, are critical for normal hearing and mutations in these can lead to nonsyndromic deafness. Various inheritance patterns exist, including autosomal recessive, autosomal dominant, and sex-linked recessive. The symptoms of hearing loss can emerge gradually or suddenly, and early detection is crucial, especially in infants. Treatment options vary based on the type and cause of hearing loss and may include medical treatments, the use of hearing aids, or surgical interventions. Preventative measures focus on avoiding exposure to loud noises and early hearing screenings for newborns to ensure timely detection and intervention.
Deafness and genetics
ALSO KNOWN AS: Hearing loss; hearing impairment
DEFINITION: Deafness means a lack or loss of the sense of hearing, which may be partial or complete. Partial loss of hearing is often called hearing loss rather than deafness. Deafness can occur in one or both ears.
There are three primary types of hearing loss. Conductive loss is hearing loss caused by the inability of the sound to reach the inner ear. This can result from outer or middle ear problems, such as ear infection, excess wax, or swelling. This type of hearing loss is most likely to respond to medical or surgical treatment. Sensorineural loss is hearing loss caused by disorders of the inner ear or auditory nerve. This type of loss is usually permanent. It can be caused by heredity or congenital problems; excess noise; old age; medications; infections, such as ear infections and meningitis; or tumors compressing the nerve of hearing, such as an acoustic neuroma. Mixed loss is a combination of both conductive and sensorineural loss.
Risk Factors
Risk factors for deafness include premature birth; increasing age for age-related hearing loss (presbycusis); taking ototoxic medications; and exposure to loud noise on the job, such as loud industrial noise or loud music. Exposure to recreational loud noise, such as guns (target practice) and music, and a family history of deafness are also risk factors.
Etiology and Genetics
Genetic factors are responsible for more than half of the children born each year with profound or partial hearing loss. Of these, between 70 and 80 percent present with deafness as the only or the major symptom, while the remaining children exhibit any one of several specific genetic syndromes. So many anatomical and neurological factors are essential for normal hearing that it is not surprising that more than thirty-five genes have been identified that, when mutated, can result in nonsyndromic deafness.
One major cause of genetic deafness is a mutation in either of two adjacent genes on the long arm of chromosome 13, known as GJB2 and GJB6 (at position 13q11-q12). These genes encode proteins called connexins, which are components of the gap junction channels that allow communication between neighboring cells in the inner ear. Another gene, known as POU3F4, is found on the X chromosome (at position Xq21.1) and helps specify shape and function of the stapes, one of the tiny bones in the middle ear. Mutations in this gene are known that cause the stapes to be fixed in place and unable to move with sound vibrations, thereby resulting in deafness. Two additional genes associated with deafness, MT-RNR1 and MT-TS1, are found on mitochondrial deoxyribonucleic acid (DNA) and therefore lack a chromosomal location entirely.
Several different patterns of inheritance are associated with nonsyndromic deafness. An autosomal recessive pattern accounts for about 75 percent of cases, which means that both copies of a particular gene must be deficient in order for the individual to be afflicted. Typically, an affected child is born to two unaffected parents, both of whom are carriers of the recessive mutant allele. About 20 percent of cases of inherited deafness show an autosomal dominant pattern, in which a single copy of the mutation is sufficient to cause full expression of the trait. An affected individual has a 50 percent chance of transmitting the mutation to each of his or her children. Most of the remaining cases exhibit a sex-linked recessive pattern in which mothers who carry the mutated gene on one of their two X chromosomes face a 50 percent chance of transmitting this disorder to each of their male children. Female children have a 50 percent chance of inheriting the gene and becoming carriers like their mothers. Fewer than 1 percent of cases result from mutations in mitochondrial genes, but in these cases deafness is transmitted from a mother to all of her children (fathers do not transmit mitochondrial genes to their offspring).
Symptoms
Hearing loss usually comes on gradually but may come on suddenly. Symptoms may include difficulty hearing, ringing in the ears (tinnitus), dizziness, ear pain in case of an infection, and feeling of ear fullness (as in earwax or fluid).
A symptom of deafness in infants who are one to four months old may be the lack of response to sounds or voices. Symptoms noted at four to eight months may include disinterest in musical toys and lack of verbalization, such as babbling, cooing, and making sounds. A symptom for infants from eight to twelve months old may be the lack of recognition of the child’s own name; lack of speech may be a symptom for children between twelve and sixteen months. According to the American Academy of Pediatrics, all children (including newborns) should be screened for hearing loss so that loss occurring before birth can be uniformly detected prior to three months of age.
Screening and Diagnosis
The doctor will ask about a patient’s symptoms and medical history and will perform a physical exam. As part of the diagnosis, the doctor may try to determine the location of the problem, the degree of loss, and the cause; it is not always possible to identify the exact cause of hearing loss. This information can help guide treatment.
Depending on the type of hearing loss, the doctor may order tests to confirm the patient’s diagnosis. Tests may include an otoscopy, the examination of the structures inside the ear; a bone vibrator (also called a tuning fork test), which helps to determine the type of hearing loss; an audiogram (also called a hearing test), which measures the degree of hearing loss; and tympanometry, which measures middle ear fluid and pressures. A brain-stem auditory evoked response measures the electrical response in the brain to sounds in order to help determine the exact location of certain hearing problems. Other tests include a computed tomography (CT) scan, a type of X-ray that uses a computer to make pictures of the inside of the body, and a magnetic resonance imaging (MRI) scan, which uses magnetic waves to make pictures of the inside of the body.
Treatment and Therapy
Treatment for deafness depends on the type of hearing loss. Options may include medical treatment, such as removal of earwax or use of antibiotics to treat an ear infection. In selected cases of sudden hearing loss, medical treatment with intratympanic steroids may be effective.
Treatment may also include the use of hearing aids—small devices that are worn in or behind the ear to help amplify sounds.
In some cases, surgery may be recommended to help improve hearing. Types of surgery include stapedectomy for treatment of otosclerosis; tympanoplasty for a perforated eardrum; tympanoplasty tubes for persistent middle ear infections or fluid; and cochlear implant, a surgically implanted electronic device that helps provide sound to a person with severe sensorineural hearing loss (although the devices do not completely restore hearing, improvements in implant technology continue to be made).
Prevention and Outcomes
To help prevent deafness, individuals should avoid loud noises. In cases when loud noises cannot be avoided, individuals can reduce exposure to loud noises by wearing earplugs, earmuffs, or ear protectors. In addition, taking steps to reduce injuries or disease may prevent certain types of deafness.
There is currently no effective way to prevent congenital or genetic deafness. Hearing screening for newborns can help insure that hearing loss in young babies is detected and treated at the earliest possible stage.
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