Deafness: Description, Causes and Risk Factors:
Alternative Name: Hearing loss.
Deafness is the inability to hear sound. It has many causes and can occur at any age. People can go deaf suddenly as a complication of a virus, or lose their hearing over time because of disease, nerve damage, or injury caused by noise. About 1 in 800 babies is born deaf, often because of genetic factors. Over 2 million people in North America cannot hear at all and are considered profoundly deaf. Another 30 million have serious hearing problems
Hearing is a complex process, so it should be no surprise that the causes of hearing loss are also complex. Hearing loss can occur because of damage to the ear, especially the inner ear. For example, infants may be born with hearing loss caused by a viral infection that was acquired during pregnancy. At other times the cause is genetic and therefore due to changes in the genes involved in the hearing process. Sometimes, hearing loss is due to a combination of genetic and environmental factors. There is, for example, a genetic change that makes some people more likely to develop hearing loss after taking certain antibiotic medications.
The different types of hearing loss are classified according to what part of the hearing system is affected. Sound is picked up by the outer ear and then passes through the ear canal to the middle ear. Problems in these places cause conductive hearing loss. After passing through the middle ear, the sound then travels to a part of the inner ear called the cochlea, where it is changed to a signal that can be sent down the hearing nerve to the brain. Problems here cause sensorineural hearing loss.
Infections that can damage the brain or nerves of the ear measles, meningitis, mumps, and Scarlet fever.
Trauma from explosions, fireworks, gunfire, rock concerts, and earphones.
Other Genetic Causes May Include:
Mutations in the connexin 26 (Cx26) gene are the most common cause of hearing loss. Cx26 hearing loss is most often seen in a person with:
Hearing loss that was found at birth or in early childhood.
Hearing loss that is mild, moderate, severe or profound.
Hearing loss without any other medical problems (nonsyndromic).
Hearing loss with no identified cause.
Although these are the most common characteristics of hearing loss due to a mutation in Cx26, there can be variations, even within a family. There have been several occasions when skin disorders have been found in people with deafness due to dominant Cx26 mutations. Furthermore, there have been instances when a child's deafness was originally thought to be due to non-hereditary factors (e.g., an infection or exposure to antibiotics) and then the child was later found to have mutations in Cx26. In such cases, it is more likely that the child's deafness was caused by the Cx26 mutations than by environmental agents. It should also be noted that many children with Cx26 mutations have no family history of hearing loss.
SLC26A4 hearing loss is most often seen in a person with:
Temporal bone abnormalities (observed on a CT scan or MRI of the inner ear) including enlarged (dilated) vestibular aqueduct and/or Mondini malformation.
Hearing loss that was found at birth or in early childhood.
Hearing loss that is severe or profound.
Thyroid enlargement (goiter) that appears during childhood or early adulthood.
A positive perchlorate discharge test (for thyroid function).
Although these are the most common characteristics of hearing loss due to mutations in SLC26A4, there can be variations, even within a family. SLC26A4 mutations are usually thought to cause Pendred Syndrome, a genetic condition associated with hearing loss and goiter (thyroid enlargement). However, mutations in the SLC26A4 gene can sometimes cause hearing loss in people who do not have any thyroid problems. Also, some persons with SLC26A4 hearing loss do not have temporal bone abnormalities and some can have mild-to-moderate hearing loss.
There are many forms of hearing loss that can be caused by mutations in mitochondrial genes. Some of the more common forms include:
Isolated (nonsyndromic) sensorineural hearing loss.
Hearing loss that began after antibiotic treatment.
Hearing loss that is associated with neuromuscular disease.
Hearing loss that is associated with diabetes.
There are two mitochondrial genes known to cause nonsyndromic hearing loss (12S rRNA and tRNAser(UCN)). There are several mitochondrial genes known to cause syndromic forms of hearing loss. For example, mutations in the tRNAleu(UUR) and tRNAlys genes can both cause hearing loss associated with neuromuscular diseases such as MELAS and MERRF. Mutations in the tRNAleu(UUR) gene can also be associated with diabetes. All forms of hearing loss due to mutations in mitochondrial genes are inherited only from the mother and not the father.
COCH hearing loss is most often seen in a person with:
Isolated (nonsyndromic) sensorineural hearing loss (initially worse at high frequencies).
Hearing loss that begins between ages 15 and 65.
Hearing loss that progresses to anacusis (complete loss).
Variable vestibular problems (e.g. dizziness, trouble with balance, etc.).
A family history of dominantly inherited hearing loss.
Although these are the most common characteristics of hearing loss due to a mutation in COCH, there can be variations, even within a family. Some individuals do not report vestibular problems and it is also possible that a family history may not be present. It should be noted that some people with COCH-associated hearing loss may initially be diagnosed with M
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