Hearing loss can be categorized by which part of the auditory system is damaged. There are three basic types of hearing loss: conductive loss, sensorineural loss, and mixed loss.
Conductive Hearing Loss
Conductive loss occurs when sound is not conducted efficiently through the outer ear canal to the eardrum and the tiny bones (ossicles) of the middle ear. Conductive hearing loss usually involves a reduction in sound level or the ability to hear faint sounds. This type of loss of hearing can often be corrected medically or surgically.
Some possible causes of conductive hearing loss:
- Fluid in the middle ear from colds
- Ear infection (otitis media)
- Allergies (serous otitis media)
- Poor eustachian tube function
- Perforated eardrum
- Benign tumors
- Impacted earwax (cerumen)
- Infection in the ear canal (external otitis)
- Swimmer’s Ear (otitis ecxterna)
- Presence of a foreign body
- Absence or malformation of the outer ear, ear canal, or middle ear
Sensorineural Hearing Loss
Sensorineural hearing loss (SNHL) occurs when there is damage to the inner ear (cochlea), or to the nerve pathways from the inner ear to the brain. Most of the time, SNHL cannot be medically or surgically corrected. This is the most common type of permanent loss of hearing.
SNHL reduces the ability to hear faint sounds. Even when speech is loud enough to hear, it may still be unclear or sound muffled.
Some possible causes of SNHL:
- Drugs that are toxic to hearing
- Hearing loss that runs in the family (genetic or hereditary)
- Head trauma
- Malformation of the inner ear
- Exposure to loud noise
Mixed Hearing Loss
Sometimes a conductive loss occurs in combination with a sensorineural loss (SNHL). In other words, there may be damage in the outer or middle ear and in the inner ear (cochlea) or auditory nerve. When this occurs, the loss of hearing is referred to as a mixed hearing loss.
Infant Hearing Loss
If your newborn child
- does not startle, move, cry or react in any way to unexpected loud noises,
- does not awaken to loud noises,
- does not turn his/her head in the direction of your voice,
- does not freely imitate sound, or
- has failed a newborn hearing screening test,
then he or she may have some degree of hearing loss.
More than three million American children have a hearing loss, and an estimated 1.3 million of them are under three years of age. Parents and grandparents are usually the first to discover hearing loss in a baby, because they spend the most time with them. If at any time you suspect your baby has a hearing loss, discuss it with your doctor. He or she may recommend evaluation by an otolaryngologist—head and neck surgeon (ear, nose and throat specialist) and additional hearing tests.
Hearing loss can be temporary, caused by ear wax, middle ear fluid, or infections. Many children with temporary hearing loss can have their hearing restored through medical treatment or minor surgery.
However, some children have sensorineural hearing loss (sometimes called nerve deafness), which is permanent. Most of these children have some usable hearing, and children as young as three months old can be fitted with hearing aids.
Early diagnosis is crucial in the management of pediatric hearing loss. When diagnosis is delayed, there can be significant impact on speech and language development. Early fitting of hearing or other prosthetic aids, and an early start on special education programs can help maximize a child’s existing hearing. This means your child will get a head start on speech and language development.
“Information from the American Academy of Otolaryngology-Head and Neck Surgery.”
Sudden Sensorineural Hearing Loss (SSHL)
Sudden Sensorineural Hearing Loss (SSHL), or sudden deafness, is a rapid loss of hearing. SSHL can happen to a person all at once or over a period of up to 3 days. It should be considered a medical emergency. A person who experiences SSHL should visit a doctor immediately.
A doctor can determine whether a person has experienced SSHL by conducting a normal hearing test. If a loss of at least 30 decibels in three connected frequencies is discovered, it is diagnosed as SSHL. A decibel is a measure of sound. A decibel level of 30 is half as loud as a normal conversation. A frequency is another way of measuring sound. Frequencies measure sound waves and help to determine what makes one sound different from another sound.
Hearing loss affects only one ear in 9 out of 10 people who experience SSHL. Many people notice it when they wake up in the morning. Others first notice it when they try to use the deafened ear, such as when they make a phone call. Still others notice a loud, alarming “pop” just before their hearing disappears. People with SSHL often experience dizziness or a ringing in their ears (tinnitus), or both.
Some patients recover completely without medical intervention, often within the first 3 days. This is called a spontaneous recovery. Others get better slowly over a 1 or 2 week period. Although a good to excellent recovery is likely, 15 percent of those with SSHL experience a hearing loss that gets worse over time.
Approximately 4,000 new cases of SSHL occur each year in the United States. It can affect anyone, but for unknown reasons it happens most often to people between the ages of 30 and 60.
Though there are more than 100 possible causes of sudden deafness, it is rare for a specific cause to be precisely identified. Only 10 to 15 percent of patients with SSHL know what caused their loss. Normally, diagnosis is based on the patient’s medical history. Possible causes include the following:
- Infectious diseases.
- Trauma, such as a head injury.
- Abnormal tissue growth.
- Immunologic diseases such as Cogan’s syndrome.
- Toxic causes, such as snake bites.
- Ototoxic drugs (drugs that harm the ear).
- Circulatory problems.
- Neurologic causes such as multiple sclerosis.
- Relation to disorders such as Ménière’s disease.
People who experience SSHL should see a physician immediately. Doctors believe that finding medical help fast increases the chances for recovery. Several treatments are used for SSHL, but researchers are not yet certain which is the best for any one cause. If a specific cause is identified, a doctor may prescribe antibiotics for the patient. Or, a doctor may advise a patient to stop taking any medicine that can irritate or damage the ear.
The most common therapy for SSHL, especially in cases with an unknown cause, is treatment with steroids. Steroids are used to treat many different disorders and usually work to reduce inflammation, decrease swelling, and help the body fight illness. Steroid treatment helps some SSHL patients who also have conditions that affect the immune system, which is the body’s defense against disease.
Another common method that may help some patients is a diet low in salt. Researchers believe that this method aids people with SSHL who also have Ménière’s disease, a hearing and balance disorder.
Hearing Loss Research
Two factors that help hearing function properly are good air and blood flow inside the ear. Many researchers now think that SSHL happens when important parts of the inner ear do not receive enough oxygen. A common treatment for this possible cause is called carbogen inhalation. Carbogen is a mixture of oxygen and carbon dioxide that seems to help air and blood flow better inside the ear. Like steroid therapy, carbogen inhalation does not help every patient, but some SSHL patients taking carbogen have recovered over a period of time.
Information from the NIDCD.
Genes and Hearing Loss
One of the most common birth defects is hearing loss or deafness (congenital), which can affect as many as three of every 1,000 babies born. Inherited genetic defects play an important role in congenital hearing loss, contributing to about 60 percent of deafness occurring in infants. Although exact data is not available, it is likely that genetics plays an important role in hearing loss in the elderly. Inherited genetic defects are just one factor that can lead to hearing loss and deafness, both of which may occur at any stage of a person’s lifespan. Other factors may include: medical problems, environmental exposure, trauma, and medications.
The most common and useful distinction in hearing impairment is syndromic versus non-syndromic.
Non-syndromic hearing impairment accounts for the vast majority of inherited hearing loss, approximately 70 percent. Autosomal- recessive inheritance is responsible for about 80 percent of cases of non-syndromic hearing impairment, while autosomal-dominant genes cause 20 percent, less than two percent of cases are caused by X-linked and mitochondrial genetic malfunctions.
Syndromic (sin-DRO-mik) means that the hearing impairment is associated with other clinical abnormalities. Among hereditary hearing impairments, 15 to 30 percent are syndromic. Over 400 syndromes are known to include hearing impairment and can be classified as: syndromes due to cyotgenetic or chromosomal anomalies, syndromes transmitted in classical monogenic or Mendelian inheritance, or syndromes due to multi-factorial influences, and finally, syndromes due to a combination of genetic and environmental factors.
Variable expression of different aspects of syndromes is common. Some aspects may be expressed in a range from mild to severe or different combinations of associated symptoms may be expressed in different individuals carrying the same mutation within a single pedigree. An example of variable expressivity is seen in families transmitting autosomal dominant Waardenburg syndrome. Within the same family, some affected members may have dystopia canthorum (an unusually wide nasal bridge due to sideways displacement of the inner angles of the eyes), white forelock, heterochromia irides (two different-colored irises or two colors in the same iris), and hearing loss, while others with the same mutation may only have dystopia canthorum.
Genes are a road map for the synthesis of proteins, which are the building blocks for everything in the body: hair, eyes, ears, heart, lung, etc. Every child inherits half of its genes from one parent and half from the other parent. If the inherited genes are defective, a health disorder such as hearing loss or deafness can result. Hearing disorders are inherited in one of four ways:
Autosomal Dominant Inheritance: For autosomal dominant disorders, the transmission of a rare allele of a gene by a single heterozygous parent is sufficient to generate an affected child. A heterozygous parent has two types of the same gene (in this case, one mutated and the other normal) and can produce two types of gametes (reproductive cells). One gamete will carry the mutant form of the gene of interest, and the other the normal form. Each of these gametes then has an equal chance of being used to form the offspring. Thus the chance that the offspring of a parent with an autosomal dominant gene will develop the disorder is 50 percent. Autosomal dominant traits usually affect males and females equally.
Autosomal Recessive Inheritance: An autosomal recessive trait is characterized by having parents who are heterozygous carriers for mutant forms of the gene in question but are not affected by the disorder. The problem gene that would cause the disorder is suppressed by the normal gene. These heterozygous parents (A/a) can each generate two types of gametes, one carrying the mutant copy of the gene (a) and the other having a normal copy of the gene (A). There are four possible combinations from each of the parents, A/a, A/A, a/A, and a/a. Only the offspring that inherits both mutant copies (a/a) will exhibit the trait. Overall, offspring of these two parents will face a 25 percent chance of inheriting the disorder.
X-linked Inheritance: A male offspring has an X chromosome and a Y chromosome, while a female has two copies of the X chromosome only. Each female inherits an X chromosome from her mother and her father. On the other hand, each male inherits an X chromosome from his mother and a Y chromosome from his father. In general, only one of the two X chromosomes carried by a female is active in any one cell while the other is rendered inactive. This is why when a female inherits a defective gene on one X chromosome, the normal gene on the other X chromosome can usually compensate. As males only have one copy of the X chromosome, any defective gene is more likely to manifest into a disorder.
Mitochondrial Inheritance: Mitochondrias, small powerhouses within each cell, also contain their own DNA. Interestingly, the sperm does not have any mitochondria, and consequently, only the mitochondria in the egg from the mother can be passed from one generation to the next. This leads to an interesting inheritance pattern where only affected mothers (and not affected fathers as their sperms do not have mitochondria) can pass on a disease from one generation to the next. Sensitivity to aminoglycoside antibiotics can be inherited through a defect in mitochondrial DNA and is the most common cause of deafness in China!
In the last decade, advances in molecular biology and genetics have contributed substantially to the understanding of development, function, and pathology of the inner ear. Researchers have identified several of the various genes responsible for hereditary deafness or hearing loss, most notably the GJB2 gene mutation. As one of the most common genetic causes of hearing loss, GJB2-related hearing loss is considered a recessive genetic disorder because the mutations only cause deafness in individuals who inherit two copies of the mutated gene, one from each parent. A person with one mutated copy and one normal copy is a carrier but is not deaf. Screening tests for the GJB2 gene are available for at risk individuals to help them determine their risk of having a child with hearing problems.
“Information from the American Academy of Otolaryngology-Head and Neck Surgery.”