Hearing loss can be categorized in many different ways, including location of the damage in the auditory system, whether the condition is temporary or permanent, and whether the hearing loss affects one or both ears.
There are three general types of hearing loss: conductive, sensorineural, and mixed.
Conductive hearing loss occurs when sound waves are not conducted through the outer ear and middle ear effectively. This type, which often can be corrected medically or surgically, causes a reduction in sound level and an inability to hear soft sounds.
Conductive hearing loss may be caused by the following:
- Build up of fluid in the middle ear (e.g., resulting from a cold virus, allergies, or middle ear infection [otitis media])
- Genetic malformation of the outer ear, ear canal, or middle ear
- Impacted earwax, scarring, or a foreign object in the ear canal
- Outer ear infection (external otitis)
- Perforated (ruptured) eardrum
Sensorineural hearing loss results from damage to the inner ear. Although this type of hearing loss is permanent, it often can be treated using hearing aids. Sensorineural hearing loss involves a reduction in sound level that results in the inability to hear soft sounds, understand speech, or hear clearly.
Causes for sensorineural hearing loss include the following:
- Diseases (e.g., vascular, infectious, or nerve disorders, HIV/AIDS)
- Exposure to loud noises
- Genetic conditions (e.g., Waardenburg syndrome, Branchio-otorenal syndrome, Usher syndrome, neurofibromatosis type 2; can cause hearing impairment that is present at birth or that develops later in life)
- Head trauma (injury)
- Medications that damage the auditory system (called ototoxic drugs; e.g., aspirin, nonsteroidal anti-inflammatory drugs [NSAIDs], antibiotics, diuretics, chemotherapy drugs)
- Ménière's disease (causes progressive hearing loss, ringing in the ears [tinnitus], and dizziness [vertigo])
- Tumor (of nervous or auditory systems)
- Viral infections (e.g., meningitis, mumps, measles, scarlet fever)
Mixed hearing loss results from a combination of damage to the outer ear or middle ear and the inner ear or auditory nerve. In some cases, treatment for mixed hearing loss provides some improvement in hearing.
Central hearing loss results from damage to the brain or spinal cord (i.e., the central nervous system) or to nerves that lead to the brain.
Hearing loss associated with certain genetic disorders (e.g., Usher syndrome, Waardenburg syndrome) often causes additional symptoms, such as loss of vision (e.g., retinitis pigmentosa).
Hearing impairments also can be classified according to the severity of the conditionthe degree of hearing loss. There are several different classification systems that can be used to assess hearing loss. Many of these systems classify hearing according to the softest sound that the person is able to perceive.
The pressure and power of sound, which indicates intensity and loudness, is expressed in units called decibels. Normal hearing ranges from -10 to 15 decibels hearing level (dB HL); moderate hearing loss ranges from 41 to 55 dB HL; and severe hearing loss ranges from 71 to 90 dB HL.
In some cases, hearing loss only affects certain frequencies, such as high-pitched tones or low-pitched tones. Hearing loss that is the same for both high and low frequencies is called flat hearing loss.
Hearing loss also can be classified according to whether the onset is acute (sudden) or progressive (worsens slowly over time) or whether the condition fluctuates (irregularly varies in severity). Unilateral hearing loss affects only one ear and bilateral hearing loss affects both ears. In symmetrical hearing loss, the location of the damage and the degree of hearing loss is the same in both ears and in asymmetrical hearing loss, the damage and/or severity is different in each ear.
In the fall 2012 issue of our sister publication, Diabetes Focus, we reported that according to a recent study, as many as 72 percent of teens have hearing problems after attending a rock concert. To reduce this risk, kids should pop in foam earplugs next time they rock out. The plugs block damaging high-pitched frequencies, but let melodies through, says the House Research Institute in Los Angeles, California.