Understanding hearing loss

Hearing
Your ear consists of three major areas:

  • Outer ear – The outer ear is the part of the ear that can be seen and includes the ear canal.
  • Middle ear – The middle ear includes the eardrum, three small bones (ossicles) and the eustachian tube.
  • Inner ear – The inner ear includes the cochlea, hair cells and nerve fibers.

Hearing occurs when sound waves reach the structures inside your ear, where the sound wave vibrations are converted into nerve signals that your brain recognizes as sound. Sound waves pass through the outer ear and cause vibrations at the eardrum. The eardrum and three bones – the hammer, anvil and stirrup – amplify the vibrations as they travel to the inner ear. The vibrations pass through fluid in the cochlea, a snail-shaped structure in the inner ear. Attached to nerve cells in the cochlea are thousands of tiny hairs that help translate sound vibrations into electrical signals that are transmitted to your brain. The vibrations of different sounds affect these tiny hairs in different ways, causing the nerve cells to send different signals to your brain. That’s how you distinguish one sound from another.

Causes of Hearing Loss
Many different things can happen in the ear to cause a hearing loss. For example, problems with the outer or middle ear can prevent the conduction of sound waves to the inner ear. Problems with the inner ear can prevent electrical signals from being transmitted to the brain efficiently. Factors that can contribute to hearing loss include ear infections, earwax buildup, diseases, tumors, trauma, noise exposure, ototoxic drugs, and the aging process.

Hearing Tests
Audiologists are professionals who specialize in evaluating hearing loss and conduct hearing tests. The results from the tests are recorded on an audiogram. An audiogram is a graph showing hearing sensitivity.

Hearing evaluations for adults are conducted in sound-proof booths where they respond to sounds presented by the audiologist. Test methods can include the following:

  • Earphones / Speakers- Earphones or speakers are used to send sounds through the ear canal and the middle ear to reach the inner ear.
  • Bone Vibrator- A bone vibrator sends sound directly to the inner ear, bypassing the ear canal and middle ear.
  • Word Recognition Testing- Word recognition tests show how clearly words are heard when they are presented at different listening levels and repeated.
  • Tympanometry – Tympanometry is performed to screen for middle ear problems.

Special test methods are used with infants, toddlers and young children.

  • Auditory Brainstem Response (ABR)ABR uses electrodes on the scalp to record the child’s responses to sounds played through earphones.
  • Otoacoustic Emissions (OAE)OAE utilizes a tiny microphone that is placed in the ear canal. Responses are analyzed to determine hearing capacity.
  • Visual Reinforcement Audiometry (VRA) or Conditioned Play Audiometry (CPA)VRA and CPA utilize gamelike activities for an audiologist to observe a child’s responses to sounds, such as head turning and hand raising.

Degree of Hearing Loss
The degree of hearing loss is determined by measuring hearing threshold, the levels in decibels (dB) at which a signal is just barely heard. The louder sounds must be made to be heard, the greater the degree of hearing loss. Thresholds are measured at several frequencies (pitches) and graphed on the audiogram.

  • Mild Hearing Loss- A person with a mild hearing loss (26-45 dB), may have difficulty hearing and understanding someone who is speaking from a distance or has a soft voice. Understanding conversation in noisy backgrounds also may be difficult.
  • Moderate Hearing Loss- A person with moderate hearing loss (45-65 dB) may find conversational levels of speech difficult to hear and understand, even in quiet backgrounds. Listening in noise is extremely difficult.
  • Severe Hearing Loss- A person with severe hearing loss (66-85 dB) may find hearing is difficult in all situations. Speech may only be heard if a speaker is talking loudly or at close range.
  • Profound Hearing Loss- A person with a profound hearing loss may not hear even loud speech or environmental sounds.

Configuration of Hearing Loss
The frequencies tested and graphed on the audiogram are those important for hearing and understanding speech and other environmental sounds. Frequency is noted in Hertz (Hz). The configuration of hearing loss refers to the pattern or shape of the hearing loss on the audiogram. Both the degree and the configuration of hearing loss will affect the ability to detect certain speech sounds. More hearing loss in one frequency region than another will affect the ability to hear sounds which have most of their energy in that region. For example, an “s” sound has energy between 4000 and 8000 hertz (Hz) at an intensity of approximately 35 dB hearing level (HL). A person with hearing thresholds greater than 35 dB HL in that region may not hear the “s” sound.

Type of Hearing Loss
The type of hearing loss depends on what part of the ear is affected.

  • Conductive Hearing Loss- A conductive hearing loss occurs when the problem is located in the outer or middle ear. With conductive hearing loss, sounds can be heard at softer levels when listening through the bone vibrator than through the earphone. This type of hearing loss is more likely to respond to medical or surgical treatment.
  • Sensorineural Hearing Loss- A sensorineural hearing loss occurs when the problem is in the inner ear. With sensorineural hearing loss, sounds will be heard at similar levels through both the bone vibrator and the earphone. This kind of hearing loss is usually permanent.
  • Mixed Hearing Loss – A mixed hearing loss occurs when there are problems involving both the inner ear and the outer or middle ear.

Sources:
Boys Town National Research Hospital
Mayo Clinic