Audiometry

Audiometry at Chandan Hospital is a branch of audiology and the science of measuring hearing acuity for variations in sound intensity and pitch and for tonal purity, involving thresholds and differing frequencies. Typically, audiometric tests determine a subject's hearing levels with the help of an audiometer, but may also measure ability to discriminate between different sound intensities, recognize pitch, or distinguish speech from background noise. Acoustic reflex and otoacoustic emissions may also be measured. Results of audiometric tests are used to diagnose hearing loss or diseases of the ear, and often make use of an audiogram.

Auditory system

Components:

The auditory system is composed of epithelial, osseous, vascular, neural and neocortical tissues. The anatomical divisions are external ear canal and tympanic membrane, middle ear, inner ear, VIII auditory nerve, and central auditory processing portions of the neocortex.

Hearing process:

Sound waves enter the outer ear and travel through the external auditory canal until they reach the tympanic membrane, causing the membrane and the attached chain of auditory ossicles to vibrate. The motion of the stapes against the oval window sets up waves in the fluids of the cochlea, causing the basilar membrane to vibrate. This stimulates the sensory cells of the organ of Corti, atop the basilar membrane, to send nerve impulses to the central auditory processing areas of the brain, the auditory cortex, where sound is perceived and interpreted.

Audiograms:

An audiogram showing typical slight hearing variation.The result of most audiometry is an audiogram plotting some measured dimension of hearing, either graphically or tabularly.

The most common type of audiogram is the result of a pure tone audiometry hearing test which plots frequency versus amplitude sensitivity thresholds for each ear along with bone conduction thresholds at 8 standard frequencies from 250 Hz to 8000 Hz. A PTA hearing test is the gold standard for evaluation of hearing loss/disability. Other types of hearing tests also generate graphs or tables of results that may be loosely called 'audiograms', but the term is universally used to refer to the result of a PTA hearing test.

Hearing assessment:

Apart from testing hearing, part of the function of audiometry is in assessing or evaluating hearing from the test results. The most commonly used assessment of hearing is the determination of the threshold of audibility, i.e. the level of sound required to be just audible. This level can vary for an individual over a range of up to 5 decibels from day to day and from determination to determination, but it provides an additional and useful tool in monitoring the potential ill effects of exposure to noise. Before carrying out a hearing test, it is important to obtain information about the person's past medical history, not only concerning the ears but also other conditions which may have a bearing on possible hearing loss detected by an audiometric test. Hearing loss may be unilateral or bilateral, and bilateral hearing loss may not be symmetrical. The most common types of hearing loss, due to age and noise exposure, are usually bilateral and symmetrical. Wax in the ear can also cause hearing loss, so the ear should be examined to see if syringing is needed; also to determine if the eardrum has suffered any damage which may reduce the ability of sound to be transmitted to the middle ear.

Hearing loss classification:

The primary focus of audiometry is assessment of hearing status and hearing loss, including extent, type and configuration.

  • There are four defined degrees of hearing loss: mild, moderate, severe and profound.
  • Hearing loss may be divided into four types: conductive hearing loss, sensorineural hearing loss, central auditory processing disorders, and mixed types.
  • Hearing loss may be unilateral or bilateal, of sudden onset or progressive, and temporary or permanent.

Hearing loss may be caused by a number of factors including heredity, congenital conditions, age-related (presbycusis) and acquired factors like noise-induced hearing loss, ototoxic chemicals and drugs, infections, and physical trauma.

Audiograms and diagnosing types of hearing loss:

Most commonly, "conventional" audiometry (utilizing audiograms up to 8 kHz) is used to measure hearing status. For research purposes, or early diagnosis of age-related hearing loss, ultra-high frequency audiograms (up to 20 kHz), requiring special audiometer calibration and headphones, can be measured.

Ideally the audiogram would show a straight line, but in practice everyone is slightly different, and small variations are considered normal. Larger variations, especially below the norm, may indicate hearing impairment which occurs to some extent with increasing age, but may be exacerbated by prolonged exposure to fairly high noise levels such as by living close to an airport or busy road, work related exposure to high noise, or brief exposure to very high sound levels such as gunshot or music in either a loud band or clubs and pubs. Hearing impairment may also be the result of certain diseases such as CMV or Ménière's disease and these can be diagnosed from the shape of the audiogram.

Otosclerosis results in an audiogram with significant loss at all frequencies, often of around 40 dB(HL). A deficiency particularly around 2 kHz (termed a Carhart notch in the audiogram) is characteristic of either otosclerosis or a congenital ossicular anomaly.

Meniere's disease results in a severe loss at low frequencies.

Noise induced deafness or sensorineural loss results in loss at high frequencies, especially around 4 kHz and above, depending on the nature of the exposure to loud noise.

Typical examples of audiograms showing conductive, noise induced and age-related hearing loss can be found here.