Audiometry

The Audiometry serves to examine and measure functional parameters of the hearing organ and to differentiate between sound conduction and sound sensation disorders. The multitude of methods used covers a wide spectrum from simple tuning fork tests to complex subjective and objective tone and speech audiometric methods. The electrical brain stem audiometry for the objective measurement of sound sensations is also counted among the objective methods.

What is Audiometry?

Audiometry is primarily used to identify and measure hearing impairments. Since hearing disorders can have multiple causes, it is not only sufficient to determine and measure the hearing impairment in simple hearing parameters such as frequency response and sound pressure, but rather the causes must be found out as far as possible in the sense of a targeted therapy.

Hearing impairments can either be due to problems with the external auditory canal or eardrum, or there are sound conduction problems in the middle ear, or there are disturbances to sound perception due to weaknesses in the conversion of mechanical sound waves into electrical impulses in the cochlea.

The same symptoms of sensorineural disorders can also be caused by lesions or diseases of the auditory nerve (vestibulocochlear nerve) or by problems with the processing of nerve impulses in the central nervous system (CNS). Therefore there are a number of methods and technical aids with which the hearing problems can be narrowed down to sound conduction or hearing perception problems.

In the case of a diagnosed sensorineural hearing loss, so-called recruit measurements can be used to determine whether the problems are in the inner ear, the auditory nerve or in the processing centers in the CNS. In recruitment audiometry, the reactions of the sensory cells in the cochlea to loud and soft sounds are measured. Quiet sounds are usually amplified by their own emissions and loud sounds are attenuated to protect the hearing.

Function, effect & goals

Audiometric methods are mainly used when there is a suspicion of impaired hearing. In special cases, an audiogram also serves as proof of a minimum hearing ability such as B. with pilots during their medical proficiency test. Tuning fork tests, each named after their inventor, such as the Weber, Rinne or Bing test, are relatively simple procedures. Most tuning fork tests are based on the subjective comparison between air and bone conduction of sound.

In the experiments, the tuning fork is placed either with the base on the skull or on the bone process behind the auricle, or alternately the vibrating fork tip is held in front of the auricle.Depending on the subjective hearing sensation, differences in hearing between the left and right ear can be identified and whether there is a sound conduction problem with restricted function of the ossicles in the middle ear. In principle, this is the case when the tuning fork is perceived better via bone conduction than via airborne noise.

Another subjective form of audiometry that is frequently used is tone audiometry, in which the sound pressure of the individual hearing threshold is recorded as a function of the frequency in a diagram for the left and right ears. The hearing thresholds for airborne sound and bone sound are measured. If the curves for the bone conduction show lower values ​​(sound pressures), i.e. better hearing, there is a sound conduction problem in the middle ear.

In addition to hearing range tests (whispered language) and examinations of the discomfort threshold, Langenbeck's noise audiometry offers options for localizing problems with a sound sensation disorder. The procedure is comparable to tone audiometry, but the pure tones for determining the hearing threshold are underlaid by noise of varying intensity. A relatively simple, objective measurement method is tympanometry, which measures the elasticity and reactivity of the eardrum.

Small pressure fluctuations are generated in the external auditory canal, the reaction of the eardrum is measured and conclusions can be drawn about the acoustic resistance. The measurement method requires an intact eardrum. The examination of the stapedius reflex is usually also included. The stapedius reflex is triggered by a loud popping noise to protect the hearing ability. When the reflex is activated by a loud bang, a tiny muscle on the stapes contracts and tilts the stirrup plate, so that the noise is only processed further in a reduced amplitude (attenuated).

Measurements of otoacoustic emissions and brain stem audiometry come into consideration especially for speech development disorders and for patients after strokes that have affected hearing. Otoacoustic emissions arise in the sensory cells of the cochlea as a reaction to soft tones, which are practically amplified, and to very loud tones, which are attenuated when translated into electrical nerve signals.

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Risks, side effects & dangers

With one exception, audiometric examinations are always non-invasive. Medicines or other chemical substances are also not involved. In this respect, audiometric examinations can be classified as free of side effects and as safe. Theoretically, there is a negligible risk of injury if the tuning fork is handled improperly during the tuning fork test.

An equally negligible technical risk exists with audiometers if the sound from headphones suddenly reached a level that could damage the hearing. The greatest danger in provoking and measuring otoacoustic emissions and in measuring brain stem activity is possible misdiagnosis, which can occur especially in the screening of newborns. A misdiagnosis - if it is not unmasked as such by further clarification - can unnecessarily stress the parents concerned and possibly trigger unnecessary therapy for the infant or toddler.

The only procedure that can be described as invasive is electrocochleography, which measures the currents generated by the sensory cells in the cochlea just a few milliseconds after receiving a sound as amplification. The procedure is particularly accurate if the electrodes are not attached from the outside, but rather placed directly in the inner ear in the form of electrode needles through the eardrum, i.e. invasive in this case.