Acoustic reflex

With acoustic reflex is called a reaction mechanism ( reflex) of the ear, which protects the inner ear from damage due to loud sound pressure level.

In the middle ear there are two small muscles:

• The tensor tympani muscle inserts on the hammer and tightens the eardrum.
• The stapedius muscle inserts on the stapes and tilted the stapes in the oval window.

On Stapediusreflex only the stapedius muscle is involved, he contracted as a result of the reflex, which is triggered by loud sound. Thereby, the coupling of the tympanic membrane to the inner ear is deteriorated, there is no longer the entire sound pressure transmitted to the inner ear, but a part is reflected at the tympanic membrane. This hearing can protect themselves to a certain extent from damage due to excessive sound levels. The afferent limb of the reflex is from the auditory nerve ( cochlear nerve ), the efferent nerve from the stapedius formed.

The acoustic reflex is at sound levels of 70 to 95 dB ( stapedius reflex threshold ) and is about 50 ms after application of the sound effect. The acoustic reflex acts on both ears even if only one ear is sonicated.

Stapediusreflexschwellen measurement

By an impedance measurement of the external auditory canal can be observed the use of stapedius reflex and the use for diagnostic purposes.

To this end, the probe of a measuring instrument ( Tympanometers ) is used airtight in the outer ear canal. The reflex is elicited by tones and adjustable volume can be measured at different frequencies. Due to the acoustic reflex, the acoustic resistance ( acoustic impedance ) of the ossicular chain, including tympanic membrane is increased. Characterized more sound is reflected from the eardrum, which can be measured by a microphone in the instrument. The collected information is output as impedance or compliance of the tympanic membrane and displayed graphically.

In conductive hearing loss, the reflex threshold is increased depending on the degree of hearing loss, or reflex not triggered. In a sensorineural hearing loss may be normal or even be reduced as a result of Recruitments the reflex threshold. This is called after the discoverer and describer Metz recruitment. In a neural hearing loss ( damage to the auditory nerve ) reflex is usually undetectable or the reflex threshold is increased. Typical of a hearing loss is also the rapid fatigability of the reflex ( reflex decay ).

In paralysis of the facial nerve ( facial nerve ), the Stapediusreflexschwellen measurement for topical diagnosis, are so used to localize the injury. If the damage in the peripheral part of the facial nerve after the departure of the stapedius nerve, the reflex is detectable despite the paralysis, with a damage centrally from the disposal of the stapedius nerve lacks the acoustic reflex. Even with diseases in the region of the hindbrain, where the facial nerve arises, the acoustic reflex may be absent.

After surgery for otosclerosis ( stapes surgery ), in which the tendon of the stapedius muscle is severed, no more acoustic reflex is also detectable. Under direct vision, however, can be seen in PA a movement of the tendon. This can be demonstrated under certain circumstances even with a large tympanic membrane ( for example, a chronic Schleimhauteiterung ).

The Stapediusreflexschwellen measurement does not require active cooperation of the patient, therefore, this method can also be applied to small children. Even in cases in which suspects that the patient only simulates a hearing, this method may provide a clue.