Impedance

Alternative names

Acoustic impedance, impedance test, tympanometry.

Definition

A sound is a set of vibratory waves. The acoustic waves pass through the outer and middle ear to the inner ear. Among the structures that participate in the transmission of sound we find at the level of the middle ear the tympanic membrane, the Eustachian tube and the ossicular chain: hammer, stapes and anvil. In a healthy patient, without hearing problems, these structures articulate with each other without presenting stiffness or friction between them.

Impedance is a physical term that designates the set of factors that oppose a resistance. Therefore, impedance measurement is an otorhinolaryngological examination whose objective is to measure the impedances or resistances of the auditory phenomenon, specifically of the middle ear. It is an objective examination, so unlike audiometry, the active participation of the patient is not required. The measurement of hearing resistances is carried out using a specially designed instrument. It will allow obtaining information about the pressure of the middle ear, the integrity and mobility of the tympanic membrane and the continuity of the ossicular chain. We speak of maximum impedance when the mobility of the ossicular complex and the membrane is minimal and is therefore altered.

The complete impedance study includes the study of the stapedial reflex and tympanometry. The stapedial reflex is a protection of the inner ear against loud noises. It consists of a contraction of the stapes muscle when we stimulate the ear with a loud sound, leading to a decrease in the mobility of the ossicular chain, therefore, an increase in impedance. Tympanometry is the specific study of the mobility of the tympanic membrane. The data obtained during the examination are represented by curves that reflect the mobility of the eardrum in relation to pressure variations.

How is the study done?

A probe is inserted into the external ear canal, which in turn seals it, turning it into an airtight cavity. The interior of the probe is made up of three conduits that run through it from side to side:

• One of these conduits is connected to a pressure pump that allows the pressure in the airtight cavity created to be changed and calibrated.
• The second conduit is connected to a loudspeaker that emits a tone, generally of low frequency, and at a constant intensity. This tone will be absorbed by the tympanic membrane and by the chain of ossicles of the inner ear. In a normal ear, under basal conditions, when the pressure inside the artificial cavity is equal to that of the tympanic cavity, the mobility of the ossicular tympanic complex is maximum, that is, the impedance is minimum, and the amount of the test tone reflected is minimal.
• The third conduit is connected to a microphone that receives the reflected test tone and transforms it into an electrical signal that can be quantified by an instrument called a voltmeter.

Preparation for the study

The practice of acoustic impedance is usually preceded by other studies that complement the assessment of hearing deficits, such as otoscopy and audiometry. Before the exam, the doctor will look inside the ear canal to make sure there is a clear pathway to the eardrum. This examination does not require any specific preparation.

What does it feel like during and after the study?

It is a painless examination. The sensation of occupation in the ear is common, as well as the perception of the tones emitted by the impedance meter. The sounds can be loud and potentially alarming. Exceptionally, in susceptible or elderly patients, vertiginous crises that resolve rapidly can be induced. It is necessary that during the examination the patient does not speak, move or swallow, since these movements could produce pressure changes in the middle ear and alter the results.

Risks and contraindications

There are no risks. No contraindications Reasons why the study is carried out: it is a useful exploration to complete the study of hearing deficits. It is of special interest in pediatrics, since, unlike audiometry, it does not require the active collaboration of the patient. The study can reveal the following conditions: tumor in the middle ear, otitis media, alteration of contact between the chain of bones, otosclerosis, perforated eardrum, scarring of the tympanic membrane and conditions of the Eustachian tube.