Tympanometry

When Tympanometry is an objective measurement method of Audiology. It belongs to the Impedanzaudiometrie. Of the impedance is to be understood that the acoustic system as a whole, such as the middle ear of the receiving sound waves is opposite in this case, the degree of resistance. An acoustic system with a high impedance absorbs little sound energy and reflects a large proportion. A system of low impedance, however, takes up a lot of sound energy in the form of vibrations. The degree of sound absorption is described by the term "compliance" and refers to the compliance or stiffness of the eardrum. The aim is to be measured by the measurement of the acoustic impedance of the tympanic membrane by indirect means the pressure in the middle ear, the middle ear demonstrate pathological content or to assess the condition of the ossicular chain. The measurement method is based on the work of Schuster ( 1934), Metz ( 1946), Zwislocki (1957) and Terkildsen and Nielsen ( 1960). Often the acoustic reflex with the measuring system is also determined.

Principle of measurement

When tympanometry in the ear canal pressure fluctuation is generated. In this case, a negative pressure followed by a slight excess pressure. These pressure changes are reflected from the eardrum and then measured by means of a tight -fitting probe. Tympanometry is a practical, standardized diagnostic procedure for examination of the eardrum and middle ear. The measured parameters in the tympanometry is the resilience or stiffness of the tympanic membrane ( eardrum so-called compliance), the reciprocal of the impedance.

Operation

When a sound of the tympanic membrane with a defined amount of sound energy, a portion of this energy is reflected and the remaining portion transmitted to the middle ear. The amount of reflected energy depends on the acoustic resistance of the acoustic impedance of the tympanic membrane, from. Under natural pressure conditions, when there is the same atmospheric pressure before and behind the tympanic membrane, the transmission of the sound is most related to the physical properties of the tympanic membrane.

Prerequisite for the implementation of this test is an intact tympanic membrane and an air -tight closure of the ear canal by the probe.

To carry out the Tympanometry is a separate device, a tympanometer required. The measurement is performed with a probe that has three holes with small tubes. About the first hole is a tone generator, a continuous tone ( " probe tone " ) at a frequency of 220 Hz usually given (95 dB SPL). The second hole includes a microphone that is connected to a measuring instrument. Defined pressures are built up in the ear canal using a pressure pump on the third hole. Under normal conditions, ie when the pressure conditions in the ear canal and middle ear are the same and the eardrum in " normal position " is, there is a certain acoustic resistance from Tympanometers as a reference point (zero point) is used. If now an over-or generated by the pressure pump vacuum, the eardrum is stretched and it's changing the acoustic resistance of the eardrum. Thus, the amount of reflected sound energy of the probe tone is increased, the sound pressure level in the ear canal increases, which can be determined by measuring the connected microphone. The altered reflections are shown graphically on the tympanogram as compliance of the tympanic membrane (elasticity, the inverse of the acoustic resistance ).

Clinical Application

Clinical applications include the following diseases or problems:

• Otitis
• Perforation of the tympanic membrane
• Dysfunction of the auditory ossicles
• Malfunction of the Eustachian tube
• Otosclerosis
• Tympanosclerosis
• Cholesteatoma

Clinical Evaluation

Depending on which disease is present, the normal curve in the tympanogram, which can be used for disease detection changed. In an inconspicuous middle ear function, the maximum of the tympanic membrane mobility is about 0 Pa, in other words, the eardrum can have a maximum swing out, as prevail in the outer ear canal, the same pressure as in the tympanic cavity. The measured curve is called the A- curve, see Figure I. If, in the tympanic cavity to a negative pressure, which may be the consequence of a tube ventilation disorder that occurs during the measurement of such patients to a shift of the maximum in the negative pressure range, see Figure II The measured curve is referred to as a C- curve.

A middle ear effusion, caused for example by an acute otitis media, acute otitis media results in measurement typically a flat tympanogram, see Figure III. This is due to fluid accumulation in the middle ear. The measured curve is referred to as a B- curve. Interrupting the sound transmission chain, such as a tympanic membrane Gehörknöchelchenluxation but also atrophic scars, lead to an open-ended curve, not shown.

In a tubal catarrh, for example, the eardrum is due to illness pulled inwardly retracted. If now the pressure pump, a negative pressure is generated in the ear canal, the tympanic membrane is in the normal position and the normal compliance is achieved. The measured values ​​are recorded in the Tympanogrammformular and show a shift in the negative pressure region.

II negative pressure in the middle ear (eg tubal catarrh ), maximum in the negative pressure range

III. Fluid in the middle ear (eg effusion )

The vertical or y-axis of the tympanogram represents information about the compliance of the middle ear space, more precisely the eardrum mobility in ml or in relative volume units. The horizontal or x-axis shows the corresponding pressure change in Pa or Deka Pascal (1 daPa = 10 Pascal = 0.1 mbar ) to, by some device manufacturers, the pressure data are also plotted in mm H2O.