声导抗知识剖析.ppt

* * When the probe tone is sent into the ear, some will be reflected back out, and some will be admitted by the middle ear system depending on how well the middle ear functions. Each moment any element of the middle ear is reached by the probe tone, some energy will be admitted by the element, and some will be reflected. We measure how much of the probe tone that is reflected and returns to the probe microphone. Consequently, the rest of the probe tone energy tells us the admittance of the ear. Immittance measurements include Tympanometry and Acoustic reflex tests In immittance measurements, we alter the middle ear admittance in two different ways: Tympanometry: we apply air pressure from the outside to see if the admittance is what is expected with the varying pressure, or if the admittance varies according to any typical pathological pattern. Acoustic reflex testing: we stimulate with loud sound, to activate the stapedius muscle reflex to see if the muscle and related nerves work, and if the middle ear mechanism is mobile enough to show any difference in admittance. * * * * Classifying different tympanogram curves comes in handy when e.g. describing the tympanogram in a patient’s journal. “Type” is a traditional way of referring to a tympanogram. Different curve shapes are indicative of different middle ear conditions. The classification is carried out in relation to a normal range displayed as a rectangle in the tympanometry graph. Type A is a normal tympanogram with a single SA peak within the rectangle. Type Ad is a high admittance tympanogram reaching above the rectangle. Type As is a low admittance tympanogram with the peak below the rectangle. Type B is a flat tympanogram with no discernible peak. If ECV is abnormally high, there is a perforation. If ECV is low, there is effusion. The latter can also occur if the probe is blocked. Type C is a tympanogram with the SA peak appearing to the left of the rectangle, thus displaying a negative TPP (more neg