Suzanne de Bruijn

25 General introduction Clinical characteristics of hearing loss HL can be broadly categorized into three different types: conductive, sensorineural and mixed. 38 Conductive HL results from abnormalities in the outer or middle ear, such as obstructions or damaged structures due to trauma. In this type of HL, the efficiency of sound transmission to the inner ear is lowered. In case of sensorineural HL, the problem resides within inner ear structures or the auditory nerve, which impairs the conversion of sound to a signal that is detected in the auditory cortex of the brain. 25 In mixed HL, there is a combination of conductive and sensorineural HL. 38 Additionally, there are several other clinical characteristics that can be used to classify and describe the type of HL including age of onset, the sound frequencies for which thresholds are elevated, severity and (a)symmetry, which are summarized in Table 1 . Clinical evaluation Different tests can be performed to determine the hearing status of an individual. These assessments can be divided into physical, electrophysiological and audiometric tests. Ideally, a combination is used to allow comparison and validation of the results. 4 Often, first a general physical otoscopic examination will be performed to evaluate the condition of the ear canal, the tympanic membrane and the middle ear. In some instances, additionally a basic balance test (e.g. head-impulse test), and CT or MRI will be performed. Electrophysiological tests provide an objective measure of the functional status of an individual’s auditory system. They include measurements of otoacoustic emissions (sounds generated by the outer hair cells) and auditory brain stem responses. 4,38 Audiometric tests, on the other hand, provide a more subjective measure of the hearing status of an individual. The most widely applied test is pure-tune audiometry (PTA), which determines the sensitivity of the auditory system for different sound frequencies. Pure-tone thresholds indicate the lowest sound intensity that can be perceived by an individual when seated in a sound attenuating booth. 4,38 Both air conduction audiometry (sound is presented via earphones), and bone conduction audiometry (sound is presented through a vibrating device placed on the skull) which bypasses the outer andmiddle ear, can be performed. This allows the distinction between conductive and sensorineural HL. 38 After completion of a PTA, hearing sensitivity can be plotted on an audiogram which displays sound intensity (decibel hearing level (dB HL)) as a function of sound frequency (hertz (Hz)). Besides PTA, also word-recognition testing (speech audiometry test) can be performed. 37,38