AAO-HNSF Primary Care Otolaryngology Handbook

HEARING LOSS

can barely hear 50 percent of the time. Normal hearing levels span the 0–20-dB range. The higher the hearing threshold, the poorer the patient’s hearing. Thresholds higher than 25 dB are considered abnormal. During the audiogram, independent thresholds are determined for each ear for both air conduction (conductive hearing) and bone conduction (sensorineural hearing) . Air conduction measures the ability of the external and middle ear to transmit sound to the cochlea. Conductive hearing loss can result from any barrier that could block sound transmis- sion in this pathway (cerumen, drum perforation, middle ear fluid). This will create an air-bone gap between the air and bone conduction thresh-

olds on the audiogram (Figure 5.1). Sensorineural hearing loss can be diagnosed if the air conduction and bone conduction thresholds are equal but higher than 25 dB. Our ability to hear is more complex than just listening to single pure tones in a sound-proof booth. Therefore, a test of the patient’s ability to understand spoken words is performed as well. In a speech discrimination test, the patient is presented with phonetically balanced words (e.g., pool, raise, boat, love, sell) that are amplified to a comfortable hearing level as necessary. The results of this test, the speech discrimination score , should be between 90 and 100 percent in normal hearing individ-

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Figure 5.1. A conductive hearing loss in the left ear caused by otitis media with effusion. Note that bone conduction thresholds are normal in both ears, but air conduction on the left is 30 dB poorer than that measured on the right. Remember that 0 dB does not refer to absence of sound, but rather represents an average threshold for young healthy adults.

uals. This test of speech clarity also assesses the function of the auditory division of the eighth cranial nerve. The ability to understand speech is important, especially when determining to what degree a hearing aid will help a particular patient. Amplifying speech perceived as abnormally garbled with a hearing aid has limited benefit for patients with poor speech discrimination. Tympanometry is commonly used to evaluate the TM and Eustachian tube. This test assesses the mobility of the TM and its response to pres- sure changes in the EAC. Three common tympanogram patterns are shown in Figure 5.2. Type A tympanogram indicates the EAC is patent,

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