Disclosure
Purpose The usual primary purpose of pure-tone tests is to determine the type, degree, and configuration of hearing loss. Pure-tone audiometry Pure-tone audiometry is a behavioral test measure used to determine hearing sensitivity. This measure involves the peripheral and central auditory systems. Pure-tone thresholds (PTTs) indicate the softest sound audible to an individual at least 50% of the time. Hearing sensitivity is plotted on an audiogram, which is a graph displaying intensity as a function of frequency. Audiogram The audiogram is a chart of hearing sensitivity with frequency charted on the abscissa and intensity on the ordinate (see Images 1-6). Intensity is the level of sound power measured in decibels; loudness is the perceptual correlate of intensity. For threshold testing intensity, decibels are measured in hearing level (HL), which is based on the standardized average of individuals with normal hearing sensitivity. HL is not equivalent to sound pressure level (SPL), but the American National Standards Institute (ANSI) has defined a relationship between SPL and HL for each audiometric frequency from 250-8000 Hz. Frequency Frequency is cycles per unit of time. Pitch is the perceptual correlate of frequency. Frequency is measured in hertz, which are cycles per second. Usually frequencies of 250-8000 Hz are used in testing because this range represents most of the speech spectrum, although the human ear can detect frequencies from 20-20,000 Hz. Some children can detect even higher frequencies. For excellent patient education resources, visit eMedicine's Ear, Nose, and Throat Center. Also, see eMedicine's patient education article Tinnitus.
Air conduction This test assesses sensitivity when the signal is transmitted through the outer, middle, and inner ear and then through the brain to the cortex. Testing may be performed using headphones, insert earphones, or sound fields. Headphones are placed over the outer ear. Circumaural headphones have a large cushion and fit around the ear, contacting the head. These generally are used to reduce ambient noise. Supra-aural headphones are more common and rest on the ear or pinna, but they typically provide no ambient noise reduction and may collapse the ear canals. Insert earphones are transducers housed in a small box approximately 2" by 3" by 0.5". The signal is transmitted down a tube to foam tips, which fit in the ear canal. Insert earphones help reduce collapsing ear canals, and they reduce some ambient noise and crossover of auditory stimuli to the nontest ear via skull transmission. Sound-field (free-field) testing signals are presented via speakers, usually at a 45° azimuth to the patient's face. This form of testing is used with infants, toddlers, and other individuals with special needs for whom earphone use may be problematic. During sound-field testing, an individual sits in the center of the room, facing forward, halfway between each speaker. Typically, visual-reinforcement audiometry (toys light and animate when the child responds to sound); conditioned-orientation response audiometry (toys on both sides test localization); or play audiometry (various games, eg, dropping a block in response to sound) are used. These conditioned responses to auditory stimulus provide reinforcement that allows for measurable responses and longer interest in the test situation. In a sound field, the auditory signals are warble tones or bursts of narrow-band noise. Pure tones cannot be used because they can create standing waves in a sound field, which can alter signal intensity. Sound-field testing also may assess hearing aid benefit. Placing the person in the center of the room (facing the speakers) yields aided thresholds. The difference between aided and unaided thresholds is termed functional gain. Crossover Crossover occurs when sound presented to the test ear travels across the head to the nontest ear. This occurs at approximately 40 dB for circumaural earphones across all frequencies. When hearing sensitivity is much poorer in the test ear than the nontest ear, the signal may cross over and be perceived in the ear with better hearing, thus yielding a false impression of the intended test ear's sensitivity. Insert earphones reduce the crossover by reducing surface contact area. Bone conduction This technique assesses sensitivity when the signal is transmitted through the bones of the skull to the cochlea and then through the auditory pathways of the brain. This type of testing bypasses the outer and middle ear. Bone-conduction oscillator A small oscillator is placed on the forehead or mastoid. The device stimulates the bones of the skull, which in turn stimulates both cochleae. The oscillator may produce a vibration that the patient may feel, thus eliciting a vibrotactile response instead of a response to the auditory stimulus. Masking Masking presents a constant noise to the nontest ear to prevent crossover from the test ear. The purpose of masking is to prevent the nontest ear from detecting the signal (line busy), so only the test ear can respond. Thresholds obtained using masking are called masked thresholds and should represent the true threshold of the test ear. A masking dilemma occurs when sufficient masking from the nontest ear crosses over to the test ear and affects threshold testing for the test ear. In this case, a reliable masked threshold cannot be obtained. This generally occurs only in the presence of a substantial conductive component to the hearing loss. Pure-tone average Pure-tone average (PTA) is usually the average of hearing sensitivity at 500, 1000, and 2000 Hz. This average nearly should match the speech reception threshold (SRT), within 5 dB, and the speech detection threshold (SDT), within 6-8 dB. Speech reception threshold The SRT is the softest intensity spondee word that an individual may repeat at least 50% of the time. Spondees are bisyllabic words equally emphasizing both syllables. In some cases (eg, patients with poor word recognition), a limited set of words may be used. Speech detection threshold The SDT, also termed the speech awareness threshold (SAT), is the lowest intensity speech stimulus that an individual can detect at least 50% of the time. Word recognition Word recognition (formerly called speech discrimination) is the ability to repeat correctly an open set of monosyllabic words at suprathreshold intensity. Word lists are phonetically balanced (PB), meaning that the speech sounds used occur with the same frequency as in the whole language.
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The 3 types of hearing loss can be differentiated as follows:
Conductive This type of hearing loss is secondary to outer ear or middle ear abnormality, which can include abnormalities of the tympanic membrane. The abnormality reduces the effective intensity of the air-conducted signal reaching the cochlea, but it does not affect the bone-conducted signal that does not pass through the outer or middle ear. Examples of abnormalities include perforated tympanic membranes, fluid in the middle ear system, or scarring of the tympanic membrane. Pure-tone air-conduction thresholds are poorer than bone-conduction thresholds by more than 10 dB (see Image 1). Sensorineural This type of hearing loss is secondary to cochlear abnormality and/or abnormality of the auditory nerve or central auditory pathways. Because the outer ear and middle ear do not reduce the signal intensity of the air-conducted signal, both air- and bone-conducted signals are effective in stimulating the cochlea. Pure-tone air- and bone-conduction thresholds are within 10 dB (see Image 2). Mixed This type of hearing loss has sensorineural and conductive components. Pure-tone air-conduction thresholds are poorer than bone-conduction thresholds by more than 10 dB, and bone-conduction thresholds are less than 25 dB (see Image 3). Common audiogram/audiologic assessment abbreviations
Presbyacusis This condition usually manifests as a bilateral and symmetric sensorineural hearing loss. Word recognition may be poorer than predicted from the audiogram. A person with age-related hearing loss may have more difficulty with hearing aids than a younger patient with equivalent hearing loss. Onset of presbyacusis typically occurs in middle-aged or older patients. Hearing loss is secondary to damage in the cochlea, cranial nerve VIII, and/or the central auditory system. The condition usually is progressive. Otitis media This condition is marked by fluid in the middle ear space, which may be caused by inflammation of the middle ear lining or inadequate aeration of the middle ear space. Otitis media frequently results in flat or up-sloping conductive hearing loss. Word recognition usually is excellent when speech is sufficiently loud. Onset may be slow or acute. With acute onset, otalgia or fever may accompany the condition. Without medical intervention, otitis media can lead to mastoiditis, facial paralysis, meningitis, labyrinthitis, or brain abscess. Onset can occur at any age, but otitis media is most common in young children. Without intervention, the fluid can thicken, and greater hearing loss develops as the middle ear mechanism stiffens. Noise-induced hearing loss Exposure to high-intensity noise may cause temporary or permanent hearing loss. Repeated exposure to noise trauma may change a temporary threshold shift (TTS) to a permanent threshold shift (PTS). Degree and configuration of hearing loss depends on time exposure, sound intensity, and sound frequency characteristics. Noise-induced hearing loss is typically greatest in the 4000- to 6000-Hz region. With more intense sound, the exposure time before hearing loss may result is shorter (Occupational Safety and Health Administration [OSHA] standard 29 CFR 1910.95 addresses exposure limits). Noise-induced hearing loss is sensorineural except in certain blast injuries with possible tympanic membrane and middle ear damage. Onset may occur at any age, and intersubject variability is high even for the same exposure. Hearing loss may be unilateral or bilateral but is usually bilateral. Asymmetric noise exposure (eg, from firearms) may yield asymmetric hearing loss. Degree of hearing loss can vary. Complaints of muffled hearing or tinnitus are common. Hearing loss is secondary to cochlear damage (see Image 4). Otosclerosis Otosclerosis develops over time, causing a progressive conductive hearing loss that is generally flat. Word recognition is usually excellent when speech is sufficiently loud. The condition is caused by stapedial fixation in the oval window, stiffening the middle ear system. Onset usually occurs when patients are aged 15-45 years, and otosclerosis is more common in women than in men. One half of patients report family history of otosclerosis. Approximately one half of patients report tinnitus (see Image 5). Ménière disease Ménière disease affects the cochlear and vestibular systems. Attacks lasting from 20 minutes to 24 hours generally include some combination of vertigo, hearing loss, sensation of aural fullness, and tinnitus. Tinnitus and hearing loss may persist between attacks. Hearing loss is usually unilateral, at least in the early stages, and fluctuant, but it typically develops into a permanent sensorineural hearing loss. Many patients report increased sensitivity to loud noises (recruitment) in addition to the listed symptoms. Word recognition is frequently poorer than predicted from pure-tone audiogram. Onset for approximately one half of patients occurs when aged 40-60 years. The disease is rare in children (see Image 6).
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