Continuing Education Activity

Hearing loss is a common condition that affects millions of people regardless of age, ethnicity, and health status. Appropriate treatment for hearing loss depends greatly on the underlying pathophysiology and the severity of the condition. Pure-tone audiometry is a critical means of obtaining the necessary diagnostic information to guide healthcare providers toward appropriate treatment. This activity reviews the details of pure-tone audiometry evaluation, including clinical appropriateness and utility, and reviews the role of the interprofessional team in the evaluation and management of patients with otologic complaints.

Objectives:

• Explain the difference between air conduction and bone conduction pure-tone evaluation.
• Describe the clinical utility of pure-tone audiometric threshold assessment.
• Identify when a clinically significant asymmetry of pure-tone thresholds is present.
• Review when pure-tone audiometry evaluation is or is not an appropriate method to assess hearing status.

Introduction

Hearing loss is a widespread chronic condition that affects over 25 million people in the United States aged 12 years old or older and may affect 1 in 5 children, temporarily or permanently, by the age of 18.[1][2] Despite the high prevalence of hearing loss, many adults do not receive appropriate or timely evaluation and treatment for their hearing concerns.[3] Appropriate treatment for hearing loss depends greatly on the pathophysiology and the severity of the condition, and a detailed hearing assessment provides the healthcare team the necessary diagnostic information to guide treatment.

While Weber and Rinne tuning fork tests are useful for clinical screening and identifying the type of hearing loss, they may miss any nuances such as mild hearing loss, bilateral hearing loss, or mixed conductive and sensorineural hearing loss.[4][5] Pure-tone (isolated frequency) audiometry evaluation over the range of frequencies important for everyday listening can determine the degree, configuration, and type of hearing loss in a manner detailed enough to assist the healthcare team in determining the etiology and prognosis for the hearing loss as well as the optimal treatment strategy.[6]

Anatomy and Physiology

Pure-tone testing assesses the auditory system through two pathways. In air conduction (AC) testing, sound waves enter the external auditory canal and pass through the tympanic membrane, ossicular chain, and cochlea to reach the cochlear nerve (part of cranial nerve VIII), and then travel through the brainstem to the auditory cortex. In bone conduction (BC) testing, sound waves are introduced directly to the cochlea through the vibration of a bone conduction oscillator on the mastoid process.[7]

By utilizing both methods of pure-tone testing, hearing loss is determined to be conductive (involving the pinna through the ossicular chain), sensorineural (involving the cochlea to the auditory cortex), or mixed (both conductive and sensorineural involvement).[8] For some patients, a false "conductive" component may appear in the presence of "third-window" pathologies like semicircular canal dehiscence because of changes in sound transmission from the dehiscence.[9] Third window phenomena are referred to as such because they function as third connections between the inner and middle ears when counted along with the oval and round windows.

Indications

Pure-tone evaluation should be performed for any patient concerned about abnormal auditory perception, ear trauma, or otologic disease.[10] This includes, but is not limited to: subjective hearing loss, tinnitus, hyperacusis, ototoxic monitoring protocols, dizziness or vertigo, middle ear dysfunction, traumatic brain injury or temporal bone fracture, loud noise exposure, blast injury, failed hearing screening, speech delay in children, or conditions with risk factors for hearing loss.[11][12]

Contraindications

A reliable pure-tone evaluation cannot be completed if a patient cannot provide reliable behavioral responses.[13] This is particularly true of children under six months of age but is also seen in patients with developmental delays or neurological impairment. Behavioral pure-tone testing may also be contraindicated if unreliable responses are given by patients who are malingering or who have factitious disorders.[14] In these circumstances, objective electrophysiologic or otoacoustic emission testing instead of behavioral responses is necessary to build a complete clinical picture.[15]

Equipment

Pure-tone evaluation is typically performed in a sound-treated booth to reduce the impact of external sounds, with the booth environment and electroacoustic equipment calibrated to American National Standards Institute (ANSI) standards to optimize inter-test and intra-test reliability. Audiometers generate the sounds presented for testing, controlling the pitch, loudness, transducer type, and ear of presentation.[16]

Various transducers may be used depending on the auditory pathway being assessed and the anatomical limitations of the ears. Standard electroacoustic transducers for pure-tone audiometry are supra-aural headphones, insert earphones, circumaural headphones, and bone conduction oscillators.[17][18] Speakers aligned at 90 or 45 degrees from the testing seat are used for children who cannot tolerate headphones. In such a sound-field environment, the presented tones are warbled to avoid standing waves from a single pure-tone frequency.[19] Sound field testing evaluates the composite best hearing ear, which means that the threshold recorded will be the quietest sound audible by either ear at each frequency presented. A normal sound-field audiogram, therefore, does not indicate normal hearing across all tested frequencies in both ears, but rather that at each tested frequency, at least one of the ears could hear the tone normally. On the other hand, because recorded thresholds in sound-field audiometry indicate the acuity of the better-hearing ear, if a hearing loss is encountered, that elevated threshold represents the hearing loss in the better-hearing ear; there may or may not be worse hearing in the other ear. For this reason, individual ear testing to rule out unilateral hearing loss cannot be obtained with sound-field audiometry.

Regardless of the method of presenting the tones, patient responses are typically given with a button, a hand raise, or a verbal acknowledgment, with adaptations made for children unable to give a deliberate response with reliability.[20][21] These adaptation techniques are called visual reinforcement audiometry and conditioned play audiometry, both of which use structured play-like stimuli-and-response with toys to obtain more reliable information.[22]

Pure-tone screenings, such as in schools or inpatient hospital environments, are performed with calibrated portable audiometers that may test air and bone conduction or air conduction only, depending on the model.[23] Screenings assess for awareness of a minimum response level at a limited set of frequencies to determine if further diagnostic evaluation is warranted.[24] Standard pure-tone assessment with a portable audiometer using a precise transducer set-up and performed in a sufficiently quiet environment can obtain comparable threshold accuracy to sound booth assessments.[16]

Personnel

When pure-tone testing is indicated for evaluation of a specific complaint, the procedure is typically performed by an audiologist; however, screening audiometry (as is commonly used for school-aged children and military personnel) may be performed using automated testing equipment by an audiology technician.

Preparation

For diagnostic pure-tone testing, patients are seated in a sound booth and counseled regarding the method of testing and expected response. With young children, parents are instructed on how the behavioral testing will occur, typically with the patient seated in the parent's lap. A young child may receive basic instructions to assist in understanding and comfort with the test procedure. 

The audiologist will place appropriate transducers on the patient's head or ears before closing the sound booth door and relocating to an adjacent booth to control the audiometer. The audiologist can observe the patient through an insulated window and hear feedback from the patient via a microphone inside the booth.

Technique

Results are recorded on a graph known as the audiogram, which logarithmically plots sound frequency in cycles per second (Hertz or Hz) from low pitch to high pitch along the X-axis and loudness in decibels of hearing level (dB HL) from soft to loud along a reversed Y-axis.[25] Decibels of hearing level differ from decibels of sound pressure level because the use of hearing level reflects the fact that normal human ears hear better in the middle frequency ranges than at the higher and lower frequencies and thus normalizes the scale using an isophonic curve. For example, a 0 dB hearing level (normal hearing) at 500 Hz is equivalent to 13.5 dB sound pressure level, according to ANSI S3.6-1996. At 1000 Hz, 0 dB HL equals 7.5 dB SPL, and at 2000 Hz, 0 dB HL equals 9 dB SPL. Down at 125 Hz, however, 0 dB HL equals 45 dB SPL, and up at 8000 Hz, 0 dB HL is 15.5 dB SPL.

A standard audiogram graphs pitch in octaves and most inter-octaves from 250 Hz to 8000 Hz; the full testable range includes 125 Hz and ultrahigh frequencies of 9, 10, 11.2, 12.5, 14, 16, and 20 kHz.[20][26] A common method of summarizing standard pure-tone audiometric findings is to use the pure-tone average (PTA), which is the arithmetic mean of each ear's thresholds at 500, 1000, and 2000 Hz. The PTA is often reported in the research literature as an outcome measure. Still, it is important to remember that it does not reflect hearing in the lowest and highest frequencies, which may be affected by certain exposures, such as loud noises and ototoxins. Ultra-high frequencies are typically tested for ototoxic monitoring protocols or if the standard audiogram does not reflect the patient's reported auditory concerns.[27][28]

Pure-tone sounds are presented through the appropriate transducers to the better-hearing ear, if one exists, starting at 1000 Hz. Initially, the tone is presented at a level estimated to be easily audible to confirm the patient understands the task of perception and response. If no response occurs, the volume is increased in 20 dB increments until the patient does respond or reinstruction is deemed necessary.

Once the initial response is obtained, a threshold search is performed in steps by descending 10 dB in volume for every response to a presentation and ascending 5 dB for every presentation without a response.[29] A threshold is recorded as the softest ascending presentation a patient responds to at least 50% of the time (determined by a minimum of 2 out of 3 responses and a maximum of 2 out of 4 responses).[10] 

The standard testing order is 1000, 2000, 3000, 4000, 6000, and 8000 Hz, then repeating 1000 Hz to assess intra-test reliability and account for practice effects before proceeding to 500 and 250 Hz. If a patient demonstrates a high number of false positives (responding without stimuli) or false negatives (not responding to stimuli that should be audible), reinstruction may be warranted. Patients with constant tinnitus may have difficulty distinguishing the pure-tone stimuli from their tinnitus, and modification of the tone stimuli to a warbled tone or a series of two or three pulses may be done to improve response accuracy.[20]

Hearing loss is generally categorized by the degree of difficulty detecting sound in 5 dB increments, from normal hearing sensitivity at ≤25 dB HL to mild (26 to 40 dB HL), to moderate (41 to 55 dB HL), moderately severe (61 to 70 dB HL), severe (71 to 90 dB HL), or profound (91+ dB HL) hearing loss.[8] The pediatric cutoff for normal hearing sensitivity is ≤15 dB HL, with 20 and 25 dB HL categorized as slight hearing loss due to the higher need for sound access for appropriate development in children.[30] 

Once AC and BC thresholds are determined, hearing loss is categorized as sensorineural if the frequencies falling outside normal limits have AC and BC thresholds within 10 dB of each other. The hearing loss is considered conductive if BC is within normal limits, but at least one frequency has a difference between BC and AC of ≥15 dB; the loss is described as "mixed" if there are some sensorineural loss frequencies and some conductive loss frequencies, or if the hearing loss has both conductive and sensorineural aspects in one or more frequencies.[31] 

The configuration of a hearing loss refers to the shape of the plotted audiogram, as some configurations are strongly correlated with specific causes of hearing loss, such as sloping high-frequency loss with presbycusis, notching mid-frequency loss with genetic causes, or low-frequency hearing loss rising to normal hearing with cochlear hydrops.[8]

Due to the way sound transmits through the skull, a sufficiently loud presentation of AC or BC to one ear may cross to the cochlea of the other ear to elicit a response, which may confuse audiometry results.[32] The volume required before this crossover between ears occurs is called interaural attenuation (IA) and varies between individuals and transducers.[33] When an asymmetry between ears is present or a conductive component is suspected, masking noise is introduced to the non-test ear (NTE) via AC to artificially raise that ear's threshold and prevent a response from that ear.[34] The minimum volume possible to overcome IA is used to calculate when masking is necessary by comparing the questionable threshold of the test ear to the BC threshold of the NTE for that frequency. For BC testing, the IA is 0 dB, the supra-aural headphone IA is 40 dB, and the insert earphone IA is 55 dB. A complete pure-tone assessment includes both bone and air conduction for the left and right ear, with masking completed as appropriate.

Complications

Multiple factors can affect the outcomes or accuracy of pure-tone evaluation. Pure-tone thresholds can vary slightly with different transducers and transducer placement, or with variations in patient alertness and attention. In adults, standard diagnostic test-retest variability is typically ±5 dB across frequencies.[20] The pediatric population's test-retest variability may be as great as ±10 dB.[35]

A masking dilemma occurs when the amount of masking noise necessary to prevent sound crossover of a tone to the NTE is loud enough to cross back to the test ear and falsely elevate the evaluated threshold.[36] This occurs most commonly with severe bilateral conductive hearing loss.[37]

Otalgia, particularly of the pinna or auditory canal, may prevent the appropriate placement of headphones for air conduction. Supra-aural headphones are necessary in cases of atresia, but when stenotic or partially-occluded canals are involved, the pressure they apply may cause obstruction of the canal and introduce a false conductive component.[38] 

Insert earphones can prevent external auditory canals from collapsing during testing and reduce the incidence of masking dilemmas.[39] Additionally, due to the differences in calibration between inserted earphones and headphones, the larger cavity created by tympanostomy tubes can cause low-frequency thresholds to be falsely elevated when using inserted earphones.[40]

Behavioral complications include false positive responses, which can result from confusing tinnitus with the presented tone, responding to a regular rhythm of tone presentation, or over-eagerness to "do well" on the evaluation, particularly when a "passing score" is required for employment qualification. False negatives may occur when a patient does not provide consistent responses to audible tones either due to a lack of understanding of testing procedures, a deficit in central processing, or a non-organic deficit caused by malingering or factitious disorders.[41] A Stenger test may be used to assess for malingering in a unilateral or asymmetric hearing loss by observing how the patient responds to a pure tone presented simultaneously to both ears when one tone is easily audible in the better-hearing ear but is quieter than the apparent threshold in the ear with greater hearing loss.[42] If the patient responds because they hear the quiet tone in the better-hearing ear, the Stenger test is negative, and the hearing loss may be considered accurate. If the patient does not respond, which typically occurs because the patient deliberately does not respond to the louder tone in the tested ear and does not realize there is a quieter tone in the NTE, the Stenger is positive and indicates at least some degree of exaggeration in the responses. Stenger testing has a high degree of sensitivity for detecting unilateral non-organic hearing loss.[43]

Clinical Significance

Pure-tone audiometry is the standard gold method of determining type, degree, and configuration of hearing loss due to its widespread availability, inter-test reliability, and relative ease of execution. The results of pure-tone audiometry provide context for diagnosis, reassurance, monitoring, or further investigation of ear concerns. Repeated assessments track changes in hearing as an indicator of changes in ear health over time.

When a sudden change in hearing is reported, immediate pure-tone evaluation is warranted to determine whether a significant hearing loss has occurred and if it meets the criteria for sudden sensorineural hearing loss (SSNHL). Identification of SSNHL and rapid initiation of therapy is critical due to the limited time window for SSNHL treatment to be effective.[44] In SSNHL, a hearing asymmetry is considered significant if there is a ≥30 dB difference over three consecutive frequencies.[45]

Outside of sudden hearing loss, a significant threshold shift, according to the American Academy of Otolaryngology-Head and Neck Surgery, is ≥10 dB increase in the PTA, or at 3000, 4000, and 6000 Hz, in either ear; the Occupational Safety and Health Administration standards for a shift for noise-related hearing loss are an average of ≥10 dB threshold increases for 2, 3, and 4 kHz.[46] 

The American Speech-Language-Hearing Association considers a threshold shift significant with respect to ototoxicity monitoring when there is a ≥20 dB threshold increase at a single frequency, ≥10 dB at two adjacent frequencies, or loss of response at three or more consecutive previously-obtained frequencies in the ultra-high frequency range where maximum volume outputs are limited.[47]

Hearing asymmetry is a common symptom of retrocochlear pathologies, such as vestibular schwannoma.[48] The definition of a significant hearing asymmetry varies throughout the literature, but an asymmetry of ≥20 dB at two consecutive frequencies or ≥15 dB at two frequencies between 2,000 and 8,000 Hz should raise suspicion for retrocochlear pathology.[49]

Aside from hearing deficits, pure-tone audiometry is utilized in the test battery to assess loudness discomfort levels, which indicate the presence of hyperacusis when the discomfort threshold is ≤85 dB HL or softer.[50][51] Pure-tone thresholds also help determine the etiology of tinnitus, and pure-tone presentations are also used as part of pitch and loudness matching assessment for characterizing tinnitus handicap and guiding auditory-based treatment for tinnitus.[52]

It is important to remember that pure-tone audiometry does not provide a complete diagnostic picture for auditory neuropathy or central auditory processing concerns, such as difficulty understanding speech in noise, which requires speech audiometry or other objective tests to assess accurately.[53][54] Central auditory processing disorder testing may be performed on patients of school-age or older with sufficient attention span for extended assessment.[55]

Enhancing Healthcare Team Outcomes

Hearing loss is the third most common chronic health condition in aging adults and affects one in five children to some degree by the age of 18. Relationships between the severity of hearing loss and the severity of ear disease, as well the relationships between hearing loss and communication, social, and educational deficits in children, make catching hearing loss early a critical healthcare task.

Pure-tone audiometry is a low-risk, low-cost procedure but can provide crucial information regarding differential diagnosis, symptom etiology or pathophysiology, and guidance for appropriate treatment. Any time a hearing loss or abnormal auditory perception is reported as a symptom, a referral to audiology for testing that includes pure-tone evaluation should be part of the evaluation process.[1][8] If a concern about hearing is not addressed, negative outcomes range from developmental delays to poorer quality of life to  debilitating disability.[56][57][58]

Due to the close correlation between ear health and hearing abnormalities, coordination between otolaryngologists and audiologists is critical. Pure-tone testing is also key for pre- and post-treatment evaluation when outcome measures include the preservation or improvement of hearing as a significant indicator of efficacy, such as for idiopathic sudden sensorineural hearing loss and otologic surgeries, including tympanoplasty, stapedotomy, semicircular canal dehiscence resurfacing, or mastoidectomy.[59][60][61]

Due to the myriad causes of hearing loss, coordination of care among healthcare providers relying on pure-tone audiometry may include general practitioners, pediatricians, otolaryngologists, oncologists, neurologists, speech-language pathologists, and audiologists. Inter-professional communication and care coordination are essential for evidence-based, timely management of patients and optimal treatment outcomes. [level IV]