Approach to Audiograms
Our 20th episode provides some useful tips to interpret audiograms, the most commonly used technique to objectively identify and describe hearing loss. Tag along to learn more about this useful tool!
Show Notes
Intro
Hey everyone and welcome to The Oto Approach, a podcast created by medical students for medical students to teach you about all things otolaryngology. I'm your host Kalpesh, and today we're going to talk about a basic approach to audiograms. Tag along for a discussion about this important topic.
Hearing loss is more prevalent than you may think… Thirty-eight percent of Canadian adults experience hearing loss [1]. The prevalence of hearing loss tends to increase with age, and 94% of Canadian adults aged 70-79 experience hearing loss [2]. Hearing loss also impacts 8% of Canadian children [1].
Sound & Hearing
Before jumping into audiograms, let’s learn a bit about sound!
Sound is the result of vibrating air particles. Sound frequency is the number of cycles per second that these particles vibrate back and forth, creating a “sound wave” [3-6]. Sound frequency is measured in Hertz, with one Hertz equaling 1 cycle of vibration per second. Sound frequency determines the pitch that we hear [3-6]. Sound is described in terms of its basic physical attributes: frequency, intensity, and time/phase of the vibration; which correlate with psychological attributes of pitch, loudness, and quality, respectively [7].
Sound amplitude is measured by the pressure acting on air particles [3-6]. Sound pressure is the loudness or intensity of sound [3-6]. This intensity is usually measured in decibels (dB), after the renowned Alexander Graham Bell [3-6]. Since humans are capable of hearing such a wide range of sound volumes, decibels require a logarithmic scale of measurement [3-6].
The hearing process can be summarized by sound energy being captured by the outer ear, transformed by the middle ear, and transduced by the inner ear. The external ear consists of the auricle and external ear canal [3-6]. The middle ear consists of the tympanic membrane and the ossicles, while the inner ear contains the cochlea [3-6]. Sound is perceived once sound information is sent from the organ of Corti to the auditory cortex in the temporal lobe via the cochlear nerve. It’s important to have a good understanding of this pathway because dysfunction anywhere along this pathway can cause hearing loss.
Audiometry
Hearing loss can be identified and described by simple bedside tuning fork testing. However, audiometry is the most commonly used tool to objectively identify and describe hearing loss. To learn more about tuning fork testing, listen to our previous episodes on adult sensorineural and conductive hearing loss.
Pure-tone audiometry assesses hearing by emitting various sound pressures (or loudness) over a range of pure-tone frequencies [4-6]. Essentially, audiometry will tell you the degree of hearing loss measured in decibels at these pure-tone frequencies. Pure-tone frequencies are those with sinusoidal wave patterns and are measured at 250, 500, 1000, 2000, 4000, and 8000 Hz [4-6]. Pure-tone audiometry is based on obtaining a series of thresholds at various pure-tone frequencies. The pure tone average is the average hearing decibel threshold at 500, 1000, and 2000 hertz [5,6]. These are included because they are the frequencies at which human speech is typically heard [5,6]. The pure tone average should be within 10 decibels of the speech reception threshold; otherwise, this may be indicative of retrocochlear pathology [5,6].
Pure tones are emitted through both air conduction and bone conduction during audiometry [4,5]. Testing a patient’s hearing when delivering the sound via air conduction assesses the ability of sound to travel from the external ear to the inner ear, also known as conductive hearing [4,5]. Testing hearing when a sound is delivered via bone conduction assesses the function of the inner ear, cochlea, and nervous system, also known as sensorineural hearing [4,5]. Air conduction is assessed by transmitting sound through earphones and is essentially the same as how we hear sound day-to-day [4,5]. Bone conduction is assessed by transmitting sound through a transducer placed behind the ear on the mastoid process of the temporal bone. This vibrates the cochlea and bypasses the conductive portion of hearing, only assessing sensorineural hearing [3-6]. Air conduction relies on conductive and sensorineural hearing, so bone conduction will never be lower than air conduction. Air conduction and bone conduction hearing are measured at each of the pure tones in each ear until a threshold is reached [4,5]. A threshold is the lowest decibel sound level that a patient can discern at least 50% of the time at a specific pure tone [4-6].
Air-bone gap
An air–bone gap reflects the degree of the conductive component contributing to the overall hearing loss. An air-bone gap will present with reduced air conduction compared to bone conduction, suggesting an issue with conductive hearing [4-6]. The opposite would not occur as air conduction relies on both conductive and sensorineural hearing.
If there is a significant difference in hearing between a patient’s ears, sound aimed to assess the “worse” ear may be erroneously interpreted by the contralateral “better” ear [4-8,10]. This is termed “cross-over”. To counter this “cross-over”, “masking” can be implemented [4-8,10]. Masking is when a sound is delivered to the contralateral, or non-tested ear to ensure that it does not interpret the tones aimed at the ear being tested [4-8,10].
In addition to the pure tone audiogram, speech testing is often conducted. The speech reception threshold is the lowest decibel at which a patient can repeat a two-syllable word, also known as a spondee, 50% of the time [5,6]. Similarly, word recognition score is the percentage of monosyllabic words, also known as phonemes, that a patient can repeat when heard at 20-40 decibels above their speech reception threshold [5,6].
Audiograms
There are 3 main components to the audiogram: the pure tone audiometry, the speech audiometry, and the tympanogram. This episode focuses on the pure-tone audiometry. Pure-tone audiometry is represented by a graph with sound frequency in hertz on the X-axis and sound volume in decibels on the Y-axis. The frequencies of pure tones increase from left to right along the X-axis. Decibels increase from top to bottom of the Y-axis. Higher decibels at a given frequency indicate that the patient requires a louder sound to hear at that pitch.
Interpreting Audiograms
When reading an audiogram, the right ear is often denoted by red symbols and the left ear by blue symbols [4,7,9]. Right ear air conduction is represented by circles, while the left is represented by Xs [4,7,9]. Right ear bone conduction is denoted by a square bracket with its open side to the right, and left ear bone conduction is a square bracket with its open side facing the left [4,7,9]. An easy way to remember this is to imagine the brackets as over-the-ear headphones on a person facing toward you. When interpreting an audiogram you should consider 1. The degree of hearing loss and 2. The pattern of hearing loss, and 3 The type of hearing loss. The degree and pattern of hearing loss can be assessed solely by looking at the air conduction line (the circles and X’s). The type of hearing loss requires both the bone conduction and air conduction lines for assessment.
The degree of hearing loss is described based on the level of decibels required to achieve the hearing threshold. Hearing plotted between 0-20 decibels for any given frequency is considered within normal limits [3,4,11]. Hearing loss can be classified by severity based on the sound threshold level. Thresholds reported over 20 decibels are considered to reflect impaired hearing. 20-40 decibels is mild hearing loss; 40-55 decibels is moderate hearing loss; 55-70 decibels is moderately severe hearing loss; 70-90 decibels is severe hearing loss, and >90 decibels is profound hearing loss [3,4,11]. The pattern of hearing loss is described based on the frequency at which the hearing loss is experienced [12]. This can be described as low, mid, or high-frequency hearing loss [3,12].
There are three types of hearing loss, conductive, sensorineural and mixed.
Conductive hearing loss is caused by disorders of the outer ear and/or middle ear, with normal bone-conduction threshold responses, and air-conduction thresholds falling outside the normal limits. The air conduction line will be impaired, meaning it will require higher decibel sounds to reach the threshold, while the bone conduction line will be normal. Such an audiogram would suggest that the conductive component of hearing is impaired while the sensorineural hearing is normal. Conductive hearing loss suggests middle or external ear pathologies [8]. Common causes of conductive hearing loss include cerumen impaction, otitis externa or otitis media, middle ear effusions, foreign bodies in the ear canal, exostoses or abnormal bony growths in the ear canal, dysfunction of the ossicles, cholesteatomas which are abnormal keratin deposits, and tympanic membrane sclerosis or perforation [3-6,9].
The type of hearing loss can be conductive, sensorineural, or mixed [3,4,10]. Conductive hearing loss is the impairment of sound transmission from the external auditory canal to the inner ear [3-5,9]. This suggests an impairment in the outer or middle ear, while the inner ear and nervous system are not impacted.
Sensorineural hearing loss is impairment of the inner ear, or a retrocochlear pathology, including etiologies relating to cranial nerves 7 and 8 [3-5,9]. If both sensorineural and conductive aspects are present, this is mixed hearing loss [3-5,9].
Sensory or cochlear hearing loss involves damage to portions of the cochlea, such as the outer or inner hair cells or stria vascularis. Sensorineural hearing loss is indicated by impaired air conduction and bone conduction, but the lines and their impairment will be equal [4,5,14]. This audiogram pattern suggests hearing is only limited by the sensorineural component and conduction is normal [14]. Sensorineural hearing loss suggests inner ear or nervous system pathologies [14]. Common causes of sensorineural hearing loss include congenital hearing loss, intracranial infections, presbycusis or age-related hearing loss, noise-induced hearing loss, ototoxic drugs, and cerebellopontine angle tumours [3-6,14].
Mixed hearing loss will have an air-bone gap, but the bone conduction line will not be normal, suggesting impairments in both conductive and sensorineural hearing [3-6]
Putting all of this together, you can describe audiograms, and therefore describe a patient’s hearing loss! For example, you could say a patient has right-sided moderately severe high-frequency sensorineural hearing loss.
For more information on hearing loss, check out our previous episodes on sensorineural and conductive hearing loss!
Common Findings
There are common patterns you might see on audiograms which are important to be familiar with!
Presbycusis
Presbycusis is age-related hearing loss [15]. It is the most common type of hearing loss and classically presents with bilateral, symmetric, sensorineural hearing loss, worsening or “downsloping” in the higher frequencies [4,5,14,15].
Noise-induced Hearing Loss
Hearing loss due to noise exposure classically presents with a “noise notch,” an audiogram with a V-shaped notch or dip in hearing typically at 4000 hertz [3-5,14,15]. Often patients will have occupational exposure to loud noise, such as construction workers, musicians, and bartenders. Hearing protection and education are important preventative health strategies [3-5,14,16].
Otosclerosis
Otosclerosis is abnormal bone growth in the middle ear which results in conductive hearing loss due to dysfunction of the ossicular chain [3-6,17]. Most often, this is due to immobility of the stapes footplate in the cochlea's oval window. Otosclerosis is classically associated with a “Carhart’s” notch on an audiogram, a dip or V-shaped notch in hearing typically at 2000 hertz [4-6,17]. Treatment for this is a stapedotomy, which is essentially, removing the diseased stapes and replacing them with a prosthesis [3-6,17].
Vestibular Schwannoma
Assymetrical, or unilateral, hearing loss without an identified cause warrants a referral for a head MRI [3-6,18]. Asymmetrical hearing loss is classically defined as a difference of 15 or greater dB at three contiguous frequencies. Although relatively uncommon, assymetry can be due to a vestibular schwannoma, a benign tumour arising from the Schwann cells of the vestibular nerve, which compromises sensorineural hearing [3-6,18]. Although this typically presents as unilateral hearing loss, bilateral vestibular schwannomas are common in patients with neurofibromatosis type 2 [3-6,18].
Meniere’s Disease
On the other hand, unilateral low-frequency sensorineural hearing loss is suggestive of Meniere’s disease [3-6,19]. The audiogram for Meniere’s disease is described as upsloping with worse hearing in the low frequencies, which improves in the higher frequencies [3-6,19]. Clinically, Meniere’s disease is described by a vertigo, tinnitus, aural fullness, and unilateral hearing loss. The vertigo attacks in Meniere’s can last hours and are quite debilitating [3-6,19]. The pathophysiology is still up for debate, but the predominant theory is overaccumulation of endolymph in the semicircular canals resulting in endolymphatic hydrops [3-6,19]. Treatment includes antiemetics, low salt diets, thiazide diuretics, Serc a betahistine medication, and in severe cases intratympanic gentamicin to destroy the inner ear vestibular organ is used, but this also results in complete sensorineural hearing loss [3-6,19].
Sudden Sensorineural Hearing Loss
Sudden sensorineural hearing loss is a unilateral sensorineural hearing loss of greater than 30 decibels over at least three contiguous pure-tone frequencies on an audiogram [3-6,20]. This must represent an acute change in hearing occurring within 72 hours [3-6,20]. Although the exact pathophysiology is debated, it is thought to be a viral infection of the inner ear, resulting in inflammation and acute sensorineural hearing loss [3-6,20]. Patients should be treated with oral or intratympanic steroids to reduce inflammation [3-6,20]. The sooner the treatment, the better. Resolution is not guaranteed in these patients [3-6,20]. However, steroids are currently the best treatment option [3-6,20].
Congenital/Hereditary Hearing Loss
Congenital or hereditary hearing loss is classically described as having a “cookie bite” pattern on the audiogram [3-6,21]. It is so named, because there is a decrease in sensorineural hearing in the mid frequencies, resulting in a “U” shaped pattern, resembling a cookie bite [3-6,21].
It is important to note that audiograms should be interpreted in the context of the patient’s clinical picture! Please listen to our previous hearing loss episodes for more information on otologic assessments! A future episode will focus on the basics of hearing aids, so be sure to check back in!
This script was written by Kalpesh Hathi, it was revised by Aileen Feschuk, Gizelle Francis, Hannah Brennan, Dr. Emily Cheng, Dr. Katie Oxford, Dr. Chad Purcell, and Dr. Christopher Chin.
We would like to extend our sincerest thanks to the Saint John Regional Hospital Department of Surgery within the Horizon Health Network for their generous support.
Thank you so much for listening to our podcast! We hope you’ll tune in to our next episode! Please head to our website at www.theotoapproach.com for our show notes, and to sign up for our newsletter to stay up to date with our latest episodes.
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