Pure tone audiometry is the main hearing test used to identify a patient's hearing thresholds and determine the degree, configuration, and type of hearing loss. It serves as the basis for diagnosing and managing hearing loss. Pure tone audiometry is a behavioral and subjective measure of a patient's hearing threshold, as it depends on the person's response to pure sound stimuli. This is why it is only performed on children and adults who are old enough to cooperate during the test. As with many other clinical examinations, the equipment, the general calibration of the test environment, and the stimuli must be in place before starting. Pure tone audiometry measures the threshold of hearing ability and does not evaluate other aspects of hearing, such as speech recognition or sound localization. However, it offers many advantages over other types of hearing tests. In this article, we will focus on pure tone audiometry in relation to conductive hearing loss. How does pure tone audiometry detect conductive hearing loss? We will answer this question, and many others.
What is pure tone audiometry used for?
As its name suggests, the pure tone audiometry test uses pure sounds to determine if a patient hears all frequencies in the audible spectrum. It is a subjective test, performed using a device called an audiometer. The audiometer records the test results on an audiogram, which then shows whether the patient's hearing is impaired at the level of sound transmission in the middle ear - this is called conductive hearing loss - or if the inner ear is affected, which corresponds to sensorineural hearing loss. The most appropriate treatment for the problem is thus identified more easily and quickly. Pure tone audiometry provides the physician with precise thresholds. It uses pure sounds specific to each frequency in order to obtain specific responses and identify the configuration of the patient's hearing loss. As pure tone audiometry combines bone conduction and air conduction, the practitioner can also determine the type of hearing loss present thanks to the audiometric Rinne test (the air-bone gap).
Is pure tone audiometry the perfect test for detecting hearing loss?
While pure tone audiometry has many clinical benefits, it is not perfect for identifying all types of hearing loss. For example, it cannot detect cochlear dead regions or neuropathies in conditions such as auditory processing disorder.
Types of pure tone audiometry tests
There are two main types of pure tone audiometry tests:
Threshold hearing test
The threshold hearing test is used to determine the patient's ability to perceive sounds at a given volume or frequency.
During this test, headphones are used to establish the hearing threshold, which corresponds to the last sound the person hears and therefore represents their minimal hearing level.
Suprathreshold or high-frequency tone test
The high-frequency tone test is used to evaluate the patient's hearing ability as well as any discomfort or impairment above their hearing threshold. The suprathreshold test is based on three main parameters: duration, intensity, and frequency. These hearing tests are generally performed in an anechoic chamber or a soundproof booth, covered with absorbent materials that limit the reflection of standing waves and noise.
What you need to know about the audiogram
Hearing is examined by a practitioner who plays different pure tone signals to the patient through in-ear headphones or a headset to measure air conduction. The audiometer assesses the patient's hearing ability by testing the hearing threshold with a sound signal at specific frequencies (in hertz, or pitch in cycles per second). The hearing threshold can be defined as the lowest level a sound can reach before becoming inaudible. Practitioners measure thresholds in decibels, with a normal threshold between 0 and 25 decibels in adults and between 0 and 15 decibels in children. This threshold is plotted on a graph called an audiogram. The audiogram displays the sound frequency (from low to high frequencies) on the horizontal axis and the sound intensity in decibels on the vertical axis. The thresholds for the right ear are shown as red circles, and those for the left ear in blue. Bone conduction testing can be performed by bypassing the middle ear (also called the air conduction pathway) and the outer ear, to determine the threshold when sound is transmitted directly to the cochlea. This is done using a bone vibrator, which sends slight vibrations to the patient's inner ear through the mastoid process. Comparing the results of the bone conduction test and the air conduction test (which uses sound as a stimulus) helps to better determine whether the hearing loss is due to nerve damage or a transmission problem.
What you need to know about pure tone audiometry and conductive hearing loss
Hearing loss is the most common sensory deficit in humans. It is a total or partial inability to hear sounds, in one or both ears. When it affects both ears, it is called bilateral hearing loss; when it affects only one, it is called unilateral loss. Hearing loss can also be sudden or progressive, slowly worsening over time. And depending on its cause, it can be mild or severe, permanent or temporary. In some patients, hearing loss fluctuates (it varies over time), improving or worsening at certain times. In others, it is stable and does not change over time. That said, hearing loss has many causes: diseases, noise exposure, physical trauma, chemicals, age, and genetics. It can affect all ages - delaying learning and language in children, and causing professional and social difficulties in adults.
Types of hearing loss
The human ear is generally divided into two main parts:
The conductive part: it concerns the segments of the ear responsible for air conduction, i.e., the outer ear and the middle ear.
The sensorineural part: it corresponds to the inner ear.
From these two parts, three main types of hearing loss are distinguished: mixed hearing loss (a combination of conductive and sensorineural loss), sensorineural loss, and conductive hearing loss. Since the latter is what interests us here, let's look in detail at what it covers.
Conductive hearing loss
Conductive hearing loss is a type of hearing loss characterized by better hearing thresholds for signals transmitted by bone conduction than for those transmitted by air conduction. It is generally related to a problem in the outer or middle ear, while the inner ear functions normally. An audiogram of conductive hearing loss, obtained during a pure tone audiometry test, shows normal bone conduction, between 0 and 25 decibels. It also reveals abnormal air conduction thresholds, above 25 decibels. In general, a difference greater than 10 decibels is considered a significant air-bone gap and requires masking to eliminate any response from the untest ear. This allows for reliable thresholds for the ear being examined. Conductive hearing loss can affect all frequency ranges. But it is most often the mid and low frequencies (250 Hz to 2 kHz), or even only the low frequencies (250 to 500 Hz), that are affected. In severe cases of conductive hearing loss, the loss reaches 60 decibels or more. In the total absence of sound reaching the ears, sound waves can still reach the cochlea through fluid movements and skull vibrations. Most conductive hearing losses can be treated with amplification, medication, assistive devices, surgery, or a combination of these solutions.
Causes of conductive hearing loss
A common cause of conductive hearing loss is a malformation or absence of the ear canal, middle ear structures, or outer ear. Microtia and atresia are examples. Causes also include cholesteatoma and otosclerosis. The former is a cystic mass of cholesterol and epithelial cells that obstructs the middle ear and produces enzymes capable of destroying adjacent bones. Another cause is tympanosclerosis, a consequence of chronic otitis media. This condition of the tympanic cavity results in calcium deposits on the ossicular chain and the tympanic membrane, leading to conductive hearing loss due to stiffening and reduced mobility. Other common causes of conductive hearing loss include obstruction of the ear canal by earwax or a foreign body, a damaged or perforated eardrum, otitis externa (inflammation of the outer ear), otitis media (inflammation of the middle ear), trauma damaging the eardrum or ossicles, allergies, fluid accumulation, benign tumors, and Eustachian tube dysfunction (the tube responsible for draining fluid from the ear to the back of the throat).
Hearing loss configuration
Hearing loss can be classified according to the audiometric configuration, i.e., the pattern or shape of the audiogram across the frequency spectrum. This configuration indicates to the audiologist which sounds the patient perceives best. A hearing loss that is roughly constant across all frequencies appears as a straight horizontal line on the audiogram: this is called a flat configuration. In this configuration, the thresholds for different frequencies do not deviate by more than 20 decibels from each other. In other words, a patient with this type of hearing loss will need a constant intensity to perceive sound, regardless of the pitch. A patient with a downward sloping configuration has little or no hearing loss in the low frequencies, severe loss in the mid frequencies, and profound loss in the high frequencies. This configuration is also called a "ski slope," because the audiogram resembles it: the "top of the slope" on the left and the "descent" to the right. Conversely, an upward sloping configuration indicates that the patient hears high-pitched sounds better than low-pitched sounds. This type of audiogram is quite rare; an example would be a person unable to hear a gunshot or thunder, but able to perceive whispers in a room. Patients with a U-shaped or "trough" configuration have one or more adjacent thresholds between 500 and 4000 Hz, equal to or greater than 20 decibels. Such a patient will likely have difficulty perceiving mid-frequency sounds, while retaining the ability to hear low and high-pitched sounds. Most often, this type of hearing loss is genetic and can evolve over time. Next is the notching configuration, which indicates hearing loss generally located between 3 and 6 kilohertz, with lower and higher frequencies unaffected. This configuration is found in noise-induced hearing loss, because the sensory cells of the cochlea are more vulnerable to acoustic trauma in the 3-6 kilohertz range than in lower or higher frequencies. Finally, the high-frequency configuration shows normal hearing in the low frequencies and poor hearing in the high frequencies.
Usual treatments when conductive hearing loss is revealed by a pure tone audiometry test
If your doctor identifies conductive hearing loss through a pure tone audiometry test, here are the treatments they may recommend:
Surgery
Surgery can correct conductive hearing loss due to a congenital absence of the ear canal or the inability of the ear canal to open at birth, dysfunction, malformation, or congenital absence of middle ear structures, as well as otosclerosis. ENT surgeons can improve hearing by repairing eardrums, replacing damaged ossicles, or creating better drainage pathways for the ear. Sometimes, the insertion of a tympanostomy tube significantly improves hearing; in adults, the procedure can be performed directly in the ENT doctor's office.
Amplification
Amplification can be a solution for treating conductive hearing loss, using a bone conduction hearing aid, a surgically implanted bone-anchored implant (e.g., the Ponto system or the Baha), or a conventional hearing aid, depending on the patient's auditory nerve status.
Antibiotic or antifungal treatments
Antibiotic or antifungal treatments can cure ear infections or chronic middle ear effusions, which often lead to conductive hearing loss. In case of a tumor, the patient may need surgery.
Conclusion
If you notice signs of hearing loss, consult your general practitioner, an ENT doctor, or an audiologist without delay. The practitioner will perform a pure tone audiometry test to assess your hearing and determine the exact cause of your hearing difficulties.
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