Audiometry tests are essential assessments used to evaluate an individual’s hearing capabilities. Various types of audiometry tests are employed, including Pure Tone Audiometry, Speech Audiometry, Bekesy Audiometry, and Impedance Audiometry. Among these, Pure Tone Audiometry (PTA) is considered the gold standard for measuring hearing thresholds.
Pure Tone Audiometry (PTA). PTA is a subjective test designed to determine the hearing threshold for pure tones, which are sinusoidal waves characterized by a single frequency, amplitude, and phase. The test utilizes an instrument called an audiometer, which is an electronic device which produces pure tones. The intensity of audiometer can be increased or decreased in 5 dB steps. During the test, the threshold intensity for each frequency is measured and recorded in a graphical representation known as an audiogram.
Components of an Audiometer. An audiometer is composed of several key parts:
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- Electronic Oscillator: This component generates pure tones across a range of frequencies, from low to high.
- Intensity Dial: This dial allows the examiner to adjust the threshold intensity of hearing for each tone.
- Headphones: These deliver pure tones of various frequencies to each ear separately.
Prerequisites for PTA. For accurate results, PTA should be conducted under specific conditions:
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- Sound-Proof Environment: The test should take place in a sound-proof room, with ambient noise levels accepted at less than 35 dB.
- Minimizing Distractions: Transient noises, such as sneezing or coughing, should be avoided, as they can interfere with the test results.
- Examiner-Patient Interaction: The patient sits in a sound-proof booth while the examiner conducts the audiometry from outside. Communication can occur via intercom or face-to-face to observe the patient’s reactions.
Testing Procedure
The testing process typically begins with the better ear. The thresholds for pure tones are measured in the following order: 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz, followed by 500 Hz and 250 Hz. If there is a difference of 20 dB or more between contiguous frequencies, inter-octave frequencies (750 Hz, 1500 Hz, 3000 Hz, and 6000 Hz) will also be tested. The procedure is then repeated for the other ear, starting with air conduction (AC) and, if necessary, followed by bone conduction (BC) testing. Test frequencies for BC are those in the range 500–4000Hz, with no retest at 1000Hz required.
Bracketing Technique
The bracketing technique, also known as the 10 dB down, 5 dB up method, involves delivering each test signal at an audible level, then reducing the volume in 10 dB steps until the signal becomes inaudible. The volume is then increased in 5 dB increments until the patient can perceive the sound again.
Pure Tone Audiogram is a graphic representation of hearing detection thresholds in each ear.
| MASKING. Masking is done to produce inaudibility of one sound by the presentation of another sound. During masking, one ear is kept busy by a narrow band noise when the other is being tested. Masking of the non-test ear is done in all bone conduction tests because BC signals are transmitted to both cochleae, regardless of the side on which the transducer is placed. However, for air conduction tests, a continuous masking noise (or masker) is required only when the interaural hearing difference (AB gap) exceeds 40 dB or more (when using supra or circum-aural headphones), or 55 dB or more (when using insert earphones). This is done to avoid transcranial transmission/ interaural attenuation/ cross-hearing and accurately measure hearing in each ear. |
Interpretation of PTA Results. The results of PTA can indicate different types of hearing loss:
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- Conductive Hearing Loss: Identified when there is a difference of more than 10 dB between air conduction and bone conduction thresholds at any frequency.
- Sensorineural Hearing Loss: Indicated when bone conduction thresholds are raised above 20 dB HL.
- Mixed Hearing Loss: Occurs when bone conduction thresholds are raised above 20 dB HL, accompanied by a significant air-bone gap (ABG).
Utilization of PTA. PTA serves multiple purposes in audiology:
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- Determining Hearing Loss: It helps identify the type, degree, and configuration of hearing loss. The air conduction pure tone audiogram primarily measures the degree of hearing loss, while the bone conduction audiogram differentiates between conductive and sensorineural hearing loss.
- Medicolegal Assessment: PTA can confirm the degree of hearing impairment for legal and insurance purposes.
- Hearing Aid Prescription: The audiogram is crucial for accurately prescribing hearing aids.
- Speech Reception Threshold: PTA also assists in determining the speech reception threshold of the subject.
Disadvantages of PTA. Despite its advantages, PTA has some limitations:
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- Subjectivity: The test results are based on the patient’s responses, which can introduce variability.
- Potential for Malingering: Results may be inaccurate if the patient is feigning hearing loss.
- Age Considerations: The test may yield unreliable results in children under the age of seven, as it depends on cognitive development and cooperation.
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Learning resources.
- Scott-Brown, Textbook of Otorhinolaryngology Head and Neck Surgery.
- Glasscock-Shambaugh, Textbook of Surgery of the Ear.
- Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
- Rob and smith, Textbook of Operative surgery.
- P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
- Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
- Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck surgery.
- Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
Author:
Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India