2.3 Pure-Tone Audiometry & Threshold Determination

Pure-Tone Audiometry & Threshold Determination

Introduction and Test Environment

Pure-tone audiometry is the cornerstone of hearing assessment. Its purpose is to determine the patient's auditory thresholds—the softest sounds they can hear 50% of the time.

To ensure accurate results, testing must be conducted in a controlled acoustic environment. The test environment must adhere to the ANSI S3.1 standards for maximum permissible ambient noise levels. This is typically achieved using a specially constructed, sound-treated booth. If ambient noise exceeds ANSI standards, it can artificially elevate (worsen) the patient's thresholds, particularly in the lower frequencies, leading to an inaccurate diagnosis.

Transducer Types

Transducers are the devices that convert electrical signals from the audiometer into acoustic or vibratory energy.

1. Supra-aural Earphones (e.g., TDH-39/49): These sit over the pinna. They are historically common but can cause collapsed ear canals in the elderly, leading to false conductive hearing loss in high frequencies.
2. Insert Earphones (e.g., ER-3A): Foam tips are inserted into the ear canal. These are highly preferred because they prevent collapsed canals, provide superior interaural attenuation (preventing crossover to the non-test ear), and reduce background noise.
3. Bone Conduction Oscillator: A vibrator placed on the mastoid process (or forehead) that directly stimulates the cochlea via bone conduction, bypassing the outer and middle ear.

The Hughson-Westlake Modified Technique

The standard procedure for threshold determination is the Hughson-Westlake modified ascending technique. This method ensures consistency and reliability across clinicians.

The Procedure:

1. Familiarization: Present a tone at a level easily audible to the patient (e.g., 30 dB HL at 1000 Hz. If no response, increase to 50 dB HL).
2. Down 10 dB: After a positive response, decrease the intensity in 10 dB steps until the patient stops responding.
3. Up 5 dB: When the patient fails to respond, increase the intensity in 5 dB steps until they respond again.
4. Threshold Criterion: A threshold is defined as the lowest decibel level at which the patient responds to at least 50% of ascending presentations (specifically, 2 out of 3 ascending responses).

Test Frequencies

• Air Conduction (AC): Tests the entire auditory system. Standard test frequencies are 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz. Inter-octave frequencies (e.g., 750, 1500 Hz) should be tested if there is a difference of 20 dB or more between adjacent octaves.
• Bone Conduction (BC): Tests inner ear function directly. Standard test frequencies are 250, 500, 1000, 2000, 3000, and 4000 Hz. Bone conduction is not reliably tested above 4000 Hz due to transducer limitations and vibrotactile responses.

Audiometric Symbols

Standardized symbols are used to plot thresholds on the audiogram (Frequency on the X-axis, Intensity in dB HL on the Y-axis).

(Mnemonic Aid: Right = Red = Round (O). Left = Blue = X).

Pure Tone Average (PTA) Calculation

The Pure Tone Average (PTA) is a crucial metric used to cross-check the reliability of Speech Reception Thresholds (SRT) and to provide a quick summary of the degree of hearing loss in the speech frequencies.

The traditional PTA is calculated by averaging the air conduction thresholds at 500 Hz, 1000 Hz, and 2000 Hz. Formula: (Threshold at 500 + Threshold at 1000 + Threshold at 2000) / 3

Example: If thresholds are 30 dB (500 Hz), 40 dB (1000 Hz), and 50 dB (2000 Hz), the PTA = (30+40+50)/3 = 40 dB HL.

Degrees of Hearing Loss

Hearing loss severity is categorized based on pure-tone thresholds. While slight variations exist among organizations, the standard classification is:

• -10 to 15 dB: Normal hearing (Children)
• -10 to 25 dB: Normal hearing (Adults)
• 26 to 40 dB: Mild hearing loss
• 41 to 55 dB: Moderate hearing loss
• 56 to 70 dB: Moderately-Severe hearing loss
• 71 to 90 dB: Severe hearing loss
• 91+ dB: Profound hearing loss

Types of Hearing Loss

Comparing Air Conduction (AC) thresholds to Bone Conduction (BC) thresholds reveals the type of hearing loss.

1. Sensorineural Hearing Loss (SNHL): Damage is in the cochlea or auditory nerve.
   • AC and BC thresholds are equally elevated (both abnormal).
   • No significant air-bone gap (ABG is 10 dB or less).
2. Conductive Hearing Loss (CHL): Sound is blocked in the outer or middle ear; the inner ear is normal.
   • BC thresholds are normal (0-25 dB).
   • AC thresholds are abnormal (elevated).
   • Significant air-bone gap (ABG is 15 dB or greater).
3. Mixed Hearing Loss: A combination of conductive and sensorineural pathology.
   • BC thresholds are abnormal (elevated).
   • AC thresholds are worse than BC thresholds.
   • Significant air-bone gap (ABG is 15 dB or greater).

Understanding these relationships allows the clinician to pinpoint the location of the auditory deficit, guiding medical referrals and hearing aid candidacy decisions.