Understanding Audiometric Thresholds and Hearing Loss Classification

Audiogram Symbols and Mapping

An audiogram uses specific symbols to represent different types of testing. These symbols are color-coded to prevent confusion between the ears: Red represents the Right ear, and Blue represents the Left ear.

Air Conduction vs. Bone Conduction

Testing is divided into two main categories:

1. Air Conduction (AC): This tests the entire auditory system (outer, middle, and inner ear) using headphones or insert earphones.
   • Right Ear: Marked with an 'O'.
   • Left Ear: Marked with an 'X'.
   • No Response: If a patient cannot hear a sound even at maximum output, an arrow is drawn from the symbol pointing in the opposite direction of the ear (e.g., an 'X' with a right-pointing arrow indicates no response in the left ear).
2. Bone Conduction (BC): This tests the inner ear (cochlea) directly using an oscillator placed behind the ear.
   • Symbols: Uses the '>' symbol.
   • The "Alligator Mouth" Rule: The symbol faces the frequency line it marks. To identify the ear, imagine the symbol is an alligator mouth; if it "eats" to the right, it is the right ear.
   • Unmasked Bone Conduction: When these symbols are used without masking (noise in the non-test ear), they are referred to as unmasked thresholds.

Classifying the Degree of Hearing Loss

The degree describes how significant the hearing loss is, based on the decibel (dB) threshold level. Because thresholds can vary by frequency, a patient might have normal hearing in low pitches but severe loss in high pitches.

The standard classifications are:

• Normal Hearing: 0 to 25 dB.
• Mild Hearing Loss: 35 dB.
• Moderate Hearing Loss: 45–55 dB.
• Moderately Severe Hearing Loss: 60–70 dB.
• Severe Hearing Loss: 75–90 dB.
• Profound Hearing Loss: Above 95 dB.

Clinical Tip: When a threshold falls exactly on a boundary (e.g., 45 dB), clinicians typically classify it within the lesser (milder) degree.

The Pure Tone Average (PTA)

To provide a single representative number for a patient's hearing, clinicians calculate the Pure Tone Average (PTA). This is the mean of the air conduction thresholds at 500, 1000, and 2000 Hz.

The calculation is performed as follows: $\text{PTA} = \frac{\text{Threshold}_{500} + \text{Threshold}_{1000} + \text{Threshold}_{2000}}{3}$

Configuration and Type of Hearing Loss

Once the degree is known, the clinician looks at the configuration (the shape of the graph) and the type (the physiological cause).

Configuration: The "Shape" of Hearing

Configuration describes the pattern of thresholds across the frequency spectrum:

• Sloping: Thresholds are better in low frequencies and get worse in high frequencies; this is typical of age-related loss.
• Precipitous Sloping: A very sharp drop in hearing, defined as a loss of more than 15–20 dB per octave.
• Cookie Bite: A dip in the mid-frequencies, often associated with genetic factors.
• Reverse Cookie Bite: A "mountain-like" shape where mid-frequencies are worse than low and high frequencies.
• Noise Notch: A specific dip, typically at 3 or 4 kHz, often caused by exposure to loud noise.
• Flat: Thresholds remain consistent across all frequencies.

Type: The Physiological Cause

The type of hearing loss identifies where the problem is located in the auditory system:

1. Conductive: A blockage or issue in the outer or middle ear.
2. Sensorineural: Damage to the cochlea (inner ear) or the auditory nerve.
3. Mixed: A combination of both conductive and sensorineural components.

By combining these three descriptors, a clinician can provide a precise diagnosis, such as a "mild sloping to moderate sensorineural hearing loss".