Audiology - Chapter 4

Introduction

• Speaker identifies as a speech-language pathologist (SLP) with a Ph.D. focused on speech and hearing sciences.
• Clarification that they are not a clinical audiologist and will reference external materials for specialized depth.
• Inclusion of short instructional videos to provide visual modeling of audiological procedures and anatomical processes.

Chapter Four: Pure Tone Audiometry and Masking

Overview of Pure Tone Audiometry

• The Testing Environment: Conducted in a sound-treated booth designed to minimize ambient noise.
• Audiology Schematic: Includes an audiologist controlling the audiometer from a separate workspace and the patient receiving stimuli.
• Patient Participation: Detection of pure tones across a frequency range (typically $250$ Hz to $8000$ Hz).
• Developmental Considerations:
  • Adults: Typically provide a physical response (button press or hand raise) and may participate in speech testing (repeating words).
  • Children: Responses are often conditioned through play or visual reinforcement to maintain engagement.

Equipment Used in Audiometry

• Audiometers: Devices used to deliver precise frequencies and intensities. Modern clinics increasingly utilize computer-based audiometers for automated data collection and integration with Electronic Health Records (EHR).
• Transducers: Components that convert electrical energy into sound, including supra-aural headphones, insert earphones, and bone oscillators.

Air Conduction Audiometry

• Pathways: Sound travels through the outer ear, middle ear, and finally to the inner ear and auditory nerve.
• Objective: To determine the Threshold, defined as the lowest intensity level in decibels (dB HL) where a patient can detect a sound $50\%$ of the time.
• Transducer Options:
  • Supra-aural earphones: Placed over the pinna; standard but can cause ear canal collapse in some patients.
  • Insert earphones: Placed inside the canal; better at reducing ambient noise and preventing ear canal collapse.
• Limitations: Results identify the degree of hearing sensitivity but cannot distinguish the specific site of lesion (outer/middle vs. inner ear).

Interpretation of the Audiogram

• Audiometric Zero: The level at which a healthy young adult can just barely hear a sound ($0$ dB HL).
• Classification of Hearing Loss (Adults):
  • Normal: $-10$ to $25$ dB HL
  • Mild: $26$ to $40$ dB HL
  • Moderate: $41$ to $55$ dB HL
  • Moderately Severe: $56$ to $70$ dB HL
  • Severe: $71$ to $90$ dB HL
  • Profound: > 90 dB HL
• The Air-Bone Gap (ABG): The numerical difference between air conduction and bone conduction thresholds. An ABG greater than $10$ dB suggests a conductive component.

Bone Conduction Audiometry

• Mechanism: A bone oscillator (placed on the mastoid process or forehead) vibrates the skull to stimulate the cochlea directly, bypassing the outer and middle ear.
• Diagnostic Utility:
  • Conductive Hearing Loss: Bone thresholds are normal ($\le 25$ dB HL), while air thresholds are abnormal, indicating the problem is in the outer/middle ear.
  • Sensorineural Hearing Loss: Both air and bone thresholds are equally abnormal (within $10$ dB of each other), indicating damage to the cochlea or auditory nerve.
  • Mixed Hearing Loss: Both air and bone thresholds are abnormal, but bone is significantly better than air (ABG exists).

Masking in Audiometry

• The Necessity of Masking: Prevents "cross-hearing," where a sound presented to the test ear is loud enough to vibrate the skull and be detected by the non-test ear.
• Clinical Rules for Masking:
  • Interaural Attenuation (IA): The loss of energy of a sound as it travels from one side of the head to the other.
  • For supra-aural headphones, IA is $40$ dB; for insert earphones, IA is roughly $60$ dB.
• Procedure: Narrowband noise is introduced to the non-test ear to "keep it busy" while the test ear is evaluated.

Behavioral Responses in Audiometry

Behavioral Observation Audiometry (BOA)

• Target Group: Infants from birth to approximately $6$ months.
• Responses: Passive reflexive behaviors, such as the sucking reflex changes, startle responses (Moro reflex), or eye-widening.
• Limitations: High variability; thresholds are often higher than true hearing sensitivity.

Visual Reinforcement Audiometry (VRA)

• Target Group: Toddlers between $6$ months and $2$ years.
• Method: The child is conditioned to turn their head toward a sound source; upon doing so, a visual reward (flashing toy or video) is activated.
• Effectiveness: Provides highly reliable ear-specific thresholds if performed with insert earphones.

Conditioned Play Audiometry (CPA)

• Target Group: Children aged $2.5$ to $5$ years.
• Method: The child performs a motor task every time they hear a sound (e.g., dropping a block in a bucket, putting a peg in a board).
• Engagement: Relies on social reinforcement and turning the test into a repetitive game to ensure cooperation.

Conclusion

• Audiological assessment is a multifaceted process requiring different strategies depending on the patient's age and developmental level.
• Integration of air conduction, bone conduction, and behavioral observations allows the SLP and Audiologist to form a comprehensive picture of a client's hearing health.