Summary of ABR Testing: The Importance of Correction Factors

Summary of "ABR Testing: The Importance of Correction Factors"

This video explains the critical role of correction factors in Auditory Brainstem Response (ABR) testing, particularly in infants, to accurately estimate hearing thresholds that relate to behavioral pure-tone audiometry. It covers the origin, derivation, and clinical application of these correction factors, supported by case examples illustrating the consequences of neglecting them.

Main Ideas and Concepts

• Purpose of Correction Factors in ABR Testing
  • ABR reflects neural activity up to the brainstem, whereas pure-tone audiograms measure behavioral thresholds involving higher brain regions.
  • Correction factors convert ABR thresholds (measured in NHL - Normal Hearing Level) into Estimated Hearing Level (EHL), which more closely approximates behavioral hearing thresholds.
  • Correction factors are essential because evoked potential systems are calibrated to adult data but are often used in infants whose ear anatomy and auditory processing differ.
• Calibration and Standards
  • Evoked potential systems must be calibrated using electrical peak equivalent sound pressure level (PESPL).
  • RetSpell values (from ISO standard 389-6) convert PESPL to NHL.
  • Different stimuli (CE chirps, tone bursts/pips, clicks) and transducers (insert earphones, headphones, bone conduction) require distinct correction factors.
  • The British Society of Audiology (BSA) guidelines provide comprehensive correction factors by stimulus type, transducer, and age group.
• Types of Stimuli and Transducers
  • Stimuli include CE chirps, frequency-specific tone bursts, and clicks, each eliciting different ABR responses.
  • Transducers include insert earphones, supra-aural headphones, circumaural headphones, and bone conduction vibrators.
  • Insert earphones require significant age-related correction factors due to differences in infant ear canal size and sound delivery.
  • Headphones require minimal or no correction factors because the acoustic load is less affected by ear size.
  • Bone conduction correction factors account for smaller infant head size leading to stronger stimulus perception.
• Derivation of Correction Factors
  • Based on a meta-analysis by Stapells (2000) including 32 studies and over 1,200 participants.
  • Adult data preferred for behavioral thresholds due to consistency; pediatric data less consistent.
  • Additional 5 dB correction added to account for variability in threshold definitions and testing conditions.
  • Ontario Newborn Hearing Screening Program correction factors align closely with BSA values.
  • CE chirp offsets derived from Firm, Lightfoot, and Stevens (2013) showing chirps produce larger ABR amplitudes, leading to a 5 dB lower offset than tone bursts.
• Age-Related Corrections
  • Infants under 12 weeks require the largest correction factors, especially with insert earphones (e.g., +10 dB at 4 kHz).
  • Correction factors reduce in two steps: at 12 weeks and 24 weeks, reflecting ear canal growth and maturation.
  • No correction factors are applied for children over 2 years old for inserts or bone conduction.
  • Bone conduction corrections similarly decrease with age due to changes in head size and mass.
• Applying Correction Factors in Clinical Practice
  • Correction factors are added algebraically to NHL thresholds to get EHL (positive values add, negative values subtract).
  • Use the combined correction factor tables from BSA to avoid manual calculations.
  • Case examples demonstrate how to:
    • Select appropriate correction factors based on stimulus, transducer, frequency, and corrected age.
    • Convert ABR NHL thresholds to EHL for air and bone conduction thresholds.
    • Use corrected thresholds for diagnosis, counseling, hearing aid fitting, and management planning.
  • The ERA masking calculator tool simplifies correction factor application by automating calculations based on input parameters.
• Consequences of Not Using Correction Factors
  • Misdiagnosis of hearing loss type (e.g., misinterpreting sensorineural loss as mixed or conductive).
  • Underestimation or overestimation of hearing thresholds, leading to inappropriate management decisions.
  • Risk of discharging infants with undetected mild hearing loss.
  • Potential over-amplification of hearing aids causing discomfort or masking critical speech frequencies, impacting speech and language development.

Methodology / Instructions for Applying Correction Factors

1. Gather Patient Information:
   • Corrected age (gestational age + postnatal age).
   • Stimulus type used in ABR (CE chirp, tone burst, click).
   • Transducer type (insert earphones, headphones, bone conduction).
   • Frequency tested (e.g., 500 Hz, 1 kHz, 4 kHz).
   • Recorded NHL threshold from ABR Testing.
2. Select Appropriate Correction Factors:

Category

Educational

Video