Hearing Measurement Nine - Electrophysiology (Posted)

ABR Threshold Estimation

• ABR threshold estimation is always based on wave V
• Wave V's robustness with level change makes it ideal for assessing thresholds
• Wave V facilitates recording due to its robustness at high rates
• It displays a distinguishable pattern and a well-defined Latency-Intensity function

Threshold Assessment

• The use of Tone-ABR allows for prediction of pure-tone behavioural thresholds
• The predictions are useful when assessing infants and young children

Auditory Development Milestones

• Infants aged 0-4 months startle or blink in response to loud sounds, such as one at 80 dB
• Infants aged 4-7 months turn their head toward sounds
• Children aged 7-24 months turn their head toward soft levels, approaching threshold
• Children aged 24+ months can engage in play

Basic Electrical Principles

• I stands for Current
• V stands for Voltage
• R stands for Resistance

Voltage

• Voltage represents a difference in potential
• Electrical current flows down the potential gradient, much like water flows down a height gradient

The Electric Neuron

• Extracellular fluid has high sodium (Na+) levels
• When a neuron is aroused, neurotransmitters are let go, bridge the synaptic gap, and attach to the focus neuron, resulting in the of Na+ channels opening
• Na+ inflow into the dendrite makes the extracellular fluid more negatively charged because it reduces Na+ in that space
• A "sink" (low potential) is created in the extracellular space
• Inflow of Na+ into the dendrite makes the intracellular fluid more positively charged (more Na+ in that region)
• A "source" in the intracellular space (a high potential) is created as a result

Measuring with Electrodes

• Extracellular potential differences can be recorded when electrodes are attached to the scalp

Near-Field vs. Far-Field

• Near-field and far-field recordings capture electrical activity at different distances from the source

Recording in the Far Field

• Highly synchronized activity is needed for recording in the far field

Cortex Layout

• The cortex measures 3-5 mm thick
• The cortex is made up of 6 layers
• Large pyramid cells are aligned in the same direction
• The pyramidal cells are packed perpendicularly to the cortex
• There are roughly 100,000 cells in 1 mm²

Spatial Summation and Dipoles

• Spatial summation of neuronal activity creates an equivalent dipole that generates recordable signals

Volume Conduction

• The volume conductor is the extracellular space of the brain
• When a potential difference is present it acts like a battery
• High point (+) travels through the volume conductor to the low point (-)
• When several of these are active in the same direction at the same moment in time, the summation happens
• This sum becomes as one large battery

Basic Principles for Measuring ABR

• Spatial summation required to measure an ABR
• Synchrony required to measure an ABR
• Open field (same direction) required to measure an ABR

Recording Methodology

• Electrodes are attached to the head using conductive paste
• Outer layer of skin needs to be removed
• The voltage difference between electrodes is captured by an amplifier
• A ground is established for reference
• Voltage is measured over time, and data is sent to a computer for analysis

Evoked Response Averaging

• Signal and noise are summed over multiple sweeps
• Responses are consistent on each sweep, the sum grows larger because noise changes on each trial
• Signal to noise ratio rises with averaging

Transient Potentials

• Transient potentials represent time-locked electrophysiological responses to stimuli

Three Response Separations

• Auditory Brainstem Response occurs at 10 ms
• Middle Latency Response is recorded at 50 ms
• Slow Auditory response occurs at 500 ms

Auditory Nerve Potentials

• Brainstem and auditory nerve potentials aid in neurodiagnostics

Amplitude Variation and Latency Stability

• Amplitudes are variable, but latencies are stable
• Waves VI and VII may be difficult to see
• Waves I-V are fused and usually identifiable
• Waves I, III, and V are the best ones in the series

Absolute Latencies

• Normal latencies for a 60 dB nHL click have been measured

Interpeak Latencies

• Normal latencies for a 60 dB nHL click have been measured with interpeak latencies

Vestibular Schwannomas

• Delayed or absent ABR peaks are what usually occur
• This is discovered ~95% with tumors ≥1 cm
• Happens unilaterally roughly 95% of the time
• Asymmetric sensorineural hearing loss is the initial symptom!
• Abnormal reflexes manifest
• Poor speech perception occurs often at high levels

I-III Interval Evaluation

• A normal I-III interval measures 2-2.1 ms with an SD of 0.2 ms
• Abnormal for VIIIth nerve tumours, about 85-100% of the time;
  • Musiek (1986) showed abnormal if > 2.4 ms
  • Lightfoot (1992) defined abnormal as > 2.52 ms

III-V Interval Evaluation

• Normal at 1.9 ms, SD measures 0.2 ms
• Sometimes abnormal for those with VIIIth nerve tumors

I-V Interval Measurement

• Normal: 4.0 ms (.2 ms SD)
• May be simpler to use than I-III for detection of abnormalities
• Does not differentiate AN from brainstem tumours

Absolute Wave V Latency Assessment

• Abnormal at greater than 6.3 ms (to a click)
• Hearing loss influences results, interpret with caution
• May be the only measure feasible

Interaural Wave V Latency

• Abnormal if greater than .3 or .4 ms.
• Only wave V is needed
• Adjust presentation level for each ear as needed when there is an Asymmetric hearing loss

V/I Amplitude Ratio

• Abnormal if less than .75; e.g. wave V is less than 3/4 of the amplitude of wave I
• Interpret only stable amplitudes
• Must make interpretations regarding near-field recordings of wave I of horizontal recordings