ECOG: Assessing Cochlea & Cranial Nerve VIII

• Auditory Evoked Potentials (AEPs) assess the auditory system's function in response to sound

ECOG (Electrocochleography)

• Assesses the status of the cochlea and proximal Cranial Nerve VIII
• Aids in differential diagnosis of Meniere’s disease, perilymphatic fistula, Superior Semicircular Canal Dehiscence (SSCD), Inner ear malformation (IOM), and auditory nerve status
• Optimal stimuli: 85 dBnHL broadband clicks with alternating polarity
• Cochlear Microphonic: No measurable latency, generated by outer hair cells at the basal turn of cochlea
• Summating Potential (SP): Shorter latency than Action Potential (AP), generated by inner hair cells and nonlinear cochlear transduction elements
• Action Potential (AP): ~1.5 ms latency (Wave I on ABR), generated by summed activity of synchronously firing cochlear nerve fibers
• Interpretation: SP/AP amplitude ratio reflects cochlear mechanics, specifically basilar and Reissner's membrane movement during stimulation, which indicates the pressure gradient between endolymph and perilymph

ABR (Auditory Brainstem Response)

• Assesses auditory system function from the outer ear through the lower brainstem
• Can be used for neurodiagnostic purposes or to estimate auditory thresholds
• Stimuli: Broadband click, tonebursts, and chirps; selection depends on the test purpose
• Wave I: 1.5 ms latency, generated by the distal portion of CN VIII
• Wave II: 2.5 ms latency, generated by the proximal portion of CN VIII
• Wave III: 3.5 ms latency, generated by the cochlear nucleus
• Wave IV: 4.5 ms latency, generated by the Superior olivary complex (SOC) and lateral lemniscus (LL)
• Wave V: 5.5 ms latency, generated by the LL as they enter the inferior colliculus (IC)
• Interpretation: Absolute latencies of Waves I, III, and V; Interwave latencies of I-III, III-V, I-V; Interaural wave V latency differences
• Absence of a wave indicates a lesion at the corresponding site
• Threshold estimation involves identifying the measurable presence of wave V at varying intensities until it disappears

ASSR (Auditory Steady-State Response)

• Middle latency potentials determine lesion site and estimate auditory thresholds
• Useful for diagnosing severe to profound hearing losses and Auditory Neuropathy Spectrum Disorder (ANSD)
• Detects modulation, not neural synchrony
• Stimuli: Continuous carrier tones or chirps with amplitude modulation (AM) or frequency modulation (FM)
• Neural Generators: Auditory nerve, brainstem for mid-to-high-frequency modulations (500-4000 Hz), cortical regions for low-frequency modulations (less than 60 Hz)
• Interpretation: Computer performs statistical analysis of replicability in terms of phase/amplitude, which generates an estimated audiogram

MLR (Middle Latency Response)

• Helps determine lesion site, CI candidacy, CI neuron survival, CAPD, and provides better low-frequency information than ABR
• Does not require neural synchrony
• Stimuli: Toneburst stimuli
• Na: 18-20 ms latency, neural generator is Heschl’s gyrus
• Pa: 28-32 ms latency, neural generators are Heschl’s gyrus, planum temporale, and superior temporal gyrus
• Nb: 35-45 ms latency, neural generators are the reticular formation, superior temporal gyrus, and primary auditory cortex
• Interpretation: Absolute Na/Pa latencies, Peak-to-peak Na-Pa amplitude, Interaural differences

ALR (Auditory Late Response)

• Determines lesion site, functional impact of lesions, and estimates auditory thresholds
• Stimuli: Tonebursts or speech
• P1/N1 Complex: 100-200 ms latency, neural generator is the auditory cortex
• Evaluated by presence/absence, absolute latency, and replicability

Frequency Specificity Considerations

• Neurodiagnostic ABR uses a broadband click, which is not frequency-specific, but triggers synchronous neuronal firing along a large portion of the basilar membrane
• Frequency-specific information can be obtained through tonebursts, masking methods, derived band procedure, and toneburst gating
• Cochlear structure and function can make it difficult to elicit a response without triggering responses from other nearby fibers
• When the traveling wave reaches the 4000-2000 Hz region, hair cells are activated, triggering neurons and the ABR
• Pure tones cannot be used because they aren’t abrupt enough to trigger synchronous neural firing
• Tonebursts: Are representative of about an octave range of frequencies, with the highest frequency dominating the ABR response
• Chirps: Present low frequencies before high frequencies so each travels to its respective area of the cochlea at the same time, resulting in synchronous firing of neurons, represents an octave band

Key Principles

• Frequency Specificity: Neurons respond most strongly to a specific frequency
• Neural Tuning: Neurons respond selectively to a specific stimulus
• Place Specificity: Neurons are tonotopically organized, with different frequencies stimulating different regions of the cochlea
• AEPs manipulate these abilities to determine the function of the auditory system

AEP Application of Key Principles

• ABR: Targets frequency specificity with tonebursts, uses the place specificity of the basilar membrane
• ASSR: Uses modulated tones at specific frequencies to estimate an audiogram; examines phase-locked responses to modulated stimuli
• Phase Locking: Neurons fire at a specific phase of a sound wave, reflecting neural tuning
• Frequency Following Response (FFR): Measures neural activity in response to sound to reveal the integrity of sound processing in the brain; tracks neural phase locking to frequency components of complex sounds
• Each test provides information about specific pathways and structures, helping to evaluate the site of lesion (cochlea, auditory nerve, brainstem, or higher cortical areas)

Clinical Approach

• Can start with neurodiagnostic ABR to rule out any retrocochlear pathologies
• Then use ASSR to estimate thresholds
• Consider additional tests based on the suspected issue