Auditory brainstem response Top Predicted Questions and Correct Answers 2024

What is an ABR? - Electrical potential (far field) generated by a transient stimulus which generates synchronous firing of auditory neurons in auditory nerve and brainstem. What is recorded, how long does it take for the ABR waves occur and how long is test time? - Seven waves - wave I, III and V are most important Occur withn 15ms of stimulus onset (generally within 12ms) Test time is 20-30 mins in a cooperative subject. Name the first five wave generator sites. - Wave I - distal portion of auditory nerve Wave II - proximal end of auditory nerve Wave III - auditory neurons in cochlear nuclei Wave IV - superior olivary complex Wave V - fibres of the lateral lemniscus as they enter the inferior colliculus What are four clinical applications of ABR? - 1. Differential diagnosis between cochlear and retrocochlear lesion 2. Neurological ie. extent of brain damage, detection of vascular lesions of brainstem, brain death 3. Estimation of hearing thresholds in difficult to test patients and pseudohypacusis 4. Intraoperative monitoring of auditory nerve and brainstem during neuro-otological surgery. When do you need to refer for an ABR? - 1. Positive retrocochlear signs ie. reflexes absent/raised, abnormal reflex decay at 500 and 1000Hz, poor speech discrimination or rollover 2. Assymmetrical sensorineural hearing loss 3. Tinnitus/dizziness 4. Significant sensorineural loss with normal OAEs What are the subject factors for ABR? - 1. Age: Infants - latency decreases with increasing age (wave V 7.5ms at birth and 5.6ms adult) - nerve fibres are immature. Adults: Little variability in latency and amplitude. Elderly: Increase in latency and decrease in amplitude - age-related loss of neurons. 2. Gender: Females - shorter latencies & larger amplitudes 3. Body temperature: Hypothermia - increase in latency - decreased axonal conduction velocity/delayed synaptic transmission. 4. State of arousal: Not affected by sleep, even narcolepsy & coma have no serious effects on latency/amplitude. 5. Muscular artifact: ABR wave components can be completely obscured by excessive muscle artifact (neck/jaw muscles). 6. Myogenic noise (100-500 Hz) can affect quality of results with long testing time. 7. Drug effects: Resistant to sedatives and anesthetic agents What extraneous factors may affect ABR? - 1. Mobile phones 2. Electrical equipment 3. Magnetic fields 4. Power lines What is the diagnostic protocol for the ABR test? - Time window = 12ms Active (+) electrode = Forehead (Fz) Reference (-) electrode = Earlobe of test ear Ground electrode = Earlobe of non-test ear Impedance = less than 5 kohMs at each electrode, with a difference of up to 2kohMs between active and reference electrodes Stimulus = 2000 alternating wideband clicks Stimulus rate = 11.1/sec