Auditory Brainstem Response Exam Questions with Correct Answers

What is an ABR? Correct Answer 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? Correct Answer 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. Correct Answer 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? Correct Answer 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? Correct Answer 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? Correct Answer 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