History of research[edit]
In 1967, Sohmer and Feinmesser were the first to publish ABRs recorded with surface electrodes in humans which showed that cochlear potentials could be obtained non-invasively. In 1971, Jewett and Williston gave a clear description of the human ABR and correctly interpreted the later waves as arriving from the brainstem. In 1977, Selters and Brackman published landmark findings on prolonged inter-peak latencies in tumor cases (greater than 1 cm). In 1974, Hecox and Galambos showed that the ABR could be used for threshold estimation in adults and infants. In 1975, Starr and Achor were the first to report the effects on the ABR of CNS pathology in the brainstem.[4]
Long and Allen were the first to report the abnormal brainstem auditory evoked potentials (BAEPs) in an alcoholic woman who recovered from acquired central hypoventilation syndrome. These investigators hypothesized that their patient's brainstem was poisoned, but not destroyed, by her chronic alcoholism.[8]
In certain cases where behavioral thresholds cannot be attained, ABR thresholds can be used for hearing aid fittings. New fitting formulas such as DSL v5.0 allow the user to base the settings in the hearing aid on the ABR thresholds. Correction factors do exist for converting ABR thresholds to behavioral thresholds, but vary greatly. For example, one set of correction factors involves lowering ABR thresholds from 1000 to 4000 Hz by 10 dB and lowering the ABR threshold at 500 Hz by 15 to 20 dB.[16] Previously, brainstem audiometry has been used for hearing aid selection by using normal and pathological intensity-amplitude functions to determine appropriate amplification.[17] The principal idea of the selection and fitting of the hearing instrument was based on the assumption that amplitudes of the brainstem potentials were directly related to loudness perception. Under this assumption, the amplitudes of brainstem potentials stimulated by the hearing devices should exhibit close-to-normal values. ABR thresholds do not necessarily improve in the aided condition.[18] ABR can be an inaccurate indicator of hearing aid benefit due to difficulty processing the appropriate amount of fidelity of the transient stimuli used to evoke a response. Bone conduction ABR thresholds can be used if other limitations are present, but thresholds are not as accurate as ABR thresholds recorded through air conduction.[19]
Advantages of hearing aid selection by brainstem audiometry include the following applications:
Disadvantages of hearing aid selection by brainstem audiometry include the following applications:
Sedation protocols[edit]
Common sedative used[edit]
To achieve the highest-quality recordings for any recording potential, good patient relaxation is generally necessary. However, many recordings can be filled and contaminated with myogenic and movement artifacts. Patient restlessness and movement will contribute to threshold overestimation and inaccurate test results. In most cases, an adult is usually more than capable to provide a good extratympanic recording. In transtympanic recordings, a sedative can be used when time-consuming events need to take place. Most patients (especially infants) are given light anesthesia when test transtympanically.
Chloral hydrate is a commonly prescribed sedative, and most common for inducing sleep in young children and infants for AEP recordings. It uses alcohol to depress the central nervous system, specifically the cerebral cortex. Side effects of chloral hydrate include vomiting, nausea, gastric irritation, delirium, disorientation, allergic reactions and occasionally excitement – a high level of activity rather than becoming tired and falling asleep. Chloral hydrate is readily available in three forms – syrup, capsule and suppository. Syrup is most successful for those 4 months and older, proper dosage is poured in an oral syringe or cup. The syringe is used to squirt in the back of the mouth and then the child is encouraged to swallow. To induce sleep, dosages range anywhere from 500 mg to 2g, the recommended pediatric dose is equal to 50 mg per kg of body weight. A second dose no greater than the first dose, and an overall dose not exceeding 100 mg/kg of body weight can be used if the child does not fall asleep after the first dose. Sedation personnel should include a physician and a registered or practical nurse. Documentation and monitoring of physiologic parameters is required throughout the entire process. Sedatives should only be administered in the presence of those who are knowledgeable and skilled in airway management and cardiopulmonary resuscitation (CPR).
Increasingly, Propofol is used intravenously via infusion pump for sedation.
Procedures[edit]
A consent form must be signed and received from the patient or guardian indicating the conscious sedation and the procedure being performed. Documented medical evaluation for pre-sedation purposes including a focused airway examination either on the same day as the sedation process or within recent days that will include but not limited to:
All orders for conscious sedation for patients must be written. Prescriptions or orders received from areas outside of the conscious sedation area are not acceptable. There has to be a single individual assigned to monitor the sedated patient's cardiorespiratory status before, during and after sedation.
If patient is deeply sedated, the individual's only job should be to verify and record vital signs no less than every five minutes. All age and size appropriate equipment and medications used to sustain life should be verified before sedation and should be readily available at any time during and after sedation.
The medication should be administered by a physician or nurse and documented (dosage, name, time, etc.). Children should not receive the sedative without supervision of a skilled and knowledgeable medical personnel (at home, technician). Emergency equipment including crash cart must be readily available and respiration monitoring should be done visually or with stethoscope. Family member needs to remain in room with patient, especially if tester steps out. In this scenario, respiration can be monitored acoustically with a talk-back system microphone placed near patient's head. Medical personnel should be notified of slow respiration state.
After procedure is over, patient must be continuously observed in the facility that is appropriately equipped and staffed because patient's typically "floppy" and have poor motor control. Patients shouldn't stand on their own for the first few hours. No other medications with alcohol should be administered until patient is back to normal state. Drinking fluids is encouraged to reduce stomach irritation. Each facility should create and use their own discharge criteria. Verbal and written instructions should be provided on the topics of limitations of activity and anticipated changes in behavior. All discharge criteria must be met and documented before the patient leaves the facility.
Some criteria prior to discharge should include: