AUDITORY BRAIN STEM EVOKED RESPONSE NEUROLOGICAL (PABRN)

	AUDIOLOGY UNIT AUDITORY BRAIN STEM EVOKED RESPONSE NEUROLOGICAL (PABRN)
	Last updated 6th September 2000

DESCRIPTION: A diagnostic type neurological test. The test is based upon utilising auditory stimuli to evoke an electrical response generated from the cochlea to the upper brainstem. The response is obtained by extraction from the EEG recording by the principle of averaging. The response to stimuli at different intensities is then presented graphically to be interpreted and scored. Provides an effective measure of the integrity of the auditory pathways up to the upper brainstem level. A useful tool for differentiating between sensory and neural impairment. Results are interpreted and report produced by an Audiological Scientist.

LOCATION: The test will take place in a sound-proof room with suitable lighting (or equivalent) and with low levels of electro-acoustic interference.

PATIENT CATEGORY: Suitable for investigating that category of patient with suspected peripheral or brainstem neurological dysfunction (e.g. resulting from an acoustic neuroma) and with pure-tone hearing thresholds (in the test ear) less than 80dB at 1-4kHz. Referrals received from E.N.T. Medical Staff.

PROTOCOLS: HUMAN RESOURCES: Audiological Scientist (Grade B or above) or trained M.T.O. (Grade 3 or above).

APPOINTMENT TIME: 80% of patients should be given a first appointment for a date within four weeks of receipt of referral.

WAITING TIME: 80% of patients attending for this procedure should be seen within 20 minutes of their appointment time with an explanation given for any delay.

SET-UP EQUIPMENT: The equipment collection parameters should be such as to optimise the ABR with appropriate masking.

SET-UP ENVIRONMENT: The patient should be positioned lying comfortably on a couch with adequate neck support. The couch should be positioned to reduce the effects of any electro-magnetic interference.

(1) BRIEFING: The following information or equivalent will be given to the patient at the start of the procedure:-

"We are going to perform a test today to see how well signals travel from your ear to your brain. This involves measuring your ears and brain responses to sounds which you will hear through some headphones. To do this, we need to put three electrodes on the surface of your head. We will then ask you to lie on the couch. The test is very safe and does not hurt".

(2) INVOLVEMENT OF OTHERS: One adult relative or friend may accompany the patient.

(3) PATIENT INSTRUCTION: Instructions, or their equivalent, should comprise of the following information:-

That the patient relax as much as possible, with eyes closed, neck rested and, if possible, go to sleep.

(4) PATIENT DEBRIEFING: The patient will be informed that responses obtained need to be analyzed and that this takes some time. They will be informed that results will be passed onto the E.N.T. Medical Staff who will discuss those results with them at their next clinic visit.

Electrode placement sites (listed below) at the vertex and mastoid/forehead should be suitably cleaned. Paediatric type silver/silver chloride electrodes will then be applied to these positions, using appropriate methods. Electrode gel is then applied to the electrode cups, using a disposable syringe and blunt needle.

(2) Earphones will be applied followed by an electrode impedance check. Individual impedances should be less than 2K and all impedances should be within 0.5K of each other. Post test, glued electrodes should be removed and electrode sites should be cleaned with cotton wool.

(3) Pre-test Requirements: For female patients at least one of the test workers should be female.

(4) Order of Testing: The best ear (as identified by pure-tone audiometry or by symptoms) should be tested first. Stimulus intensity should initially be set at the recommended starting level ca. 80dBn(HL). If no waveform is identifiable in the initial record (collection of responses) stimulus intensity should be raised to the maximum output of the equipment with at least one replication per intensity. Each record will be subjected to digital filtering prior to the visual analysis. Once the latencies of the principal components of the waveforms (Waves I and V) have been identified reliably (through repetition if necessary) stimulus intensity will be lowered to 80dBn(HL). Records will normally continue to be collected until Waves I, III and V are identified reliably (i.e. high degree of repeatability and/or good relationship to higher stimulus intensity waveforms) or until three repeat records have been obtained. If Wave V is the only identifiable component of a waveform at any test level, the test will normally be repeated twice at that level at a repetition rate of 80 clicks/seconds and the resulting Wave V latency measured (averaged over the two records).

(5) Operator Interpretation of Results: This should be performed or viewed by an Audiological Scientist (Grade B or above) who should be at hand, if not actually present, throughout the test procedure. The principal components of a waveform which should be identifiable at consistent latencies to effect most reliable test conclusions are Waves I, III and V.

(6) Criteria for Normality: Absolute latencies and interlatencies (i.e. test values) should be compared with 'normative values'. Test values outside normative value confidence intervals (two sd from mean) should be considered as abnormal. Wave V latency shift, for a ten-fold increase in stimulus presentation rate should be <1.0 ms in normals at 80dBnHL. Adjustments should be made when comparing test and normative data for the degree of hearing loss (measured by pure-tone thresholds) in the test ear.

RESULTS: Test result latencies and interlatencies with associated waveforms should be presented to complement a written test report. Test data should be labelled at minimum with the patient's name, hospital number and test date.

Individuals interpreting results should be aware that research generally indicates, in screening for acoustic BERA, a hit rate of 97% and false alarm rate of 30% (Turner et al 1984). It should be indicated, however, that the diagnostic potential of the procedure is somewhat reduced when the test ear demonstrates the following (particularly at the higher frequencies):

REPORT : An individual test report will be produced for each patient comprising a summary of results and conclusions. The conclusions presented will consider results from other tests. If so indicated, the conclusion will state the recommendation of further means and methods of assessment (e.g. imaging techniques). The report will be produced by an Audiological Scientist (Grade B8 or above).

ERA equipment should be calibrated to the local specifications of North Wales Medical Physics Departments.

INTERNAL: Once, in a six months' period, the length of time patients wait for attention will be monitored and the results recorded and made available for external audit.

An Audiological Scientist (Grade B8 wutg CAC qualification or above) will monitor annually staff (a) performing ERA tests and (b) those analysing results/producing reports. Monitoring will take place while the test is being performed and following report production to ensure that correct test practices and protocols are being observed by test workers. Monitoring of procedures will be according to local protocols. The results will be logged and made available for external audit.