AUDITORY BRAIN STEM EVOKED RESPONSE THRESHOLD ESTIMATION (PABRT)

	AUDIOLOGY UNIT AUDITORY BRAIN STEM EVOKED RESPONSE THRESHOLD ESTIMATION (PABRT)
	Last updated 6th September 2000

PRODUCT NAME: AUDITORY BRAINSTEM EVOKED RESPONSE AUDIOMETRY (ABR) THRESHOLD ESTIMATION

DESCRIPTION: An 'objective' type threshold estimation test for co-operative subjects. 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 scored and threshold estimated. Results are interpreted and report produced by an Audiological Scientist.

LOCATION: The test will take place in a sound-proof room (min. 3m x 3m) with low levels of electro-acoustic interference.

PATIENT CATEGORY: Suitable for that category of patient suspected of presenting non- organic hearing loss. Referrals received from E.N.T. Medical Staff.

PROTOCOLS: HUMAN RESOURCES: Audiological Scientist (Grade B or above) or trained Audiologist (Grade 3 or above) plus, ideally, an Audiologist (Student or above) to assist.

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 appointment time.

SET-UP ENVIRONMENT: The patient will be positioned lying comfortably on a couch with adequate neck support. The couch should be positioned as far as practicable from the main test equipment within the test room. Non-test electrical equipment within the room should be switched off and lighting reduced or dimmed 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: For patients aged 16 years or under, one adult relative or friend should accompany him/her.

(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, try to sleep.

That the patient indicate by raising his/her hand, if he/she requires assistance.

Patients will be asked if they are comfortable in the test position. They will also be informed that the test worker is close at hand throughout the test procedure and that they should indicate that they require assistance during the test procedure by raising their arm.

(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 the results will be passed on to E.N.T. Medical Staff who will discuss the results with them at their next clinic visit.

(1) Earphone/Electrode Application and Removal: Electrode placement sites (listed below) at the vertex and mastoids/forehead should be cleaned with alcohol swabs. Paediatric type silver/silver chloride electrodes will then be applied to these positions using approved methods. Electrode gel is then applied to the electrode cups using a disposable syringe and blunt needs.

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

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

(3) Order of Testing: The best ear (as identified by pure-tone audiometry) will normally be tested first. Stimulus intensity should initially be set at 80dBn(HL). If no waveform is identifiable stimulus intensity will be raised in 10dB steps until a waveform is identifiable or the maximum output of the equipment is reached (98dB). Waveform response will be traced down in 10, 15 or 20dB steps to a level at least 10dB below that level at which a response is identifiable prior to digital filtering. The estimated threshold level will, if possible, be bracketed in 5dB steps and be repeatable.

(4) Operator Interpretation of Results: This will be performed or viewed by an Audiological Scientist (Grade B or above). The principal component of a waveform which must be identifiable to confirm a response is Wave V. This must be demonstrated to be present, following digital filtering, at descending stimuli levels down to threshold, following digital filtering.

RESULTS: Thresholds to clicks will be presented in a report and entered on a standard audiogram form at 3kHz, with the intensity scale labelled 'dBn(HL)' and with the letters 'ERA' written on the audiogram. Test data should be labelled at minimum withthe patient's name, hospital number and test date.

REPORTS: An individual test report will be produced for each patient comprising summary of results and conclusions. Estimated thresholds to clicks will be presented for each ear in the test report and the associated sequence of waveforms attached. The conclusions presented will compare thresholds estimated by ERA with those obtained by behavioural means and consider other test results. An indication of the likelihood of non-organic hearing loss will be given and will be produced by an Audiological Scientist (Grade B8 or above).

ERA equipment will be calibrated to the local specifications of North Wales Medical Physics Department.

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