Auditory brainstem evoked potentials (BSEPs) provide an objective measure of auditory function. In paediatrics their most common application is for assessment of peripheral auditory function to exclude a hearing loss for infants and difficult to test children. They also provide information about the maturity of central auditory brainstem pathways. In cases of VIIIth nerve tumours, BSEPs are sensitive in detecting involvement of the nerve. BSEPs also offer a simple non-invasive means of monitoring central auditory brainstem function in cases of demyelination or compression of the brainstem. This sensitivity to central auditory neural pathway integrity means that interpretation of results in terms of hearing capacity must assume normal auditory brainstem function. In some cases of brainstem malformation e.g.Arnold Chiari Malformation. and intrinsic lesions e.g.Glioma, this is clearly not possible. Conversely, hearing loss may complicate interpretation of central auditory brainstem waveforms when attempting to identify auditory brainstem dysfunction. The following information is provided as a description of the protocol for our department and is not presented as the definitive methodology.
Sedation: Experience has shown us that the best result is obtained for infants and children over 1mth of age if testing is performed while the patient is sedated. Neonates are usually tested during natural sleep following a feed. Older uncooperative children who do not sedate satisfactorily are tested under General Anaesthetic if this is considered justified by the parents and the clinician.
Patients for sedation fast from 5:00am
Out-patients report to Paediatric Admissions at 8:30am. Admission is usually to Day Ward.
Sedation is prescribed by the neurology registrar and is administered at 9:00am.
Sedation is usually Chloral Hydrate (50mg/kg body weight for patients from 1 to 3mths of age or who are under 4kg weight or who suffer from cardiopulmonary disease or other significant illness, 75mg/kg body weight for healthy children over 3mths of age and weighing more than 4kg unless there are clinical contra-indications.)
The patient is brought to the Neurology Dept. when asleep.
Parents remain with the patient and the procedure and results are explained as testing progresses.
Restless patients are occasionally nursed by their parent.
Gold cup recording electrodes are taped on each mastoid close to the pinna, at the vertex and a ground is on the fore-head. The ear-lobe may be used in stead of the mastoid, but tends to provide a less stable sub-strate on which to affix the electrodes. Skin impedances are reduced using OMNI-PREP (or similar) and maintained below 5kohms. Low balanced impedance values are as in all evoked potential work, essential to good recordings. We display our responses vertex positive polarity up.
Initial screening is performed using 100us clicks followed by further recordings using tone-pips, if time permits and if a hearing loss is suggested by the click responses.
Click duration: 100us, Rate: 16.6/sec, Condensation Polarity (We avoid alternating polarity as changing the polarity can have dramatic effects on response configuration). Click intensity is calibrated against normal hearing thresholds for a group of audio metrically assessed volunteers. Wide-band masking noise is applied to the contra-lateral ear at 40dBHL.
Pip parameters: Envelope for 1kHz, 2kHz and 4kHz linear rise/plateau/fall of 2/2/2 cycles, for 0.5Hz, 1/1/1 cycles due to stimulus artefact limitations.
Cadwell Spectrum32: Filters 5Hz - 3kHz., Gain: 20, Sensitivity:28 (Split screen mode). Sweep Duration:15ms except for 500Hz and 1kHz, 35ms.
Testing is routinely begun using 70dBHL clicks for the right ear. Each response is replicated at least once. If a good response is recorded, the stimulus intensity is reduced to 30dBHL then 20dBHL and 10dBHL. The threshold is determined by two supra-threshold responses separated by 10dBHL and one sub-threshold response. The left ear is then tested in a similar manner. The maximum stimulus intensity possible in the case of a profound hearing loss, is limited by the equipment used. The use of a range of stimulus intensities can help differentiate conductive, sensori-neural and central auditory disorders.
Tone-pip testing then follows depending on the suspected nature of the hearing loss: i.e. wether it is low or high frequency, mild or severe.
Where hearing loss is detected, follow-up is arranged immediately through the hospital Audiology Department with whom we have a good working liaison.
Patients with suspected middle-ear dysfunction are referred to Audiology for tympanometry.
Patients with more severe hearing losses are counselled by the hospital Audiologist and if not already arranged, an ENT appointment and possibly an assessment by the Australian Hearing Services for fitting of hearing aids is organised.
Where hearing appears to be normal, we leave it to the referring doctor to arrange any follow-up.
This is often performed on older children who do not require sedation. The main areas of variation from the above protocol are in the use of additional recording electrodes, a slower stimulation rate and a more limited range of click intensities.
Montage: M2-Cz, M1-Cz, M2-M1, Cv7-Cz.
Stimulus rate: 8.3/s
Stimulus Intensity: 70dBHL and greater.
Right and left ears are tested monaurally. Where there is a suspected hearing loss, the stimulus intensity may be increased.
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