Evoked potentials are the responses of the nervous system to special stimulation of receptors or peripheral nerves. Their measurement is a part of complex electrophysiological examination methodology. They are used in diagnostics to determine the cause and degree of impairment of the nervous system, monitor the time course of disabilities and also during operations to monitor the status of the sleeping patient.
We obtain somatosensory evoked potentials (SSEP, VEP) by electrical stimulation of sensory fibres of the peripheral nerve. We detect damaged neural pathways to guide sensitivity and position sensitivity from the periphery through the lateral strands of the spinal cord into brain centre. The examination is performed from the upper and lower extremities. In the upper extremity we stimulate the median nerve in the wrist with registration above the brachial plexus in the clavicle, above the spinal cord (above the 5th cervical vertebrae) and above the cerebral cortex. We obtain SSEP from lower extremities by stimulating the tibial nerve behind the medial malleolus with registration above the lumbar spinal cord and above the cerebral cortex. This examination is by repeated stimulus with a frequency of 3-7Hz in a total of 300 to 1,000 pulses twice in a row for each limb. Registration electrodes are on the surface, stuck to the skin. The patient lies down during the examination, which has a total duration of around 45 minutes.
During examination of motor evoked potentials (VEP), we detect damage to the motor pathways in the frontal and lateral strands of the spinal cord. A magnetic coil attached above the head and above the spine is used for stimulation. Irritation leads to muscle responses that we detect with surface electrodes over the muscles of the arms and legs. The patient sits during the examination, in more severe cases they lie on the bed. Overall the examination lasts around 15 minutes.
Contraindications are pacemakers, hearing aids, epileptic status, or any electromagnetic metals in the body (e.g. aneurysm clips from older operations).
BAEP are stem auditory evoked potentials, which are used mainly in ENT patients with hearing disorders. Examination detects whether these are caused by peripheral impairment during inflammation, degenerative diseases, or whether there is central damage in the cerebellopontine angle or brainstem caused, for example, by a tumour. Auditory stimulus (clicking) is used for this examination. The patient wears headphones shielding them from ambient noise and sensory electrodes stuck to their head. The total duration of the examination is around 20 minutes.
BAEP testing is also used by the Department of Anaesthesiology intensive care unit to confirm brain death in patients after particularly traumatic intracranial injuries, and as part of the transplantation programme.
IOM (intraoperative monitoring)
Monitoring the functions of the nervous system during some neurosurgical operations under general anaesthetic is a part of neuro electrophysiology. This involves monitoring the conductivity of important pathways during surgery in the brain convexity, in the cerebellopontine angle and brainstem areas, during carotid disobliteration and during spinal cord operations. The benefit is easier orientation for the neurosurgeon in a difficult surgical field, reducing the risk of intraoperative injury to the patient. Because the operation is performed under general anaesthesia, spiral