Evoked potential tests
What is it?
Evoked potentials (EP) are a particular neurophysiological technique that allows to record at scalp level the brain bioelectric activity induced ("evoked") by the activation of the main sensory systems (visual, acoustic and somatosensory) through appropriate stimuli. The generators of PE are represented by visual stimuli for optical pathways (Visual Evoked Potentials, VEP), sound stimuli for acoustic pathways (Acoustic Evoked Potentials) and tactile stimuli, consisting of electrical impulses just above the perceptual threshold for somatosensory pathways (Somatosensory Evoked Potentials, SSEP). In contrast, the Motor Evoked Potentials (MEP) are potentials generated at the muscular level by stimulation of motor areas of the cerebral cortex, entered into clinical practice thanks to non-invasive magnetic stimulation of the motor cortex (Transcranial Magnetic Stimulation, TMS).
When is this exam indicated?
Evoked potentials are indicated in all neurological and extraneurological pathologies, congenital or acquired, involving specific sensory, somatosensory and motor systems. So, they find primary indication in visual and acoustic deficits, helping to differentiate receptor (or peripheral) deficits from those of the central projections both of the skull base and occipital cortex (ENP), and truncated brain (AEP). Similarly, SSEP, which allow to assess how the nerve stimulus is conducted from the periphery to the cortex through the medulla, are indicated in nervous system dysfunctions characterized by positive sensory symptoms (paresthesias) or negative (hypo/anesthesia), as well as in pain syndromes. MEPs are useful for functional exploration in all systemic motor diseases, such as motor neuron diseases, in which they allow to differentiate between the extent of central and peripheral involvement. Both SEPP and MEP are also highly sensitive methods in extrinsic compressive pathologies of the spinal canal capable of damaging the long efferent corticospinal pathways (cervical myelopathy, lumbar canal stenosis, neoplasms). In addition to specific systemic diseases, EPs are widely used in the multisystem disease, i.e. multiple sclerosis, representing an instrumental support useful for the diagnosis and functional monitoring of the disease, as well as the effect of therapy. A very interesting application of EPs is intraoperative monitoring (IOM), where their high temporal resolution is utilized, i.e., their ability to rapidly change following damage to sensory and motor systems as a result of both direct (neurosurgery) and indirect (thoracoabdominal aneurysm vascular surgery) surgical manipulations.
How is it performed?
In outpatient clinical practice, evoked potentials are recorded with surface electrodes from areas of the scalp corresponding to the primary cortical visual, acoustic, and somatosensory areas for VEP, AEP, and SEPP, respectively. VEPs are evoked in the cortex by the fixation of a monitor that reproduces particular visual checkerboard patterns; in case of major refractive defects or impossibility of fixation, it is possible to obtain a visual potential by means of light flashes. AEPs are obtained with acoustic stimuli (clicks) emitted through the use of a headset, of varying intensity depending on the acoustic threshold of each subject. SEPPs are instead obtained by stimulating the peripheral nerves to the 4 limbs through an electrical stimulus just above the threshold of perception. MEPs are obtained by direct stimulating the motor cortex through a magnetic stimulator and recording the motor potential through electrodes placed on the distal muscles of the 4 limbs.
Contraindications
Transcranial magnetic stimulation motor evoked potentials are contraindicated in cardiac pacemakers and in patients with epilepsy (risk of seizure triggering). There are no particular contraindications, however, with regard to VEP, SEPP and AEP
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