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Evoked potentials (EP)

Overview

Evoked potentials are electrical responses in the brain or nervous system that are elicited by specific stimuli such as sound or touch. EP tests the intactness and conductivity of sensory conduction pathway from peripheral receptor organ to the brain.

  • Electrodes are placed over cortical areas
  • Stimulus is given as described above
  • The electrical response is recorded from the scalp electrodes
  • Latency and amplitude of stimulation is analysed.

  1. Visual evoked potential (VEP)

    Visual stimulation is given in the form of inverse checker board pattern or flash stimulation and the response is recorded from the occipital areas.

    Indications

    • Optic neuritis
    • Demyelinating disorders like multiple sclerosis
    • In head injury/ encephalopathy to check the visual pathway functioning

  2. Auditory evoked potentials (BAER)

    BAER, also known as brainstem auditory evoked response, measures the electrical activity generated in response to an auditory stimulus.

  3. Somatosensory evoked potentials (SSEP)

    SSEP measures the responses to touch or pressure stimulation. They enable to check the integrity of large sensory pathways through peripheral nerves, spinal cord, and brain.

    Indications

    • Differentiate between central and peripheral causes of large fibre sensory dysfunction
    • Study proximal peripheral nerves, when standard sensory NCV are normal
    • Confirmation of non-organic peripheral sensory loss
    • SSEPs in specific conditions
    • Multiple sclerosis (increased sensory latencies).
    • Need for localizing the level of lesion in spinal cord
    • Coma (bilateral absence of the thalamo-cortical (N19–P22) waveforms indicate poor prognosis)
    • Brain death (Absence of N19–P22 waveforms)
    • Cortical myoclonus (progressive myoclonic epilepsy, CJD, post-hypoxic myoclonus)

  4. Motor evoked potentials

    Measures the electrical response of muscles to electrical stimulation of brain or spinal cord.

    Indications

    • Assess motor pathway function
    • Spinal cord injury or multiple sclerosis
    • Intra-operative monitoring in spinal cord surgeries
    • Nerve injury evaluation
    • Assess muscle function in muscle weakness/ paralysis
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