Evoked Brain Potentials

Evoked Brain Potentials

Synonym/acronym: Brainstem auditory evoked potentials (BAEP), brainstem auditory evoked responses (BAER), EP studies.

Common use

To assist in diagnosing sensory deficits related to nervous system lesions manifested by visual defects, hearing defects, neuropathies, and cognitive disorders.

Area of application

Brain.

Contrast

None.

Description

Evoked brain potentials, also known as evoked potential (EP) responses, are electrophysiological studies performed to measure the brain’s electrical responses to various visual, auditory, and somatosensory stimuli. EP studies help diagnose lesions of the nervous system by evaluating the integrity of the visual, somatosensory, and auditory nerve pathways. Three response types are measured: visual evoked response (VER), auditory brainstem response (ABR), and somatosensory evoked response (SER). The stimuli activate the nerve tracts that connect the stimulated (receptor) area with the cortical (visual and somatosensory) or midbrain (auditory) sensory area. A number of stimuli are given, and then responses are electronically displayed in waveforms, recorded, and computer analyzed. Abnormalities are determined by a delay in time, measured in milliseconds, between the stimulus and the response. This is known as increased latency. VER provides information about visual pathway function to identify lesions of the optic nerves, optic tracts, and demyelinating diseases such as multiple sclerosis. ABR provides information about auditory pathways to identify hearing loss and lesions of the brainstem. SER provides information about the somatosensory pathways to identify lesions at various levels of the central nervous system (spinal cord and brain) and peripheral nerve disease. EP studies are especially useful in patients with problems and those unable to speak or respond to instructions during the test, because these studies do not require voluntary cooperation or participation in the activity. This allows collection of objective diagnostic information about visual or auditory disorders affecting infants and children and allows differentiation between organic brain and psychological disorders in adults. EP studies are also used to monitor the progression of or the effectiveness of treatment for deteriorating neurological diseases such as multiple sclerosis.

This procedure is contraindicated for

    N/A

Indications

    VER (potentials)

  • Detect cryptic or past retrobulbar neuritis
  • Detect lesions of the eye or optic nerves
  • Detect neurological disorders such as multiple sclerosis, Parkinson’s disease, and Huntington’s chorea
  • Evaluate binocularity in infants
  • Evaluate optic pathway lesions and visual cortex defects

    • ABR (potentials)

  • Detect abnormalities or lesions in the brainstem or auditory nerve areas
  • Detect brainstem tumors and acoustic neuromas
  • Screen or evaluate neonates, infants, children, and adults for auditory problems
  • EP studies may be indicated when a child falls below growth chart norms

    • SER (potentials)

  • Detect multiple sclerosis and Guillain-Barré syndrome
  • Detect sensorimotor neuropathies and cervical pathology
  • Evaluate spinal cord and brain injury and function
  • Monitor sensory potentials to determine spinal cord function during a surgical procedure or medical regimen

    • ERP (potentials)

  • Detect suspected psychosis or dementia
  • Differentiate between organic brain disorder and cognitive function abnormality

Potential diagnosis

Normal findings

  • VER and ABR: Normal latency in recorded cortical and brainstem waveforms depending on age, gender, and stature
  • ERP: Normal recognition and attention span
  • SER: No loss of consciousness or presence of weakness

Abnormal findings related to

  • VER (potentials):
    • P100 latencies (extended) confined to one eye suggest a lesion anterior to the optic chiasm.
    • Bilateral abnormal P100 latencies indicate multiple sclerosis, optic neuritis, retinopathies, spinocerebellar degeneration, sarcoidosis, Parkinson’s disease, adrenoleukodystrophy, Huntington’s chorea, or amblyopias.
  • ABR (potentials):
    • Normal response at high intensities; wave V may occur slightly later. Earlier wave distortions suggest cochlear lesion.
    • Absent or late waves at high intensities; increased amplitude of wave V suggests retrocochlear lesion.
  • SER (potentials):
    • Abnormal upper limb latencies suggest cervical spondylosis or intracerebral lesions.
    • Abnormal lower limb latencies suggest peripheral nerve root disease such as Guillain-Barré syndrome, multiple sclerosis, transverse myelitis, or traumatic spinal cord injuries.

Critical findings

    N/A

Interfering factors

  • Factors that may impair the results of the examination

    • Inability of the patient to cooperate or remain still during the procedure because of age, significant pain, or mental status. (Note: Significant behavioral problems may limit the ability to complete the test.)
    • Improper placement of electrodes.
    • Patient stress, which can affect brain chemistry, thus making it difficult to distinguish whether the results are due to the patient’s emotional reaction or to organic pathology.
    • Extremely poor visual acuity, which can hinder accurate determination of VER.
    • Severe hearing loss, which can interfere with accurate determination of ABR.

Nursing Implications and Procedure

Pretest

  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this procedure measures electrical activity in the nervous system.
  • Obtain a history of the patient’s complaints or symptoms, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s neuromuscular system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Address concerns about pain related to the procedure and explain that the procedure is painless and harmless. Inform the patient that the procedure is performed in a special laboratory by a health-care provider (HCP) and takes approximately 30 min to 2 hr, depending on the type of studies required.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Instruct the patient to clean the hair and to refrain from using hair sprays, creams, or solutions before the test.
  • Instruct the patient to remove jewelry and other metallic objects from the area to be examined.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.
  • Make sure a written and informed consent has been signed prior to the procedure and before administering any medications.

Intratest

  • Potential complications: N/A
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient.
  • Ensure the patient is able to relax; report any extreme anxiety or restlessness.
  • Ensure that hair is clean and free of hair sprays, creams, or solutions.
  • Ensure the patient has removed all external metallic objects from the area to be examined prior to the procedure.
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Visual Evoked Potentials
  • Place the patient in a comfortable position about 1 m from the stimulation source. Attach electrodes to the occipital and vertex lobe areas and a reference electrode to the ear. A light-emitting stimulation or a checkerboard pattern is projected on a screen at a regulated speed. This procedure is done for each eye (with the opposite eye covered) as the patient looks at a dot on the screen without any change in the gaze while the stimuli are delivered. A computer interprets the brain’s responses to the stimuli and records them in waveforms.
  • Auditory Evoked Potentials
  • Place the patient in a comfortable position, and place the electrodes on the scalp at the vertex lobe area and on each earlobe. Earphones are placed on the patient’s ears, and a clicking noise stimulus is delivered into one ear while a continuous tone is delivered to the opposite ear. Responses to the stimuli are recorded as waveforms for analysis.
  • Somatosensory Evoked Potentials
  • Place the patient in a comfortable position, and place the electrodes at the nerve sites of the wrist, knee, and ankle and on the scalp at the sensory cortex of the hemisphere on the opposite side (the electrode that picks up the response and delivers it to the recorder). Additional electrodes can be positioned at the cervical or lumbar vertebrae for upper or lower limb stimulation. The rate at which the electric shock stimulus is delivered to the nerve electrodes and travels to the brain is measured, computer analyzed, and recorded in waveforms for analysis. Both sides of the area being examined can be tested by switching the electrodes and repeating the procedure.
  • Event-Related Potentials
  • Place the patient in a sitting position in a chair in a quiet room. Earphones are placed on the patient’s ears and auditory cues administered. The patient is asked to push a button when the tones are recognized. Flashes of light are also used as visual cues, with the client pushing a button when cues are noted. Results are compared to normal EP waveforms for correct, incorrect, or absent responses.

Post-Test

  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • When the procedure is complete, remove the electrodes and clean the skin where the electrodes were applied.
  • Recognize anxiety related to test results, and be supportive of perceived loss of independent function. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be needed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include acetylcholine receptor antibody, Alzheimer’s disease markers, biopsy muscle, CSF analysis, CT brain, CK, EEG, ENG, MRI brain, plethysmography, and PET brain.
  • Refer to the Musculoskeletal System table at the end of the book for related tests by body system.