SOMATOSENSORY EVOKED POTENTIALS

Sensory Evoked Potential Responses are minute electrical signals generated by the brain and spinal cord when transmitting and processing responses to sensory stimuli. These sensory stimuli may be something seen (visual), heard (auditory) or felt (somatosensory). Since these electrical signals are very small they are normally obscured by random electrical activity in the environment. In order to measure them dozens of responses are collected sequentially and then averaged together. The random electrical signals tend to cancel each other out, leaving the evoked response to be seen and measured. Delays or reductions in these responses help define or locate any problem in the system of nerves and nerve pathways that transmit and process the responses.

Somatosensory Evoked Potentials (SSEP, SEP) can be elicited by virtually any sensory stimuli, such as touch or temperature change. The preferred method of eliciting responses is by repeated minute electrical stimulation of the peripheral nerves (in the arms or feet) since this is easily controlled and tends to produce larger, better defined and hence more easily measured and compared responses.

History of SSEP and IONM:

•1913 – first sensory evoked response - attributed to Richard Caton (Liverpool, England)

•1947 - first scalp recording

•1954 - first signal averager – Dawson

•1980s - IOM use of SSEPs

•1990 - Therapeutic and Technology Assessment Subcommittee of the AAN – “Considerable evidence favors the use of monitoring as a safe and efficacious tool in clinical situations where there is significant nervous system risk, provided that it’s limitations are appreciated”

•1991 – 1995 (European Scoliosis Group, Nuwer et al.) large multi-center trials of SEP during scoliosis surgery showing efficacy in preventing surgical injury

•Current use of SSEP - SSEP for spinal cord monitoring now includes multiple recording sites and waveforms. For ex ANAle recordings of SSEPs from stimulation of the posterior tibial nerve produce potentials from (see diagram on right):

–Peripheral potentials from popliteal fossa
–Spinal potentials from caudal or rosteral cord
–Subcortical potentials from brainstem and thalamus
–Cortical waveforms from multiple cranial montages

USES:

SSEPs have been used in the operating room to measure integrity of the sensory nervous system for almost two decades. They have been shown to be sensitive in detecting or predicting injury to the sensory pathway and adjacent structures, especially in spinal surgeries, in several level 2 studies. They have now become the recommended standard of care for corrective scoliosis surgery (*), are used frequently in cervical (neck) surgeries and are recommended in any lumbar surgery where the surgeon wishes additional information about spinal integrity during the procedure(**).

Criteria For Change in Waveforms:

•‘Traditional’10/50 rule: The accepted threshold criteria for significant changes in waveforms (in the absence of anesthesia related or other non surgical causes) that indicate neurological dysfunction are a 10% increase in latency or a 50% reduction in ANAlitude.
•Subcortical waveforms are favored for measuring changes since they are less susceptible to anesthetic effect (Wolfe and Drummond 1988, Abel et al. 1990, Bernard et al. 1996) and ANAlitudes vary directly with size of incoming volley (Burke and Hicks 1998)
•An 80% reduction in the ANAlitude of the dorsal column mediated incoming volley is required before the cortical SEP is reduced to <50% of baseline (“Cortical ANAlification”) (Eisen et al. 1982)

•For subcortical (cervical) waves, 60% reduction in ANAlitude carries up to a 10X risk of neurological complications

Summary:

Upside of SSEP Monitoirng

•Well established criteria for reporting (gold standard)
•Well understood effects of anesthesia
•Can be done with paralytics
•High sensitivity (false negatives as low as 0.063%, Nuwer et al 1995)
•High negative predictive value (as high as 99.93 % Nuwer et al 1995)

Downside of SSEP MOnitoring:

•Require time consuming averaged signals (delayed results)
•Monitor only the dorsal columns / dorsal cord
•Not recordable in presence of peripheral neuropathies
•Very sensitive to electrical noise

Last Word on SSEPs

SSEPs remain the 'gold standard' for spinal monitoring and are likely the most frequently used monitoring modality

R. O'Brien MD

Select references:

Agarwal R, Roitman KJ, Stokes M. Improvement of intraoperative somatosensory evoked potentials by ketamine. Paediatr Anaesth 1998;8:263-6.

American Electroencephalographic Society. Guideline eleven: guidelines for intraoperative monitoring of sensory evoked potentials. American Electroencephalographic Society. J Clin Neurophysiol 1994;11(1):77-87.

Celesia GG, Allison T, Bodis-Wollner I, et al. American electroencephalographic society committee on guidelines for intraoperative monitoring of sensory evoked potentials. Guideline eleven: guidelines for intraoperative monitoring of sensory evoked potentials. J Clin Neurophysiol 1994;11:77-87.

Chatrian GE, Berger MS, Wirch AL. Discrepancy between intraoperative SSEP's and postoperative function. Case report. J Neurosurg 1988;69:450-4.

Chaves-Vischer V, Brustowicz R, Helmers SL. The effect of intravenous lidocaine on intraoperative somatosensory evoked potentials during scoliosis surgery. Anesth Analg 1996;83:1122-5.

Choudhry DK, Stayer SA, Rehman MA, Schwartz RE. Electrocardiographic artefact with SSEP monitoring unit during scoliosis surgery. Paediatr Anaesth 1998;8:341-3.

Daube JR. Spine surgery. In: Mayo clinic course on monitoring neural function during surgery. 1989.

Ecker ML, Dormans JP, Schwartz DM, Drummond DS, Bulman WA. Efficacy of spinal cord monitoring in scoliosis surgery in patients with cerebral palsy. J Spinal Disord 1996;9:159-64.

Grant GA, Farrell D, Silbergeld DL. Continuous somatosensory evoked potential monitoring during brain tumor resection. Report of four cases and review of the literature. J Neurosurg 2002;97(3):709-13.

Grundy BL. Intraoperative monitoring by evoked potential techniques. In: Aminoff ML, editor. Electrodiagnosis in clinical neurology. 3rd ed. New York: 1992:649-82.

Gugino V, Chabot RJ. Somatosensory evoked potentials. Int Anesthesiol Clin 1990;28:154-64.

Helmers SL. Intraoperative neurophysiological monitoring in pediatrics. In: Chiappa KH, editor. Evoked potentials in clinical medicine. 3rd ed. Philadelphia: Lippincott-Raven Publishers, 1997:661-74.

Helmers SL, Carmant L, Flanigin D. Anterior neck recording of intraoperative somatosensory-evoked potentials in children. Spine 1995;20:782-6.

Helmers SL, Hall JE. Intraoperative somatosensory evoked potential monitoring in pediatrics. J Pediatr Orthop 1994;14:592-8.

Johnson RM, McPherson RW, Szymanski J. The effects of stimulus intensity on somatosensory evoked potentials during intraoperative monitoring. Anesthesiology 1983;59:A365.

Kothbauer K, Deletis V, Epstein FJ. Intraoperative spinal cord monitoring for intramedullary surgery: an essential adjunct. Pediatr Neurosurg 1997;26:247-54.

Kumar A, Bhattacharya A, Makhija N. Evoked potential monitoring in anaesthesia and analgesia. Anaesthesia 2000;55(3):225-41.

Lesser RP, Raudzens P, Luders H, et al. Postoperative neurological deficits may occur despite unchanged intraoperative somatosensory evoked potentials. Ann Neurol 1986;19:22-5.

Linden DR, Zappulla R, Shields CB. Intraoperative evoked potential monitoring. In: Chiappa KH, editor. Evoked potentials in clinical medicine. 3rd ed. Philadelphia: Lippincott-Raven Publishers, 1997:601-38.

MacEwen GD, Bunnell WP, Sriram K. Acute neurological complications in the treatment of scoliosis. A report of the Scoliosis Research Society. J Bone Joint Surg Am 1975;57:404-8.

Mauguiere F. Somatosensory evoked potentials. In: Niedermeyer E, Lopes da Silva F, editors. Electromencephalography: basic principles, clinical applications and related fields. 4th ed. Baltimore: Williams and Wilkins, 1999.

Norcross-Nechay K, Mathew T, Simmons JW, Hadjipavlou A. Intraoperative somatosensory evoked potential findings in acute and chronic spinal canal compromise. Spine 1999;15;24(10):1029-33.

Nuwer M. Spinal cord monitoring. In: Nuwer M, editor. Evoked potential monitoring in the operating room. New York: Raven Press, 1986:126.

Nuwer MR. Spinal cord monitoring with somatosensory techniques. J Clin Neurophysiology 1998;15(3):183-93.

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Sala F, Krzan MF, Deletis V. Intraoperative neurophysiological monitoring in pediatric neurosurgery: why, when, how? Childs Nerv Syst 2002;18(6-7):264-87.

Seyal M, Mull B. Mechanisms of signal change during intraoperative somatosensory evoked potential monitoring of the spinal cord. J Clin Neurophysiol 2002;19(5):409-15.

Stephen JP, Sullivan MR, Hicks RG, et al. Cotrel-dubousset instrumentation in children using simultaneous motor and somatosensory evoked potential monitoring. Spine 1996;21:2450-7.

Thornton C, Sharpe RM. Evoked responses in anaesthesia. Br J Anaesth 1998;81(5):771-81.

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modalities

SSEP
EMG
TceMEP
BAER
Deep Brain Stimulation
Mapping

SSEP Waveforms

Upper extremity SSEP waveforms