Sensory Evoked Potential Responses (SSEPs) 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 visual, auditory or 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 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.

SSEP Waveforms

Upper extremity SSEP waveforms

Examples of amplitude recordings of SSEPs from stimulation of the posterior tibial nerve produce potentials (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. They have now become the recommended standard of care for corrective scoliosis surgery, are used frequently in cervical 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 amplitude.
• 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 amplitudes vary directly with size of incoming volley (Burke and Hicks 1998).