Monitoring Modalities

Somatosensory Evoked Potentials (SSEPs) are used to measure the functional integrity of the sensory nervous system. They have also proven to be an accurate and indirect measure of vascular function within and around the spinal column.

Evoked Potentials are electrical signals generated by the brain and spinal cord. Recurrent stimulation of strategic peripheral nerves elicits an electrical signal that travels via the dorsal pathways of the spinal cord. This electrical signal is then captured by recording electrodes, generally placed on the scalp or at the end point of a peripheral nerve recording. The electrical signal or “evoked potential” is then measured in terms of changes in the signal, with a 10% increase in latency (delay in the signal) or a 50% decrease in amplitude (loss of voltage or power of the signal) being the standardized criteria for alarm. The peripheral nerves commonly used to perform this test include Posterior Tibial Nerve (PTN), Common Peroneal Nerve (CPN), Ulnar (UN) or Median (MN).

With their simplicity of application and relative ease of use in regard to anesthetic constraints, SSEPs are the gold standard of monitoring utilized in the operating room today. Changes in SSEPs may stem from either mechanical or vascular compromise.

Motor Evoked Potentials (MEPs) are responses that are recorded following stimulation of the motor pathways within the central nervous system. Transcranial electric motor evoked potentials (TceMEP) are the most commonly used method of monitoring the lateral corticospinal tract or motor pathway. TceMEP involves electrical stimulation of the brain’s motor strip through the skull and recording the MEP responses distally in the small muscles of the arms and legs.

While SSEP monitoring provides a direct assessment of the spinal cord’s dorsal column (sensory pathway), TceMEP provides a direct and real time assessment of the spinal cord’s lateral corticospinal tract (motor pathway). While in some cases, mechanical or vascular compromise to the lateral corticospinal tract may be detected in the form of changes in SSEPs, there is much evidence to conclude that lone SSEP monitoring may often fail to detect neurological complications. And while it is possible to have mechanical injury to separate portions of the spinal cord, this failure is most likely due to the fact that there are separate vascular supplies feeding the motor and sensory pathways.

Electromyography (EMG) is a measure of the electrical activity within muscle tissue.

With specific muscles being attached to specific nerves, nerve function is interpreted from the type of activity captured in an EMG recording. In the surgical environment, EMG monitoring allows for the direct measurement of the particular nerves that are at risk during surgery.

In the surgical environment, EMG is useful where there is the possibility of injury to peripheral or cranial nerves that have muscle innervations. With resting muscle and functionally normal nerves being electrically silent, injured or irritated nerves will fire spontaneously, activating the muscle fibers. This activation of muscle fibers manifests itself in the form of “motor unit firing,” which can occur in increasing degrees of nerve injury:

  • spikes (individual discharges)
  • bursts (brief but larger patterns of discharge)
  • train activity (repeating discharge patterns)
  • neurotonic discharges (persistent and prolonged bursts of activity)

In spinal fusion surgery, pedicle screws are placed in the vertebra in order to fixate the spine into a desired corrective position. Pedicle screw placement may be accurately assessed by utilizing a triggered form of EMG. After the screw or screw hole is placed, the screw or screw tap is stimulated. When a certain threshold of current is attained the electricity escapes the bone, subsequently activating the adjacent nerves thereby eliciting a corresponding response in the muscle of which it is innervated. This level of current attained which causes a response helps to determine the “safe” proximity of the screw from the nerves to ensure that no breach of the pedicle has occurred.

Brainstem Auditory Evoked Responses or “BAERs” are a type of sensory response generated by the brain when transmitting and processing responses to auditory stimuli. In the operating room, evoked responses are obtained by utilizing cochlear implants to stimulate and monitor the auditory pathway.

BAERs monitoring has proven especially uselful in surgeries that involve the posterior fossa and approaches that involve operating close to the great structures of the auditory pathway such as microvascular decompression. Often used in conjunction with other modalities, in surgeries when the auditory pathway is at risk, BAERs have been shown to aid in the reduction of hearing loss that may often occur during these procedures.

It is difficult to judge anatomically which areas of the brain control which functions. Brain Mapping is achieved by identifying functional areas of the brain by use of either evoked electrical responses or by identifying physiological responses after direct brain stimulation.

There are two generally utilized methods of Mapping:

  • Identifying the central sulcus, which separates the motor vs sensory portions of the brain by looking for phase reversal
  • Direct brain stimulation which elicits responses distally in the arms and legs, thereby identifying specific brain function at the time of surgery before tumor resection.

By identifying the functional areas of the brain, such as the motor strip components and language regions, surgeons are more equipped to accurately judge which critical areas of the brain may be preserved during surgery.

Electroencepholography represents a graphical record of the electrical activity of the brain. EEG is utilized in a number of procedures in which either vascular compromise or monitoring of the brain’s generalized electrical activity is desired. Such cases include functional brain mapping and carotid endarterectomy.