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Intraoperative monitoring

Stimulating and Recording Electrode Placement

The neurotechnologist (NT) applies a series of 30 to 50 stimulating and recording electrodes in the pre-operative holding area or when the patient is brought into the operating room. Needle and stimulating electrodes may be applied alternately to avoid interference with intubation, placement of a foley catheter, or introduction of central and arterial lines, post-induction. The type of surgery will dictate the number of electrodes to be attached.

Patient Transfer

After the patient is brought into the operating room, he or she must be moved to the operating table. The NT verifies that the integrity of the neural structures associated with the brachial plexus is not compromised.

The NT must be aware of any prior orthopedic procedures (e.g., joint replacement or joint fixation due to surgery or disease) that may limit or require modification of technique during patient transfer.

Anesthesia and Post Induction Protocol

The NT must be aware of all anesthetic agents administered, including the approximate time of administration and duration of action. Handling this situation diplomatically becomes extremely important, since cooperation with the anesthesiologist is imperative to the smooth monitoring of any surgical procedure.

The operation moves into the actual surgical phase once anesthesia has been administered to the patient. There are brief periods during this time when the NT can check the electrical integrity of the recording electrodes and obtain basic information relative to the nerve structures placed at risk. At these points in the procedure, information provided by the NT’s biomedical computer is most helpful to the surgeon, and the NT can make the strongest contribution to the surgery’s success and patient’s welfare.

Anesthetic concentrations are monitored closely for their effect on the recordings that will be present for the remainder of the surgery. The anesthesiologist, working in conjunction with the NT, must be prepared to alter the anesthetic regime as needed for accurate monitoring of various modalities. Continuous monitoring alerts the NT to what may be happening as a result of anesthetic variations as opposed to surgical intervention itself. This minimizes the chance of making false judgments during the surgery when increased concentrations of anesthetic agents cause the responses to cease (as in the case of EMG activity and use of neuromuscular blockade), or when latencies and amplitudes are prolonged and decreased respectively.

Diminished blood flow to the spinal cord is detectable through changes in the somatosensory and motor evoked responses. Significant consequences can result if the NT does not alert the surgeon that a change has occurred.

Baseline Studies

Once patient baselines have been obtained, it is imperative that the NT inform the surgeon and document any abnormalities observed (e.g., when latency of a dermatome evoked potential is delayed bilaterally or on the right side when compared to the left). As a result, the surgeon may be more aggressive as decompression is performed to ensure that a particular nerve has no additional impingements or is not compromised by underlying pathology. The NT can thus provide useful information to aid the surgeon’s dissection and decompression.

Surgical Protocol

Knowledge of surgical instruments selected bythe surgeon will inform the NT of critical monitoring junctures or required alterations in recording or stimulating techniques. Monitoring the particular modalities that may be most affected at this juncture in the surgical procedure becomes imperative.

In general, critical monitoring periods are going to occur intermittently once the surgeon has finished the dissection and exposure. During the surgery, the NT must:

  • Understand what is being done surgically, so that the appropriate structures may be monitored at any given point during the procedure.
  • Make preparations for electrical stimulation of an implant such as a pedicle screw.
  • Verify intensities to avoid injury to other nerve structures in proximity to the implant stimulated.

Of course, ongoing communication between the NT and both the anesthesiologist and the surgeon remains critical.

It is advisable to verify that adequate stimuli are being delivered by observing a muscle contraction, (e.g., the small finger for the ulnar nerve; the big toe for the tibial nerve).

Improvement(s) in response(s) and deterioration of response(s) will generally occur in the following procedures:

  • Laminectomy
  • Discectomy
  • Foraminotomy
  • Distraction/Compression
  • Graft impaction
  • Tumor removal

Spontaneous EMG generally occurs during:

  • Electrocautery
  • Foraminotomy
  • Discectomy
  • Pedicle screw fixation
  • Lysis of adhesions
  • Lateral mass plating
  • Tumor removal

Electrically elicited EMG may occur:

  • With stimulation of all implanted devices.
  • With direct electrical stimulation of neural structures involved in surgical procedures.

Post-Surgical Protocol and Monitoring Duration

Atclosure, all needle electrodes that were placed in the sterile field should be shown to the circulating nurse or surgical technician. The nurse will verify that all needles are intact, the needle count is validated and nonreusable sharps have been properly disposed. Standards, Recommended Practices, and Guidelines by the Association of Perioperative Registered Nurses (AORN) serves as an excellent reference manual.

Continuous monitoring should be performed during the remainder of the procedure and throughout closure. The NT must be aware of any electrophysiological abnormalities occurring during closure, since a condition may develop that compromises the spinal cord. If the NT fails to notice a change in evoked potentials, premature transfer of the patient to the recovery room and ensuing deterioration in status can prompt a return to the operating room, reintubation and surgical re-exploration to correct the problem, a chain of events that may result in increased morbidity or even mortality.