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THE IONM PROCESS
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 by the 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
At closure, 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.
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