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MONITORING YOUR SURGERY
Prior to your surgery, the neurotechnologist (NT) who will
conduct intraoperative neurophysiological monitoring (IONM)
during your procedure will meet you in the preoperative holding
area. After obtaining pertinent information from your chart,
the NT will attach multiple electrodes to stimulate the nerve
structures involved with your procedure. The number of electrodes
depends on the type of surgery you are having. Tape is used
to secure the electrodes carefully because once the procedure
is underway, it is difficult and sometimes impossible for
the NT to add more electrodes.
When you are brought into the operating
room:
- The electrodes will be attached to a
biomedical computer designed specifically for the purpose
of intraoperative monitoring.
- You will be anesthetized and positioned
on the operating room table.
- Then, sterile disposable needle electrodes
will be inserted under the skin of the scalp and along the
nerve structures to record responses obtained from the stimulating
electrodes.
Before the surgery begins, the NT makes
a baseline recording of all the nerves involved and stores
the information for comparison to subsequent recordings. Depending
on the length of your surgery, any combination of the nerves
monitored may be stimulated several thousand times. The stimulations
will not cause injury to the nerves themselves.
When your surgical procedure has been completed,
but before you recover from anesthesia, all stimulating and
recording electrodes will be removed.
 How
Monitoring Works
The NT will notify the surgeon if the latency (or time frame)
of the response changes by 10 percent and if the amplitude
(or intensity) of the response decreases by 50 percent. For
example, the evoked response obtained with stimulation of
the tibial nerve at your ankle may take 40 milliseconds to
reach the brain. If this response were to increase by four
milliseconds when compared to your baseline recording, the
NT would have sufficient cause to notify the surgeon of the
change. The amplitude and latency of a response equal each
other in importance.
A change in amplitude may occur independently
of a change in latency, or they may happen at the same time.
Additional modalities (or diagnostic tests)
of monitoring used may include:
- Monitoring of the electromyographic response
(or motor activity) produced by irritation of a nerve that
can cause the muscle to contract (called spontaneous electromyographic
activity).
- Electrically stimulating an implant such
as a pedicle screw to ensure the implant has been placed
in the proper position (called triggered electromyographic
activity).
- Monitoring of blood oxygen saturation
(SPO2) from the big toe to see if blood flow to the lower
extremities decreases (as with anterior lumbar surgery).
We are now capable of stimulating the motor
cortex of the brain and recording responses from appropriate
muscle groups. This technique proves very valuable in corrective
surgeries such as the reduction of a scoliotic deformity.
The NT checks both motor evoked potentials (MEPs) and somatosensory
evoked potentials (SSEPs). MEPs monitor the motor pathways
located in the front of the spinal cord, while SSEPs monitor
the sensory pathways located in back of the spinal cord.
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