Intraoperative Neuromonitoring

Today on Anesthesiology, I was lucky to intubate a patient undergoing a laminectomy and lumbar spinal fusion. It was my first time participating in a neurosurgical case and I came across wealth of new information from my attending and resident! What I want to focus on is the topic of neuromonitoring during surgery.

First of all, why is neuromonitoring so important?

Patients undergoing neurologic procedures may be at increased risk from hypoxia and ischemia to vital neurologic structures. Since the patient is under anesthesia and is paralyzed for the procedure, one cannot simply check reflexes and ask a patient if they can feel this or that, move their toes and fingers etc. Intraoperative neuromonitoring is an excellent way to monitor the function of the central nervous and peripheral nervous system and may allow the early detection of ischemia and hypoxia before irreversible damage to the nervous system occurs. If a patient's action potential has diminished in amplitude or prolonged in duration, it is time to look at all of the factors that could be contributing. A good anesthesiologist will check all of the vitals (BP, HR, O2 saturation), the amount of volatile anesthestic the patient is receiving and if all of the monitors are in place. If everything looks good from the Anesthesiology side, the problem may be with the procedure itself (blood loss, swelling and compression of the spinal cord or peripheral nerves). 

What are the types of neuromonitoring used?

This can be divided into essentially the three "realms" that need to be monitored: motor, somatosensory and the overall brain function. 

Motor Evoked Potentials (MEP)

Through direct stimulation, motor pathways are assessed directly and measure the integrity of the motor neuron output by way of the action potential amplitude and duration. A nerve is stimulated and an outcome is measured from a muscle or a group of muscles. This is actually not monitored by the anesthesiologist but rather, a surgical tech who dedicates their entire time sitting by a machine and monitoring the evoked potentials (at least that's what was going on in our room). There are multiple different sites on the body where one could evoke action potentials such as...

• spinal cord stimulation - rarely used anymore
• transcranial motor evoked potentials - the cortex is stimulated through the scalp. This is what we used today. The pt had electrodes over his scalp (specifically over the appropriate area) and had evoked action potentials for the upper and lower extremities
• Cortical motor evoked potentials - the cortex is stimulated directly

Sensory Evoked Potentials (SEP) - Can be divided into somatosensory, visual and auditory

Somatosensory Evoked Potential (SSEP) is a very common form of neuromonitoring. Similar to MEP, it is measured action potential in response to stimulation of a cranial or peripheral sensory nerve (only in MEP it is motor and not sensory nerve stimulation). Common nerves stimulated are median, ulnar and posterior tibial nerves (our patient had ulnar stimulation).  SSEP  will help monitor the integrity of the sensory pathway (dorsal and spinothalamic tracts). 

Visual Evoked Potential (VEP) measures the action potential in response to stimulation of the optic nerve. A light is flashed into the patient’s eyes, and recordings are taken off the occipital area (need an EEG to record). An interesting point...our patient needed to be in the prone position for the procedure and his head was resting in a soft foam block with a hole in the middle for his face and eyes. My attending pointed out that we need to frequently check the patient's face to make sure that his eyes were not being compressed. The patient cannot move so if their eyes become compressed, they could swell and cause damage to the optic nerve or artery! It obviously wouldn't be a good thing if your patient woke up blind (hello, malpractice!). 

Auditory Evoked Potential (AEP) are used to monitor the integrity of CN VIII. We didn't use this today but I read that they can be very helpful in procedures such as during resection of acoustic neuromas.

Brain

Electroencephalography (EEG) is the summation and recording of postsynaptic potentials from the pyramidal cells of the cerebral cortex. The EEG is typically classified by frequency (alpha, beta, theta and delta waves). The EEG can be recorded off the scalp and forehead using surface and needle electrodes and reflects the overall metabolic activity of the brain. Why would this be helpful during surgery? Metabolic activity of brain cells requires energy so if there are problems or alterations with energy supply or production (hypoxia or ischemia) the brain cells will affect the readings on the EEG. 

A subset of the EEG that I have been seeing a lot of in all of the surgery rooms (not just neuro) are the Bispectral (BIS) monitors. BIS monitor is a derived EEG parameter used to measure the depth of hypnosis under anesthesia. A number above 80 indicates emergence from anesthesia. A value between 40-80 usually implies adequate hypnosis without possible recall. Recall is something that the anesthesiologist wants to prevent a patient from experiencing as it can be a very traumatic event psychologically. I have sensed from the residents and attendings that BIS is a very "rough" estimate and should not be used to assess blood flow to the cortex but only be used to get a general idea of "consciousness" of the patient. 

That's all for today! This got me even more excited for my Neurology rotation! I only have almost 1 more week left of Anesthesiology!