A 73-year-old man with spinal cord compression presented for posterior cervical fusion (C3-6). For the past year, he has experienced neck pain and increasing weakness and “tingling” in his right arm. His medications included sustained-release oxycodone, 40 mg twice a day, and oxycodone 5 mg/acetaminophen 650 mg, 2 tablets every 4 hours, although he did not take his medication the morning of surgery. Radiology studies showed severe cervical spinal cord compression. The neurophysiologic monitoring team monitored somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), and electromyography (EMG) beginning after induction to obtain a baseline before positioning.
Does severe cervical stenosis necessitate awake fiberoptic intubation?
The decision to perform an awake fiberoptic intubation for patients with cervical spine disease depends on the location of the injury or disease, the risk of aspiration (full stomach), the anticipated relative ease of mask ventilation, and the patient’s ability to cooperate with airway anesthesia. Awake fiberoptic intubation is the “gold standard” to minimize neck movement caused by mask ventilation and direct laryngoscopy. Additionally, any patient who has risk factors for difficult mask ventilation (i.e., obesity, >55 years old, history of snoring, lack of teeth, the presence of a beard, Mallampati class III or IV, abnormal mandibular protrusion test) should be considered for awake intubation. In an uncooperative patient, one must weigh the risk of induction versus movement from agitation or inadequate airway anesthesia. Induction before tracheal intubation is an option for patients who are not at increased risk for aspiration and for whom mask ventilation is not predicted to be difficult.
Videolaryngoscopes may be useful to secure the airway in patients with limited mobility, although they can cause more movement of the cervical spine than a well-executed fiberoptic intubation. Some channel-type videolaryngoscopes (e.g., Airway Scope (AWS) S-100; [Pentax-AWS system]; AWS; Pentax, Tokyo, Japan, and AirTraq [ATQ; Prodol Meditec, Vizcaya, Spain]) have been shown to produce less cervical spine movement relative to direct laryngoscopy. Glidescopes (Glidescope® videolaryngoscope (Verathon, Bothell, WA) have been shown to cause less cervical spine movement at C2-5 but similar movement to direct laryngoscopy at other levels, which means that Glidescopes are not a better choice for cases with more rostral cervical instability. However, the Glidescope does produce improved views, even with a cervical immobilization collar in place. The advantage of these alternative devices is their larger field of view, making them better choices for an airway with a large amount of secretions or blood or debris, such as in a trauma patient.
How is chronic pain managed during major spine surgery?
Although transdermal systems (e.g., fentanyl patch) should not be initiated, they should be continued if already in use. Patients should be instructed to take their oral medications, with the exception of nonsteroidal antiinflammatory drugs, which should be discussed with the surgeon. For opioid-tolerant patients who refrain from pain medication on the morning of surgery, the daily opioid consumption can be calculated and converted to the equivalent intravenous morphine dosage. This calculation provides an approximation of the daily opioid intake, some of which should be administered intravenously during surgery. Converting oral opioids to an intravenous dose is controversial; however, the underlying principle is that surgical patients who are opioid-tolerant require opioid dosing based on their usual consumption.
Oral gabapentin can reduce morphine consumption and pain scores in the first 12 hours after surgery. The optimal preoperative dose to maximize pain control and minimize side effects ranges from 600 to 900 mg. The most common side effects are nausea and vomiting, lightheadedness, and ataxia. Because many patients with chronic pain have experience with gabapentin, it is prudent to ascertain effective dosing for each individual patient. In some cases, side effects limit use of gabapentin. Based on this information, a decision can be made for or against administering gabapentin preoperatively and at what dose.
Methadone can be an important part of the intraoperative opioid regimen in opioid-tolerant patients undergoing major spine surgery. A single bolus dose of methadone (0.2 mg/kg) may reduce postoperative opioid use and visual analog scale scores for 48 hours postoperatively. Ketamine can also be a useful adjunct during spine surgery because it is profoundly analgesic and may improve the quality of intraoperative monitoring. Especially for opioid-dependent patients, the use of intraoperative ketamine (0.5 mg/kg on induction of anesthesia, with continuous infusion at 10 μg/kg/min terminated at wound closure) can reduce opioid requirements and pain scores up to 48 hours to 6 weeks later without an increase in side effects. The mechanism of action of ketamine includes N -methyl-d-aspartate receptor antagonism.
Explain the mechanism by which intraoperative neurophysiologic monitoring helps detect evolving spinal injury; are there any contraindications to its use?
Three major categories of neurophysiologic monitoring are routinely performed for spine surgery: SSEPs, MEPs, and EMG.
SSEPs monitor the integrity of the dorsal columns of the spinal cord. An electrical or magnetic impulse is delivered to the periphery, producing electrical potentials that propagate through the spinal cord to the brain. The waveform that is generated can be measured at the level of the spinal cord proximal to the surgical field or at the brain ( Figure 18-1 ).