Author(s): John J. A. Marota, Zachary Achen

Pathologic conditions of the spine treated with surgical intervention include intervertebral disk disease, spinal stenosis, scoliosis, spondylosis, spondylolisthesis, kyphosis and lordosis, tumors, and trauma. In the United States, the demand for these procedures has increased dramatically, owing to the widespread prevalence of low back pain, an aging demographic, and advancements in minimally invasive methods of surgical intervention. It is important that anesthetists be familiar with the challenges associated with these procedures including the prone position, fluid shifts, prolonged surgeries, and manipulations adjacent to nerves and major blood vessels. While general anesthesia is the predominant choice for anesthetic management of these procedures, both regional and local anesthesia are potential options for lumbar microdiscectomy or laminectomy.

1. Spinal cord injury

   1. Acute spinal cord injury may present with neurogenic shock, more commonly with lesions above T6. Presentation is hemodynamic instability due to sympathectomy with resultant arterial and venous vasodilation; hypotension and increased heat loss are common. Weeks to months after the acute phase of spinal shock, autonomic dysreflexia may develop, characterized by hypertension and bradycardia in response to stimulation below the level of loss of sensation; hypertension may result in myocardial ischemia, retinal/cerebral hemorrhage, and seizures. While a variety of physical stimulations can trigger this autonomic hyperresponse, the majority are associated with the most caudal root levels below the region of injury. Bladder distention is responsible for up to 75% to 82% of episodes of autonomic dysreflexia.

   2. Cervical spine injury is commonly associated with head trauma. Injury to the spinal cord at C5 presents with weakness of the deltoid, biceps brachialis, brachioradialis, and partial diaphragmatic paralysis. Cervical cord injury above C6 can compromise ventilatory function with either partial or total loss of ventilation. Approximately 55% of all spinal cord injuries occur in the cervical region; the most common cervical spine fracture occurs at C2 and accounts for ≈24% of injuries.

   3. Prolonged immobility secondary to paraplegia/quadriplegia leads to increased proliferation of extrajunctional acetylcholine receptors at the neuromuscular junction. Accordingly, succinylcholine is contraindicated starting at 48 hours after spinal cord injury due to risk of resultant hyperkalemia.

2. Airway

   1. In addition to standard concerns for securing the airway, additional caution is required for patients with cervical spine injuries to prevent or not worsen damage to the spinal cord. Hard cervical collars permit 72% to 73% of normal extension and flexion of the cervical spine. Movement

      of the cervical spine during direct laryngoscopy predominantly occurs at the occipital/C1 and C1/C2 levels with much less potential for movement at lower levels.

   2. Video laryngoscopy has emerged as a useful technique for intubation of patients with cervical spine injury because it requires less cervical spinal motion during laryngoscopy.

   3. Awake fiberoptic intubation followed by awake positioning for surgery should be considered for any patients with severe cervical spine instability after injury; directed neurologic examination is performed immediately before and after intubation to confirm that no change has occurred with manipulation. This approach should be discussed in advance with the surgical team as well as the sequence of events should be explained to the patient before proceeding with awake intubation; this approach requires a cooperative patient. Intubation is achieved with minimal or no sedation and adequate topicalization of the upper and lower airway with local anesthetic, after which the brief neurologic examination is repeated. The patient can be positioned on the operating (OR) table awake and then general anesthesia induced after confirming postposition neurologic examination.

   4. Manual inline stabilization of the cervical spine can be performed to limit cervical motion. With the patient supine, a second provider stands by the top of the head and provides traction on the head to maintain alignment and immobility relative to the shoulders; this limits motion of the neck during intubation and positioning. The laryngoscopist usually stands at the patient’s side by the left shoulder. Although this technique has gained wide acceptance and is a component of Advanced Trauma Life Support, its effectiveness has been questioned due to potential increase in craniocervical motion and worsening view during laryngoscopy. This technique can be performed with or without cervical collar in place and with direct laryngoscopy, video laryngoscopy, or fiberoptic intubation.

   5. In emergency situations where there is no evidence of facial/basal skull fractures, blind nasotracheal intubation is an acceptable option.

3. Prone position

   1. For patients undergoing posterior procedures, induction of general anesthesia and intubation is performed supine, typically on a stretcher or hospital bed, and then the patient is rolled prone onto OR table. Before turning prone, eyes are protected with tape or clear plastic adhesive; bite blocks, orogastric tube, and temperature probe are placed. Nasogastric or nasal temperature probes are discouraged because of the potential for bleeding when prone; the nasal approach is not recommended in cases with basal skull fracture or evidence of CSF leak from the nose.

   2. There are several options to support the head in the prone position. It can be placed on a commercially available foam head and/or face rest in which there are cut-out spaces for eyes and nose to remain free from compression; endotracheal tubes may be positioned either straight down through a hole in the rest or accessed to the side. Alternatively, the head can be supported by sharp pins screwed into the outer table of skull; Gardner–Wells tongs have two-pin fixation; Mayfield skull clamp system has a three-pin fixation. Gardner–Wells tongs system provides continuous traction; Mayfield skull clamp system holds the head rigidly in place, attached to the OR table. Alternatively, the halo of a halo vest immobilizer can be used either with traction, fixed to the OR table, or with the vest fixation.

   3. The physiologic changes associated with prone positioning may include depressed cardiac index from reduced filling pressures, inferior vena cava obstruction with decreased venous return to the heart, peripheral pooling of blood volume, increase in airway ventilation pressure with increase in intrathoracic pressure, increased functional residual capacity, and redistribution of pulmonary blood flow and lung ventilation to dependent areas. The prone position predisposes to atelectasis; PEEP may be necessary to maintain oxygenation.

   4. Care should be taken so that the eyes, abdomen, genitalia, and breasts are free from compression; the stomach can be decompressed with an orogastric tube and bladder with a urinary catheter. Improper positioning of arms can result in vascular compromise or brachial plexus injury, as well as increased pressure within the cubital tunnel with elbow flexion greater than 130 degrees. Complications such as shoulder dislocation, facial and laryngeal edema, eye injuries, and peripheral nerve palsy have been reported as well as hip dislocation with rolling into position. Bite injuries can occur during motor-evoked potential (MEP) monitoring due to jaw muscle contractions; bilateral molar bite blocks will decrease the likelihood of forceful apposition of the teeth. Turning from supine to prone presents the greatest risk for injury to the spinal cord with or without instability; care should be taken to maintain good alignment of the spine across the area of instability.

4. Monitoring

   1. Intraoperative monitoring of spinal cord functional integrity involves electrophysiologic monitoring of the sensory and/or motor transmission pathways; changes in functional activity could result from transection, direct compression, or ischemia produced by compromise or distortion of blood vessels. Different techniques to monitor integrity of spinal cord function include somatosensory-evoked potentials (SSEPs), MEPs, epidural electrodes, direct stimulation of spinal roots, F-responses, H-reflexes, testing specific reflexes, electromyography, transcranial electrical stimulation with screw electrodes, neuromuscular junction monitoring, and electrical impedance testing. Combining multiple methods increases sensitivity when the risk of spinal cord, nerve root, or peripheral nerve damage is high. Use of intraoperative monitoring has been shown to reduce postoperative neurologic morbidity and may identify functional compromise caused by surgical manipulation (eg, retractor or pedicle screw placement) so as to permit immediate correction of a reversible deficit.

   2. SSEPs and MEPs are the two most commonly utilized modalities for monitoring integrity of the spinal cord and peripheral nervous system; amplitude and latency of the electrophysiologic complex wave generated by depolarization of either nerve or muscle, respectively, are detected from multiple electrodes placed on the patient. Peripheral nerves at sites distal to the surgical site are stimulated repetitively to obtain SSEPs; interruption of the blood supply of the posterior spinal arteries or nerve compression causes loss of these signals. Transcranial electrical stimulation of the motor cortex or direct stimulation of the spinal cord or nerve roots generates MEPs detected as action potentials within specific muscles; spinal cord motor pathways are supplied by the single anterior spinal artery.

   3. Intraoperative “wake-up test” can be used to assess motor function. Patients are coached about testing before induction of general anesthesia. At the appropriate time, typically immediately after distraction

      of the spinal cord, the anesthetic is reduced sufficiently to permit the patient response to command to move the hands and feet; the anesthetic is deepened after confirming adequate neurologic function. The procedure requires adequate analgesia and reversal of muscle relaxation before the “wake-up.” The test carries the potential complications of coughing, displacement or loss of the endotracheal tube, venous air embolism (VAE), and awareness.

   4. All general anesthetics decrease the effectiveness of neuromonitoring in a dose-dependent manner by increasing latency and/or decreasing amplitude of the electrophysiologic signal detected from neuronal depolarization; these same changes are suggestive also of ischemia. Hypothermia and cortical burst suppression on EEG will suppress both detection of SSEPs and evoked neuronal depolarization necessary for MEPs; hypothermia and neuromuscular blockers suppress MEP detection. Low concentrations of isoflurane, desflurane, and N₂O do permit intraoperative monitoring but with reduced signal. The goal of anesthetic management is to maintain patient immobility for surgery; both inhalational agents as well as intravenous anesthetics including propofol and dexmedetomidine at sufficient dose can suppress EEG activity and decrease effectiveness of SEP and MEP to detect integrity of function. Opioids have a minor impact on evoked potentials. Total intravenous anesthetic with propofol and opioid infusions is a widely used technique with the caveat that the dose of propofol is adjusted to maintain adequate EEG activity. Ketamine increases EEG activity and may mask suppression of activity produced by propofol that obscures detection of SSEP and MEP; alternatively, etomidate increases SSEP signal amplitude, but variations in dose can lead to alterations in signal inappropriately interpreted as decrement of function. Nonetheless, the combination of propofol infusion, opioids (including remifentanil), ketamine, lidocaine, and dexmedetomidine can provide adequate patient immobility for surgery and permit adequate signal detection for monitoring evoked potentials. Bolus doses of anesthetic agents and sudden increases in inhaled agent concentration should be avoided when possible to minimize the deleterious impact on the detection of evoked potential signal changes.