Pathological conditions of the spine treated with surgical intervention include intervertebral disk disease, spinal stenosis, scoliosis, spondylosis, spondylolisthesis, kyphosis and lordosis, tumors and trauma. The number of these procedures has increased exponentially in the United States as a consequence of the low back pain epidemic, aging population, and development of less invasive surgical techniques. It is imperative that anesthetists be familiar with the challenges associated with these procedures including prone position, fluid shifts, prolonged surgeries and manipulations adjacent to nerves and major blood vessels. General anesthesia is the most common technique for surgery of the spine, although regional anesthesia is a potential option for lumbar microdiskectomy 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 and resultant 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; these may result in myocardial ischemia, retinal/cerebral hemorrhage, and seizures.
   2. Cervical spine injury is commonly associated with head trauma. Notable lesions are C5 injury presenting with weakness of the deltoid, biceps, brachialis, brachioradialis and partial diaphragmatic paralysis; C4 lesions can result in paralysis of respiratory muscles including the diaphragm and necessitate positive pressure ventilation.
   3. Prolonged immobility secondary to paraplegia/quadriplegia leads to increased extrajunctional receptors at the neuromuscular junction. Succinylcholine is, therefore, contraindicated from 48 hours after spinal cord injury due to risk of hyperkalemia.
2. Airway
   1. In addition to the 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.
   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. For patients with severely unstable cervical spine injury, awake fiber-optic intubation followed by awake positioning should be considered; pointed neurological examination is performed immediately before and after intubation to confirm that no change has occurred with manipulation. The sequence of events should be explained to the patient in advance. Intubation is achieved with minimal or no sedation and adequate topicalization of the upper and lower airway with local anesthetic, after which a brief neurological examination is repeated. The patient can be positioned on the operating (OR) table awake and then general anesthesia induced after confirming postposition neurological examination.
   4. Manual inline stabilization can be performed to limit cervical motion; a second provider holds the shoulders and head to limit motion of the neck during intubation and positioning. The position is standing aside the patient with forearms resting on the chest. Although this technique has gained wide acceptance and is a component of Advanced Trauma Life Support, effectiveness has been questioned due to potential increase in craniocervical motion and worsening view during laryngoscopy.
   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 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 potential for bleeding when prone.
   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 the 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 use two pins; Mayfield skull clamp system uses three pins. Gardner-Wells tongs system provides continuous traction; the Mayfield skull clamp system holds the head rigidly in place attached to the OR table. Alternatively, the halo of a halo vest can be used either with traction or fixed to the OR table.
   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.
   4. Care should be taken so that the eyes, abdomen, genitalia, and breasts are free from compression; the stomach and bladder can be decompressed with an orogastric tube and urinary catheter. Improper positioning of arms can result in vascular compromise or brachial plexus injury, specifically increased pressure within the cubital tunnel with elbow flexion greater than 130°. Complications such as shoulder dislocation, facial and laryngeal edema, eye injuries, and peripheral nerve palsy have been reported. The greatest risk for injury to the spinal cord occurs during turning from supine to prone; 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 pathway transmission; changes in functional activity could result from direct compression or ischemia produced by compromise or distortion of blood vessels. Different methods to monitor spinal cord function include somatosensory-evoked potentials (SSEPs), motor-evoked potentials (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 risk of ischemia is high. Use of intraoperative monitoring has been shown to reduce postoperative neurologic morbidity and may identify in real-time compromise with surgical manipulation (ie, retractor or pedicle screw placement) to permit 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 electrophysiological 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 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 patient response to command to move the hands and feet; the anesthetic is deepened after confirming adequate neurological 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 electrophysiological signal detected from depolarization; these same changes are suggestive also of ischemia. Hypothermia and burst suppression on EEG suppress SSEPs; hypothermia and neuromuscular blockers suppress MEPs. Low concentrations of isoflurane, desflurane, and N₂O permit intraoperative monitoring but with reduced signal. Intravenous anesthesia with propofol, remifentanil, ketamine, midazolam, and etomidate, or some combination of these drugs, also permit adequate signal for neuromonitoring with less suppression of signal; etomidate increases SSEP amplitude.