Anesthesia for Neurosurgery - Surgery on the Spine and Spinal Cord
Surgery on the spine and spinal cord is undertaken for a variety of conditions, including intervertebral disk disease, spondylosis, stenosis, neoplasm, scoliosis, and trauma. The physiology of the spinal cord and brain is similar, even though absolute rates of blood flow and metabolism are lower in the spinal cord. Maintaining spinal cord perfusion pressure (which equals MAP minus extrinsic pressure on the cord) and reducing cord compression are clinical management objectives.
The prone position
The prone position is frequently used. Most patients can be anesthetized on a stretcher and “logrolled” onto the operating room table after endotracheal intubation. Awake intubation should be considered for patients with tenuous neurologic conditions that may be worsened by laryngoscopy/intubation or positioning (e.g., patients with unstable cervical or thoracic spine injuries). Under these circumstances, an abbreviated neurologic examination should be performed after intubation and transfer to ensure that injury has not occurred. The anesthetist should ensure that all pressure points are padded; neck and extremities are in neutral positions; eyes, ears, nose, and genitalia are free from pressure; and all monitors and lines are secured in place and functioning. Special attention should be paid to the endotracheal tube, since it can move or kink in the process of positioning. Ischemic optic neuropathy is a potential complication of prone cases associated with length of procedure (usually >5 hours), blood loss (usually >2 L), hypotension, and fluid resuscitation. Increased facial swelling may alter venous hemodynamics in the globe, leading to optic nerve ischemia and postoperative visual deficits. There are no standard preventive guidelines, but maintenance of systemic blood pressure near baseline values, frequent eye checks to assess for direct pressure on the globe, and maintenance of adequate perfusion are likely beneficial.
Surgery to correct scoliosis
Surgery to correct scoliosis can be accompanied by significant blood loss. Various techniques can be used to reduce homologous blood transfusion, including preoperative autologous donation, intraoperative hemodilution, use of intraoperative blood-scavenging techniques, and meticulous patient positioning to prevent increased abdominal and intrathoracic pressures that can increase venous bleeding. Because of concern of neurologic sequelae, induced hypotension may not be advantageous in this procedure. Scoliosis surgery is accompanied by a 1% to 4% incidence of serious postoperative neurologic complications. Spinal instrumentation and distraction can cause spinal cord ischemia and result in paraplegia. Intraoperative monitoring of spinal cord function is used routinely.
- SSEP and MEP monitoring provide continuous evaluation of spinal cord function.
- The selective wake-up test. Intraoperatively, if there is uncertainty about the neurophysiologic monitoring, the presence of neuromuscular function is assured when patients are awakened briefly and asked to move their legs. If there is no leg movement, the spine distraction is released until movement is observed. Patients should be prepared for this event preoperatively. Wake-up tests can be performed in older children.
- Total IV anesthesia with remifentanil and propofol is often selected because it is less likely to interfere with neurophysiologic monitoring than volatile anesthetics; however, it does not provide for a reliably fast intraoperative wake-up test. Alternatively, children, who often have robust nerve conduction, may be anesthetized with desflurane, with or without nitrous oxide and short-acting narcotics to achieve a much faster intraoperative wake-up. Communication of anesthetic interventions with the clinical neurophysiologist or technician is important.
After acute spinal cord injury
After acute spinal cord injury, surgery may be required to decompress and stabilize the spinal cord. The primary goal in the initial management of acute spinal cord injury is to prevent secondary damage to the injured cord. This is accomplished by stabilizing the spine and correcting circulatory and ventilatory abnormalities that can exacerbate the primary injury. The presence of cervical cord injury should lead one to suspect associated head, face, or tracheal trauma; thoracic and lumbar spine injuries often are associated with chest or intra-abdominal trauma.
- Spinal shock is characterized by vasodilation and hypotension. If the lesion involves the sympathetic cardiac accelerator nerves (T1–T4), bradycardia, bradyarrhythmias, atrioventricular block, and cardiac arrest can occur due to unopposed vagal activity. Spinal shock occurs because of functional transection of sympathetic innervation below the level of the injury and may persist for days to weeks. Bradycardia can be treated by atropine. Hypotension can be treated by fluid, vasopressors, or both. A pulmonary artery catheter may be helpful when other injuries are present and volume status is uncertain. Patients with high spinal cord injury may be unusually sensitive to the cardiovascular depressant effects of anesthetics because of an inability to increase sympathetic tone.
- Lesions above C3–C4 necessitate intubation and mechanical ventilatory support because of loss of innervation to the diaphragm (C3–C5). Lesions below C5–C6 may still cause as much as 70% reduction in vital capacity and FEV1 with impaired ventilation and oxygenation.
- Atony of the gastrointestinal tract and urinary bladder necessitates a nasogastric tube and indwelling urinary catheter, respectively. These patients are also prone to heat loss because of inability to vasoconstrict.
- Methylprednisolone (30 mg/kg IV loading dose, followed by an infusion of 5.4 mg/kg/h for 23 hours) may improve the functional recovery of patients with acute spinal cord injuries if treatment is begun within the first 3 hours after injury. There is some controversy surrounding this therapy for spinal cord injury, and some centers do not follow this protocol.
- Chronic spinal cord injuries are discussed in Chapter 27.
- Airway management of patients with cervical spine injury is discussed in section VI.I.