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In general, spinal surgery may be thought of as degenerative for degenerative disk disease or for deformity. Patients undergoing deformity (or scoliosis) surgery may have congenital scoliosis or acquired scoliosis. Patients undergoing surgery for degenerative spine disease are generally older, and their anesthetic plans must take into account the comorbidities generally associated with aging.
Patients undergoing surgical treatment for congenital scoliosis may have restrictive lung disease and should be carefully evaluated for other congenital anomalies of cardiac, pulmonary, and neurologic origin. Patients with acquired scoliosis may be of idiopathic or neuromuscular origins and may also have restrictive lung disease. Some of the neuromuscular conditions that may accompany scoliosis are muscular dystrophy, dysautonomia, cerebral palsy, and neurocutaneous syndromes such as neurofibromatosis. For patients with neuromuscular scoliosis, it is important to be aware of the nature of the disease and associations with malignant hyperthermia.
Regardless of the specific indication for spinal surgery, perioperative considerations must include attention to positioning, an anesthetic choice compatible with neurologic monitoring, blood conservation, and adequate intravenous access. Postoperatively, these patients often require ventilatory support, present complex pain-management problems, and are subject to many metabolic and hematologic derangements, including hyponatremia and anemia.
Problems of surgery and anesthesia in the prone position are cited in Table 61-3 . Particular attention should be focused on positioning of the neck, arms, and eyes to protect pressure-sensitive areas. Patients who have an anterior approach to the spine may be positioned in the lateral decubitus position (if not supine) and require a heightened level of sensitivity to positioning. Regardless of how well a patient is positioned at the start of a procedure, ongoing vigilance with regard to position is essential because a patient's situation may change after movement during a wake-up test or manipulation of the operating table.
SSEPs, electromyograms (EMGs), motor evoked potentials (MEPs), and wake-up tests are commonly used to help safeguard spinal cord and nerve root function during surgery. Distraction of the spine, placement of pedicle screws, and bony decompression are intraoperative events in which the spinal cord itself or nerves may be injured. Successful monitoring efforts depend on
SSEPs, MEPs, and EMGs are sensitive to a number of anesthetic, patient, and environmental factors. Most notably, volatile anesthetics impair the reliability of SSEPs.[128] SSEPs monitor the integrity of the posterior aspect of the spinal cord for ischemia and injury. Consequently, isolated anterior spinal injury may go undetected. Although volatile anesthetics decrease the amplitude and increase the latency of evoked potential signals, low doses (<0.4% isoflurane) can be used. Nitrous oxide has a mild depressant effect, and can be used even in concentrations of 50% or greater. Intravenous anesthetic agents, including propofol, ketamine, and narcotics, have no clinically relevant effect on SSEP signals. EMGs, which are increasingly used to monitor nerve root injury, can be ablated with complete neuromuscular relaxation. However, partial relaxation monitored with a peripheral nerve stimulator can be used to provide clinical relaxation and maintain the integrity of EMGs.
The wake-up test remains the most reliable assessment of the intact spine for several reasons. Anesthetic agents may suppress SSEP signals; certain patient conditions such as neuromuscular degeneration may make SSEPs impossible to obtain; and anterior cord injury may go completely undetected with SSEPs. This becomes particularly important in deformity surgery in which the spine may be distracted significantly, which can affect blood flow to the anterior cord.
A wake-up test must be planned for well in advance. Because of the neuromonitoring concerns mentioned earlier, a predominantly nitrous oxide and narcotic technique typically is used. Small doses of volatile anesthetics, if used, should be discontinued an hour before wake up is anticipated. Two or three twitches on a train of four nerve stimulator are sufficient to allow the patient to move her or his toes. After discontinuation of nitrous oxide and ventilation with 100% oxygen, patients should be able to follow commands to "wiggle their toes" within 10 minutes. It is not advisable to reverse neuromuscular blockade or narcotics to speed a wake-up test, because this may result in violent movements that can damage instrumentation or hurt the patient. As soon as satisfactory movement is observed, anesthesia is reestablished, and the patient's positioning reconfirmed. A successful wake-up test suggests an intact cortex and spinal cord. Before deepening anesthesia, the anesthesiologist should check with the surgeon that another wake-up test will not be needed too soon. Although patient recall is very unlikely, it is advisable to discuss the test with the patient as part of the preoperative discussion.
The amount of blood loss varies somewhat with the type of surgery, but major blood loss can be expected. Particularly with large anterior-posterior deformity cases, blood loss of more than 3 to 5 L can occur. Autologous blood donation (some of which may be frozen if the donations start significantly before surgery), deliberate hypotension, intraoperative autologous salvage (i.e., cell-saver technique), [129] acute normovolemic hemodilution,[130] and antifibrinolytic therapy[131] are useful to reduce the need for homologous transfusion.
Intraoperative blood salvage, although increasingly common, is expensive and may not be appropriate in cases involving infection or tumor. Estimates of the cost vary, but it does not appear to achieve cost-effectiveness in cases of less than 1000 mL of blood loss[132] or cases in which less than 500 mL of red blood cell transfusion is required.[129] The use of aprotinin and aminocaproic acid have become common in open heart surgery. Aprotinin has been shown in spinal surgery[131] to reduce blood loss, diminish transfusion requirements, and preserve clotting as measured by thromboelastogram.
In any case of complex spinal surgery in which large blood losses can be anticipated, invasive monitoring should be used. Arterial lines should be paced because deliberate hypotension is a reliable method of blood conservation and blood gas determinations may be required to evaluate pulmonary function perioperatively. [133] Foley catheter placement should be standard for any surgery of significant duration and with fluid shifts. Central venous catheters also provide excellent and reliable venous access.
Pulmonary complications are the most common complications during the postoperative period. The incidence of pulmonary complications increases among patients who have anterior thoracic or thoracolumbar procedures. Vigilant monitoring, incentive spirometry, and aggressive pulmonary toilet are essential for reducing morbidity. In patients with preexisting pulmonary disease, the concern is even greater.
Many of the patients presenting for degenerative spinal surgery are already narcotic tolerant, and pain management in the postoperative period may be extremely difficult. The use of low-dose ketamine perioperatively has demonstrated efficacy in general and in orthopedic surgery.[134] [135] [136] [137] An initial dose of approximately 0.25 mg/kg, followed by an infusion of 2 to 2.5 µg/kg/min, improves pain scores, decreases nausea, reduces narcotic requirements, and is not associated in these low doses with hallucinations.
Postoperative hyponatremia can occur.[59] Although the mechanism is not clearly defined, inappropriate secretion of antidiuretic hormone has been implicated. Because of the massive fluid shifts, postoperative monitoring of all electrolytes and hematologic variables is essential.
Visual loss under anesthesia is an uncommon but devastating complication. Although well described in spinal surgery,[138] postoperative blindness is in no way limited to posterior spinal surgery (see Chapter 82 ). Anterior ischemic optic neuropathy also occurs after cardiopulmonary bypass, anterior spinal surgery, and even during sleep.
Visual loss occurring under anesthesia may occur by a number of mechanisms. Central retinal artery occlusion and venous occlusion are pressure-dependent phenomena[139] and generally preventable by meticulous attention to the eyes in the prone position. Ischemia of the occipital lobe can cause cortical blindness, and patients with risk factors for cerebrovascular disease are at increased risk.
Anterior ischemic optic neuropathy is a devastating cause of postoperative blindness and has a poor prognosis for recovery. Cases of anterior ischemic optic neuropathy have been associated with hypotension, anemia, surgery of long duration, or large intraoperative blood losses. The blood supply to the retina is from branches of the short posterior ciliary arteries, and vision loss occurs with infarction in areas of watershed blood supply between these arteries. Unfortunately, these same circumstances exist frequently in major spinal surgery without visual sequelae, making it impossible to identify those at risk preoperatively. Because the cause of ischemic optic neuropathy is unknown, there are no specific precautions. However, a discussion of the phenomenon should be included as part of informed consent before spinal surgery or other operations in which postoperative visual loss is more common.
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