Anesthesia for Surgery on the Spine
Spinal surgery is most often performed for symp-tomatic nerve root or cord compression secondary to trauma or degenerative disorders. Compression may occur from protrusion of an intervertebral disk or osteophytic bone (spondylosis) into the spinal canal or an intervertebral foramen. Prolapse of an intervertebral disk usually occurs at either the fourth or fifth lumbar or the fifth or sixth cervical levels in patients 30–50 years old. Spondylosis tends to affect the lower cervical spine more than the lumbar spine and typically afflicts older patients. Operations on the spinal column can help correct deformities (eg, scoliosis), decompress the cord, and fuse the spine if disrupted by trauma. Spinal surgery may also be performed to resect a tumor or vascular malforma-tion or to drain an abscess or hematoma.
Preoperative evaluation should focus on any exist-ing anatomic abnormalities and limited neck movements due to disease, traction, or braces that might complicate airway management and neces-sitate special techniques. Neurological deficits should be documented. Neck mobility should be assessed in all patients presenting for spine sur-gery at any level. Patients with unstable cervical spines can be managed with either awake fiber-optic intubation or asleep intubation with in-line stabilization.
For many of these procedures, anesthetic manage-ment is complicated by the use of the prone position. Spinal operations involving multiple levels, fusion, and instrumentation are also complicated by the potential for large intraoperative blood losses; a red cell salvage device is often used. Excessive distrac-tion during spinal instrumentation (Harrington rod or pedicle screw fixation) can additionally injure the spinal cord. Transthoracic approaches to the spine require one-lung ventilation. Anterior/posterior approaches require the patient to be repositioned in the middle of surgery.
Most spine surgical procedures are carried out in the prone position. The supine position may be used for an anterior approach to the cervical spine, making anesthetic management easier, but increasing the risk of injury to the trachea, esophagus, recurrent laryngeal nerve, sympathetic chain, carotid artery, or jugular vein. A sitting (for cervical spine pro-cedures) or lateral decubitus (most commonly for lumbar spine procedures) position may occasionally be used.
Following induction of anesthesia and tra-cheal intubation in the supine position, the patient is turned to the prone position. Care must be taken to maintain the neck in a neutral position. Once in the prone position, the head may be turned to the side (not exceeding the patient’s normal range of motion) or (more commonly) can remain face down on a cushioned holder. Caution is necessary to avoid corneal abrasions or retinal ischemia from pressure on either globe, or pressure injuries of the nose, ears, forehead, chin, breasts (females), or genitalia (males). The chest should rest on parallel rolls (of foam, gel, or other padding) or special supports—if a frame is used—to facilitate ventilation. The arms may be tucked by the sides in a comfortable posi-tion or extended with the elbows flexed (avoiding excessive abduction at the shoulder).
Turning the patient prone is a critical maneuver, sometimes complicated by hypotension. Abdominal compression, particularly in obese patients, may impede venous return and contribute to exces-sive intraoperative blood loss from engorgement of epidural veins. Prone positioning that permits the abdomen to hang freely can mitigate this increase in venous pressure. Deliberate hypotension has been advocated in the past to reduce bleeding associ-ated with spine surgery. However, this should only be undertaken with a full understanding that con-trolled hypotension may increase the risk of periop-erative vision loss (POVL).
POVL occurs secondary to:
· Ischemic optic neuropathy
· Perioperative glaucoma
· Cortical hypotension/embolism
Prolonged surgery in a head-down position, major blood loss, relative hypotension, diabetes, obesity, and smoking all put patients at greater risk of POVL following spine surgery.
Airway and facial edema can likewise develop after prolonged “head-down” positioning. Reintubation, if required, will likely present more difficulty than the intubation at the start of surgery.
When patients are placed in the prone posi-tion, the face must be checked periodically to determine that the eyes, nose, and ears are free of pressure. Even foam cushions can exert pressure over time on the chin, orbit, and maxilla. Turning the head is not easily accomplished when the head is positioned on a cushion; therefore, if prolonged procedures are planned, the head can be secured with pins keeping the face free from any pressure.
When major blood loss is anticipated or the patient has preexisting cardiac disease, intra-arterial and possibly central venous pressure moni-tors should be considered prior to “positioning” or “turning.” Massive blood loss from injuries to the great vessels can occur intraoperatively with tho-racic or lumbar spine procedures.
Instrumentation of the spine requires the abil-ity to intraoperatively detect spinal cord injury. Intraoperative wake-up techniques employing nitrous oxide-narcotic or total intravenous anes-thesia allow the testing of motor function following distraction. Once preservation of motor function is established, the patient’s anesthetic can be deepened. Continuous monitoring of somatosensory evoked potentials and motor evoked potentials provides alternatives that avoid the need for intraoperative awakening. These monitoring techniques require substitution of propofol, opioid, and/or ketamine infusions for volatile anesthetics and avoidance of neuromuscular paralysis.