Spine surgery

1

What are the common indications for spine surgery?

By-and-large, spinal deformity is the most common reason a patient requires surgery. Causes of the deformity are usually developmental, such as scoliosis, or degenerative processes. Other reasons for spine surgery include injury from trauma, mass resection, osteomyelitis, and neurological decompensation. These procedures are usually elective but can be urgent or emergent in certain situations.

2

What are the anesthetic considerations for spine surgery?

The site of surgery (i.e., lumbar, cervical, thoracic) and the planned surgery (fusion, microdiscectomy, number of levels, etc.) are the primary determinants of developing an anesthetic plan. Surgeons can approach the spine anteriorly, laterally, or posteriorly depending on the patient’s pathology.

• Airway: Surgery on the cervical spine may be associated with challenges in airway management because of significant limitations in neck range of motion. Moreover, with an anterior approach to the cervical spine, a specialized endotracheal tube that monitors vocal cord activity is typically used to provide information about possible recurrent laryngeal nerve injury. In the setting of cervical spine injury, great care is required to secure the airway in a safe manner. Direct laryngoscopy is rarely used for endotracheal tube placement. Instead, video laryngoscopy and fiberoptic bronchoscopy are commonplace, as they require less neck motion during laryngoscopy. In certain circumstances, an awake fiberoptic intubation is necessary. Postoperatively, airway swelling must be considered and the anesthesiologist needs to proceed with extubation cautiously. Airway edema is more likely to be encountered in cervical spine procedures, long procedures, and procedures with large volume of fluid administration.

• Positioning: The surgical approach dictates patient positioning and the anesthesiologist is instrumental in ensuring the patient is properly positioned. Most often a posterior approach is used; thus the patient is positioned prone. In this instance, great care must be taken to ensure that pressure points are padded, that joints are in neutral positions, and that the face is protected from excessive pressure. Vision loss is a rare but devastating complication of spine surgery and proper prone positioning is an essential part of prevention. Anterior approaches to the spine require the patient to be supine and positioning is usually straightforward. In cervical spine surgery, anterior approaches may require coordinated positioning of the head and neck between the anesthesiologist and surgeon. Lateral approaches to the spine require the patient to be in a lateral decubitus position. In this position, an axillary roll on the dependent side is needed to prevent brachial plexus injury and the neck and arms need careful attention to be in neutral positions.

• Blood loss: The amount of blood loss can be extremely variable, which requires the anesthesiologist to maintain vigilance and to frequently assess the extent of blood loss. A preoperative blood type and screen should be performed, when transfusion risk is low, and a blood type and cross, when transfusion risk is moderate or high. In most instances, at least two peripheral intravenous catheters are needed. When high-volume blood loss is expected, central venous catheters (i.e., an introducer sheath) should be considered. Placement of an arterial line should be considered when access to the arms during surgery is limited (cervical and thoracic spine surgery), when the volume of blood loss is anticipated to be high, when the duration of surgery is long, or when patient comorbidities warrant. On one end of the spectrum, spine surgery can be limited to one or two levels with a goal to relieve compression of the spinal cord or spinal nerve roots. These cases typically have less than 250 mL of blood loss. On the other end of the spectrum, surgery can span the entirety of the spine from the head to the pelvis. These procedures often require osteotomies, or cuts in the bone, to accomplish repair of extensive spinal deformities. Osteotomies can result in significant bleeding and a large surgery, such as the one described can have several liters of blood loss.

• Anesthetic agents: Surgery on the spine requires general anesthesia. Anesthetic agents chosen for surgery often depend on whether the surgeon requests intraoperative neuromonitoring (IONM) for the procedure. If IONM is not used, then volatile anesthetic maintenance with muscle relaxation is often sufficient. However, when IONM is used, total intravenous anesthesia is commonly used, and patients often cannot be paralyzed.

• Special considerations: Surgery on the thoracic spine, particularly with a lateral approach, requires one-lung ventilation. A double-lumen endotracheal tube or bronchial blocker is needed to facilitate surgical exposure.

3

What adverse event is more common after spine surgery compared with other surgical procedures?

Postoperative visual loss (POVL) can occur following spine surgery and may appear with or without obvious signs of ocular trauma. Visual deficits range from blurring to complete blindness. The four types of visual loss encountered are central retinal artery occlusion, central retinal vein occlusion, cortical blindness, and ischemic optic neuropathy (ION). ION is the most common form of POVL after spine surgery.

4

What are the causes of POVL?

Occlusions of the central retinal artery or vein are most commonly associated with direct trauma to the globe of the eye (direct pressure on the globe) and less commonly with embolic events. Proper positioning of the patient with careful attention to protection of the eyes is important to prevent this injury. Cortical blindness is rare and is caused by ischemia of the visual cortex of the brain or the optic tracts within the cranium. The precise causes of ION are unknown, but the optic nerve and its blood supply are at risk within the orbit and at the lamina cribrosa where it penetrates the thick sclera. The blood supply is variable among individuals, and a watershed area exists along the midsection of the nerve, between the zones of perfusion from the more posterior hypophyseal branches of the carotid artery and the short posterior ciliary artery anteriorly. The nerve is damaged when there is a decrease in perfusion pressure to the optic nerve below the threshold of autoregulation, and severity and duration of the ischemia will influence the resulting injury. Although ION has occurred following other surgical procedures, most commonly it follows prone spine surgery. The estimated incidence of ION after spine surgery ranges from 0.01% to 0.2%.

5

What are the risk factors for POVL?

In an analysis of 80 patients from the American Society of Anesthesiologists POVL registry, patients undergoing prone spine surgery were studied with the goal to identify independent risk factors. Those factors are increasing duration of surgery, male sex, use of the Wilson frame for patient positioning, obesity, and lower percentage of colloid to crystalloid fluid replacement. Anemia, intraoperative blood pressure, and the presence of chronic hypertension, atherosclerosis, smoking, or diabetes were not found to affect risk. There is no known treatment for POVL.