A detailed medical history should be obtained to rule out comorbidities including alcoholism and clotting disorders. Anticoagulants and medications such as aspirin or other anti-inflammatories that modify the clotting cascade should be stopped several days before surgery. Patients on coumadin for life-threatening conditions such as cardiac arrythmia may require hospital admission for preoperative heparinization, which may then be stopped a few hours before surgery. Adequate platelet function and availability of blood products, including fresh frozen plasma in selected cases, should be ensured.

Close attention should be paid to preoperative MRI or CT images to visualize different anatomic characteristics of the vertebrae (i.e., rotational component, angle of kyphosis, size of lateral masses, or vertebral body). This information assists with intraoperative placement of hardware for rigid fixation. Furthermore, the advent of image guidance systems, including frameless stereotaxy, can minimize the incidence of injury in both anterior and posterior approaches, as well as improve placement of implants, including atlantoaxial transarticular screws.

Proper positioning is paramount to safe and meticulous execution of surgical technique. Patients with unstable cervical spines, whether caused by traumatic or inflammatory conditions, and those with compressive myelopathy are at risk for injury during positioning and intubation. Care must be taken when moving the patient, and extreme positions should be avoided (7). Because direct compression of the cervical cord may occur with excessive extension or rotation in patients with myelopathy, a safe range of motion, particularly extension, should be determined preoperatively. Conscious fiberoptic intubation should be gently performed in these patients. Furthermore, baseline motor-evoked potentials (MEPs) and somatosensory-evoked potentials (SSEPs) should be obtained to monitor the spinal cord and spinal nerves during intubation and positioning. The loss and reappearance of SSEPs during positioning may occur (8). It may be more beneficial to position the neck in slight flexion and, once the spinal cord is decompressed, the neck can be repositioned to bring it to a more normal position.

The head should be securely held in position by an appropriate headrest. Ophthalmic complications such as permanent blindness have occurred with headrests that put pressure on the eyes (9). A headrest should eliminate all pressure points and securely hold the head and neck in the desired position. A headrest may sometimes lead to skull fracture or perforation (10), and other complications such as infection, scalp lacerations, and pressure necrosis of surrounding skin. Avoiding unnecessary tightening of the clamp and careful use in thin or soft skulls will prevent fracture or perforation of the inner table of the skull in elective cases. Furthermore, in traumatic cases, the existence of traumatic fractures should be known and therefore avoided to prevent subsequent dural penetration, brain abscess formation, or epidural hematomas. For many unstable traumatic injuries, preoperative immobilization in a halo device is the best option, as long as it does not interfere with the operative procedure.

The padding and positioning of the extremities should also not be overlooked. Extra foam padding or cushioning of bony prominences is important. Every patient should be positioned and all areas padded appropriately with the understanding that the surgery may take much longer than expected. The ulnar and anterior interosseus nerves are most at risk in the upper extremity, whereas the common peroneal nerve and its branches along with the lateral femoral cutaneous nerve are at risk in the lower extremities during patient positioning.

The cervical spinal cord is very sensitive to any manipulation and does not tolerate any external compression or retraction. Adequate visualization with appropriate magnification with surgical loupes and a light source, or the use of an operative microscope, is mandatory for all cervical procedures to minimize the potential for injury to the dura or spinal cord. Hemostasis and a relatively bloodless surgical field with maintenance of adequate mean arterial blood pressure also aid in improving visualization within the tight confines of the neck. Furthermore, we cannot stress enough
that high-quality neurophysiologic monitoring is quintessential to safe performance of the operative procedure.

Flynn reviewed 36,657 cases and noted 311 neurologic injuries, with 100 of these permanent myelopathy or radiculomyelopathy. He observed that 53 of 70 postoperative myelopathic complications were immediate in onset. However, actual intraoperative trauma was recorded in only 38 cases (3). In other reports, the incidence of myelopathic complications is low, ranging from 0% to 2.1% (3,11,12). Plunging of an instrument with direct spinal cord damage during anterior cervical surgery has been reported in the literature (13).

During the procedure, direct manipulation of the delicate spinal cord is to be strictly avoided because permanent neurologic injury can result. Distraction of the vertebral segment should be performed with the utmost care, especially in patients with spinal stenosis (14). If the canal is severely stenotic, distraction should be applied incrementally. During anterior cervical discectomy surgery, distraction of the disc space with a vertebral body spacer or Caspar retractor facilitates visualization of the posterior half of the disc space. Stretching also tends to separate the posterior annulus from the posterior longitudinal ligament (PLL), making it easier to perform a complete discectomy. At this depth, care should be taken to do side-to-side curetting and to avoid plunging. Application of posterior pressure on the PLL during removal of the disc material should be avoided to minimize the risk of spinal cord injury. A burr can facilitate osteophyte removal, as it can be used to thin the offending osteophytes before they are removed with curettes in a posterior-to-anterior motion. Furthermore, frequent irrigation while using a burr is important as it helps to diminish the local thermal effects and improves visualization. It should also be mentioned that nerve root injury is possible while removing osteophytes and disc material in the lateral corner near the uncovertebral joints. Thus, the removal of such material should be performed with care and vigorous attempts to remove these osteophytes should be avoided. However, takedown of the PLL with an angled curette under direct visualization can facilitate removal of a sequestered disc or large osteophyte. With that in mind, there is an increased risk of direct spinal cord or dural injury whenever the PLL needs to be removed in anterior cervical surgery. The speed of correction of spinal deformity or reduction also may play a role in cord injury. Recognizing the viscoelastic properties of the cord and slowly performing realignment maneuvers with corrective and then relaxive cycles reduces the risk of cord injury.

The use of high-quality spinal cord monitoring is mandatory for patients with myelopathy. Intraoperative spinal cord monitoring, however, may not be reliable in as many as 6.1% patients (15). Monitoring of somatosensory-evoked potentials alone does not provide enough safeguard against an adverse event, as it may not be able to detect motor weakness, even when profound. Jones et al. reported two cases of temporary quadriparesis following anterior cervical decompression and fusion (ACDF) where SSEPs did not reveal any intraoperative abnormality (16). Motor-evoked potentials recorded postoperatively to transcranial magnetic stimulation were absent in these patients.

Addition of motor-evoked potentials should significantly decrease the risk of false negative monitoring in the presence of anterior cord dysfunction. Somatosensory-evoked potentials and transcranial motor-evoked potentials can help monitor the cord function, while spontaneous electromyography can be used to monitor individual nerve function. A latency increase of 10% and an amplitude drop of more than 50% in SSEPs are generally considered to be the warning signs of an acute spinal cord injury (17). One study of 1,168 consecutive patients using 50% amplitude drop as the cutoff found a false negative rate of 0% (18). However, it has been reported that at lower than normal temperatures, maintenance of more than 50% of baseline evoked potentials is no guarantee of normal postoperative neural function (19).

If there is neurophysiologic evidence of neurologic injury intraoperatively, the exact time of the changes should be noted and its correlation with intraoperative actions, such as spinal column distraction, instrumentation, or decompression, should be identified. The first action should be to check the leads and make sure that the wires have not been disconnected. While the neurophysiologist is trying to establish the integrity of the circuitry, the anesthesiologist should raise the mean arterial blood pressure and ensure that the systolic blood pressure is 100 mmHg or higher to facilitate spinal cord perfusion. The anesthesiologist should also address potentially adverse anesthetic agents immediately. Volatile agents such as halothane should not be used (20), but agents such as isoflurane appear to be safe in low concentration and do not significantly alter the evoked potentials.

If there is no improvement in the monitoring after a few minutes, the surgeon should try to reverse the surgical steps undertaken immediately preceding the detection of loss of spinal cord function. This may include relieving distraction of the spinal cord by removing the distractor, bone graft, or instrumentation.

If the SSEPs do not return to normal after the above actions, a Stagnara wake-up test may be indicated, although this test is not always effective in detecting neurologic complications (21). If the patient is able to move his extremities, then the surgery may continue; if he is not able, then any hardware still present should be removed and all correction should be removed. Additionally, no further correction, reduction, or manipulation of the spine should be attempted and the administration of IV steroids should be considered. The dose of IV steroids to be given in acute spinal cord injury cases, as recommended by the National Acute Spinal Cord Injury III spinal cord injury protocol
(22), is a methylprednisolone bolus of 30 mg/kg followed by 5.4 mg/kg per hour for 23 or 48 hours, depending on if the patient received the bolus within three or eight hours of the injury, respectively. The efficacy of steroids in treating this type of injury is not well-established, and the surgeon should be aware of the possible complications, as well as the possible benefits of giving high-dose IV steroids.

Krause and Stauffer published a report of ten patients with iatrogenic spinal cord injuries. Faulty bone graft insertion technique or improper handling of surgical instruments were identified as the underlying causes of surgically induced cord injuries in half of these patients. One patient developed transverse myelitis, presumably due to use of electrocautery on the PLL. Although the etiology was unclear in the remaining patients, vascular compromise to the anterior arterial system was believed to be pathogenic (23). The anterior spinal artery is essentially independent and does not have any collateral circulation. An important radicular artery to the cervical cord enters at the C5-C6 or C6-C7 foramen and is second only in size to the artery of Adamkiewicz. Damage to this radicular artery or a significant reduction in blood flow through this vessel may produce ischemic changes in the cervical cord.

Vascular compromise of the spinal cord can cause intra-and postoperative neurologic injury. Ischemic factors that may cause neurologic compromise include hypoperfusion, overdistraction, and surgical edema. Vascular ischemia is more pronounced in combined anterior and posterior operations, especially if performed during the same anesthetic (24). Furthermore, if anterior vessels are compromised, the cord may still be well-perfused until the cord is stressed via posterior distraction or manipulation. Also, spinal cord ischemia can occur from operative hypotension (intentional via anesthesia or from volume loss) or hypothermia. Many surgeons use hypotensive anesthesia to minimize operative blood loss. When used, the mean arterial pressure should be kept at more than 60 mmHg to avoid risk of inadequate spinal cord perfusion. Spinal cord perfusion pressure is around 55 mmHg, so the overzealous use of intraoperative hypotensive anesthesia to decrease blood loss may lead to vascular insult and subsequent cord injury with resultant paralysis. Furthermore, the risk for ischemic injury to the cord may persist even in the early postoperative period; delayed paralysis has been reported and is likely caused by a combination of intraoperative vascular insufficiency and postoperative edema (25).

Spinal cord injury may also occur during graft impacting or postoperative dislodgement of an interbody or strut graft. Careful preparation of the fusion site and meticulous grafting technique are necessary to prevent this complication. The graft usually migrates anteriorly but occasionally may encroach upon the spinal canal. A ledge should be created posteriorly to prevent posterior migration, and the graft should be snug-fitting. Intraoperative radiographs should be obtained to make sure that the graft is not placed too far posteriorly, and the graft should be tested for stability. Additional posterior stabilization may be indicated if the spine is deemed to be unstable. A cage may migrate posteriorly during surgery or postoperatively following corpectomy, much like a strut graft. Similar principles apply in preventing such a complication.

Instrumentation of the cervical spine will be covered in detail in another chapter; however, it should be recognized that instrumentation can injure the spinal cord, nerve roots, and cause durotomies as well. During the instrumentation, the surgeon needs to use meticulous technique and constantly re-establish anatomical position to avoid injury (26). No matter what type of fixation is utilized, the cord should not be ignored, and diligent monitoring is crucial.

Common anterior cervical procedures include anterior cervical discectomy with fusion and instrumentation surgery, and anterior cervical corpectomy with fusion and instrumentation surgery. Anterior procedures for C1 and C2 are less frequently performed but have a greater incidence of neurologic complications when compared with posterior procedures for upper cervical spine. A recent study by Clark and White illustrated that anterior procedures are more dangerous than posterior surgery on the odontoid. Two of eight patients who underwent anterior screw fixation for an odontoid fracture had postoperative neurologic complications; one was an ocular nerve palsy and the other was postoperative quadriparesis. On the other hand, only one of 96 patients who underwent posterior fusion for an odontoid fracture developed a neurologic complication, a postoperative Brown-Sequard deficit. Both of the anterior complications were believed to have been caused by screw migration (27). Furthermore, the anterior approach for internal fixation of the odontoid may be associated with screw malposition and breakout in 1.5% to 2% of patients (28). A good reduction of the C2 fracture is necessary to avoid this complication, but this technique should not be used for unstable or irreducible fractures. Cannulated screw fixation over a guide wire or a K-wire should not be used during anterior odontoid fracture fixation because the wire may migrate into the brain stem as the screw is being advanced.