Pathoanatomy of Neural Injury

Neurologic injury may be direct or indirect. Direct causes are essentially mechanical in etiology and include contusion, laceration, traction, compression, and thermal (cautery) injuries to neural structures. Indirect causes result from ischemia, either from local compression of the neural elements or as a result of systemic hypotension, anemia, or poor oxygenation. Ischemia can lower the threshold for neural injury from mechanical etiologies and vice versa.

Untoward events may occur anatomically at the level of the brain, brainstem, spinal cord, plexus, nerve root, or peripheral nerve. Brain and brainstem involvement can present clinically as a stroke. The spinal cord terminates as the conus medullaris, typically at the level of the L1-2 disc space, and is much less tolerant of manipulation than are the nerve roots of the cauda equina. Injury to the spinal cord can result in incomplete spinal cord syndromes (anterior cord syndrome, central cord syndrome, and Brown-Sequard syndrome), as well as complete cord injury. Recovery is variable and dependent on host factors (e.g., age, comorbidities) and the degree of injury. The spinal nerve roots, although more tolerant of manipulation than the spinal cord, are less tolerant than are peripheral nerves because intradural nerve fibers of the nerve root lack the protection of the epineurium and perineurium of the peripheral nerve. Root, plexus, and peripheral nerve injury can result in varying degrees of pain, paresthesias, and weakness. Motor fibers recover more quickly than sensory fibers from deformation stressors; thus transient compression is more likely to affect sensory function and motor recovery tends to precede sensory recovery. Maximum recovery of central or peripheral neurologic injury can take up to 12 to 18 months.

The spinal cord lies protected within the spinal canal. The nerve roots exit segmentally through their corresponding neural foramina. Within the spinal canal, the pedicle is a consistent anatomic landmark that can be used to locate the exiting nerve roots and is useful especially in the presence of anomalous anatomy. In the upper cervical spine the hypoglossal nerves (cranial nerve XII) lie 2 to 3 mm lateral to the midpoint of the lateral masses of C1. They are at risk for injury from overpenetration of C1-C2 transarticular screws, which can result in tongue deviation toward the side of injury and dysarthria.^1,² The C2 nerve root exits dorsal to the C1-C2 facet joint and is at risk for injury during placement of C1 lateral mass screws, which can result in pain or paresthesias in the C2 dermatome (i.e., occipital sensation).³ In the subaxial cervical spine the nerve roots exit anterolaterally through the neural foramen and lie within a groove immediately anterior to the superior articular processes, placing them at risk for injury from improperly placed cervical lateral mass and pedicle screws. In anterior approaches to the cervical spine, dysphagia or dysphonia can result from injury to the pharyngeal plexus during approaches to C2-C5, the hypoglossal during approaches above the level of C3, the superior laryngeal nerve during exposure of the C3-4 level, and recurrent laryngeal nerves during approaches to C5-T1.⁴ The right recurrent laryngeal nerve passes around the subclavian vessels and then ascends in the neck, coursing medially at a variable level (typically C6 to C7) along with the inferior thyroid artery to its destination in the larynx. The left recurrent laryngeal nerve, in contrast, passes around the ligamentum arteriosum at the aortic arch and then predictably ascends within the tracheoesophageal groove, where it remains relatively protected before its destination in the larynx. This distinction is most relevant in surgical approaches to the lower cervical spine, where the variability in the course of the right recurrent laryngeal nerve is greatest, making it at increased risk for injury during dissection. The cervical sympathetic chains lie ventral to the longus colli and course most medially at C5-6, where they are at greatest risk for injury.⁵ Injury can result in Horner syndrome of ptosis, meiosis, anhidrosis, and enophthalmos. In the thoracic spine the ventral nerve roots continue as the intercostal nerves and lie with the costal groove on the undersurface of the rib. They are at risk for injury during surgical dissection in this area. In the lumbar spine the nerve roots lie along the inferomedial edge of the pedicle and are at risk of injury from medial and inferior pedicle breaches during pedicle screw placement. The lumbar sympathetic chain lies anterior to the psoas muscle and can be injured during retroperitoneal approaches to the lumbar spine manifesting as patient complaints of contralateral foot coolness. The superior hypogastric sympathetic plexus is at risk for injury during approaches to the L5-S1 disc space.^1,² Injury can result in retrograde ejaculation and infertility from lack of proper sphincter control, which misdirects the ejaculate into the bladder instead of through the urethra. This is less of a concern at upper lumbar levels because the fibers of the plexus lie on the anterior aortic wall and only become immediately prevertebral at the level of the L5-S1 disc space. The lateral femoral cutaneous nerve, which provides sensation to the anterolateral thigh, may be injured during harvesting of bone graft.⁶ Although the nerve typically passes anterior to the anterior superior iliac spine (ASIS), variants may cross posterior to the ASIS, placing the nerve at risk for injury with dissection within 3 cm posterior of the ASIS. Injury to the nerve can result in painful neuromas and meralgia paresthetica. The superior cluneal nerves, cutaneous branches of L1-L3 involved in sensation to the buttock, may be injured during harvesting of bone graft from the posterior iliac crest with dissection more than 6 cm lateral to the posterior superior iliac spine.⁷

The spinal cord is nourished by an anterior spinal artery and paired posterior spinal arteries. In the cervical spine, radicular branches of the vertebral arteries pass through the neuroforamina to form the anterior and posterior spinal arteries. In the thoracic and lumbar spine, segmental vessels arising off the aorta form radicular branches that then pass through the neuroforamina to form the anterior and posterior spinal arteries. The largest of the segmental vessels, the arteria radicularis magna, more commonly referred to as the artery of Adamkiewicz, most commonly arises on the left side between T9 and T11, although it may be found anywhere between T7 and L4, and is occasionally identified on the right side.

Intraoperative Complications and Their Prevention

A systematic approach to avoiding intraoperative complications includes careful preoperative evaluation of the patient, attention to detail during positioning, meticulous surgical technique, and the use of intraoperative neuromonitoring techniques to increase the margin for safety in selected cases with a coordination of efforts between the surgical, anesthesia, and neuromonitoring teams.

Preoperative Patient Evaluation

Preoperative evaluation of the patient begins with an accurate diagnosis obtained after a thorough history and physical and review of imaging studies. Preexisting neurologic deficits should be identified, and a baseline neurologic examination obtained and accurately documented. These should be communicated with other health care providers involved in the care of the patient. The preoperative imaging studies should be thoroughly reviewed for anatomic anomalies that may increase the chance for neurologic injury. Dysraphic states can increase the risk of durotomy and complicate exposure. Nerve root anomalies increase the risk of injury if not identified preoperatively (Fig. 94–1). Areas of significant neurologic compression increase the risk of injury to neural structures from surgical decompressive maneuvers because there is less room for error. In these situations, significant stenosis results in loss of the normal subarachnoid space. Normally, the subarachnoid space provides a protective buffer between the dura mater and neural structures. Loss of the normal subarachnoid space increases the chance of contusing neural tissue with surgical maneuvers performed immediately adjacent to the thecal sac (e.g. use of a Kerrison rongeur, sublaminar wiring, epidural bipolar electrocautery during anterior cervical discectomy). Abnormal osseous anatomy (e.g., abnormal pedicle morphology, compressive osteophytes, ossified ligamentum flavum) can complicate decompression, instrumentation, or both and should be identified before surgery because these affect intraoperative tactics. Patients should be instructed to preoperatively discontinue anticoagulant medications to minimize intraoperative and postoperative blood loss and the potential for epidural hematoma formation.

FIGURE 94–1 A-B, Axial T1 and T2 magnetic resonance images at the caudal aspect of the L5 foramen (i.e., just cephalad to the L5-S1 disc) from a patient with a right L5-S1 disc herniation. Note in this image that the right L5 nerve root is branching off the thecal sac while the left L5 nerve root is already exiting the foramen. The anomalous location of this conjoined nerve root puts it at risk for injury during exposure for right-sided access to the L5-S1 disc space and may preclude safe intraoperative nerve root retraction. When performing a neurologic decompression, one needs to be able to recognize anomalous anatomy before surgery to know what to expect during surgery.

Proper choice of the surgical exposure takes into consideration a variety of factors. An important consideration is the approach-related risk of neurologic injury because in general, the approach-related neurologic risks of anterior surgery are different from those associated with posterior surgery. In addition, anatomic side-to-side differences in the course of neurologic structures may play a role in choosing to approach the spine from one side versus the other. During anterior approaches to the lower cervical spine, for example, left-sided approaches may be theoretically more appealing because of the more consistent protected course of the left recurrent laryngeal nerve within the tracheoesophageal groove in contrast to the more variable course of its right-sided counterpart.

In addition to the specific neurologic risks of a surgical approach, patient factors (e.g., gender, history of prior surgery, preexisting neurologic deficits) and their tolerance for specific injuries may play a role in determining the risk-to-benefit ratio of a given approach; thus it is important to have a preoperative discussion with patients regarding the risks for neurologic injury and determine how willing they are to accept these risks. For example, retrograde ejaculation and male infertility can result from injury to the superior hypogastric plexus during an anterior lumbar approach with transabdominal approaches at especially increased risk in comparison with retroperitoneal approaches.⁸ Although blunt dissection anterior to the L5 and S1 vertebrae can decrease the chances for injury, it cannot eliminate this risk. Although preoperative cryopreservation (i.e., sperm banking) is an option to retain the possibility of procreation in the event of postoperative infertility, a young male interested in having children may not want to bear this risk. History of prior surgery can also significantly alter the risk-to-benefit ratio of using a given approach because operating through a prior surgical incision (especially during anterior approaches to the spine) considerably increases the risk for complication. This increased risk may be acceptable, however, if the relevant structures have already been compromised. For example, in revising an anterior cervical fusion in a patient with acquired left recurrent laryngeal nerve palsy from a previous left-sided approach, repeat use of a left-sided approach may be preferable because this minimizes the risk of bilateral recurrent laryngeal nerve injury and consequent exacerbation of postoperative dysphagia and/or dysphonia.

Surgical Phases

Avoiding neurologic injury during the surgical phases of patient care requires meticulous attention to detail. Preoperative imaging studies should be accessible within the operating room for intraoperative review. Preparation of the operating environment should include adequate lighting of the surgical field and may require loupe or microscope magnification for safe visualization of the neural elements. A calm, well-rested surgeon is of obvious benefit. In addition, each surgical tactic has specific considerations, which we review as follows.

Positioning

The surgical and anesthetic teams should work together to ensure proper patient positioning to minimize the chances of neurologic injury. Special attention should be paid to positioning of the neck and limbs with proper padding of superficial nerves. Abnormal cervical postures should be avoided. Patients should be positioned with their heads in a neutral or slightly flexed position without excessive rotation because neurologic injury can occur from cervical extension in myelopathic patients, which narrows the internal diameter of the spinal canal and from excessive neck rotation to one side, which narrows the ipsilateral cervical neuroforamina. Case reports exist of excessive neck rotation resulting in carotid artery occlusion and subsequent stroke.⁹ In cervical spine surgery, the shoulders are often depressed and taped in position to improve radiographic visualization. If excessive, this can result in a traction injury to the brachial plexus. Placing a patient into the prone position requires a coordinated effort directed by the individual controlling the head. The persons receiving the patient have the important task of controlling the rate of descent of the patient onto the operating room table. In patients with an unstable spine, gentle inline traction can help maintain alignment. The limbs should be positioned to minimize the potential for compression or traction injury to the peripheral nerves and the brachial plexus. Extra care should be taken in patients with stiff shoulders, morbid obesity, and other circumstances that mitigate against correct positioning of the limbs.

Proper positioning also plays an important role in the prevention of ocular complications. Postoperative visual deficits have a reported incidence as high as 0.1% in spinal surgery in the prone position but have also been described in supine patients.¹⁰ The most common cause (>80% of cases) is ischemic optic neuropathy associated with surgical blood losses greater than 4 L with a hypotensive event or relative hypotension over an extended period, either from blood loss or deliberate hypotension to minimize blood losses.¹¹ Visual deficits can also result from central retinal artery or vein occlusion from direct pressure to the globe, resulting in increased intraocular pressure and decreased retinal blood flow through the central retinal artery. Transient cortical blindness can result from an isolated stroke that affects the visual cortex typically from an embolic phenomenon or hypoperfusion. In contrast to ischemic optic neuropathy and central retinal vessel occlusion, which are typically irreversible, most cases of cortical blindness resolve. Although the risks of these complications cannot be eliminated, they can be minimized through prevention of direct pressure to the globe (e.g., Mayfield three-point head holder in posterior cervical spine surgeries) and appropriate patient-specific management of hemodynamic parameters with measures aimed at reducing central venous pressure (e.g., reverse Trendelenburg positioning, minimizing abdominal compression).¹² These considerations are especially important in high-risk patients (i.e., diabetes, hypertension, glaucoma, large estimated blood losses, systemic hypotension, and long operative times).