Traumatic and Nontraumatic Spine Emergencies

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CHAPTER 7 Traumatic and Nontraumatic Spine Emergencies

TRAUMATIC SPINE INJURY

Background and Imaging Algorithms

Cervical spine injuries are quite common, causing an estimated 6000 deaths and 5000 new cases of quadriplegia annually in the United States. Imaging is liberally applied with a positive yield of from 1% to 3% of all exams, resulting in an annual cost of approximately $3 billion. Approximately 14,000 cases of spinal cord injury occur each year in the United States, the majority affecting young adults. The cost to individuals and society is enormous due to their long life expectancy. Understandably, this is one area of medicine in which attempts have been made to develop evidence-based diagnostic algorithms. Multivariate analysis of data derived from two major clinical research initiatives, the National Emergency X-Radiography Utilization Study (NEXUS) and the Canadian C-Spine Study, has provided the basis for acute spine trauma imaging pathway development. Decision rules have been created that allow for discrimination of patients in need of imaging and those for whom imaging can be safely avoided, thereby reducing costs when possible. Once the decision to image has been made, the next step is to select the most appropriate modality. Plain film radiography has traditionally been the initial examination to evaluate for possible fracture or malalignment. It is readily available, relatively inexpensive to perform, and highly sensitive. It continues to be a cost-effective option for patients with a low probability of injury. However, it has been supplanted by computed tomography (CT) in the setting of moderate-to-high probability of injury, based on cost-effectiveness analysis that takes into account the high medical and legal costs of the rare missed fracture that leads to severe neurologic deficit. Studies of CT as the initial modality have demonstrated higher sensitivity for the detection of fractures; however, the clinical significance of many of the radiographically occult injuries is uncertain owing to the lack of studies addressing outcomes. Another clinical prediction rule that has been developed, based on data from the Harborview study, may be used to stratify risk based on injury mechanism and other clinical parameters. This type of approach is supported by trauma surgery societies and is commonly applied at trauma centers.

With the increasing availability of multidetector-row CT (MDCT), it seems that this is quickly becoming the new standard of care, even for patients with a low probability of injury. A zero-tolerance (for missed injury) approach to diagnosis using the fastest, most accurate exam is easily adopted by the emergency department or trauma team rather than an evidence-based approach. It used to be that for patients with negative radiographs but persistent pain, tenderness, or limited range of motion, symptomatic treatment with analgesics, soft collar application, and clinical follow-up were the rule. At the follow-up visit, if symptoms had not resolved, flexion-extension radiographs were obtained to evaluate for the possibility of ligamentous injury or instability. Flexion-extension views have been shown to have little utility in the acute setting, primarily due to limited range of motion secondary to muscle spasm. In low-probability settings, this approach may still be followed, but more commonly, CT is requested to exclude occult fracture. The higher direct cost of CT may be offset by the increase in emergency department throughput, albeit at a higher radiation exposure. On some occasions, after a negative CT exam, magnetic resonance (MR) imaging may be pursued to screen for signs of potential ligamentous injury before allowing the patient to be discharged.

The cost-effectiveness of MR for the detection of clinically significant ligamentous injuries has not yet been determined. This is another instance where technology is being applied because of its availability and perhaps for theoretical limitation of liability. Clearly, when a neurologic deficit is present and CT fails to identify a cause, MR may offer additional sensitivity for the detection of soft tissue injuries, including disc extrusion, hematoma within the spinal canal, cord compression or contusion, and unstable ligamentous injury. Other injuries such as unsuspected bone marrow edema (microfractures) and vascular injuries may also be detected. One study that correlated MR imaging and intraoperative findings found that MR had moderate to high sensitivity for injury to specific ligamentous structures but suggested that it may overestimate the extent of disruptive injury. MR, with its increased sensitivity, also brings with it a small false-positive rate that may lead to added costs related to treatment/workup of clinically insignificant or unrelated abnormalities, such as thyroid lesions and lymphadenopathy. Special consideration has been given to the obtunded patient, since some studies have shown a 2% incidence of unstable cervical spine injuries that were not detected by radiography and CT due to the lack of associated fracture or malalignment. Although other authors have suggested that it is not necessary, MR “clearance” of the cervical spine has become a reality. At our institution, we generally adhere to the algorithm shown below.

Regarding the thoracolumbar spine, clinical prediction rules have been evaluated but provide only a very small decrease in the number of exams performed. In those patients with blunt trauma undergoing CT of the chest, abdomen, and pelvis with thin-section CT (2.5 mm or less), sagittal and coronal reformats have been shown to be more sensitive and specific for detection of fractures, and therefore radiography can be avoided. When the viscera are not in need of examination, the role of CT for screening the spine is not as clear. Mechanism of injury is an important determinant for further workup in this category of patients. Similar to the logic applied to the cervical spine, screening is warranted if a high-energy mechanism of injury is known or suspected, including falls from significant height (greater than 10 feet), motor vehicle or bicycle crash, pedestrians struck, assault, sport or crush accident, and a concomitant cervical spine fracture. Other

valid indications are altered mental status, evidence of intoxication with ethanol or drugs, painful distracting injuries, neurologic deficits, and spine pain or palpation tenderness.

For patients with neurologic deficits referable to a thoracolumbar spine injury, current Eastern Association for the Surgery of Trauma guidelines recommend obtaining an MR exam as soon as possible after admission to the emergency department. Early decompression of mass lesions, such as traumatic herniated discs or epidural hematomas, is likely to improve neurologic outcome.

A somewhat unintuitive finding is that the absence of symptoms does not exclude injury to the thoracolumbar spine. In one study, only 60% of trauma patients with a confirmed fracture were symptomatic. In a review from Maryland’s Shock Trauma Center of 183 fractures in 110 patients who were neurologically intact with a Glasgow Coma Scale score between 13 and 15 and considered amenable to clinical examination, 31% of these patients were recorded as having no pain or tenderness, yet all had fractures. The evidence would suggest that many of these fractures were not truly asymptomatic but rather occult as a result of intoxication or an unreliable physical exam. It is clear from the literature that no imaging modality is accurate 100% of the time. Most studies have found that radiographs of the thoracolumbar spine are commonly inadequate, especially in obese patients, and provide a sensitivity and specificity of only 60% to 70%.

Separate studies to develop guidelines for the pediatric population have not been performed. The increasing use of MDCT and the long-term effects of radiation exposure are topics of concern and current research.

Patterns of Spine Injury and Imaging Findings

Following is a brief review of the many different types of spine injuries that one must be familiar with when evaluating victims of trauma. There are many texts devoted solely to the imaging of spine trauma, with a few that truly reward the reader with insight into the anatomy, physiology, biomechanics, and pathology of this extensive topic. This section should serve as a valuable aid to the radiologist on call and be used as a starting point for further study. Rather than taking a how-to approach to evaluating spinal imaging, this section relies on a working knowledge of the normal anatomy and basic principles of plain film, CT, and MR analysis. The general classifications of injuries are covered through a review of classic examples, using primarily CT with important plain film and MR correlations where appropriate.

Imaging of the spine can be thought of as a continuum, with radiography providing an overview of alignment and soft tissues, CT adding greater detail regarding fractures, and MR yielding finer detail with respect to soft tissues including the spinal cord. Attention must be paid to the technical factors necessary to achieve a satisfactory (and safe) exam, including patient immobilization and positioning, image acquisition parameters, and multiplanar analysis.

Lateral, anteroposterior, and open-mouth odontoid views are the minimum requirement for plain films. A “swimmer’s” view may be necessary to adequately demonstrate the cervicothoracic junction. Thin-section CT (section thickness of 2 mm or less) with similar-thickness sagittal and coronal reformats generally suffices. However, anecdotal cases have arisen in which hairline fractures were detected on scans performed with submillimeter thickness that were not detected prospectively with the standard technique. Clearly there is a trade-off between level of anatomic detail and number of images that must be reviewed. With isotropic voxel size now possible with modern scanners, some have proposed primary review of sagittal and coronal reformats in order to increase patient throughput. Thankfully, many of the missed fractures will be clinically insignificant due to their small size and inherent stability. In addition to the standard T1- and T2-weighted sequences used to evaluate the cervical spine, fat-suppressed T2-weighted sequences, with either chemical selective or short tau inversion recovery (STIR) technique and gradient-echo sagittal imaging, are useful in the trauma setting. MR angiographic sequences may be indicated in certain circumstances.

Careful analysis of the structures (vertebrae, intervertebral discs, spinal cord, and other soft tissues) and their normal and abnormal attributes (size, shape, alignment, density, and signal intensity) requires an understanding of mechanisms of injury, including magnitude and acuity, and underlying diseases. The mechanisms can generally be grouped into hyperflexion, hyperextension, rotation, axial loading, lateral flexion, and others. Box 7-1 attempts to categorize the injuries of the cervical spine based on these mechanisms. Combined mechanisms, such as flexion and rotation, are common and may lead to multiple injuries at different sites and vertebral levels within the same patient. Rather than relaxing after detecting an injury, the examiner should intensify the search for other lesions.

The determination of instability, which may be associated with or have the potential to progress to neurologic injury, major deformity, or incapacitating pain, is an important part of this process. There are general principles that may apply based on specific imaging findings, but the final determination is probably best made by an expert in the treatment of these injuries. One should note that the classifications of these injuries are constantly being revised based on new treatment techniques and clinical outcomes. Some of the more commonly used classifications will be mentioned. The three-column approach to stability proposed by Denis divides the spine into anterior column (anterior longitudinal ligament, annulus, and anterior two thirds of the vertebral body and disc), middle column (posterior third of vertebral body and disc, annulus, posterior longitudinal ligament), and posterior column (posterior elements, ligamentum flavum, joint capsules, intertransverse, interspinal, and supraspinal ligaments). Instability is generally based on disruption of two of the three columns.

Injuries of the Cervicocranium

The cervical spine can be subdivided into the cervicocranium (including the basiocciput, craniocervical junction, atlas [C1], and axis [C2]) and the subaxial spine (C3 through C7). Following is a top-down review of the major types of injuries and the mechanisms that cause them. It is not possible to describe all of the features of each injury in this abbreviated format; however, the general principles and commonly used classifications are described.

Atlantoaxial Dissociation

Atlantoaxial dissociation includes partial (subluxation) and complete (dislocation) disruptions of the articulations of C1 and C2. Disruption of the transverse atlantal ligament (TAL), the horizontal component of the cruciform ligament complex, allows for widening of the anterior atlantodental interval (AADI) (Fig. 7-4). Greater than 3 mm in adults or greater than 5 mm in children is considered abnormal. Conditions that may predispose to atlantoaxial dissociation include rheumatoid arthritis, Down syndrome, neurofibromatosis, and other syndromes and congenital anomalies. Rotatory dissociation (fixation) is rare and has been subdivided into four types based on extent and direction of displacement of the atlas. Type I may appear similar to physiologic rotation. Therefore, to confirm the diagnosis, CT may be repeated after voluntary contralateral rotation of the head to assess for a locked position. Torticollis refers to simultaneous lateral tilt and rotation of the head and may be caused by disorders affecting either the atlantoaxial joint or the sternocleidomastoid muscle. Since it may produce the same imaging findings as type I rotatory fixation, diagnosis may rely on clinical judgment and a trial of conservative treatment. Types II, III, and IV are determined based on direction and extent of displacement of the cranium.

C2 Fractures

Approximately 20% of cervical fractures involve the axis (C2). Of these, more than half are traumatic spondylolysis/spondylolisthesis—fracture between the superior and inferior facets (pars interarticularis) of C2—often described as the “hangman” fracture (Fig. 7-5). Due to the unique shape of C2, this fracture involves the pedicles, whereas a pars fracture of the subaxial spine is termed the “pillar” fracture. Aside from the mechanism implied by the name, other forms of hyperextension, such as motor vehicle dashboard impact, are usually to blame. At least three types have been described, based on degree of fragment distraction, angulation at the fracture site, and disruption of the C2-C3 disc.

The dens (odontoid process) may be involved in approximately 25% of C2 fractures. The classification system of Anderson and D’Alonso is commonly applied. Type I is uncommon—an avulsion of the tip by the alar ligament—and may be associated with AOD. Type II is the most common (about 60%) and involves the base of the dens (Fig. 7-6). Operative repair via transoral screw fixation or posterior arthrodesis of C1 and C2 is often necessary. Type III involves the dens and body of C2. Due to the larger surface area, this type of fracture is more likely to heal without the need for instrumentation (Fig. 7-7). As the plane may be nearly horizontal, dens fractures may be quite subtle on axial CT. Sagittal reformats therefore demand careful review. Beware of misregistration artifact of axial CT, due to patient movement between adjacent images, although this has become less of a problem since the advent of helical and MDCT techniques. Since the scanning process is so fast with these techniques, a different type of artifact can result—motion blur. Distorted images should signal the need to repeat the scan. For uncooperative patients, a lateral plain film may provide complementary demonstration of proper alignment.