Trauma to the spinal column and spinal cord

Transcription

1 NEURORADIOLOGY REVIEW SERIES NEURORADIOLOGY REVIEW SERIES Lubdha M. Shah, MD* Jeffrey S. Ross, MD *Department of Radiology, University of Utah, Salt Lake City, Utah; Department of Radiology, Mayo Clinic Arizona, Phoenix, Arizona Correspondence: Lubdha M. Shah, MD, Director of Spine Imaging, Associate Professor of Radiology and Neurosurgery, University of Utah, 30 North 1900 East, #1A071, Salt Lake City, UT Received, December 17, Accepted, May 11, Published Online, July 12, Copyright 2016 by the Congress of Neurological Surgeons. Imaging of Spine Trauma Imaging with computed tomography and magnetic resonance imaging is fundamental to the evaluation of traumatic spinal injury. Specifically, neuroradiologic techniques show the exact location of injury, evaluate the stability of the spine, and determine neural element compromise. This review focuses on the complementary role of different radiologic modalities in the diagnosis of patients with traumatic injuries of the spine. The role of imaging in spinal trauma classifications will be addressed. The importance of magnetic resonance imaging in the assessment of soft tissue injury, particularly of the spinal cord, will be discussed. Last, the increasing role of advanced imaging techniques for prognostication of the traumatic spine will be explored. KEY WORDS: Computed tomography, Diffusion tensor imaging, Magnetic resonance imaging, Spine trauma Neurosurgery 79: , 2016 DOI: /NEU Trauma to the spinal column and spinal cord are potentially devastating injuries. Imaging is a major pillar in the evaluation of the trauma patient in addition to the history and physical examination. Computed tomography (CT) and magnetic resonance imaging (MRI) are the main imaging modalities for the evaluation of traumatic spinal injury, specifically, to confirm the exact injury location, to assess spine stability, and to delineate neural element compromise. We will review the fundamental role of imaging, focusing on the complimentary role of the different modalities, in the diagnosis of patients with traumatic injuries of the spine. ROLE OF IMAGING IN THE ASSESSMENT OF THE TRAUMATIC SPINE In the setting of acute spinal trauma, imaging delineates all osseous and soft tissue injuries and helps to guide potential surgical intervention. Guidelines for the Management of Acute Cervical ABBREVIATIONS: ADC, apparent diffusion coefficient; ALL, anterior longitudinal ligament; AS, ankylosing spondylitis; ASIA, American Spine Injury Association; CTA, computed tomography angiography; CVJ, craniovertebral junction; DISH, diffuse idiopathic skeletal hyperostosis; DTI, diffusion tensor imaging; FA, fractional anisotropy; MDCT, multidetector computed tomography; MRA, magnetic resonance angiography; SCI, spinal cord injury; STIR, short tau inversion recovery; VAI, vertebral artery injury Spine and Spinal Cord Injuries by the Joint Section on Disorders of the Spine and Peripheral Nerve provide recommendations on the role of imaging for acute cervical spine and spinal cord injury patients. 1 In alert, asymptomatic patients without neck pain or distracting injury, with a normal neurological examination and ability to perform a functional range of motion, clinical clearance remains the standard, and radiographic evaluation is not recommended. If the patient has neck tenderness and pain, the recommendation is CT. 1 Radiographs are not recommended, because, even with the best possible technique, they underestimate the amount of traumatic spine injury. Despite repeated attempts at open-mouth odontoid and swimmers views and other variations of imaging, it is very often difficult to visualize the entirety of the cervical spine. Radiographs of the cervical spine detect only 60% to 80% of fractures, even when 3 views are obtained. 2 Major trauma centers use multidetector CT (MDCT) as the primary imaging modality for evaluating patients with blunt cervical spine injury, particularly in symptomatic patients. 3 CT has a higher sensitivity and specificity for evaluating cervical spine injury than radiographs, detecting 97% to 100% of fractures. 2-4 For those patients with persistent neck pain but without identification of fracture on CT, further imaging considerations include flexionextension radiographs and MRI. Studies have determined that spine precautions can be withdrawn without additional imaging in most blunt 626 VOLUME 79 NUMBER 5 NOVEMBER

2 SPINE TRAUMA IMAGING trauma patients with cervical spine tenderness but negative neurological evaluation and cervical spine CT. 5,6 Flexionextension radiographs have limited utility in the acute setting with high false-negative and false-positive rates. 2 A study comparing flexion-extension radiographs with CT also concluded that they are not efficacious when a negative CT has been performed in blunt trauma without neurological findings. 7 The role of MRI in alert patients with cervical spine tenderness but without identification of fracture on CT is controversial. A prospective observational study of 178 alert, neurologically intact trauma patients with CT-negative but persistent midline neck tenderness had MRI examinations that detected discoligamentous injuries. The authors found that advanced cervical spine degeneration evident on CT, minor thoracolumbar fracture, and multidirectional cervical spine forces were associated with increased injury extent but acknowledged that a larger study is required to validate which variables may reliably predict clinically important injury. 8 Clearing the cervical spine in obtunded patients is primarily by MDCT. CT identifies all unstable injuries in this subgroup of patients compared with upright radiographs obtained after alertness is restored. 2 Dynamic fluoroscopy also does not identify additional fractures or instability that are not already seen on CT. 3 Current data as to the necessity of MRI in the cervical spine clearance in obtunded/comatose blunt trauma patients are inconclusive. In a study of 180 patients, Tomycz et al 4 discovered acute traumatic findings on MRI in the cervical spine in 21.1%; however, none of these patients had amissedunstableinjuryandnopatientrequiredsurgeryor developed evidence of delayed instability. Similarly, in a study of 366 obtunded or unreliable patients, Hogan et al showed that, of the 4 patients who had negative MDCT and ligament injury on subsequent MRI, none had unstable injuries. 5 They reported in their series that the negative predictive value of MDCT was 98.9% for ligament injury and 100% for unstable cervical spine injury. Given these data, they concluded that, outside its appropriate application to patients with a neurological deficit, MRI is unlikely to uncover unstable cervical spine injuries in obtunded/comatose patients who have negative CT scans. In obtunded patients with normal CT, they suggest discontinuation of cervical spine immobilization if the MRI obtained within 72 hours of injury is normal or at the discretion of the treating physicians. 6,7 Others, however, have found that MRI is critical for the clearance of the cervical spine in the altered patient. In a study of 203 patients without neurological deficit and a negative CT but unreliable clinical examination, MRI changed the management of 7.9% of patients. 8 Two meta-analyses on the role of MRI in clearance of the cervical spine in blunt trauma patients, with symptoms suspicious for cervical spine injury or with unreliable clinical examinations who have negative CTs, have concluded that CT alone to clear the cervical spine after blunt trauma can lead to missed injuries and that a normal MRI can conclusively exclude cervical spine injury. 9,10 In keeping with current spine trauma classification systems, which incorporate the patient s neurological status, any patient with presumed spinal cord injury (SCI) should undergo MRI to disclose the location and severity of the injury and to reveal the cause of spinal cord compression (eg, disc protrusion, hematoma, osseous fragment). In particular, this is critical in the management of patients with incomplete SCI, for whom surgical intervention may prevent further deterioration. In a study of 99 patients with cervical spine fracture, a recent study found that changes in management were not related to discovery of instability but rather revelation of SCI. 11 Predisposing factors for the detection of SCI on MRI include, in older patients (older than 60 years), obtundation, cervical spondylosis, polytrauma, and neurological deficit (Figure 1). Evaluation of spinal trauma in older patients requires a lower threshold for the incorporation of advanced imaging techniques. Elderly patients experience different injury patterns because of osteopenia, degenerative change, and ankylosed spinal segments (ie, diffuse idiopathic skeletal hyperostosis [DISH] and ankylosing spondylitis [AS]). Low-energy mechanisms are not uncommonly a cause of spinal fractures, particularly in the cervical spine, in this group of patients. 12,13 Theassociatedosteopeniaof the AS spine makes it more prone to fracture than DISH. In patients with AS, fractures are seeninmorethan1regionofthe spine in greater than 10% of patients. Cervical fractures are the most common, followed by thoracic, lumbar, and sacral. SCI is present in over 20% of cases (27.5% of cervical fractures and 16.0% of thoracic fractures). 14 In early AS, fractures tend to occur through the disc space, whereas in late stages, because of the osteopenia and ossified disc space, fractures occur through the vertebral body (Figure 2). 15 In DISH, fractures occur through the mid vertebral body adjacent to the osteophyte attachment sites. Fractures in the ankylosed spine may also occur at the junction of the fused and mobile segments. 15 Although CT defines the full extent of the osseous injury in these patients, MRI should also be utilized in the evaluation of thesepatientsbecauseofthehigherprevalenceofspinalcord and ligamentous injuries. 16,17 IMAGING MODALITIES MDCT is rapid, is easily performed, and optimally delineates osseous anatomy and fractures. Multiplanar reconstructions of submillimeter axial acquisitions can be performed of all spinal segments. In addition to bone algorithms to detect fractures, soft tissue and lung algorithms enable identification of prevertebral/ paraspinal hematoma, traumatic disc herniations, and pneumothorax, respectively. An additional advantage is that CT allows more rapid radiologic clearance of the cervical spine than radiography, 18 shortening the time to removal of spine precautions. MDCT scan clearance of the spine does not appear to result in greater radiation exposure and improves resource use. Radiation exposure can be reduced with secondary reconstructions of the spine from MDCT of the chest and abdomen, which NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

3 SHAH AND ROSS FIGURE 1. A, sagittal CT demonstrates a focal disc osteophyte complex in a trauma patient (status post motor vehicle collision) who presented with central cord syndrome. No fracture or ligamentous injury was identified on the CT or MRI of the cervical spine. The DTI trace and ADC images (B, C) reveal restricted diffusion at the level of compression (white and red arrows, respectively). D, axial T2-weighted image shows impingement of the cord anteriorly by the disc osteophyte complex and posteriorly by thickened ligamentum flavum. There is subtle intramedullary T2 hyperintensity. ADC, apparent diffusion coefficient; DTI, diffusion tensor imaging. is part of the polytrauma protocol in most institutions. Accurate evaluation of the thoracolumbar spine is possible with this targeted image reconstruction with reported sensitivity and specificity of spinal fracture detection of 98% and 97%, FIGURE 2. Sagittal T1-weighted (left) and sagittal STIR (right) images show a fracture through the T8-T9 intervertebral space, vertebral bodies, and ligaments. There is disruption of the ligamentum flavum with a small posterior epidural hematoma (white arrow). The ALL is stripped off the anterior margin of the vertebral bodies with a small paraspinal hematoma (yellow arrow). ALL, anterior longitudinal ligament; STIR, short tau inversion recovery. respectively. 19 The most important limitation of CT is the inability to provide screening for ligamentous and spinal cord injury, although the pattern of injury can suggest such pathology. In addition, CT can be difficult to interpret in patients with severe degenerative disease and osteopenia. MRI is commonly used in the evaluation of acute spine trauma after the MDCT has been acquired because of its high sensitivity for soft tissue injury, and it is the preferred modality for evaluation of the spinal cord, ligaments, intervertebral discs, and paraspinal soft tissues. 20 Spinal injury patients require additional consideration before MRI with respect to patient transfer, life support, monitoring, and fixation devices, as well as choices of pulse sequences and coils. With this in mind, the potential risks of transporting a neurologically and possibly medically unstable patient must be weighed against the benefits derived from the diagnostic information attained from the MRI. The standard MRI trauma protocol includes sagittal T1- weighted, T2-weighted, and short tau inversion recovery (STIR) images, as well as axial sequences, which are T2 or T2* weighted. Sagittal T1-weighted images provide excellent anatomic delineation. STIR or T2-weighted imaging with fat saturation increases the conspicuity of edema in the bones and ligaments and depicts abnormalities in the spinal cord, discs, and epidural space (eg, hematoma). Lee et al 21 determined that fatsuppressed T2-weighted sagittal sequences were highly sensitive, specific, and accurate for evaluating posterior ligament complex injury. Proton density-weighted images may also be performed to identify ligamentous injuries. Fluid-sensitive sequences can identify hemorrhage on the basis of a low T2 signal. Although fast spin echo images are comparable in sensitivity to spin echo images for detection of intramedullary 628 VOLUME 79 NUMBER 5 NOVEMBER

4 SPINE TRAUMA IMAGING FIGURE 3. Left, sagittal T2-weighted image demonstrates a flexion-distraction injury at the thoracolumbar junction. Subtle intramedullary T2 hyperintensity reflects cord edema/contusion (yellow arrow). Right, sagittal gradient-recalled echo image has increased conspicuity of the intramedullary hemorrhage (white arrow). hemorrhage, gradient-recalled echo T2* sequences are more sensitive to soft tissue hemorrhage (Figure 3). 22 Sagittal T2* images may be included in a cervical spine trauma protocol. Recent developments in susceptibility-weighted imaging have shown that it is more sensitive than gradient-recalled echo T2* sequences in detecting hemorrhage in acute SCI. 23 Detecting injury to 1 soft tissue type should prompt scrutiny at the same and adjacent 1 to 2 levels for additional injury. Facet joint fluid and widening is suggestive of synovial capsule injury. Posttraumatic disc injury is implied when, at the level of other damaged tissue, there is asymmetric narrowing or widening of an isolated disc space and focal T2/STIR hyperintensity. There may not be an associated vertebral body fracture. Posttraumatic disc herniation has a MRI appearance similar to a nontraumatic disc herniation, and identification of acute disc herniation is even more challenging in the setting of underlying degenerative spondylotic changes. 24 Imaging findings suggesting acute disc herniation include a single level of intradiscal signal abnormality, asymmetric width of involved level, subluxation, and associated injuries. Traumatic hemorrhage can be detected in the spinal subarachnoid and epidural spaces. Epidural hematomas may be seen in up to 41% of spinal traumas, 25 with a higher incidence (up to 50%) in patients with ankylosing spondylitis. 26 The MRI appearance depends on the oxidative state of the hemorrhage. Acutely, epidural collection is isointense to the spinal cord on T1- weighted images and isointense to cerebrospinal fluid on T2- weighted/stir images and, therefore, can be very difficult to discern (Figure 4). Methemoglobin can appear within hours in an epidural hematoma but can take greater than a week to have that T1 hyperintensity within neural structures. 27,28 MRI provides optimal assessment of the ligaments, which appear as hypointense structures on all MRI sequences. When abnormally stretched or ruptured, there is intrasubstance T2/STIR hyperintensity or discontinuity, respectively. However, it is important to FIGURE 4. Sagittal CT reconstruction (A), T1-weighted (B), T2-weighted (C), and STIR (D) images demonstrate a T7 compression fracture (yellow arrow) and T6 marrow edema in a patient who had a minor fall but had a supratherapeutic international normalized ratio. The acute epidural hematoma is T1 isointense to the cord (white arrow), heterogeneously T2 hyperintense (red arrow), and STIR hyperintense to cerebrospinal fluid (blue arrow). STIR, short tau inversion recovery. NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

5 SHAH AND ROSS recognize that a ligament does not have to be torn to be mechanically incompetent. Failure of the anterior longitudinal ligament (ALL) or posterior longitudinal ligament is concerning for spinal instability. Hyperextension may result in an ALL tear, which is seen as focal discontinuity of the hypointense band that is normally adherent to the ventral aspect of the vertebral bodies. With either hyperextension or hyperflexion injuries, the posterior longitudinal ligament may be stripped off the annulus and posterior cortex by a traumatic disc herniation or epidural fluid collection, or it may be frankly ruptured. When injured, the ligamentum flavum may exhibit focal discontinuity or be displaced into the spinal canal, distorting the posterolateral thecal sac. In addition to T2/STIR hyperintensity, interspinous and supraspinous ligament injury may be associated with interspinous widening (Figure 5). Although MRI is highly sensitive for the detection of ligamentous injury, not all cases of injury may cause instability. In acute cervical spine trauma, Goradia et al 29 showed that MRI has moderate to high sensitivity for injury to specific ligamentous structures. They determined that MRI was highly sensitive for injury to disc (93%), posterior longitudinal ligament (93%), and interspinous soft tissues (100%). However, they found limited agreement between specific MRI findings and injury at surgery, concluding that MRI may overestimate the extent of disruption compared with intraoperative findings and that this may have potential clinical consequences. The challenge of MRI in the assessment of ligamentous integrity is further complicated by the fact that it can be difficult to identify complete ligaments in a substantial proportion of healthy subjects and that identification has poor interobserver reliability. 30 Despite on the advantages of MRI, it is limited in its ability to detect nondisplaced fractures through the vertebral bodies and, especially, the posterior elements. Fracture detection is particularly limited in the cervical spine. On occasion, a thin T2/STIR hyperintense, T1 hypointense band of the fracture line may be visible. MRI is able to detect marrow edema in compressive injuries even when there is no evidence of fracture deformity or cortical failure (Figure 6). The signal changes may be the result of microfractures within the medullary bone and consequent FIGURE 5. Sagittal T2-weighted (left) and STIR (right) images show a flexiondistraction injury at the C6-C7 level. There is rupture of the ligamentum flavum with an acute epidural hematoma (yellow arrow). Disruption of the supraspinous (red arrow) and interspinous ligaments is observed. Paraspinal muscle edema/ contusion is also seen (white arrow). STIR, short tau inversion recovery. FIGURE 6. Sagittal STIR image reveals hyperintensity along the superior margins of multiple vertebral bodies (T12-L3). This indicates marrow edema related to compression injury without cortical failure. STIR, short tau inversion recovery. 630 VOLUME 79 NUMBER 5 NOVEMBER

6 SPINE TRAUMA IMAGING hemorrhage. Brinckman et al 31 found variability in abnormal marrow T2/STIR signal intensity in acute spinal trauma, with only fractures derived from vertebral body compression reliably generating marrow edema. This lack of marrow edema in fractures without compression and/or fractures with distraction may lead to a false-negative MRI. Another setting in which MRI may not accurately depict fracture acuity is in older individuals with odontoid fractures. The composition of the odontoid in older individuals may be altered with osteopenia and decreased vascularity such that the fluid-sensitive STIR sequence may not accurately depict the acuity of an odontoid fracture (Figure 7). 32 SPINE INJURY CLASSIFICATION A review of the many different spine trauma classification systems is not within the scope of this article. The commonly used Denis classification of acute thoracolumbar spinal injuries 33 divides the spine into 3 columns: (1) anterior column including the 2 anterior thirds of the vertebral body and discs, (2) middle column consisting of the posterior third of the vertebral body and discs, and (3) posterior column consisting of the facet joints, lamina, spinous processes, and intricate ligamentous complex. From this model, inferences can be made about the status of the soft tissues based on the radiologic findings. The Arbeitsgemeinschaft für Osteosynthesefragen classification focuses thoracolumbar spine injuries on 3 pathomorphological criteria (compression, distraction, axial torque, and rotational deformity), which are further divided into 3 groups and 3 subgroups reflecting a progressive scale of morphological damage and the degree of instability. The more recent subaxial cervical spine injury classification and thoracolumbar injury classification systems incorporate neurological status with injury morphology and integrity of the posterior ligamentous complex. 34 The subgroups are arranged from least to most significant, with a numeric value assigned to each injury pattern. Point values from these injury categories are totaled and a comprehensive severity score is calculated. The severity scores in these systems offer prognostic information and may be helpful in decision making about surgical vs nonsurgical management. The classification schemes incorporate the mechanism of injury to anticipate associated pathologic processes and better prognosticate on patient outcome. These mechanisms can often be surmised by the radiologic traumatic pattern on CT and MRI. The craniovertebral junction (CVJ) must be carefully scrutinized for specific injury patterns on CT and MRI. High-resolution axial T2 and T2* images from the foramen magnum to C2 should be obtained if there is a question of CVJ injury to fully assess the stabilizing ligaments (ie, transverse ligament). Atlanto-occipital and atlantoaxial dissociation injuries include dislocations and subluxations due to varying degrees of vertical distraction with anterior, posterior, or lateral shear force (Figure 8). 35,36 These are combined osseous and ligamentous injuries. Recent data emphasize the major role of the CVJ ligaments and membranes in traumatic injuries with a secondary function of the osseous structures. 37 There are often associated spinal cord or other neurological injuries, such as brainstem dysfunction and cranial nerve palsies (Figure 9). 36,38 Several quantitative parameters can be used to identify patients with CVJ distraction injury. Classically described on plain film radiographs are the basiondens and basion-axial intervals, which should be less than 12 mm. 39 However, as the primary mode of spine evaluation shifts from plain radiographs to MDCT, Rojas et al 40 FIGURE 7. Left, Sagittal STIR shows an acute type 2 dens fracture in an 89-year-old man after a ground-level fall. There is linear hyperintensity along the fracture (red arrow) but no marrow edema. Right, Sagittal CT shows the irregular, sharp margins of the acute dens fracture. STIR, short tau inversion recovery. NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

7 SHAH AND ROSS FIGURE 8. CT of the craniovertebral junction in the coronal (A), sagittal (B), and parasagittal (C, D) planes shows atlantooccipital dissociation. There is widening (white arrow) and anterior subluxation (red arrows) of the atlanto-occipital articulations. The basion dens distance is abnormally increased as well (yellow arrow). demonstrated that the normal values of the CVJ on MDCT are significantly different from the accepted ranges of normal on plain radiographs. In a study of 35 patients with CVJ distraction injury, Chang et al 35 suggestthatusinganupper limit of 4.2 mm for the summed condylar distance or an upper limit of 7.8 mm for C1-C2 spinolaminar line length results in detectionofmorethan95%ofcasesofinjury.otherimaging signs of C1-C2 subluxation include widening of C1-C2 facet joints, displacement of C1 lateral masses with respect to those of C2, and widening of atlantodental interval (normally #3 mm in adults, #5 mm in children). It should be cautioned, however, that craniometric measurements may not exclude ligamentous instability. 41 Injuries to C1 may involve the anterior and posterior arches as well as the lateral masses. Vertical compression is the usual mechanism and results in 2 or more fracture lines, the pattern of which depends on the head position at impact and the degree of rotational force. Of importance in C1 injuries is the integrity of the transverse ligament, which is well delineated by the superior soft tissue resolution on MRI. Ligamentous disruption vs osseous avulsion determines surgical treatment (Figure 10). 42 Fractures of C2 may involve the dens or body and are often categorized into 3 types. 43 Type 1 is a fracture of the odontoid tip and is the least common. Type 2, the most common, is a fracture through the base of the dens, whereas type 3 involves the C2 body (Figure 11). Assessment of the transverse ligament in these fractures is important given the shear/translational and hyperextension mechanism. Types 1 and 2 can be associated with alar ligament and transverse ligament injury, which determine instability. The atlantodental interval may be widened. Dens 632 VOLUME 79 NUMBER 5 NOVEMBER

8 SPINE TRAUMA IMAGING FIGURE 9. Left, coronal T2 sampling perfection with application optimized contrasts using different flip angle evolution image illustrates atlanto-occipital dissociation with widening of the atlanto-occipital joints (white arrow) and atlantoaxial joints (red arrow). Right, Sagittal T2-weighted image shows intramedullary hemorrhage at the cervicomedullary junction (yellow arrow). displacement greater than 6 mm is suspicious for transverse ligament injury, requiring surgical fixation. 44 An entity that can be mistaken for an acute dens fracture is an os odontoideum. FIGURE 10. Axial T2-weighted image demonstrates rupture of the transverse ligament (arrow), which is seen as discontinuity of the hypointense ligament posterior to the dens. Sharp and irregular cortical margins of an acute dens fracture can help distinguish between it and an os odontoideum, which will have sclerotic, well-corticated margins and often an associated dysmorphic C1 arch. Another important traumatic injury of C2 involves coronally oriented fractures of the pars interarticularis. Although there have been several subsequent modifications, the original Effendi classification is based on angulation and translation of C2 on C3. The appearance of the fracture and associated fragment displacement and angulation indicates the mechanism. The C2- C3 disc space may be widened with intradiscal T2/STIR hyperintensity (Figure 12). Interfacet dislocation may also exhibit widening and T2/STIR hyperintense facet fluid. Traumatic atlantoaxial rotatory subluxation is an uncommon injury, seen more commonly in pediatric patients. The patient presents with torticollis: lateral neck flexion, contralateral rotation, and variable flexion (Figure 13). On imaging, there is persistent asymmetry of the odontoid with respect to the C1 lateral masses, which is uncorrected by head motion. CT coupled with radiographs has 99% sensitivity, and dynamic CT may provide functional information. 45 MRI findings suggest that failure of the alar ligament is a cause of atlantoaxial rotatory subluxation. 46,47 A practical assessment of subaxial cervical, thoracic, and lumbar fractures includes description of the morphology on imaging with interpretation of mechanism based on the pattern of injury. Vertebral morphology may be described as compression and burst. Compression fractures involve wedging of the anterior column, NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

9 SHAH AND ROSS FIGURE 11. Sagittal (A) CT reconstruction, T1-weighted (B), T2-weighted (C), and STIR (D) images illustrate a type 2 dens fracture (red arrows). Acute compression fractures of C7 and T3 are also noted (white arrows). STIR, short tau inversion recovery. usually less than 50%, with sparing of the middle and posterior columns (Figure 14). When there is a greater flexion component, not only is there anterior column involvement with.50% height loss, but there may also be posterior ligament failure, resulting in widening of the interspinous distance. This type of flexion-compression fracture pattern is potentially unstable when it involves all 3 columns. In addition to a detailed description of the fractured vertebral body and posterior element morphology, it is important to evaluate the consequent effect on the spinal canal and neural foramina. Sagittal and axial planes on CT and MRI will delineate the spinal canal narrowing from translation and retropulsion of osseous structures. Axial compression can result in a burst fracture, which is characterized by vertebral body height loss and involvement of the anterior and middle columns (Figure 15). The stability of the fracture is determined by the integrity of the posterior column. 48 There may be spinal cord or nerve root injury due to displacement of osseous fragments or vascular injury. With combined flexion and distraction forces, there is injury to the posterior column osseous and/or ligamentous structures. There are different subtypes depending on the axis of flexion. The most common subtype is with the axis of flexion located anterior to the ALL. CT coronal reconstructions are best to illustrate the horizontal fractures, particularly of the posterior elements, whereas sagittal reconstructions display the increased interspinous distance. Sagittal T2-weighted and STIR MRI images will show disruption of the interspinous and supraspinous ligaments. A diagnostic clue is the seeming dissolution of the pedicles on the axial CT images, because there is a gradual loss of FIGURE 12. Sagittal STIR (A), sagittal CT (B), and parasagittal CT (C, D) images show traumatic spondylolisthesis of C2. There are bilateral comminuted pars interarticularis fractures (white arrows). Anterolisthesis and inferior angulation of C2 on C3 are observed. A small prevertebral hematoma (red arrow) is noted. STIR, short tau inversion recovery. 634 VOLUME 79 NUMBER 5 NOVEMBER

10 SPINE TRAUMA IMAGING FIGURE 13. Coronal CT (A) and 3-D surface-rendered (B) images demonstrate atlantoaxial rotatory subluxation. B, the 3-D surface-rendered image shows widening of the atlantoaxial joint (white arrow). C, sagittal CT displays preserved atlantodental interval (,3 mm), which corresponds to a Fielding and Hawkins type 1 classification. pedicle definition with the fracture lines. 49 The axis of flexion shifts posterior to the ALL with greater flexion-distraction forces. With the more severe flexion-distraction forces, the vertebral fracture component is burst type, and there is often buckling or retropulsion of the posterior cortex (Figure 16). 49 Fracture of the pars interarticularis contributes to instability with the potential of subluxation. Hyperextension type of injury results in failure of the anterior column, which may extend through the disc and/or vertebral body with ALL disruption (Figure 17). The spine appears hyperextended with a posteriorly located hinge that precludes translational displacement. However, there may also be posterior element fracture. These types of injuries are often seen in diffuse idiopathic skeletal hyperostosis and ankylosing spondylitis and are unstable. 50 Hyperextension injury in the cervical spine, as well as fracture-dislocation and acute disc herniation, can lead to FIGURE 14. Sagittal CT reconstruction displays a compression fracture of the T6 vertebral body. The posterior vertebral body cortex is intact. FIGURE 15. Sagittal STIR (left) and T2-weighted (right) images demonstrate a T12 burst fracture. There is involvement of all 3 columns and retropulsion of the posterior cortex, which impinges on the conus medullaris (yellow arrow). Although posteriorly bowed, the PLL appears intact (white arrow). PLL, posterior longitudinal ligament; STIR, short tau inversion recovery. NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

11 SHAH AND ROSS FIGURE 16. Sagittal (A) CT reconstruction, T1-weighted (B), T2-weighted (C), and STIR (D) images show a midthoracic flexion-distraction injury. There is a burst fracture of T7 with retropulsed bone fragments. In addition to fracture of the posterior elements, there is ligamentous injury with stripping of the ALL (white arrow) and discontinuity of the PLL (red arrow) and supraspinous ligament (yellow arrow). Ligamentum flavum rupture with epidural hemorrhage is also noted (blue arrow). ALL, anterior longitudinal ligament; PLL, posterior longitudinal ligament; STIR, short tau inversion recovery. central cord syndrome, particularly when there is underlying congenital or spondylotic stenosis. 51 The fracture-dislocation type of injury results in middle and posterior column failure with varying degrees of anterior column insult (Figure 18). The mechanism is a combination of lateral flexion and rotation, resulting in disruption of the posterior ligaments and facet joints (often unilateral). There may be an additional transversely directed force. Transverse process fractures are the major indicators of a rotational component. 52 In the thoracolumbar junction, these types of injuries are associated FIGURE 17. Sagittal (A) CT reconstruction, STIR (B), and T2-weighted (C) images demonstrate a hyperextension injury at the thoracolumbar junction. There is a small avulsed fracture fragment from the L1 superior endplate (white arrow). The injury extends through the T12-L1 disc space (yellow arrow). However, the ALL appears intact (red arrow). ALL, anterior longitudinal ligament; STIR, short tau inversion recovery. 636 VOLUME 79 NUMBER 5 NOVEMBER

12 SPINE TRAUMA IMAGING FIGURE 18. Sagittal (A) CT reconstruction, T1-weighted (B), T2-weighted (C), and STIR (D) images demonstrate fracturedislocation at the C6-C7 level. There is ALL disruption with anteriorly extruded disc material (white arrow). The PLL is stripped off the posterior margin of C6 with a small amount of T1 isointense epidural hematoma (dashed arrow). Rupture of the posterior ligamentous complex is seen. There is intramedullary T2/STIR hyperintensity due to edema/contusion (circle). ALL, anterior longitudinal ligament; PLL, posterior longitudinal ligament; STIR, short tau inversion recovery. with severe neurological deficit. There is compression of the spinal cord/nerves by fragments displaced into the spinal canal and/or spinal canal narrowing due to translational displacement. There is a spectrum of traumatic SCIs from cord edema, cord contusion, intramedullary hemorrhage to cord transection. SCI without radiographic abnormality is blunt injury to the spinal cord, usually the cervical cord, without overt osseous injury. In such patients, MRI may reveal extraneural (ligament and disc) injury or neural injury. 53 The anatomic and biomechanical characteristics of the pediatric spine make young patients (typically less than 8 years of age) more at risk for this type of injury (Figure 19). 54,55 In adult patients, the term is not as widely used. If there are neurological symptoms in adults, who are likely to have underlying degenerative change, MRI is routinely used to assess for compressive pathology (ie, disc herniation). 56 Cord edema has a potential for neurological recovery, whereas cord contusion tends to be associated with an incomplete SCI. The length of the edema/contusion is directly proportional to the degree of the initial neurological deficit. 57,58 The prognosis for neurological recovery is poor in patients with intramedullary hemorrhage or cord transection. 59 Spinal cord hemorrhage is centered at the point of mechanical impact, commonly in the gray matter. A large focus of hemorrhage (.10 mm in craniocaudal dimension) is often indicative of complete neurological injury, 60 particularly in the cervical spine. 58 A literature review by Bozzo et al 61 determined that the presence of intramedullary hemorrhage is the most important factor tied to final neurological status. Flanders et al 62 found that, in addition to hemorrhage, long segments of edema and high cervical locations are imaging factors that correlate with poor functional recovery. Studies have assessed spinal canal diameter (8 mm or less) 63 and Torg-Pavlov ratio (0.7) 64 for positive predictive value of SCI (84% and 75%, respectively), but these quantitative metrics do not correlate with American Spine Injury Association (ASIA) grade. In a prospective study of 100 patients, Miyanji et al 65 evaluated the correlation of quantitative and qualitative MRI assessments after SCI with patient neurological status and FIGURE 19. Sagittal STIR image in a 2-year-old patient developed flaccid paralysis after rough-house play shows intramedullary STIR hyperintensity from the cervicomedullary junction through the C5 level. CT (not shown) was negative for fracture. STIR, short tau inversion recovery. NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

13 SHAH AND ROSS prognosis for neurological recovery, and that the extent of maximum spinal cord compression is more reliable than presence of canal stenosis for predicting the neurological outcome after SCI. The prognosis of the compression pattern of SCI has been shown to depend on the degree of the initial neurological damage. 60 VASCULAR INJURIES FIGURE 20. Left, sagittal CT image exhibits a C5-C6 fracture-dislocation. Right, the parasagittal CTA image reveals occlusion of the vertebral artery (red arrow). CTA, computed tomography angiography. whether they are predictive of outcome at long-term follow-up. They found maximum spinal cord compression, spinal cord hemorrhage, and cord swelling are associated with a poor Certain fracture patterns increase the risk of cervical arterial injury: hyperflexion, hyperextension, distraction, facet dislocation, and fractures of the cervical spine. The vertebral artery is most vulnerable to injury when there are fractures of C1-C3, when the fracture extends through the transverse foramen and in the setting of subluxation or torsional factors, which may cause overstretching of the artery. 66,67 Dissection of the vertebral artery is more common than of the carotid arteries with cervical spine fractures/ subluxations. A literature review of publications pertaining to vertebral artery injury (VAI) associated with cervical spine trauma determined that approximately 0.5% of all trauma patients have a VAI, and 70% of all traumatic VAIs will have an associated cervical spine fracture. 66 The recognition of VAI is important, not only because of the potential primary neurological damage, but also to prevent secondary injury with implementation of therapy. In series that incorporate a screening protocol for asymptomatic patients, the incidence of neurological deficits secondary to VAI ranges from 0% to 24% In a retrospective review of 632 patients, 13% of whom had VAI, Torina et al 72 found that VAI was significantly more common in motor-complete patients than in neurologically intact and motor-incomplete patients. It is noteworthy that they found that the absence of neurological symptoms in a patient with cervical spine fracture does not preclude VAI. There are few FIGURE 21. Axial MP RAGE (A), proton density (B), and enhanced 3-D time-of-flight (C) images reveal a traumatic left vertebral artery dissection. A hyperintense crescentic mural thrombus is noted (white arrows). The residual opacified lumen is decreased in caliber (yellow arrow). MP RAGE, magnetization-prepared 180-degree radiofrequency pulses and rapid gradient-echo. 638 VOLUME 79 NUMBER 5 NOVEMBER

14 SPINE TRAUMA IMAGING published data regarding the time frame of the manifestation of neurological events after trauma, but the majority of events of blunt cerebrovascular injuries seem to develop within 10 to 72 hours after the injury. 73,74 Vascular injuries of the extracranial carotid and vertebral arteries may be identified on MRI as absence of flow voids, dissection flaps, or pseudoaneurysms. However, dedicated vascular imaging can be performed to assess vascular integrity. Digital subtraction angiography is the gold standard but carries a 0.5% risk of stroke itself. CT angiography (CTA) of the craniocervical vessels is a fast, high-resolution technique for the detecting of vascular injury. The sensitivity of CTA in detecting VAI has improved over time, with 1 series showing 99% sensitivity of multislice CTA for angiographically proven VAI (Figure 20). 75 A recent clinical review concluded that the literature supports CTA as the preferred method of choice for screening of VAI. 76 CTA can be integrated into a whole-body CT protocol for patients with multiple traumas, such that additional screening technique is not necessary to identify clinically relevant vascular injuries. 77 With earlier recognition, treatment can be implemented sooner to decrease mortality and morbidity from these potentially devastating injuries. Magnetic resonance angiography (MRA) has also been shown to be a valid technique for imaging vertebral artery pathology. 78,79 On time-of-flight MRA source images, the intramural hematoma appears as a T1 hyperintense periarterial rim with the flow void in the patent lumen. This relatively high signal intensity of the intramural thrombus is due to the T1 effects of methemoglobin. The affected artery has an apparent increase in the external diameter compared with that of the contralateral side because of the intramural hematoma. Other specific sequences such as axial T1-weighted with fat saturation, proton density, and magnetization-prepared 180-degree radiofrequency pulses and rapid gradient-echo, can increase the conspicuity of intramural thrombus in a vascular dissection (Figure 21). MRA can be performed as part of the MRI evaluation of dissection, but in the context of acute polytrauma patients, this modality may be unsuitable given the long scan times and safety challenges within the scanning environment. Also, the typical and easily diagnosed T1 hyperintensity of vessel wall hemorrhage will not be present in FIGURE 22. Left, sagittal STIR image in a patient with cervical cord injury (ASIA A neurological status) shows mild cord swelling and edema at the C3 level (circle). Right, the graphs (fractional anisotropy [FA], top; apparent diffusion coefficient [ADC], bottom) illustrate decreased ADC at the level of the cord injury, which is the most sensitive marker of cord injury. The apparent increased FA is hypothesized to reflect the early stage of injury and will decrease with time (Courtesy of Adam Flanders, MD). ASIA, American Spine Injury Association; STIR, short tau inversion recovery. NEUROSURGERY VOLUME 79 NUMBER 5 NOVEMBER

15 SHAH AND ROSS the hyperacute phase, making acute evaluation on MRI alone much more difficult. ADVANCED IMAGING TECHNIQUES An important limitation of conventional MRI is the inability to delineate preserved white matter tracts at the level of injury as well as the extent of injury to distant normal-appearing white matter. Diffusion tensor imaging (DTI) can detect spinal cord changes in otherwise normal-appearing parenchyma. 80,81 DTI quantifies the multidirectional movement of water molecules. The highly anisotropic architecture of the spinal cord lends itself to the localization of the white matter from the gray matter and to the assessment of structural changes. Microstructural variations in gray and white matter at different spinal levels (cervical, thoracic, lumbar) result in different DTI indices. 82 Such DTI indices, including apparent diffusion coefficient (ADC), fractional anisotropy (FA), mean diffusivity, radial diffusivity, and axial diffusivity, can be used to analyze spinal cord structural integrity and to measure the microstructural alterations that affect diffusion of water molecules in pathology. Alterations in DTI indices often depend on the level and severity of injury. DTI indices provide quantification of microstructural neural damage, which may help guide prognostication in patients with acute or chronic spinal cord injuries. DTI reveals reduced FA, ADC, and axial diffusivity around the acutely injury site (Figure 22). 83,84 Retrograde neural injury may cause changes in the diffusivity rostral to the site of injury in regions that appear normal on conventional MRI sequences. 84,85 Current investigations are exploring whether DTI indices at the acutely injured site are associated with degree of neurological injury and clinical outcomes. Higher ADC values at the acutely injured site have been shown to be associated with better postoperative Neurosurgical Cervical Spine Scale but not with Frankel scale measures. 86 In patients with nonhemorrhagic spinal cord contusions, strong correlations are observed between ASIA motor scores and average mean diffusivity, FA, radial diffusivity, and axial diffusivity at the injury site. 85 Changes rostral to the injury may be useful to evaluate neural injury because susceptibility artifact from blood products, bone fragments, and fixation hardware may limit imaging evaluation around the injury site. In regions rostral to the location of injury, DTI metrics have correlated with functional measures and may be used as a marker of supratentorial neural organization and plasticity. CONCLUSION Radiologic examinations have an important role in the assessment of the trauma spine, with the different imaging modalities providing specific diagnostic information. 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