MRI of chronic spinal cord injury

Transcription

1 The British Journal of Radiology, 76 (2003), DOI: /bjr/ E 2003 The British Institute of Radiology Pictorial review MRI of chronic spinal cord injury 1 K POTTER, FRCR and 1 A SAIFUDDIN, MBChB, MRCP, FRCR The Department of Diagnostic Imaging, The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex HA7 4LP, UK Abstract. Patients who have suffered a spinal cord injury and who demonstrate new or changing clinical features such as increasing myelopathy, ascending neurological level, pain or increasing muscle spasms may have developed a late complication such as post-traumatic syrinx. MRI is the investigation of choice for assessment of chronic spinal cord injury. The aim of this pictorial review is to illustrate the various late appearances of the injured spinal cord. Patients who have suffered a spinal cord injury (SCI) and who demonstrate new or changing clinical features such as increasing myelopathy, ascending neurological level, pain and increasing muscle spasms require imaging, as treatable causes exist. Chronic SCI is optimally investigated using MRI. The majority of abnormalities are adequately assessed with standard T 1 and T 2 weighted spin echo sequences in the sagittal and axial planes. Spin echo sequences, especially T 1 weighted images are subject to little artefact from metallic implants since the majority of these are now titanium based (Figure 1). The use of gradient echo sequences in the presence of metallic implants should be avoided. Patients with chronic SCI also commonly have a variety of metallic implants such as sacral anterior and posterior root stimulators (SARS and SPARS), artificial urinary sphincters, urethral and ureteric stents, phrenic pacers (in patients with high cervical lesions), dorsal column stimulators and baclofen pumps. Care must be taken to question patients regarding such devices and to determine their MRI compatability. This may be possible via product information sheets or by contacting the relevant manufacturers. The most common focal post-traumatic abnormalities of the spinal cord include cyst formation, myelomalacia and syrinx. A spinal cord cyst is considered to represent the residue of an intramendullary haemorrhage, with subsequent liquefaction of necrotic cord tissue [1]. Myelomalacia is thought to occur at sites of cord oedema. Cord swelling is thought to result in ischaemia with subsequent demyelination and gliosis [1]. The aetiology of posttraumatic syrinx is not fully understood. It has been suggested that some post-traumatic cysts communicate with the cerebrospinal fluid (CSF) space via the central canal, resulting in extension of the cyst due to flow of CSF into the resulting cavity. The presence of dural adhesions Received 17 January 2002 and in revised form 30 July 2002, accepted 25 March Address correspondence to Dr Asif Saifuddin. and coalescence of foci of myelomalacia have also been suggested as causes of syrinx formation [2]. Abnormal MRI findings Cord atrophy Cord atrophy is an abnormal narrowing of the spinal cord (Figure 1). Extended atrophy is considered to be present when it reaches at least two vertebral segments beyond the vertebral injury. Such extension may be both above and below the level of injury (Figure 2). The spinal cord is considered atrophic if it measures less than 7 mm in anteroposterior (AP) dimension in the cervical region and less than 6 mm in the thoracic region. The exact extent of such atrophy can be difficult to determine, as there may be no clear demarcation of normal from abnormal cord [2]. Atrophy is seen in patients with a long history (more than 2 years) of post-traumatic myelopathy [3]. The prevalence of atrophy depends upon the time between injury and imaging and has been reported in 18% of patients. Extended atrophy is the most common spinal cord abnormality seen in patients imaged more than 20 years after injury [2], with a prevalence of 62%. It is more commonly associated with cervical injuries and approximately 75% of patients will have a complete SCI. Focal cord atrophy must be distinguished from posttraumatic subarachnoid cyst formation which may only manifest on MRI as a focal, subtle compression of the cord, resulting in asymmetric cord thinning and displacement, since the signal characteristics of the cyst will be identical to CSF [4]. Myelomalacia Myelomalacia is seen as an ill-defined area of abnormal cord, which is hypointense on T 1 (but hyperintense to CSF) and hyperintense on T 2 compared with normal cord (Figure 3) [2]. It may be associated with focal atrophy and has been described in patients as early as 2 months post injury. Myelomalacia is the second most common finding in patients imaged more than 20 years after injury, with a The British Journal of Radiology, May

2 K Potter and A Saifuddin prevalence of 55%. Approximately 75% of patients will have a complete SCI [2]. Myelomalacia may be associated with syrinx formation [2, 3]. Cyst In the context of chronic SCI, a cyst is an oval intramedullary lesion, which is confined to the vertebral level of maximum bony protrusion into the spinal canal. A cyst is most commonly associated with cervical injuries (Figure 4). It has the same signal intensity and definition as a syrinx (Figure 5) and has a prevalence of 9% in patients scanned more than 20 years after injury [2]. Although a cyst can expand the cord (Figure 6), it rarely requires intervention and is associated with a complete SCI in less than 50% of cases. Areas of myelomalacia may be identified adjacent to the cyst (Figure 6). associated with a complete SCI. It has been documented in approximately 4% of cases and most commonly occurs in the thoracic region. It is often associated with other changes, particularly extended atrophy and myelomalacia [2]. Cord tethering Tethering can be seen when the spinal cord appears attached to the spinal canal s bony wall (Figure 7). It occurs in 4% of cases, being seen in the cervical and thoracic regions with approximately equal frequency. Tethering is commonly associated with other pathologies, especially atrophy and cyst formation and is always associated with a complete SCI [2]. Syrinx The incidence of post-traumatic syrinx has been estimated at 3.4% [5]. A syrinx is a tubular, well-defined fluid-filled region within the spinal cord. It is usually tapered to one, or both ends and can appear septated. Syrinxes usually cause expansion of the cord with thinning of cord tissue. The signal characteristics are typically those of CSF (Figures 7 9) [2]. In some patients the T 1 signal is higher than CSF, possibly due to higher protein content [3]. Flow voids may result in prominent loss of signal intensity within the syrinx on T 2 weighted images (Figure 9). Such a finding is considered to represent a high-pressure syrinx, which is more likely to show a good response to surgical drainage than low-pressure syrinxes, where no flow void is identified [6]. The longitudinal extent of syrinxes ranges from 2 to 20 vertebral segments with a mean of 6 [2]. Syrinxes extend both rostrally and caudally to the level of vertebral injury with equal incidence. The incidence of asymptomatic syrinx is not known. Symptoms associated with syrinx formation include pain, sensory changes, loss of reflexes and temperature sensation, motor deficit, hyperhydrosis and increased spasticity [5]. Shunting a syrinx can improve these symptoms in up to 85% of patients. Symptoms of pain and motor deficit are most likely to show an improvement following intervention [5]. The frequency of syrinx formation in patients with severe, complete injuries of the spinal cord is twice that in patients with incomplete lesions [3]. They occur most commonly in the thoracic region [2, 5] and there appears to be an association with residual spinal deformity [7] and post-traumatic spinal stenosis [8]. Whilst considered a late complication, cavities within the cord have been described as early as 6 weeks after injury with extensive syrinxes seen at 2 months. There does not seem to be a correlation between the length of a syrinx and the time taken for it to develop. However, the longer a syrinx is, the more likely it is to be symptomatic. The prevalence of syrinx also increases with time [3]. Cord disruption This is complete absence of the spinal cord tissue at the site of or distal to the injury (Figure 10) and is always Correlation of MRI findings with progressive symptoms and signs Patients imaged for progressive signs or symptoms are most likely to show the presence of a syrinx (43%), extended atrophy (26%) or myelomalacia (21%). Elevation of sensory level is the most common finding with syrinx [2]. However, it is clear that the majority of patients with stable post-injury neurological status will have abnormal MRI findings. Therefore, it cannot be assumed that the changes seen on MRI are the cause of the neurological deterioration. Post-traumatic syrinx is the major abnormality amenable to surgical intervention and the presence of signal void on T 2 weighted sequences should be carefully sought, as it appears that such patients are most likely to respond to drainage. Figure 1. Sagittal T 1 W and T 2 W spin echo MRI in a patient who suffered a hyperextension injury at the C3/4 level. Focal atrophy is present at the disc level, associated with on-going mild cord compression and possible central cyst formation or myelomalacia, manifest as reduced cord signal intensity on T 1 W and hyperintensity on T 2 W. Note the relative lack of artefact from the anterior instrumented fusion with titanium screws. 348 The British Journal of Radiology, May 2003

3 Pictorial review: MRI of chronic spinal cord injury Figure 3. Sagittal T 1 W and T 2 W spin echo MRI in a patient who suffered a previous dens fracture, which is now well-healed. An area of myelomalacia is evident adjacent to C2, manifest as a poorly defined region of hypointensity in the cord on T 1 W and hyperintensity on T 2 W. The signal abnormality on T 1 W is slightly hyperintense to cerebrospinal fluid. Figure 2. Sagittal T 2 W fast spin echo MRI in a patient who suffered a flexion distraction injury at the T4/5 level. Extended atrophy is present both above and below the level of injury and is associated with central hyperintensity due to a small syrinx. Figure 4. Sagittal T 1 W spin echo MRI in a patient who suffered a burst fracture of C5. A cyst is present in the cord at the C5 level and is associated with focal cord atrophy. The British Journal of Radiology, May

4 K Potter and A Saifuddin Figure 5. Sagittal T 1 W and T 2 W spin echo MRI in a patient who suffered a hyperextension injury at the C3/4 level showing a well-defined post-traumatic spinal cord cyst. Figure 6. Sagittal T 1 W and T 2 W spin echo MRI in a patient who suffered a T7 and T8 burst fracture. A well defined cyst is present in the cord and is associated with slight cord expansion and adjacent myelomalacia. 350 The British Journal of Radiology, May 2003

5 Pictorial review: MRI of chronic spinal cord injury (c) Figure 7. (a, b) Sagittal T 1 W and (c) T 2 W spin echo MRI in a patient who suffered a burst fracture of L1. The syrinx is septated and isointense to cerebrospinal fluid. The conus is tethered to the posterior wall of L1. Figure 8. Axial T 1 W spin echo MRI through the mid-thoracic region in a patient who suffered a burst fracture of T9. The syrinx expands and thins the cord. The British Journal of Radiology, May

6 K Potter and A Saifuddin Figure 10. Sagittal T 2 W and T 1 W spin echo MRI in a patient who suffered a T5/6 shear injury showing a focal area of cord disruption. Figure 9. Sagittal T 2 W spin echo MRI in a patient who suffered a burst fracture of T11. The spine has been reconstructed with T11 vertebrectomy and replacement by a titanium cage/ plate construct. The syrinx expands the cord, extending caudally into the conus. Regions of flow void are identified, indicating the presence of a high-pressure syrinx. References 1. Silberstein M, Hennessy O. Implications of focal spinal cord lesions following trauma: evaluation with magnetic resonance imaging. Paraplegia 1993;31: Wang D, Bodley R, Sett P, Gardner B, Frankel H. A clinical magnetic resonance imaging study of the traumatised spinal cord more than 20 years following injury. Paraplegia 1996; 34: Curati WL, Kingsley DPE, Kendall BE, Moseley IF. MRI in chronic spinal cord trauma. Neuroradiology 1992;35: Sklar E, Quencer RM, Green BA, Montalvo BM, Post MJ. Acquired spinal subarachnoid cysts: evaluation with MR, CT myelography, and intraoperative sonography. AJR Am J Roentgenol 1989;153: El Masry WS, Biyani A. Incidence, management, and outcome of post-traumatic syringomyelia. J Neurol Neurosurg Psychiatry 1996;60: Asano M, Fujiwara K, Yonenobu K, Hiroshima K. Posttraumatic syringomyelia. Spine 1996;21: Abel R, Gerner HJ, Smit C, Meiners T. Residual deformity of the spinal canal in patients with traumatic paraplegia and secondary changes of the spinal cord. Spinal Cord 1999;37: Perrouin-Verbe B, Lenne-Aurier K, Robert R, et al. Posttraumatic syringomyelia and post-traumatic spinal stenosis: A direct relationship: Review of 75 patients with a spinal cord injury. Spinal Cord 1998;36: The British Journal of Radiology, May 2003