Traumatic Spinal Cord Injury: Acute Spinal Cord Injury and Prognosis

1.3A.1

Introduction

Prior to the 1980s a diagnosis of trauma to the spinal cord was largely inferred based on either X-rays showing misalignment of the spinal column or myelography whereby interruption of the flow of contrast medium in the cerebral spinal fluid (CSF) space indicated impingement of the spinal cord. With the widespread adoption of MRI came the visualization of the spinal cord itself, and along with it a revolution in the diagnosis and treatment of traumatic spinal cord injury. It wasn't long after the implementation of MR imaging that numerous research groups began to study the ability of MRI to determine both neurological function and prognosis following spinal cord injury. Numerous studies were undertaken with an aim of understanding the limitations of MRI to aid clinicians in treating their patients. These studies, reviewed and summarized here, introduced a host of new questions to the field such as the meaning of different signal characteristics with respect to pathological changes in the spinal cord tissue and the ability of these signal characteristics to yield meaningful clinical information such as the degree of damage to the spinal cord in terms of neurological function and the potential prognostic implications.

The advantages of being able to both determine neurological function and predict prognosis after acute traumatic spinal cord injury by reviewing MRI scans are numerous. While determining function and prognosis may seem beyond the limits of structural MR imaging, significant progress has been made in this direction. Benefits include distinguishing subclasses of patients that would benefit from different treatment options; for example, persons with severe spinal cord compression may benefit most from urgent decompressive surgery. Others, with no spinal cord compression, may benefit from delivery of neuroprotective agents, either systemically or locally. Accurate diagnostic and prognostic information could be conveyed to patients, family members, and the rehabilitation team such that efforts can be coordinated to plan and execute the best rehabilitation strategy.

Other modalities have certainly been used to predict prognosis following spinal cord injury, each with its own limitation. Physical examination, electrophysiology, computed tomography, and myelography have been studied. Of these, neurological examination has maintained its position as the gold standard for assessing patients both in the acute stage and at long-term follow-up. There are however two factors that make an alternate test worth seeking. The first is that patients who suffer from traumatic spinal cord injury often have other associated injuries, are intoxicated or are medically unstable making a thorough examination impossible. The second is that a neurological examination taken immediately after injury is often not a good indicator of prognosis.

In this chapter we aim to convey three messages. The first will describe the meaning of different MRI signal characteristics relative to the pathobiology of spinal cord injury as elucidated in animal imaging models. We will place an emphasis on animal imaging models that use MRI as a prognostic tool after traumatic spinal cord injury. The second message is to describe how quantitative measurements of maximum spinal cord compression (MSCC) and maximum canal compromise (MCC) can be used as a tool to describe the degree of spinal cord damage in a patient. The third message of this chapter is to describe how intramedullary MRI signal characteristics have been used as a diagnostic and prognostic tool. To accomplish this, we have conducted a meta-analysis of published papers and constructed receiver operator characteristics to describe the sensitivity and specificity of different MR signal characteristics.

1.3A.2

Message 1: Animal Models That Link Pathology of Acute Spinal Cord Injury to MR Signal Characteristics

Table 1.3A.1 summarizes the animal literature regarding the use of MRI as a prognostic tool in spinal cord injury. Of the eight animal studies identified, five were performed in a rat model and three in a mouse model. Each study involved a surgical procedure to induce spinal cord injury followed by serial clinical examinations and MRI studies. Attempts were then made to correlate clinical status with MRI findings.

Six of the eight studies showed a positive correlation between prognosis and MRI characteristics. Two of the eight studies showed no such correlation. These discrepancies can easily be explained by the fact that each of these studies examined novel ways of imaging the injured animal spinal cord. The negative studies examined the use of diffusion tensor metrics and high magnetic field strength (9.4 T) on recovery. Establishing these novel methods in an animal model must take place prior to using them to predict either neurological function or prognosis.

Of the studies that showed a positive correlation, two focused on vascular effects, two focused on neuronal structure and function, and two simply reported MRI characteristics as they relate to functional recovery.