Cervical Cord-Canal Mismatch: A New Method for Identifying Predisposition to Spinal Cord Injury

The risk for spinal cord injuries (SCIs) ranging from devastating traumatic injuries, compression because of degenerative pathology, and neurapraxia is increased in patients with congenital spinal stenosis. Classical diagnostic criteria include an absolute anteroposterior diameter of <12–13 mm or a Torg-Pavlov ratio of <0.80–0.82; however, these factors do not take into account the size of the spinal cord, which varies across patients, independent of canal size. Recent large magnetic resonance imaging studies of population cohorts have allowed newer methods to emerge that account for both cord and canal size by measuring a spinal cord occupation ratio (SCOR). A SCOR defined as ≥70% on midsagittal imaging or ≥80% on axial imaging appears to be an effective method of identifying cord-canal mismatch, but requires further validation. Cord-canal size mismatch predisposes patients to SCI because of 1) less space within the canal lowering the amount of degenerative changes needed for cord compression, and 2) less cerebrospinal fluid surrounding the spinal cord decreasing the ability to absorb kinetic forces directed at the spine. Patients with cord-canal mismatch have been reported to be at a substantially higher risk of traumatic SCI, and present with degenerative cervical myelopathy at a younger age than patients without cord-canal mismatch. However, neurologic outcome after SCI has occurred does not appear to be different in patients with or without a cord-canal mismatch. Recognition that canal and cord size are both factors which predispose to SCI supports that cord-canal size mismatch rather than a narrow cervical canal in isolation should be viewed as the underlying mechanism predisposing to SCI.

Introduction

Spinal cord injuries (SCIs) have a devastating impact on neurologic function and consequently can tremendously reduce the quality of life of affected individuals. Although substantial injuries to the cervical spinal cord typically occur because of high impact forces, such as from falls or motor vehicle accidents,¹ more progressive and milder forms of SCI are the most common type of injury and occur from age-related wear, referred to as degenerative cervical myelopathy (DCM).² Although the risk profiles for these types of injuries differ, it has been recognized that a common predisposition to traumatic SCIs, DCM, and neurapraxia is what has been classically referred to as congenital or developmental cervical stenosis.2, 3 This designation simply refers to a narrow anatomic cervical canal. Indeed, classical diagnostic criteria have suggested that a radiologic spinal canal diameter of 12–13 mm4, 5 or a Torg-Pavlov ratio (TPR) of 0.80–0.823, 6 is indicative of canal stenosis; unfortunately, neither method takes into account the size of the spinal cord (Figure 1). However, recent magnetic resonance imaging (MRI) studies of cervical spines have shown that there are considerable variations in the size of the spinal cord,7, 8, 9 therefore supporting the need to rethink these classical diagnostic criteria. More recent methods have looked at the subaxial space available for the cord (SAC) or assessed the spinal cord occupation ratio (SCOR), which evaluates the size of the spinal cord within the spinal canal and DCM.7, 10, 11, 12, 13, 14 In contrast with the classical diagnostic criteria, the premise for using the SCOR is based on a spinal cord-canal size mismatch. To support this pathophysiologic perspective, we first review current MRI size parameters for the cord and canal. Thereafter, through discussion of the rationale of SCOR measurements and recent findings, it is proposed that cord-canal mismatch be used as a diagnostic criterion for identifying individuals at risk for spinal cord compression and injury.