Update on spinal cord injury associated with intervertebral disc disease

Introduction

Intervertebral disc disease is an almost ubiquitous degenerative condition but will lead to spinal cord injury only when there is herniation into the vertebral canal. Classically, this can take the form of: a) protrusion, in which the annulus bulges into the canal and causes chronic compression; or, b) extrusion, in which the nucleus of the disc herniates through the annulus into the canal and cause a mixed compressive and contusive lesion. It can be considered that each acute extrusion creates a more-or-less unique injury to each individual dog: the variables of degree of compression, severity of contusion, rapidity of extrusion, volume of extrusion will combine with variables inherent to each individual animal such as the relative size of the spinal cord to the vertebral canal to create a unique event.¹

Of the two injury mechanisms, contusion is the best understood, mainly because it is more straightforward to model and there have been many thousands of experiments on laboratory animals, especially during the past 3 decades.² The initial mechanical injury to the spinal cord is considered to be a relatively minor component of the injury, creating minor blood vessel injuries and some physical discontinuities in axonal integrity. The main component of tissue injury is thought to be created by cellular responses in the injured tissue which lead to its auto-destruction over a period of days. An important initial component of this injury is impairment of blood flow, which then leads to a cascade of secondary consequences, involving free radical formation, excitotoxicity and abnormal ionic (especially calcium) fluxes. Inflammation occurs because of cellular responses by both intrinsic (astrocytes and microglia) and infiltrating cells from the circulation (neutrophils and lymphocytes). The inflammatory response can give rise to systemic complications, not all of which are fully elucidated yet. [...]