Spinal cord injury
Epidemiology and aetiology of spinal cord injury
In the UK, the annual incidence of spinal cord injury (SCI) is 19 new cases per million population, contributing to an estimated 50,000 people who are currently living with SCI in the UK. Age at the time of SCI has a bimodal distribution, with spikes in young adulthood and then in later life. Although SCI has traditionally been regarded as an affliction of young men, the lengthening of life expectancies has resulted in an increasing number of elderly patients presenting with SCI. Data from America show that the average age at diagnosis of SCI has increased from 28 years in the 1970s to 43 in recent years. There continues to be a male preponderance for SCI, with around four-fifths of traumatic SCIs in the USA occurring in men.
Aetiology of spinal cord injury
SCI can be divided into two aetiological categories e traumatic and non-traumatic SCI. Trauma is the most common cause of SCI both globally and locally. In Scotland, falls are the most common precipitator of SCI, followed by road traffic accidents. SCI can result from relatively low impact trauma in elderly patients. Additionally, a variety of non-traumatic processes can cause SCI, including spinal stroke-causing cord ischaemia, or spinal cord compression by a mass lesion such as an abscess, tumour or haematoma.
Anatomy of the spinal cord
The spinal cord is a complex network of pathways that convey information and instructions between the brain and the rest of the body. Multiple, distinct anatomical pathways within the cord are responsible for the transmission of specific information. The corticospinal tract is responsible for the transmission of information relating to motor function, while the spinothalamic tract and posterior columns are the major sensory pathways. These sensory pathways differ in the sensations they transmit – the posterior columns relay information for vibration, fine touch, and proprioception whereas the spinothalamic tracts carry pain, temperature, and coarse touch. The spinal cord extends distally as a continuation of the medulla oblongata, tapering off to form the conus medullaris usually at vertebral level L2. The cord is offered protection by meninges and cervical, thoracic and lumbar vertebral bones. Arterial blood supply to the cord is via one anterior and two posterior spinal arteries, with additional contributions from radicular arteries at various points along the cord, the most significant of which is the artery of Adamkiewicz which supplies the inferior two-thirds of the cord. Venous drainage of the cord is via a complex network of valveless venous plexuses.
Pathophysiology of spinal cord injury
Damage to the spinal cord occurs both at the time of injury (primary) and in its aftermath (secondary). The primary cord injury can involve disruption to or pressure on the cord itself, its blood supply and surrounding supportive structures such as ligaments and vertebrae. Four main mechanisms of primary injury have been established:
- impact with transient compression of the cord
- impact with persistent compression of the cord
- distraction injury
- direct laceration or transection
Secondary injury is an exacerbation of the primary injury as a result of a number of local or systemic processes including hypotension, hypoxaemia, haemorrhage and cord oedema. These mechanisms act to impair perfusion and oxygen delivery to the already damaged cord. Although often little can be done to reverse the primary cord injury, much can be done to prevent or minimize secondary injury.
Description of spinal cord injury
SCI can be described in terms of the anatomical location and functional level of the injury, and the completeness of the injury. The latter two variables are included in the ASIA International Standard for the Neurological Classification of Spinal Cord Injury scoring scale, which is an internationally adopted tool for assessing and describing SCI. Anatomical location of SCI The most common site of traumatic SCI is the cervical portion of the spinal cord, accounting for approximately 60% of all traumatic SCI, followed by lumbar and thoracic. Thoracic cord injuries are less common due to the increased biomechanical support offered to this portion of the cord by the thorax.
Level of injury
A SCI can be described as having a neurological level, which is determined by a detailed assessment of motor and sensory function. The sensory level is the most caudal, intact dermatome for both pinprick and light touch sensation. The Medical Research Council’s muscle power scale (MRC scale) is used to assess motor function. The motor level is the lowest key muscle function that has a muscle function strength of at least three (corresponding to the active movement against gravity), providing the key muscle functions represented by segments above that level are judged to be intact. The neurological level of an SCI is the most caudal level at which the individual has both intact sensation and muscle function strength of at least three.
Complete versus incomplete SCI
If the examination confirms that sensory or motor function is preserved at the S4eS5 level (sacral sparing), then the injury is described as incomplete. In contrast, loss of such function confirms a complete spinal cord injury. ASIA spinal injury classification system (Figure 1) The American Spinal Injury Association has published an International Standard for Neurological Classification of Spinal Cord Injury (ISNCSCI, or more commonly known as the ASIA score). This document grades injuries according to their neurological level and extent of SCI. It is usually performed with 72 hours of the time of injury. The ASIA system has gained widespread use as a means of accurately quantifying the severity and prognosis of SCI.
Common spinal cord injury syndromes
Paraplegia describes a reduction in motor and/or sensory function in the lower limbs with varying truncal involvement, resulting from damage to the cord at a thoracic or lumbar level. In contrast, tetraplegia refers to a reduction in motor and/or sensory function in all limbs, often resulting from a cervical cord injury. Central cord syndrome is a distinct presentation that tends to affect older individuals with the degenerative cervical vertebral disease. It classically occurs following traumatic hyperextension of the cervical spine, with transient compression of the cord resulting in damage to the central portion of the cervical cord. Clinically, this presents as an incomplete injury with sensory and motor dysfunction affecting the upper limbs more than the lower limbs. Central cord syndrome is the most common incomplete SCI.
Author: Mark Patek, Mark Stewart