SPINALCORD: Overview of Spinal Cord Injury Anatomy & Physiology

This is a summary of information from a presentation given at the Alabama Department of Rehabiliation Services conference, An In-depth Analysis of Medical Disabilities by Specialists in Their Fields held on August 29, 2000.

Overview of Spinal Cord Injury Anatomy & Physiology
by Amie B. Jackson, MD

Anatomy

The spinal cord is the largest nerve in the body, and it is comprised of the nerves which act as the communication system for the body. The nerve fibers within the spinal cord carry messages to and from the brain to other parts of the body. Thus, the spinal cord can be compared to a telephone cable which connects the central office (brain) to the individual homes. Because of its important role in the nervous system, the spinal cord is surrounded by protective bone segments, called the vertebral column. The vertebral column is comprised of seven cervical vertebrae, twelve thoracic vertebrae, 5 lumbar vertebrae and five sacral vertebrae. As the body grows, the vertebral column grows more in length than the spinal cord, causing a discrepancy between the location of the spinal cord segments and the vertebral column segments, particularly in the lower part of the spinal system. For this reason, there is often a discrepancy between the level of vertebral fracture and the level of spinal cord injury.

The term spinal cord injury refers to any injury of the neural elements within the spinal canal. Spinal cord injury can occur from either trauma or disease to the vertebral column or the spinal cord itself. Most spinal cord injuries are the result of trauma to the vertebral column causing a fracture of the bone, or tearing of the ligaments with displacement of the bony column producing a pinching of the spinal cord. The majority of broken necks and broken backs, or vertebral fractures, do not cause any spinal cord damage; however, in 10-14% of the cases where a vertebral trauma has occurred, the damage is of such severity it results in damage to the spinal cord.

Causes

Spinal cord injury primarily occurs in young men with the greatest number of injuries occurring in the 16-30 age group. Patients with a spinal cord injury are designated as having tetraplegia (preferred to quadriplegia) or paraplegia. Tetraplegia refers to injuries to the cervical spinal cord and paraplegia refers to injuries below the cervical spinal cord. Patients with tetraplegia are slightly more common (51.7%) than patients with paraplegia. The majority of spinal cord injuries, about 37.4%, are sustained during a motor vehicle accident. Acts of violence are the second most common cause at 25.9%, falls are third at 21.5% and sports injuries are fourth at 7.1%.

Spinal cord injury can occur at any level of the spinal cord or at multiple levels; however, the most common area of injury is at the lower part of the neck at the C-4, C-5 and C-6 levels. The second most common area of injury is at the bottom of the rib cage at T-12. C-7 is the third most common area of spinal cord injury. An injury to the bone at the C-1 and C-2 level may not damage the spinal cord, but if the injury is very severe it will lead to immediate death because the high level of injury interferes with breathing. Therefore, at this level medical professionals only see patients with an incomplete injury or those who have a complete injury and receive mouth to mouth resuscitation until other assistive ventilation is possible. All of these areas are particularly vulnerable injury points because a more fixed part of the skeleton is attached to a more movable part and at the moment of trauma the movable part is injured at the place of fixation.

Classification

A spinal cord injury is classified according to the American Spinal Injury Association’s (ASIA) International Standards for Neurological and Functional Classification of Spinal Cord Injury (revised 1996) An injury is a class “A” complete injury if no motor or sensory function is preserved in the sacral segments S4-S5. An injury is a class “B” incomplete injury if sensory but no motor function is preserved below the neurological level and includes the sacral segments S4-S5. A class “C” incomplete injury is given to an injury where sacral sensation is present and motor function is preserved below the neurological level, and more than half of key muscles below the neurological level have a muscle grade less than “3” (less than active movement against gravity). A class “D” incomplete injury is given where where sacral sensation is present and motor function is preserved below the neurological level, and more than half of key muscles below the neurological level have a muscle grade of “3” or more. Finally, normal motor and sensory function is classified an “E” normal. Trends over time indicate an increasing proportion of persons with incomplete paraplegia and a decreasing proportion of persons with complete tetraplegia.

Spinal cord injuries must also be distinguished as to whether the injury is within the spinal cord proper or whether it has occurred in the cauda equina which is below the lower tip of the spinal cord. The spinal cord itself ends between the L-1 and L-2 vertebrae. The level of injury will also effect the type of paralysis. A person with preservation of normal spinal cord segments below the level of injury usually has an upper motor neuron type of injury, or spastic paralysis. However, a person with a conus medullaris injury without preservation of normal spinal cord segments below the lesion or a cauda equina injury, will have a lower motor neuron injury, or flaccid paralysis.

Neurologic Assessment

A good neurologic examination, including a sensory examination and motor examination, is required to classify the level of injury. The classification standards endorsed by the ASIA is recommended for determining the level of injury. The neurologic level of injury is defined as "the most caudal segment of the spinal cord with normal sensory and motor function on both sides of the body." The injury is further classified as complete or incomplete and by the Anatomical Syndromes. The Anatomical Syndromes include the Central Cord Syndrome, the Brown-Sequard Syndrome, Anterior Cord Syndrome, Conus Medullaris Syndrome, Cauda Equina Syndrome or unclassified Syndrome. The Central Cord Syndrome is the most common type of incomplete injury. Trauma to the spinal cord initially causes hemorrhage, hypoxia and edema in the central gray matter, gradually increasing over several hours and extending peripherally from the center of the cord, and is dependent on the extent of the injury. The damage may stop with peripheral cord sparing or may extend to completely destroy the spinal cord at the level of injury. Because the corticospinal tracks are laminated with the cervical segments more central and the sacral segments more peripheral, there is a disassociation in the degree of motor weakness; lower limbs remain stronger than upper limbs along with sacral sensory sparing, giving the picture of a Central Cord Syndrome. While there are many different dermatome charts, it is important to utilize a chart which is used by other persons working in the field of spinal cord injury; the International Standards for Neurological and Functional Classification of Spinal Injury, endorsed by both the American Spinal Injury Association (ASIA) and the International Medical Society of Paraplegia (IMSOP) is recommended and attached for your information.

In addition, the sensory and motor levels need to be determined for both the right and left sides. When there is a discrepancy between the lowest normal sensory level and the lowest normal motor level, this should be described, as it is not unusual to have a sensory level which is only normal down to C-4, but with a motor level that may be normal through C-5 or C-6.

Recovery

An accurate and complete examination is vital in determining the neurologic level, and therefore, establishing realistic rehabilitation goals. In persons with a complete injury, goals can usually be established on the day of the initial examination, since only about three percent of these patients later have recovery to an incomplete injury with minimal sparing. In persons with an incomplete injury, recovery is often much more difficult to predict. The person with any preserved motor movement often has fairly good prognosis. It is important that every patient have an examination of the sacral areas for sensation, since preservation of sensation may be the only evidence of an incomplete injury and this can change the outlook for recovery. In the patient with sensory sparing only, some rather dramatic recovery may take place about 1-2 months after injury, or very little recovery may occur.

Recovery from spinal cord injury is, therefore, fairly predictable. Persons with a complete injury by neurologic examination generally are not expected to make any significant recovery except some improvement in motor strength in the zone of injury. Recovery for persons with an incomplete injury is less predictable and most recovery occurs within the first six months; however, some additional neurologic recovery may take place up to 18 months after injury. In a patient with a cauda equina injury, some recovery is possible through regeneration for up to three years following spinal cord injury since it is a peripheral nerve type of injury.

Conclusion

Understanding the anatomy and physiology of the spinal cord and the vertebral column are crucial to understanding how injury to the spinal cord causes neurologic impairment and what rehabilitation goals may be expected following injury to the spinal cord.