Spinal Cord Injury Basics
What is spinal cord injury?
Many misconceptions abound concerning spinal cord injury. For example, many people believe that the spinal cord below the injury site dies after injury. Others think that the injured spinal cord is like a cut telephone wire and can be fixed by reconnecting the cut ends. Some people think that the vertebral column is the spinal cord. Even doctors have misleading and inaccurate ideas about spinal cord injury. For example, many doctors casually use the word “transection” to refer to severely injured spinal cords. The word should only be applied to the extremely rare situation when the spinal cord has been cut and the cut ends are separated.
Spinal cord injury usually results from trauma to the vertebral column. Displaced bone or disc then compresses the spinal cord. Spinal cord injury can occur without obvious vertebral fractures and you can have spinal fractures without spinal cord injury. It can also result from loss of blood flow to the spinal cord. Many people may have had mild spinal cord injury without thinking that it is spinal cord injury. For example, over a million people per year get “whiplash” in car accidents; they often have neck pain, weakness, and sensory loss that may last days or even months. Athletes who play football or other contact sports often suffer a transient loss of function that they call a “stinger”, i.e. paralysis and sensory loss for minutes or even hours. Sometimes, people can get spinal cord injury without any obvious cause, a condition called transverse myelitis.
Spinal cord injuries are usually defined by vertebral level and neurological level, as well as severity. Vertebral levels are indicated by which bony vertebrae have been fractured or show damage. Multiple bony vertebra may be injured. For example, an injury that causes the C5 vertebra to slip relative to C4 may be called a C4/C5 injury because it compresses the C4 and C5 spinal cord. Spinal cord levels do not necessary correspond to vertebral levels. For example, the C5 spinal cord lies in the C4 vertebral segment. The cord ends at the L1 vertebral level even though the spinal roots continue and exit between the appropriate vertebral segments.
For many years, there was no standardized way of referring to spinal cord injury levels. Surgeons generally referred to the injury level by the vertebra that are damaged. Neurologists and physiatrists, however, tend to refer to the level of spinal cord injury based on the neurological loss. Neurologists identify the level of injury as the first segmental level that shows sensory or motor loss. In contrast, physiatrists identify injury level from lowest spinal cord level that has normal motor and sensory function.
What is the spinal cord?
This may seem to be a silly question, but until people get a spinal cord injury or know somebody who has, most pay little attention to their spinal cords. Most people don’t know the different parts of the spinal cord, what each part does, and how the spinal cord transmits sensory and motor information. Many think that the spinal cord conducts information like a telephone wire and the spinal cord can be fixed by reconnecting it. Some people mistakenly believe that the spinal cord is the vertebral column. While almost everybody knows that spinal cord injury causes paralysis, many are not aware that the spinal cord also controls the bladder and bowel, sexual function, blood pressure, skin blood flow, sweating, and temperature regulation.
The spinal cord connects the brain to the body. The spinal cord resides in a bony spinal or vertebral column that has 24 segments. Seven vertebra in the neck are called cervical (C1-C7), twelve chest or thoracic (T1-T12) segments form the rib cage, five segments for the lower back or lumbar (L1-L5), and five segments form the tail or sacral (S1-S5) vertebra. The vertebral bodies are in the front of the spinal column. Spinal discs are located between the vertebral bodies. The front of the spinal cord is referred to as anterior while the back is referred to as posterior. The sides of the spinal cord are called lateral. Note that in animals that walk on four legs, posterior is dorsal and anterior is ventral.
Each segment has four spinal roots (left and right, posterior and anterior) that send and receive information from each side of the body. Posterior roots receive sensation while anterior roots send motor signals to muscles. For example, the C1-C3 segments send and receive information from the back of the head and neck, C4 covers the shoulder and deltoid muscles, C5 the biceps, C6 the wrist extensors, C7 the triceps, C8 the wrist flexors, and T1 the intrinsic muscles of the hand. The spinal roots leave the vertebral column between the bony segments through openings in the vertebral column called foramina. Note that there are only 7 cervical vertebra but 8 sets of cervical roots because the C1 roots are between the skull and C1.
The spinal cord is shorter than the vertebral column and occupies the spinal canal from the C1 to L1 vertebral levels. In general, the bony vertebral segments are lower than the spinal cord levels. The spinal roots exit through the spinal column through openings between vertebral segments called foramina. The spinal cord stops just below the L1 vertebral level and only spinal roots are present from L1 to S5 vertebral spinal column. The end of the cord is called the conus. Spinal roots below the conus are called the cauda equina because they resemble a horse’s tail.
How does the spinal cord work?
Neurons (nerve cells) in the brain, spinal cord, and peripheral nerves send axons (nerve fibers) up and down the spinal cord in spinal tracts. These spinal tracts are called white matter because axons are coated with a membrane called myelin, which appears white. In the spinal cord, white matter is usually situated close to the surface of the cord, arranged into several columns called the anterior, posterior, and lateral columns. The spinal cord contains neurons located in the middle part of the spinal cord. The areas of the spinal cord that contain neurons are called gray matter. The gray matter is most abundant in the parts of the spinal cord that connect to the arms and legs, called the cervical and lumbosacral enlargements.
The spinal cord transmits signals for sensations and to control movement, as well as breathing, bladder, bowel, sweating, blood pressure, sexual, and other essential functions of the body. The spinal cord contains neuronal circuitry for reflexes that control all these functions. Over 20 million axons ascend and descend in the human spinal cord, organized into spinal tracts named according to their source and destination. For example, the spinal tract that sends axons from the cerebral cortex to the spinal cord is called the corticospinal tract. Likewise, the tract that sends axons from the red nucleus in the midbrain to the spinal cord is called the rubrospinal tract. The sensory tract that transmits pain and temperature sensation from the spinal cord to the thalamus is called the spinothalamic tract. Some tracts, however, are named by their position. For example, the posterior column transmits sensory information from the spinal roots to the brainstem.
Neurons that send axons to muscles are called motoneurons while neurons that send axons to other neurons are called interneurons. Motoneurons and interneurons receive information from descending axons and sensory axons. When you activate sensory input to the spinal cord by tapping a tendon, the activity turns on motoneurons that cause the muscle of that tendon to contract. This is called a monosynaptic reflex. To signal the muscles to move, the brain sends information directly to motoneurons or indirectly through interneurons that can either excite or inhibit other neurons.
Sensory neurons send axons from the spinal cord to the brain. Some sensory axons go from peripheral nerve neurons in posterior sensory ganglia located just outside of the spinal column. Posterior sensory ganglion neurons send a T-shaped axon to the body where it collects information like touch and movement while the other end goes into the spinal cord and branches. One branch goes into the gray matter where it activates motoneurons and the other end goes up the posterior column all the way to the brainstem.
How is spinal cord injury classified?
In 1990, the American Spinal Cord Injury Association (ASIA) proposed a uniform classification system that had five categories, defined in Table 1. Motor level is defined as the level at which the key muscle innervated by the segment has at least 3/5 of its normal strength. Sensory level is defined as the lowest spinal cord level that still has normal pinprick and touch sensation. If there is a spinal cord level below which there is no voluntary motor or conscious sensory function, the person is called a “complete” spinal cord injury. Since the S5 is the lowest spinal cord level that innervates the anal sphincter, a person that has no voluntary anal sphincter control or sensation is defined as a “complete” spinal cord injury. A person who has any anal control or sensation is an “incomplete” spinal cord injury. Some people may have a “complete” spinal cord injury but still has preserved motor or sensory function between the injury level and S5. This is called the “zone of partial preservation”. Usually, the spinal cord injury level and severity is classified between 72 hours and 7 days after injury. Note that some people have neurological loss at a given spinal cord level but partially preserved function for several or even many segments; this is called the zone of partial preservation (ZPP).
Table 1: Neurological Classification of Spinal Cord Injury
A
No motor or sensory function preserved in the lowest sacral segments
B
Sensory but not motor function preserved in the lowest sacral segments
C
Motor function present below the injury but most key muscles are <3/5
D
Motor function present below the injury but most key muscles are ≥3/5
E
Motor and sensory function normal in key muscles and dermatomes
Some patterns of spinal cord injury have special names.
• In the “Central Cord Syndrome”, arm function is affected more than the legs. This paradoxical condition is attributed to damage to the central part of the spinal cord. Recent studies of central cord syndrome suggest that the syndrome may be associated with destruction of the lateral spinal tracts.
• “Brown-Sequard Syndrome” refers to injuries limited to one side of the cord. People have weakness and loss of touch sense in one leg but loss of pain and temperature sensation in the other side.
• “Anterior Cord Syndrome” refers to the condition when sensation is preserved but motor function is absent below the injury site.
• “Posterior Cord Syndrome” refers to the condition when motor function is preserved in the absence of sensation.
• “Conus Medullaire” refers to injury of the conus or lower tip of the spinal cord. This damages the lower lumbar and sacral spinal cord segments.
• “Cauda Equina Injury” refers to the condition when the damage is limited to the spinal roots below L1.
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