Wise Young, Ph.D., M.D.
W. M. Keck Center for Collaborative Neuroscience
Rutgers University, Piscataway, NJ
When people are injured, they are often told that they have an injury at a given spinal cord level and are given a qualifier indicating the severity of injury, i.e. "complete" or "incomplete". They may also be told that they are classified according to the American Spinal Injury Association (ASIA) Classification, as a ASIA A, B, C, or D. They may also be told that they have a bony fracture or involvement of one or more spinal segments or vertebral levels. What most people do not know is doctors are frequently confused about the definition of spinal cord injury levels, the definition of complete and incomplete injury, and the classification of spinal cord injury. In the early 1990's, when I co-chaired the committee that helped define the currently accepted ASIA Classification, there was no single definition of level, completeness of injury, or classification. In this article, I will briefly address the issue of spinal cord injury levels, the definition of "complete" spinal cord injury, and the ASIA Classification approach towards spinal cord injury.
The spinal cord is situated within the spine. The spine consists of a series of vertebral segments. The spinal cord itself has "neurological" segmental levels which are defined by the spinal roots that enter and exist the spinal column between each of the vertebral segments. As shown in the figure the spinal cord segmental levels do not necessarily correspond to the bony segments. The vertebral levels are indicated on the left side while the cord segmental levels are listed for the cervical (red), thoracic (green), lumbar (blue), and sacral (yellow) cord.
Vertebral segments. There are 7 cervical (neck), 12 thoracic (chest), 5 lumbar (back), and 5 sacral (tail) vertebrae. The thoracic vertebrae are defined by The spinal cord segments are not necessarily situated at the same vertebral levels. For example, while the C1 cord is located at the C1 vertebra, the C8 cord is situated at the C7 vertebra. While the T1 cord is situated at the T1 vertebra, the T12 cord is situated at the T8 vertebra. The lumbar cord is situated between T9 and T11 vertebrae. The sacral cord is situated between the T12 to L2 vertebrae.
Spinal Roots. The spinal roots for C1 exit the spinal column at the atlanto-occiput junction. The spinal roots for C2 exit the spinal column at the atlanto-axis. The C3 roots exit between C2 and C3. The C8 root exits between C7 and C8. The first thoracic root or T1 exits the spinal cord between T1 and T2 vertebral bodies. The T12 root exits the spinal cord between T1 and L1. The L1 root exits the spinal cord between L1 and L2 bodies. The L5 root exits the cord between L1 and S1 bodies.
The Cervical Cord. The first and second cervical segments are special because this is what holds the head. The lower back of the head is called the Occiput. The first cervical vertebra, upon which the head is perched is sometimes called Atlas, after the Greek mythological figure who held up earth. The second cervical vertebra is sometimes called the Axis, upon which Atlas pivots. The interface between the occiput and the atlas is therefore called the atlanto-occiput junction. The interface between the first and second vertebra is called the atlanto-axis junction. The C3 cord contains the phrenic nucleus. The cervical cord innervates the deltoids (C4), biceps (C4-5), wrist extensors (C6), triceps (C7), wrist extensors (C8), and hand muscles (C8-T1).
The Thoracic Cord. The thoracic vertebral segments are defined by those that have a rib. These vertebral segments are also very special because they form the back wall of the pulmonary cavity and the ribs. The spinal roots form the intercostal (between the ribs) nerves that run on the bottom side of the ribs and these nerves control the intercostal muscles and associated dermatomes.
The Lumbosacral Cord. The lumbosacral vertebra form the remainder of the segments below the vertebrae of the thorax. The lumbosacral spinal cord, however, starts at about T9 and continues only to L2. It contains most of the segments that innervate the hip and legs, as well as the buttocks and anal regions.
The Cauda Equina. In human, the spinal cord ends at L2 vertebral level. The tip of the spinal cord is called the conus. Below the conus, there is a spray of spinal roots that is frequently called the cauda equina or horse's tail. Injuries to T12 and L1 vertebra damage the lumbar cord. Injuries to L2 frequently damage the conus. Injuries below L2 usually involve the cauda equina and represent injuries to spinal roots rather than the spinal cord proper.
In summary, spinal vertebral and spinal cord segmental levels are not necessarily the same. In the upper spinal cord, the first two cervical cord segments roughly match the first two cervical vertebral levels. However, the C3 through C8 segments of the spinal cords are situated between C3 through C7 bony vertebral levels. Likewise, in the thoracic spinal cord, the first two thoracic cord segments roughly match first two thoracic vertebral levels. However, T3 through T12 cord segments are situated between T3 to T8. The lumbar cord segments are situated at the T9 through T11 levels while the sacral segments are situated from T12 to L1. The tip of the spinal cord or conus is situated at L2 vertebral level. Below L2, there is only spinal roots, called the cauda equina.
A dermatome is a patch of skin that is innervated by a given spinal cord level. Figure 2 is taken from the ASIA classification manual, obtainable from the ASIA web site. Each dermatome has a specific point recommended for testing and shown in the figure. After injury, the dermatomes can expand or contract, depending on plasticity of the spinal cord.
C2 to C4. The C2 dermatome covers the occiput and the top part of the neck. C3 covers the lower part of the neck to the clavicle (the horizontal bone that goes to the shoulder. C4 covers the area just below the clavicle.
C5 to T1. These dermatomes are all situated in the arms. C5 covers the lateral arm at and above the elbow. C6 covers the forearm and the radial (thumb) side of the hand. C7 is the middle finger, C8 is the lateral aspects of the hand, and T1 covers the medial side of the forearm.
T2 to T12. The thoracic covers the axillary and chest region. T3 to T12 covers the chest and back to the hip girdle. The nipples are situated in the middle of T4. T10 is situated at the umbilicus. T12 ends just above the hip girdle.
L1 to L5. The cutaneous dermatome representating the hip girdle and groin area is innervated by L1 spinal cord. L2 and 3 cover the front part of the thighs. L4 and L5 cover medial and lateral aspects of the lower leg.
S1 to S5. S1 covers the heel and the middle back of the leg. S2 covers the back of the thighs. S3 cover the medial side of the buttocks and S4-5 covers the perineal region. S5 is of course the lowest dermatome and represents the skin immediately at and adjacent to the anus.
Ten muscle groups represent the motor innervation by the cervical and lumbosacral spinal cord. The ASIA system does not include the abdominal muscles (i.e. T10-11) because the thoracic levels are much easier to determine from sensory levels. It also excludes certain muscles (e.g. hamstrings) because the segmental levels that innervate them are already represented by other muscles.
Arm and hand muscles. C5 represents the elbow flexors (biceps), C6 the wrist extensors, C7 the elbow extensors (triceps), C8 the finger flexors, and T1 the little finger abductor (outward movement of the pinky finger).
Leg and foot muscles. The leg muscles represent the lumbar segments, i.e. L2 are the hip flexors (psoas), L3 the knee extensors (quadriceps), L4 the ankle dorsiflexors (anterior tibialis), L5 the long toe extensors (hallucis longus), S1 the ankle plantar flexors (gastrocnemius).
The anal sphincter is innervated by the S4-5 cord and represents the end of the spinal cord. The anal sphincter is a critical part of the spinal cord injury examination. If the person has any voluntary anal contraction, regardless of any other finding, that person is by definition a motor incomplete injury.
It is important to note that the muscle groups specified in the ASIA classifications represent a gross over simplication of the situation. Almost every muscle received innervation from two or more segments.
In summary, the spinal cord segment serve specific motor and sensory regions of the body. The sensory regions are called dermatomes with each segment of the spinal cord innervating a particularly area of skin. The distribution of these dermatomes are relatively straightforward except on the limbs. In the arms, the cervical dermatomes C5 to T1 are arrayed from proximal radial (C5) to distal (C6-8) and proximal medial (T1). In the legs, the L1 to L5 dermatomes cover the front of the leg from proximal to distal while the sacral dermatomes cover the back of the leg.
Differences between neurological and rehabilitation definitions of spinal cord injury levels. Doctors use two different definitions for spinal cord injury levels. Given the same neurological examination and findings, neurologists and physiatrists may not assign the same spinal cord injury level. In general, neurologists define the level of injury as the first spinal segmental level that shows abnormal neurological loss. Thus, for example, if a person has loss of biceps, the motor level of the injury is often said to be C4. In contrast, physiatrists or rehabilitation doctors tend to define level of injury as the lowest spinal segmental level that is normal. Thus, if a patient has normal C3 sensations and absent C4 sensation, a physiatrist would say the sensory level is C3 whereas a neurologist or neurosurgeon would call it a C4 injury level. Most orthopedic surgeons tend to refer to the bony level of injury as the level of injury.
EXAMPLE. The most common cervical spinal injuries involve C4 or C5. Take, for example, a person who has had a burst fracture of the C5 vertebral body. A burst fracture usually indicates severe trauma to vertebral body that typically injures the C6 spinal cord situated at the C5 vertebrae and also the C4 spinal roots that exits the spinal column between the C4 and C5 vertebra. Such an injury should cause a loss of sensations in C4 dermatome and weak deltoids (C4) due to injury to the C4 roots. Due to edema (swelling of the spinal cord), the biceps (C5) may be initially weak but should recover. The wrist extensors (C6), however, should remain weak and sensation at and below C6 should be severely compromised. A neurosurgeon or neurologist examining the above patient usually would conclude that there is a burst fracture at C5 from the x-rays, an initial sensory level at C4 (the first abnormal sensory dermatome) and the partial loss of deltoids and biceps would imply a motor level at C4 (the highest abnormal muscle level). Over time, as the patient recovers the C4 roots and the C5 spinal cord, both the sensory level and motor level should end up at C6. Such recovery is often attributed to "root" recovery. On the other hand, a physiatrist would conclude that the patient initially has a C3 sensory level, a C4 motor level, and a C5 vertebral injury level. If the patient recovers the C4 root and the C5 cord, the physiatrist would conclude that both the sensory and motor levels are C5.
Discrepant lower thoracic vertebral and cord levels. The spinal vertebral and cord segmental levels become increasingly discrepant further down the spinal column. For example, a T8 vertebral injury will result in a T12 spinal cord or neurological level. A T11 vertebral injury, in fact, will result in a L5 lumbar spinal cord level. Most patients and even many doctors do not understand how discrepant the vertebral and spinal cord levels can get in the lower spinal cord.
EXAMPLE. The most common thoracic spinal cord injury involves T11 and T12. A patient with a T11 vertebral injury may have or recover sensations in the L1 through L4 dermatomes which include the front of the leg down to the mid-shin level. In addition, such a patient should recover hip extensors, knee extensors, and even ankle dorsiflexion. However, the sacral functions, including bowel and bladder and many of the flexor functions of the leg may be absent or weak. As in the case of cervical and thoracic spinal cord injury, it is important to assess both sensory and motor function.
Conus and Cauda Equina Injuries. Injuries to the spinal column at L2 or lower will damage the tip of the spinal cord, called the conus, or the spray of spinal roots that are descending to the appropriate spinal vertebral levels to exit the spinal canal or the caudal equina. Please note that the spinal roots for L2 through S5 all descend in the cauda equina and injury to these roots would disrupt sensory and motor fibers from these segments. Strictly speaking, the spinal roots are part of the peripheral nervous system as opposed to the spinal cord. Peripheral nerves are supposed to be able to regenerate to some extent. However, the spinal roots are different from peripheral nerves in two respects. First, the neurons from which sensory axons emanate are situated in the dorsal root ganglia (DRG) which are located just outside the spinal column. One branch of the DRG goes into the spinal cord (called the central branch) and the other is the peripheral branch.
Thus, a spinal root injury is damaging the central branch of the sensory nerve whereas peripheral nerve injury usually damages the peripheral branch. The sensory axon must grow back into the spinal cord in order to restore function and they generally will not do so because of axonal growth inhibitors in the spinal cord and particular at the so-called PNS-CNS junction at the dorsal root entry zone. Second, the cauda equina contains the ventral roots of the spinal cord, through which the motor axons of the spinal cord pass to innervate muscles. If the injury to the ventral root is close to the motoneurons that sent the axons, the injury may damage the motoneuron itself. Both of these factors significantly reduce the likelihood of neurological recovery in a cauda equina injury compared to a peripheral nerve injury.
Most clinicians commonly describe injuries as "complete" or "incomplete".
Traditionally, "complete" spinal cord injury means having no voluntary motor or conscious sensory function below the injury site. However, this definition is often difficult to apply. The following three example illustrate the weaknesses and ambiguity of the traditional definition. The ASIA committee considered these questions when it formulated the classification system for spinal cord injury in 1992.
Most clinicians would regard a person as complete if the person has any level below which no function is present. The ASIA Committee decided to take this criterion to its logical limit, i.e. if the person has any spinal level below which there is no neurological function, that person would be classified as a "complete" injury. This translates into a simple definition of "complete" spinal cord injury: a person is a "complete" if they do not have motor and sensory function in the anal and perineal region representing the lowest sacral cord (S4-S5).
The decision to make the absence and presence of function at S4-5 the definition for "complete" injury not only resolved the problem of the zone of partial preservation but lateral preservation of function but it also resolved the issue of recovery of function. As it turns out, very few patients who have loss of S4/5 function recovered such function spontaneously. As shown in figure 3 below, while this simplifies the criterion for assessing whether an injury is "complete", the ASIA classification committee decided that both motor and sensory levels should be expressed on each side separately, as well as the zone of partial preservation.
In the end, the whole issue of "complete" versus "incomplete" injury may be a moot issue. The absence of motor and sensory function below the injury site does not necessarily mean that there are no axons that cross the injury site. Many clinicians equate a "complete" spinal cord injury with the lack of axons crossing the injury site. However, much animal and clinical data suggest that an animal or person with no function below the injury site can recover some function when the spinal cord is reperfused (in the case of an arteriovenous malformation causing ischemia to the cord), decompressed (in the case of a spinal cord that is chronically compressed), or treated with a drug such as 4-aminopyridine. The labeling of a person as being "complete" or "incomplete", in my opinion, should not be used to deny a person hope or therapy.
Clinicians have long used a clinical scale to grade severity of neurological loss. First devised at Stokes Manville before World War II and popularized by Frankel in the 1970's, the original scoring approach segregated patients into five categories, i.e. no function (A), sensory only (B), some sensory and motor preservation (C), useful motor function (D), and normal (E).
The ASIA Impairment Scale is follows the Frankel scale but differs from the older scale in several important respects. First, instead of no function below the injury level, ASIA A is defined as a person with no motor or sensory function preserved in the sacral segments S4-S5. This definition is clear and unambiguous. ASIA B is essentially identical to Frankel B but adds the requirement of preserved sacral S4-S5 function. It should be noted that ASIA A and B classification depend entirely on a single observation, i.e. the preservation of motor and sensory function of S4-5.
The ASIA scale also added quantitive criteria for C and D. The original Frankel scale asked clinicians to evaluate the usefulness of lower limb function. This not only introduced a subjective element to the scale but ignored arm and hand function in patients with cervical spinal cord injury. To get around this problem, ASIA stipulated that a patient would be an ASIA C if more than half of the muscles evaluated had a grade of less than 3/5. If not, the person was assigned to ASIA D.
ASIA E is of interest because it implies that somebody can have spinal cord injury without having any neurological deficits at least detectable on a neurological examination of this type. Also, the ASIA motor and sensory scoring may not be sensitive to subtle weakness, presence of spasticity, pain, and certain forms of dyesthesia that could be a result of spinal cord injury. Note that such a person would be categorized as an ASIA E.
These changes in the ASIA scale significantly improved the reliability and consistency of the classification. Although it was more logical, the new definition of "complete" injury does not necessarily mean that it better reflects injury severity. For example, is there any situation where a person could be an ASIA B and better off the ASIA C or even ASIA D?
The new ASIA A categorization turns out to be more predictive of prognosis than the previous definition where the presence of function several segments below the injury site but the absence of function below a given level could be interpreted as an "incomplete" spinal cord injury.
The ASIA committee also classified incomplete spinal cord injuries into five types. A central cord syndrome is associated with greater loss of upper limb function compared to the lower limbs. The Brown-Sequard syndrome results from a hemisection lesion of the spinal cord. Anterior cord syndrome occurs when the injury affects the anterior spinal tracts, including the vestibulospnal tract. Conus medullaris and cauda equina syndromes occur with damage to the conus or spinal roots of the cord.
Much confusion surrounds the terminology associated with spinal cord injury levels, severity, and classification. The American Spinal Injury Association tried to sort some of these issues and standardize the language that is used to describe spinal cord injury. The ASIA Spinal Cord Injury Classification approach has now been adopted by almost every major organization associated with spinal cord injury. This has resulted in more consistent terminology being used to /describe the findings in spinal cord injury around the world.