Wise Young, Ph.D., M.D.
People with spinal cord injury are often told that they have an injury at a given spinal cord level, that they are “complete” or “incomplete”, that they have a bony fracture at one or more spinal vertebral levels, and that they are classified as A, B, C, D, or E according to the American Spinal Injury Association (ASIA) Classification. What is the meaning of the different spinal cord injury levels, the definition of complete and incomplete injury, and the different classification of spinal cord injury? In this article, I will try to explain the currently accepted definitions of spinal cord injury levels and classification.
Vertebral vs. Cord Segmental Levels
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 that are defined by spinal roots that enter and exit the spinal column between vertebral segments. As shown figure 1, 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 (blue), lumbar (green), and sacral (gray) cord.
The spine has 7 cervical (neck), 12 thoracic (chest), 5 lumbar (back), and 5 sacral (tail) vertebra. The spinal cord is shorter than the spinal canal, usually ending just below the L1 vertebral body. The C1 spinal roots exit the spinal column above the C1 vertebral body. There is no C8 vertebral body and so the C8 roots exit between C7 and T1. The T1 roots exit between T1 and T2 and the L5 roots exit between L1 and S1 vertebrae.
The first and second cervical vertebra hold and pivot the head. The C1 vertebrae, upon which the head is perched, is called Atlas after the Greek god who holds the earth. The back of the head is the occiput. The junction between the occiput and atlas is thus the atlanto-occiput junction. The C2 vertebra, upon which Atlas pivots, is called Axis, The junction between C1 and C2 vertebra is the atlanto-axis junction. The cervical cord innervates the diaphragm (C3), the deltoids (C4), biceps (C4-5), wrist extensors (C6), triceps (C7), wrist extensors (C8), and hand muscles (C8-T1).
The twelve thoracic vertebrae have associated ribs. The spinal roots form the intercostal (between the ribs) nerves that run on the bottom side of the ribs and connect to the intercostal muscles and associated dermatomes. About 5% of people have a vestigial 13th rib. The spinal cord ends just below L1. The conus is the tip of the spinal cord. Below the conus, the spinal roots of L2 to S5 form the cauda equina. Injuries to the lower thoracic spinal cords generally damage the lumbar enlargement. Injuries to the lumbosacral spine invariably reults in damage to the lumbosacral enlargement.
| ||Figure 1. Spinal cord and vertebral levels. |
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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.
Motor and Sensory Examination
Each spinal cord segment innervates a patch of skin, called a dermatome. That dermatome is tested for light touch (proprioceptive) and pinprick (pain) sensation at a particular defined point. The muscles innervated by a spinal cord segment is called a myotome but this is seldom used because multiple segments innervate each muscle and there is substantial overlap of myotomes. 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. The muscle groups specificed in the ASIA classifications are a gross oversimplication. Almost every muscle receives innervation from two or more segments.
The C2 dermatome covers the occiput and the top half of the neck while C3 covers the lower neck down to the clavicle. The C4 dermatome is located below the clavicle. The dermatomes for the arms are not straightforward. The C5 to T2 dermatomes progress from proximal to distal on the lateral side of the arm (C5 lateral upper arm, C6 radial hand, and C7 middle finger) and then from distal to proximal on the medial side of the arm (C8 ulnar hand, T1 medial forearm, T2 medial upper arm). The T3 dermatome is back on the chest, above the nipples. The nipples are situated in the middle of T4. The umbilicus in the middle of T10. T12 is above the hip girdle while L1 covers the hip girdle and groin. L2 and L3 cover the front of the thighs while L4 and L5 cover the medial and lateral aspects of the lower leg. S1 covers the heel and middle back of the leg. S2 covers the back of the thighs. S3 covers the medial sides of the buttocks and S4/5 covers the perineal region.
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|Figure 2. Sensory and motor segmentation of the spinal cord. These are the dermatomes and muscles recommended by the American Spinal Injury Association. |
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Ten muscle groups represent the motor functions of the spinal cord. The ASIA motor score does not include abdomenal muscles (i.e. T10-11) and has only one muscle per segment. Thus, even though the segment innervates other muscles, the motor score uses elbow flexors (biceps) for C5, the wrist extensors for C6, the elbow extensors (triceps) for C7, the finger flexors for C8, and the little finger abductors for T1. The hip flexors (psoas) represent L2, knee extensors (quadriceps) are L3, ankle dorsiflexors (anterior tibialis) are L4, the long toe extensors (hallucis longus) are L5, and the ankle plantar flexors (gastrocnemius) are S1.
The anal examination is an essential part of the ASIA examination. If a person can perceive light touch or pinprick around the anus or can achieve voluntary anal contraction, regardless of any other finding, that person is by definition an incomplete injury. Strictly speaking, deep rectal sensation does not count. However, it may be a good sign. Unless the anal examination is done, the ASIA classification cannot be made.
In summary, the spinal cord segments serve specific motor and sensory regions of the body. The sensory regions are called dermatomes. 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 (C5) to distal (C6-8) on the radial side of the arm and from distal to proximal on the medial side of the arm 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 from distal to proximal.
Spinal Cord Injury Levels
Differences between neurological and rehabilitation definitions of spinal cord injury levels. Doctors often use different definitions for spinal cord injury levels. Given the same neurological examination and findings, neurologists, physiatrists, and surgeons may not assign the same level. Neurologists define spinal cord injury level as the first spinal segmental level that shows abnormal neurological loss whereas physiatrists define the neurological level as the lowest contiguous “intact” segment. Thus, for example, if a person has loss of bicep function, a neurologist would say that the motor level of the injury is C5 while a physiatrit would say the motor level is C4. Many surgeons refer to the bony level of injury as the level of injury. Since the spinal cord is foreshortened, e.g. the C6 cord is located at C5 vertebral level, the vertebral level usually is lower than the neurological level but the two may coincide as the spinal cord recovers a level.
- EXAMPLE. The most common cervical spinal injuries involve C4 or C5. Let’s consider a person who has had a burst fracture of the C5 vertebral body. The C5 vertebral injury may have injured the underlying C4 spinal cord, the C5 spinal roots that exit the spinal canal between the C4 and C5 vertebra, and possibly the C6 spinal roots that leave spinal canal between the C5 and C6 vertebra. Such an injury should cause a loss of sensations in C5 dermatome and weak biceps (C5) due to injury to the C5 cord and roots. Due to edema (swelling) of the spinal cord, the deltoids (C4) may be initially weak but may recover over time. Function below C5 should be compromised as well, due to the spinal cord injury. Based on the above, a neurosurgeon or neurologist would assign a C5 level. However, a physiatrist would assign a C4 neurological level because C4 is the lowest “intact” segment.
Lower thoracic vertebral and cord levels. The spinal vertebral and cord segmental levels become increasingly discrepant further down the spinal column. This is particularly true in the lower thoracic spine. For example, a T12 vertebral injury may result in a L2 neurological level. An L1 injury may damage only the conus and sacral segments. An L2 vertebral injury may not damage spinal cord at all and just cause a cauda equina injury.
- EXAMPLE. The most common thoracic spinal cord injury involves T11 and T12 vertebral. Since the spinal segments of L1 to L4 are situated the spinal canal of the T11 and T12 vertebra, a patient with a T12 vertebral injury may lose motor and sensory function in L2 through L4 dermatomes which include the front of the leg down to the mid-shin level. Such a patient may lose hip flexors, knee extensors, and even ankle dorsiflexion. Sacral functions such as anal sensation and sphincter control may be lost. Because of injury to the lumbar enlargement gray matter, many patients with T11 or T12 injuries will have flaccid paralysis due to motoneuronal damage. A with a T12 verteral injury may recover some L2 function, including hip flexors.
Conus and Cauda Equina Injury. Injuries to the spinal column at L1 or lower will damage the tip of the spinal cord, called the conus, or the cauda equina, a “horse’s tail” of spinal roots that descend in the L1-S5 spinal canal to exit at the appropriate spinal vertebral levels. The spinal roots for L2 through S5 are present in the cauda equina. Strictly speaking, the spinal roots are peripheral to the central nervous system. However, both motor and sensory recovery is usually very limited after cauda equina injuries because injury to the conus or cauda equina may damage motoneurons and sensoy fibers in the spinal roots cannot re-enter the spinal cord.
- EXAMPLE. Cauda equina injury is the most common consequence of lumbosacral vertebral injuries. The motor fibers in the spinal roots come from motoneurons situated in the spinal cord. Injury to motor axons close to the motoneurons may result in loss of some motoneurons. Sensory fibers in the spinal root come from dorsal root sensory ganglion that are situated just outside the spinal canal. These neurons send an axon out the peripheral nerve and an axon to the spinal cord. The axons will not be able to regenerate across the central nervous system and peripheral nervous system (CNS:PNS) barrier. Fibroblasts can form adhesive scars within the roots that may cause pain, contribute to further injury, and prevent regeneration.
Complete versus Incomplete Injury
Most clinicians describe spinal cord injuries as “complete” or “incomplete”. Traditionally, “complete” 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 of complete and incomplete spinal cord injury.
- Zone of partial preservation. Some people have some function for several segments below the injury site but below which no motor and sensory function was present. This is in fact rather common. Many people have zones of partial preservation. Is such a person “complete” or “incomplete”, and at what level?
- Difference in levels on each side. A person may have partial preservation of function on one side but not the other or at a different level. For example, if a person has a C4 level on one side and a T1 level on the other side, is the person complete and at what level?
- Recovery of function. A person may initially have no function below the injury level but recovers substantial motor or sensory function below the injury site. Was that person a “complete” spinal cord injury and became “complete”? This is not a trivial question because if one has a clinical trial that stipulates “complete” spinal cord injuries, a time must be stipulated for when the status was determined.
The ASIA committee recommended a change in the definition of “complete” spinal cord injury based on presence or absence of anal sensation and voluntary sphincter contraction. Most clinicians would regard a person as complete if the person has any level below which no motor or sensory function is present. The ASIA Committee took this criterion to its logical limit, i.e. if the person has any spinal level below which there is no neurological function, that person is classified as a “complete” injury. This translates into a straightforward 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 functional recovery. 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.
Figure 3. Neurological level, completeness, and zone of partial preservation
The absence of motor and sensory function below the injury site does not necessarily mean that no axons are crossing the injury site. Many clinicians equate a “complete” spinal cord injury with lack of axons crossing the injury site. Much animal and clinical data suggest that as many as 5-10% of animals or persons with initially no sensory or voluntary function below the injury site will recover some function. If a person is incomplete, recent studies suggest that nearly 90% will be recover independent locomotion again with intensive practice and exercise. The goal of regenerative and remyelinative treatment is to make a person more “incomplete”.
Classification of Spinal Cord Injury
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 (AIS) follows the Frankel scale but differs from the older scale in several important respects. First, AIS A is defined as no motor or sensory function in the sacral segments S4-S5. Second, AIS B requires preserved sacral S4-S5 sensation. Third, AIS C now has a quantitative criterion: half of the muscles below the injury level has a motor score of 3/5 or less. Since a score of 3 indicates anti-gravity, this means that half of the muscles are not capable of lifting more than their own weight.
These changes significantly improved reliability and consistency of the classification. The AIS A classification circumvented previous ambiguities concerning “complete” injuries and took the definition of complete spinal cord injury to its logical conclusion, i.e. an injury that causes loss of all voluntary motor and conscious pin and touch sensation below some spinal cord segmental level. Likewise, it made ASIA B classification more consistent by requiring anal sensation. On the other hand, the classificiation now depends on a single clinical finding, the presence of anal sensation or voluntary anal sphincter contraction. The scale cannot be applied if that examination has not been not done or extenuating circumstances such as surgery, peripheral nerve injury, or other conditions compromised anal sensation or function.
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, the ASIA committee stipulated that a patient would be an AIS C if more than half of the muscles evaluated had a grade of less than 3/5. If not, the person was assigned to AIS D. AIS E is presumably “normal” in the sense that motor and sensory scores should be normal. However, such a person may have spasticity or spasms, coordination or balance problems, or sporadic weakness. The person also may have bladder problems. The ASIA classification system does not document any of these problems.
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ASIA Impairment Scale (AIS) A indicates no motor or sensory function in the S4-S5 sacral segments. It is equivalent to a “complete” spinal cord injury.
AIS B is a sensory incomplete, including the sacral segments S4-S5.
AIS C is a motor incomplete but more than half of the key muscles have a muscle grade or less than 3.
AIS D is a motor incomplete where more than half of the key muscles have grades of 3 or greater.
AIS E is when the motor and sensory scores are “normal”. Note that such a person would be categorized as an ASIA E.
ASIA recognized five incomplete syndromes. The central cord syndrome is associated with greater loss of upper limb function than the lower limbs. The Brown-Sequard syndrome reflects greater injury to one side of the spinal cord. The anterior cord syndrome affects primarily anterior spinal tracts, including vestibulospinal tract. Conus medullaris and cauda equina syndromes signify damage to conus or spinal roots.
| ||Figure 4. ASIA Impairment Scale and Clinical Syndromes. || |
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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.
© Wise Young
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