Spinal Cord Injury Levels
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 in 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.