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What is Spinal Cord Injury

Wise Young, MD, PhD

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 vertebrae 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 vertebrae 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. 

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


No motor or sensory function preserved in the lowest sacral segments


Sensory but not motor function preserved in the lowest sacral segments


Motor function present below the injury but most key muscles are <3/5


Motor function present below the injury but most key muscles are ≥3/5


Motor and sensory function normal in key muscles and dermatomes

Some patterns of spinal cord injury have special names.

  • "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.

To read more about spinal cord injury:

What is the Spinal Cord

What is Spinal Cord Injury

Spinal Cord Injury Levels and Classification

Acute Spinal Cord Injury

Chronic Problems of Spinal Cord Injury

Recovery and Treatment

Recovery from Spinal Cord Injury

Spinal Cord Injury and Family

Ten Frequently Asked Questions Concerning Cure of Spinal Cord Injury

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