Spinal cord injury is an immediate clinical emergency, a progressive disorder and a chronic condition, all in one.
The anatomic level at which functional impairment begins and the severity depend on site and mechanics of the injury. SCI are therefore described in a variety of ways.
There are two types of injury which are known as complete and incomplete.
In a complete injury, the spinal cord is sufficiently damaged across the whole of its width such that there is no function (either sensation or muscle control) below the level of injury.
In an incomplete injury, the injury does not spread across the whole of the spinal cord; some areas away from the injury remain intact or at least intact enough to retain some function. People with incomplete injuries have some sensation and/or movement control below the level of injury. Sometimes, one side of the body is more affected than the other.
The medical profession have very specific ways to describe an injury based on internationally agreed criteria.
The higher the location of the injury in the spinal cord (ie closer to the head), the greater the proportion of the body affected. Thus, injuries in the neck area (called the cervical region) have the most extensive consequences. The cervical region contains eight vertebrae, known as C1–C8, with C1 closest to the skull. The nerves that control movement of the muscles needed for breathing emerge from the spinal column at C4, so people with injuries at C4 or higher need a ventilator to breathe. Individuals with a C6 injury have little control over arm movement, apart from some ability to move their shoulders and flex their elbows, whereas those with an injury at C7 can move shoulders, elbows and wrists.
Below the cervical level are the thoracic vertebrae (T1–T12). Injury in this region usually results in paralysis of the chest and abdominal muscles, whereas injury to the lumbar (L1–L5) and sacral (S1–S5) regions disrupts the control of leg and hip muscles.
However, the dysfunction associated with injury is not confined to tissues that are directly associated with the affected level. Injury at a particular level also disrupts the flow of signals that travel along the entire length of the cord. An injury prevents messages that originate in the brain from travelling below the damaged area and it also blocks the upward flow of sensory signals from below the wound to the brain.
Thus, injuries higher up the spinal cord cause relatively greater paralysis and dysfunction than lower spinal injuries: injuries in the cervical region cause paralysis in both the arms and then legs (known as tetraplegia or quadriplegia) whereas injuries in the thoracic region cause paralysis in the legs, which is called paraplegia.