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Complete and incomplete injuries

In a "complete" spinal injury, all functions below the injured area are lost, whether or not the spinal cord is severed. An "incomplete" spinal cord injury involves preservation of motor or sensory function below the level of injury in the spinal cord. To be classed as incomplete, there must be some preservation of sensation or motion in the areas innervated by S4 to S5, including voluntary external anal sphincter contraction. The nerves in this area are connected to the very lowest region of the spinal cord, and retaining sensation and function in these parts of the body indicates that the spinal cord is only partially damaged. Incomplete injury by definition includes a phenomenon known as sacral sparing: some degree of sensation is preserved in the sacral dermatomes, even though sensation may be more impaired in other, higher dermatomes below the level of the lesion. Sacral sparing has been attributed to the fact that the sacral spinal pathways are not as likely as the other spinal pathways to become compressed after injury due to the lamination of fibers within the spinal cord.