complete vs incomplete spinal cord injury

When a spinal cord is injured, one of the very first questions is whether the cord has lost all connection below the injury, or whether at least a thread of it still gets through. That single distinction — complete versus incomplete — shapes the whole outlook, because a pathway that still carries even a trickle of signal can sometimes be strengthened, while a fully interrupted one usually cannot.

The official dividing line is surprisingly specific. An injury is complete (AIS A) when there is no motor or sensory function in the very lowest sacral segments, S4–S5 — practically, no feeling around the anus and no voluntary squeeze of the anal sphincter. An injury is incomplete when something survives at that lowest level — any sacral sensation, or any voluntary anal squeeze, or movement preserved more than three levels below the injury. This is called sacral sparing, and it is the formal proof that the cord is not fully cut off. Notice the logic: the test hinges on the very bottom of the cord because if signals reach all the way down there, the lesion cannot be a complete block. 'Incomplete' does not mean 'a little injured' — it means the connection is partial, which can range from nearly normal to profoundly impaired.

In rehabilitation this distinction is one of the strongest predictors of recovery. Incomplete injuries, especially the motor-incomplete grades, often regain meaningful strength and sometimes walking; complete injuries rarely recover movement below the level, so therapy concentrates on compensation, independence, and preventing complications. The classification can change early on — spinal shock can make an injury look worse than it is — which is why the official complete/incomplete call is made after spinal shock resolves and is rechecked over time.