A spinal cord injury (SCI) occurs when damage to the spinal cord causes a temporary or permanent change in its function, leading to a loss of motor, sensory, or autonomic function below the injury site. A severe SCI is classified as either “complete” or “incomplete,” depending on whether any neural connections remain intact across the damaged segment. Suspicion of a complete spinal cord injury is specifically raised by the absence of certain clinical signs, which indicates a total severance of communication between the brain and the body below the injury level.
General Loss of Function Below the Injury
The first and most apparent sign of a severe spinal cord injury is the immediate, significant loss of conscious function below the level of the trauma. This acute deficit involves both motor paralysis and sensory deprivation in a bilateral pattern. For instance, a cervical injury can result in quadriplegia, affecting all four limbs, while a thoracic or lumbar injury typically causes paraplegia, impacting the lower limbs and trunk.
This motor loss is characterized by the inability to voluntarily move muscles, ranging from profound weakness to complete flaccid paralysis. The initial neurological assessment also reveals a corresponding loss of sensation, including light touch, pain, temperature, and proprioception, below the injury site.
The severity of these initial motor and sensory findings establishes the level of the injury, but it does not definitively determine its “completeness.” Even with a seemingly total loss of function, some nerve pathways may still be intact, classifying the injury as incomplete. The initial examination provides a neurological baseline, but a more specific test is required to determine if the injury is complete.
The Critical Finding: Loss of Sacral Sparing
The absence of “sacral sparing” is the most specific finding that raises suspicion of a complete spinal cord injury. Sacral sparing refers to the preservation of any sensation or motor function in the lowest segments of the spinal cord (S4 and S5), which innervate the perianal area and the external anal sphincter muscle.
The spinal cord pathways responsible for these functions are often the last to be affected by trauma. If a small portion of the spinal cord remains functional, it is usually reflected in the preservation of perianal sensation or the ability to voluntarily contract the anal sphincter. The presence of either indicates that some neural tissue is still transmitting signals across the injury site.
The absence of both perianal sensation and voluntary anal contraction below the neurological level of injury is the defining clinical indicator for a complete spinal cord injury. This finding signifies that all descending motor and ascending sensory tracts have been fully disrupted at the site of trauma. Testing these S4-S5 dermatomes is the clinical differentiator between a complete injury, which has a poorer prognosis, and an incomplete injury, which retains functional continuity and a better chance for neurological improvement.
Involuntary System Failure
Beyond the conscious loss of motor and sensory control, a complete spinal cord injury often presents with profound failure of the involuntary, or autonomic, nervous system. The autonomic nervous system controls functions like heart rate, blood pressure, and bowel and bladder activity. Severe injuries, particularly those occurring above T6, disrupt the communication of the sympathetic nervous system.
This disruption can result in neurogenic shock, characterized by low blood pressure (hypotension) and a slow heart rate (bradycardia) due to unopposed parasympathetic action. The loss of sympathetic tone causes blood vessels to dilate, leading to a drop in blood pressure and the inability to maintain body temperature.
The damage also results in the immediate loss of bladder and bowel control, as reflex pathways are disconnected from the brain. The immediate phase includes areflexia, known as spinal shock, where all reflexes below the injury level are temporarily absent, leading to flaccid paralysis. These involuntary system failures indicate the severity and widespread impact of the spinal cord damage.
Confirmation Through Immediate Testing
The suspicion of a complete spinal cord injury, raised by the absence of sacral sparing, is quickly followed by immediate diagnostic testing. The initial physical neurological examination assesses motor strength and sensory levels, establishing the precise neurological level of the injury.
Following clinical assessment, imaging studies are rapidly performed to confirm structural damage to the spine and spinal cord. Computed tomography (CT) scans are typically the initial modality used to quickly identify bony injuries, such as fractures or dislocations of the vertebrae, and assess spinal stability.
Magnetic resonance imaging (MRI) is the most useful test for visualizing the spinal cord and surrounding soft tissues. MRI provides detailed images of the spinal cord, allowing clinicians to see the extent of contusion, edema, or hemorrhage. The combination of the definitive clinical finding (loss of sacral sparing) and clear imaging provides rapid confirmation of a complete spinal cord injury.