After a spinal cord injury (SCI), one of the first determinations a clinical team makes is whether function remains in the lowest part of the spinal cord. This is known as sacral sparing, which is the preservation of sensory or motor function in the sacral segments. These segments, located at the bottom of the spine, control functions in the perianal area.
Identifying Sacral Sparing
Clinicians identify sacral sparing through a physical examination focused on the S4-S5 spinal cord segments. This assessment checks for both sensation and voluntary muscle control and is performed after the initial period of spinal shock, a temporary state where reflexes are absent below the injury level, has subsided.
The sensory examination involves testing for light touch and pinprick sensation in the perianal region. Another part of the assessment is checking for deep anal pressure, where a clinician inserts a gloved finger into the rectum and asks if the patient can feel the pressure.
For the motor component, the primary test is for voluntary anal contraction. During the digital rectal exam, the patient is asked to tighten their anal sphincter. The ability to perform this action, even slightly, demonstrates that some voluntary motor control has been preserved and confirms sacral sparing.
Significance in Spinal Cord Injury Classification
The presence or absence of sacral sparing is the defining factor that distinguishes between a complete and an incomplete spinal cord injury. If no sensory or motor function is found in the lowest sacral segments (S4-S5), the injury is classified as complete. This indicates that no nerve signals are getting past the site of the injury.
Conversely, if any sensory or voluntary motor function is preserved, the injury is classified as incomplete. This means at least some nerve fibers crossing the site of the lesion remain intact. Even if an individual has no other movement or feeling below the level of their injury, the presence of sacral sparing places them in the incomplete category.
This classification is formalized using the American Spinal Injury Association (ASIA) Impairment Scale (AIS). A complete injury with no sacral sparing is classified as AIS Grade A. If there is sensory sacral sparing but no motor function more than three levels below the injury level, the classification is AIS Grade B. The sacral examination is the element that separates AIS A from all other grades.
Implications for Recovery
The identification of sacral sparing is a positive indicator for recovery. Its presence signifies that the spinal cord was not entirely severed and that some communication pathways between the brain and body still exist below the injury level. These preserved neural connections form a bridge across the damaged area, allowing for neuroplasticity, the spinal cord’s ability to adapt.
An incomplete injury is associated with a greater likelihood of regaining motor and sensory function compared to a complete injury. For example, the nerve tracts for pinprick sensation are located near the tracts for motor function. The preservation of this sensation can suggest a higher probability of future motor recovery, and rehabilitation therapies are designed to strengthen these existing connections.
While sacral sparing improves the prognosis, it does not guarantee a specific outcome. The extent of recovery is highly individual and depends on factors like the severity of the initial injury and engagement in intensive rehabilitation. However, it allows medical teams and patients to set realistic goals for regaining function.