Sacral sparing refers to the preservation of some sensory or motor functions controlled by the lowest segments of the spinal cord, specifically segments S4-S5. This is particularly relevant in spinal cord injuries. Its presence indicates that not all neural pathways have been completely severed, suggesting an incomplete spinal cord injury.
Understanding the Sacral Region
The sacral region of the spinal cord is located at the lowest part of the spinal column, comprising segments S2 through S5. Although the spinal cord ends around the L1-L2 vertebral level, nerve roots from these sacral segments descend within the vertebral canal before exiting. These nerves form part of the sacral plexus, controlling motor functions and transmitting sensory information for the pelvic region, buttocks, groin, and lower limbs.
Functions associated with the S2-S5 segments include perianal sensation, voluntary contraction of the anal sphincter, and some control over toe flexion. The S4 and S5 nerves specifically affect the perineal area, which includes the anus. The integrity of these lowermost segments provides insight into the extent of a spinal cord injury.
Assessing Sacral Sparing
Clinical assessment for sacral sparing involves specific neurological tests, part of the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), also known as the ASIA Impairment Scale (AIS). These tests determine if any sensory or motor function is preserved in the S4-S5 dermatomes, which are the skin areas supplied by these nerve segments.
Common tests include assessing perianal sensation (light touch and pinprick around the anal region) and evaluating voluntary anal sphincter contraction (VAC). Deep anal pressure (DAP) is also assessed by applying gentle pressure to the internal anorectal wall. These examinations help classify the spinal cord injury as either complete or incomplete.
Why Sacral Sparing Matters
The presence of sacral sparing after a spinal cord injury has clinical implications. It indicates that some neural connections below the level of injury remain intact. This is a favorable indicator for neurological recovery, particularly for regaining motor function and the ability to walk.
Even minimal sacral sparing suggests a greater likelihood of recovering some motor and sensory function below the injury site. This is because the spinothalamic tract, which carries pain and temperature sensation, is located close to the corticospinal tract, responsible for motor function. Preservation of perianal sensation, especially pinprick, is a strong predictor of improved outcomes and functional recovery. Sacral sparing helps guide rehabilitation planning and sets expectations for a patient’s potential for recovery.