What is the effect of an injury at the level between S3 and S5?

Injuries at the level between S3 and S5 primarily affect the sacral region of the spinal cord, which is responsible for innervating the pelvic organs and perineal area. As a result, an injury in this region typically leads to numbness or altered sensation in the perineum due to the disruption of nerve pathways that carry sensory information from that area to the brain.

The sacral nerves are involved in both voluntary and involuntary control of bladder, bowel, and sexual functions, along with providing sensation to the perineum. Therefore, an injury here may not necessarily lead to complete loss of lower limb function or strength, as higher segments of the spinal cord (such as lumbar or thoracic) primarily control those functions. Additionally, loss of sensation in the lower back relates more to higher spinal injuries, not those confined to the sacral region. This makes numbness in the perineum the most accurate effect of an injury at the level between S3 and S5, as it directly corresponds to the sensory distribution of the sacral nerves.