Sacral sparing is the preservation of pain and temperature sensation in sacral dermatomes. This is a late, unusual, but diagnostic feature of an intrinsic (intramedullary) spinal cord lesion. Spastic paraparesis below the level of the lesion due to corticospinal tract involvement is invariably present by this stage.
Sacral sparing is explained by the lamination of fibers within the spinothalamic tract: ventrolateral fibers (of sacral origin), the most external fibers, are involved later than the dorsomedial fibers (of cervical and thoracic origin) by an expanding central intramedullary lesion (e.g., glioma, ependymoma, syringomyelia).
Although sacral sparing is rare, sacral sensation should always be checked in any patient with a spastic paraparesis.