The first sacral nerve root (S1) is primarily responsible for skin sensation behind the calf, heel, and sole, while the second sacral nerve root (S2) covers the sensation behind the thigh, upper thigh, and posterior calf. The skin sensation of the buttocks, anus, and perineum is mainly innervated by the third, fourth, and fifth sacral nerve roots (S3, S4, S5).
Sacral cysts can compress nerve roots, leading to varying symptoms depending on the pressure. Typically, prolonged standing or sitting increases pressure and exacerbates pain, which improves upon resting in a supine position.
Large sacral cysts can compress nerve roots on both sides due to the limited width of the sacral canal (approximately 1.5cm). For instance, a cyst originating from the left side can affect the right-side nerve roots if sufficiently large. Similarly, cysts originating from upper nerve roots may compress both the corresponding and lower nerve roots, causing pain in various areas such as the heel, posterior calf, anus, and perineum. However, cysts from lower nerve roots typically do not affect upper nerve roots. These principles contribute to the wandering pain sites and fluctuating pain levels observed in sacral cysts.
It’s crucial to note that painful skin segment distribution varies significantly among individuals and should not be interpreted mechanically. Instead, it requires comprehensive evaluation alongside other neurological signs.