The ganglion impar is the terminal, unpaired sympathetic ganglion located anterior to the sacrococcygeal region—typically at or just anterior to the sacrococcygeal joint—formed by the convergence of the right and left sympathetic chains.
It carries sympathetic efferents and visceral afferents, with somatic afferent overlap, that relay nociceptive and autonomic input from the perineum, distal rectum and anal canal, distal urethra, vulva/scrotum, and distal vagina.
The ganglion impar is a pain target for pelvic and perineal pain syndromes (e.g., coccygodynia, malignancy-related perineal pain), approached most commonly via a trans-sacrococcygeal or transcoccygeal percutaneous route under image guidance (fluoroscopy, CT, ultrasound, or MRI) for diagnostic block, neurolysis, or radiofrequency ablation.