Perineural cysts

These perineural cysts are of unknown etiology and arise in sacral nerve roots, in an extradural location and communicate with the thecal sac. They are seen on pelvic ultrasound when they extend through adjacent foramina with erosion of the bone. These cysts are often multiple and bilateral. They are usually an incidental finding on asymptomatic patients, although they may cause pelvic or lower back pain.

The sonographic appearance of Tarlov cysts includes cystic masses (often bilateral) in the posterior part of the pelvis, fixed to the pelvic side wall. Careful scanning will reveal that the uterus and ovaries are separate from these masses, which lie along the posterior pelvic side wall. Tarlov cysts are avascular on color Doppler.