Tarlov / Perineural Cyst
Tarlov cysts, also called perineural cysts or sacral nerve root cysts, are fluid-filled sacs that form on the nerve root sheaths, most commonly in the sacral region of the spine near the tailbone. These cysts are located between the perineurium and endoneurium layers of the nerve root sheath and are filled with cerebrospinal fluid. While most Tarlov cysts are asymptomatic and discovered incidentally on MRI scans, larger cysts can compress nerve roots or erode into surrounding bone, causing pain and neurological symptoms. They affect an estimated 4-9% of the population based on imaging studies.
What is it?
Tarlov cysts were first described by neurosurgeon Isadore Tarlov in 1938. These cysts develop when cerebrospinal fluid (CSF) accumulates within the layers of the nerve root sheath, creating a balloon-like expansion. Unlike other types of spinal cysts, Tarlov cysts communicate with the subarachnoid space (the CSF-filled area around the spinal cord) and contain nerve fibers within or on their walls. The exact mechanism of formation remains debated, but theories include congenital weakness of the nerve root sheath, trauma or hemorrhage causing nerve root sheath damage, inflammation leading to blockage of normal CSF drainage, or increased CSF pressure creating a ball-valve effect that allows fluid in but restricts outflow.
Tarlov cysts occur most commonly at the S2 and S3 nerve root levels in the sacrum (tailbone region), though they can develop at any spinal level including lumbar, thoracic, and rarely cervical regions. The sacral location is most frequent because nerve roots in this area have longer dural sleeves, making them more susceptible to cyst formation. Most cysts are small (less than 1.5 cm) and remain stable over time. However, some cysts gradually enlarge, potentially causing erosion of the surrounding sacral bone visible on CT scans. Tarlov cysts are more commonly identified in women than men and can occur at any age, though they are most often discovered in middle-aged adults. The cysts may be single or multiple, unilateral or bilateral.
Important to Know
The majority of Tarlov cysts are asymptomatic and discovered incidentally during lumbar or pelvic MRI performed for other reasons. When symptoms do occur—typically only with larger cysts (greater than 1.5 cm) or those causing significant bone erosion—they result from nerve root compression or irritation. Common symptoms include sacral or lower back pain that may radiate to the buttocks, legs, or pelvis; sciatica-like pain down the legs; numbness, tingling, or weakness in the legs or feet; bladder dysfunction including urinary frequency, urgency, retention, or incontinence; bowel dysfunction including constipation or fecal incontinence; sexual dysfunction; perineal pain or discomfort; and pain that worsens with sitting, standing, coughing, or straining and may improve when lying down. The relationship between Tarlov cysts and symptoms can be challenging to establish, as many people with cysts have no symptoms, while symptoms attributed to cysts may have other causes. Diagnosis is made through MRI, which clearly shows the fluid-filled cysts along nerve roots and their relationship to surrounding structures. CT scan is useful for demonstrating bone erosion. CT myelography can confirm communication between the cyst and subarachnoid space. Treatment for asymptomatic cysts is observation only. For symptomatic cysts, options include conservative management with pain medications, physical therapy, and activity modification; CT-guided cyst aspiration and fibrin glue injection to seal the cyst (though recurrence is common); and surgical intervention including cyst excision, nerve root decompression, or cyst fenestration with or without filling, reserved for severely symptomatic cases refractory to other treatments. Treatment of Tarlov cysts remains controversial, as surgery carries risks of nerve damage, CSF leak, and infection, while outcomes are unpredictable. Many patients find adequate symptom control with conservative management. The prognosis varies—small, asymptomatic cysts typically remain stable and require no intervention, while symptomatic cysts may improve with treatment but can also persist or recur, requiring ongoing management.