Tarlov cysts, also known as perineurial cysts, are abnormal fluid-filled sacs that typically develop along the nerve roots of the spine. These cysts are most frequently found in the sacral region, the triangular bone at the base of the spine, often near the S1 through S4 nerve roots. The fluid inside these sacs is cerebrospinal fluid (CSF). While they are relatively common, affecting up to 9% of the population, the majority of Tarlov cysts remain asymptomatic and are discovered incidentally during imaging for unrelated issues.
Anatomy and Measurement of Tarlov Cysts
Tarlov cysts form as dilatations of the nerve root sheath, specifically within the perineural space near the dorsal root ganglion. They are classified as extradural cysts, meaning they are outside the dura mater, the protective membrane surrounding the spinal cord. A defining feature is that the walls of the cyst often contain actual spinal nerve fibers, which distinguishes them from other types of meningeal cysts. The presence of these neural elements makes interventions complex, as any procedure risks damaging the embedded nerve tissue.
The primary tool for identifying and measuring these cysts is Magnetic Resonance Imaging (MRI). MRI provides detailed images of soft tissues and clearly shows the size, location, and fluid composition of the cyst. Measurement is generally taken by determining the maximum diameter, often reported in the craniocaudal dimension.
There is no single, universally standardized size that defines a “large” Tarlov cyst across all medical literature. However, cysts that are clinically considered large often exceed a diameter of 1.5 to 2.0 centimeters. Some sources define “giant” cysts as those measuring over 3.0 centimeters, especially if they extend into the pelvic cavity. Clinicians track whether a cyst is large enough to exert a mass effect on surrounding structures, which is more important than the absolute size alone.
Clinical Presentation of Large Cysts
A Tarlov cyst is considered symptomatic when its size causes a mass effect, physically compressing the adjacent sacral nerve roots or eroding the bone of the sacrum. This compression generates the neurological symptoms experienced by the patient. The most common complaint is localized pain in the sacral or coccygeal area, often exacerbated by sitting, standing, or prolonged walking due to increased pressure.
Pain can radiate down the back of the leg, presenting as sciatica, described as a shocking or burning sensation. Large cysts frequently cause localized numbness or paresthesia, a pins-and-needles feeling, in the buttocks, perineum, or the legs and feet. Symptoms are variable and can be intermittent, often depending on changes in cerebrospinal fluid pressure within the cyst.
Because the sacral nerve roots (S2-S4) control function for the bladder, bowel, and sexual organs, a large cyst can cause dysfunction in these systems. Patients may experience difficulties such as urinary frequency, bladder incontinence, constipation, or, in rare cases, loss of bowel control. Sexual dysfunction is also a possible symptom resulting from the pressure on these nerves.
Treatment Pathways for Symptomatic Cysts
For a cyst confirmed to be large and symptomatic, the initial approach involves conservative management aimed at alleviating discomfort. This includes non-steroidal anti-inflammatory drugs (NSAIDs) or medications designed to target nerve pain, such as gabapentinoids. Physical therapy may also be recommended to maintain mobility and strengthen supporting musculature without placing excessive strain on the sacrum.
If conservative measures fail to provide adequate relief, interventional procedures are considered to reduce the size and pressure of the cyst. One common minimally invasive option is CT-guided aspiration, where a fine needle is used to drain the cerebrospinal fluid from the cyst. This procedure is often immediately followed by an injection of fibrin glue to seal the connection and prevent the cyst from refilling.
The goal of the fibrin glue injection is to occlude the one-way valve mechanism that allows CSF to enter and become trapped in the sac. However, these minimally invasive procedures carry a risk of recurrence, as the cyst may eventually refill. Surgical intervention is typically reserved for cases involving intractable pain, progressive neurological deficit, or failure of other treatments.
The most common surgical technique is microsurgical fenestration and imbrication. This involves opening the cyst, draining the fluid, and then carefully suturing the cyst wall to prevent fluid re-entry. This procedure requires an experienced neurosurgeon due to the presence of nerve fibers within the cyst wall.