Tarlov cysts show up on MRI as fluid-filled sacs along spinal nerve root sleaths, most often in the sacrum, and a large meta-analysis puts their global prevalence at roughly 4% of all people who get a spinal MRI.1PubMed Central. Global incidence of spinal perineural Tarlov’s cysts and their morphological characteristics: a meta-analysis of 13,266 subjects The overwhelming majority are incidental findings that never cause symptoms, which makes the radiologist’s job twofold: recognizing them confidently and deciding when further workup is warranted. Because these cysts can mimic other lesions and occasionally produce real clinical problems, their imaging features deserve a closer look than the standard one-line radiology report usually gives them.
What Tarlov Cysts Look Like on MRI
On standard MRI sequences, Tarlov cysts behave almost exactly like cerebrospinal fluid, because that is essentially what fills them. They appear dark on T1-weighted images and bright on T2-weighted images, matching the signal of the surrounding spinal fluid. The key distinguishing detail is what sits inside the cyst: a nerve root, which shows up as an intermediate-signal structure on T1 and a low-signal strand on T2. On a sagittal view the cyst often looks elongated, while on an axial slice you can sometimes spot the nerve root as a small dark dot within the bright fluid.2Asian Journal of Medical Sciences. Symptomatic Tarlov Cysts: an MRI evaluation of case series and literature review
Another reliable feature is the absence of contrast enhancement. After gadolinium administration, the cyst wall does not light up, which helps separate Tarlov cysts from cystic tumors that typically show at least some peripheral enhancement.3Asian Journal of Medical Sciences. Symptomatic Tarlov Cysts: an MRI evaluation of case series and literature review Larger cysts can compress adjacent structures and erode bone, widening the spinal canal or neural foramen on the affected side. These secondary signs are visible on MRI but often more clearly appreciated on CT.
Where They Tend to Appear
Tarlov cysts have a strong preference for the sacrum. In the large meta-analysis pooling over 13,000 subjects, sacral cysts overwhelmingly predominated, with the S2 nerve root level being the single most common site, accounting for about 47% of sacral cysts.4PubMed Central. Global incidence of spinal perineural Tarlov’s cysts and their morphological characteristics: a meta-analysis of 13,266 subjects A study of back-pain patients undergoing spinal MRI similarly found the vast majority of cysts clustered at the S2/S3 level.5PubMed Central. Tarlov cysts in back pain patients: prevalence, measurement method and reporting points
That said, they are not exclusively sacral. One study categorizing cyst locations found about 40% in the lumbosacral junction and roughly 13% in the lumbar spine alone, with the L4–L5 and L5–S1 levels being the most common lumbar sites.6Journal of Health and Rehabilitation Research. Clinical findings of Tarlov cyst according to its location and level in lumbosacral spine on magnetic resonance imaging When cysts grow large enough, they can extend beyond the bony sacrum entirely. One reported case featured giant, bilateral cysts originating from multiple nerve root sleeves that pushed into the presacral space and pelvis, initially mimicking a gynecological mass on imaging.7PubMed Central. Giant multiple and bilateral presacral Tarlov cysts mimicking adnexal mass – imaging features That kind of presentation is rare, but it highlights how a cyst’s location and size can change the radiological picture dramatically.
How Radiologists Classify Them
Tarlov cysts fall under Type II in the widely used Nabors classification of spinal meningeal cysts, which divides these lesions into three categories. Type I cysts sit outside the dura and contain no nerve root fibers. Type II cysts, which include Tarlov cysts, are also extradural but critically contain nerve root fibers within or along their wall. Type III cysts are intradural.8PubMed. Updated assessment and current classification of spinal meningeal cysts This classification matters for more than taxonomy: the presence of nerve tissue within the cyst wall is the reason Tarlov cysts can produce neurological symptoms and the reason surgeons cannot simply drain or excise them without risking nerve damage.
On imaging, the Type II designation translates to a specific finding: the nerve root should be visible entering or traversing the cyst. If a radiologist sees a sacral cystic lesion that matches CSF signal but cannot identify a nerve root inside, the differential shifts toward a simple meningeal (Type I) cyst or an entirely different pathology.
Most Are Incidental, So When Do They Matter?
The disconnect between how often Tarlov cysts show up and how rarely they cause trouble is one of the bigger headaches in spinal radiology. Estimates put the incidence at around 4–5% of MRI scans, yet symptomatic cases are considered rare.9PubMed Central. Tarlov cyst: Case report and review of literature When symptoms do arise, they typically include sacral or lower back pain, sciatica, and occasionally bladder or bowel dysfunction consistent with nerve root compression.
Size is probably the most practical radiological clue to symptomatic potential. The literature on interventional treatment commonly uses a threshold of 15 mm or larger to identify cysts worth investigating further.10World Neurosurgery. Myelographic CT, A Check-Valve Mechanism, and Microsurgical Treatment of Sacral Perineural Tarlov Cysts Small cysts found incidentally in someone getting an MRI for unrelated back pain are almost never the cause of that pain. The challenge is the patient who has a medium-to-large cyst and symptoms that could plausibly be explained by it but could also come from a disc or facet joint. In those cases, standard MRI alone often cannot settle the question, and additional imaging comes into play.
Myelography and CT for the Tough Calls
When a radiologist needs to determine whether a Tarlov cyst is actively filling with cerebrospinal fluid, which would suggest it communicates with the subarachnoid space through a one-way valve-like mechanism, myelographic CT is the go-to study. The technique involves injecting contrast into the spinal fluid and then scanning at two time points: immediately and again several hours later.
The hallmark pattern for a symptomatic Tarlov cyst is a “filling defect sign” on the immediate scan, meaning the cyst does not take up contrast right away, followed by a “delayed filling sign” or “retention sign” on the delayed scan, where contrast has slowly seeped in and stays trapped.11PubMed Central. Diagnosis and Treatment of Symptomatic Multiple Sacral Perineural Cysts-Technical Note This delayed-inflow, delayed-outflow pattern supports the check-valve theory of Tarlov cyst pathophysiology: fluid enters the cyst under pressure but cannot exit freely, causing the cyst to expand over time. One group used this myelographic criterion, along with cyst size over 15 mm and symptom correlation, as part of their standard surgical selection protocol.12World Neurosurgery. Myelographic CT, A Check-Valve Mechanism, and Microsurgical Treatment of Sacral Perineural Tarlov Cysts
This two-phase approach is especially valuable when a patient has multiple sacral cysts. Not all of them may be symptomatic, and myelographic CT can help distinguish the cyst that is actively trapping fluid from a neighboring cyst that is simply sitting there.13PubMed Central. Diagnosis and Treatment of Symptomatic Multiple Sacral Perineural Cysts-Technical Note Targeting the right cyst matters enormously for surgical planning.
What Else Could It Be
The differential diagnosis for a cystic sacral lesion on MRI is relatively short, but the stakes of getting it wrong can be high. The most commonly discussed mimic is a sacral schwannoma, a nerve sheath tumor that can undergo cystic degeneration and look remarkably similar to a Tarlov cyst on standard sequences. One case report described a giant cystic schwannoma in the sacral canal with bony erosion that was initially diagnosed radiologically as a Tarlov cyst. The distinction mattered because the schwannoma required surgical excision, a very different management path.14PubMed. Giant cystic sacral schwannoma mimicking tarlov cyst: a case report
Clues that should raise suspicion for a tumor rather than a cyst include solid components, contrast enhancement of the wall or internal nodules, and an irregular shape. A pure Tarlov cyst follows CSF signal perfectly across all sequences and does not enhance. Any deviation from that pattern warrants further characterization, often with post-contrast imaging or biopsy. Other lesions that enter the differential include simple meningeal cysts (Type I, lacking the nerve root component), dermoid or epidermoid cysts, and, in very rare cases, metastatic disease involving the sacrum with secondary cystic change.15Insights Imaging. Extramedullary cystic lesions of the spine – pictorial review
Bone Erosion and Structural Damage
A feature that can catch both radiologists and clinicians off guard is the degree of bony remodeling a Tarlov cyst can produce. Large, longstanding cysts slowly erode the sacral bone through persistent pressure, a process sometimes described as scalloping. On CT, this appears as smooth, well-corticated bone loss along the margins of the cyst, distinct from the irregular destruction you would expect with infection or malignancy.
In extreme cases, this erosion becomes clinically significant. One reported case involved a large cyst that eroded enough sacral bone to produce a sacral insufficiency fracture in a 38-year-old woman, causing severe back and buttock pain with radiculopathy. The bone loss was extensive enough that surgical fixation of the sacroiliac joints was deemed necessary in addition to addressing the cyst itself.16PubMed. Tarlov Cyst Causing Sacral Insufficiency Fracture While this is an uncommon scenario, it illustrates why radiology reports on larger Tarlov cysts should comment on the degree of surrounding bone loss. A cyst that has eroded most of a sacral segment is a different clinical entity from a small, neatly contained one.
CT-Guided Treatment and Radiology’s Expanding Role
Interventional radiology plays a growing role in managing symptomatic Tarlov cysts, particularly through CT-guided percutaneous approaches. The most studied technique involves advancing two needles into the cyst under CT guidance, aspirating the fluid, and injecting fibrin sealant to collapse the cyst and prevent refilling. In the largest published series, 213 consecutive patients underwent this procedure. At one year, about 82% were satisfied with the outcome, and at three to six years, roughly 74% remained satisfied, with no clinically significant complications reported.17PubMed Central. Treatment of 213 Patients with Symptomatic Tarlov Cysts by CT-Guided Percutaneous Injection of Fibrin Sealant
CT guidance is essential here because the cysts sit in close proximity to nerve roots, and blind aspiration would risk nerve injury. Real-time imaging allows the operator to position needles within the cyst while avoiding the nerve fibers traversing it. Individual case reports have confirmed marked cyst reduction on follow-up imaging after fibrin glue injection.18PubMed Central. Symptomatic lumbar Tarlov cyst resolution after computed tomography-guided percutaneous trans-sacral fibrin glue intracystic injection: A case report and literature review
For smaller cysts, particularly those under 15 mm without motor symptoms, less invasive options exist. Epidural steroid injection has been reported to relieve symptoms in select patients, with one case showing sustained benefit at one year follow-up.19PubMed Central. Conservative Management of Symptomatic Sacral Tarlov Cyst – A Rare Case Report Beyond direct treatment, percutaneous cyst aspiration also serves a diagnostic purpose: if draining the cyst temporarily eliminates the patient’s symptoms before the cyst refills, that confirms the cyst as the pain generator and helps identify who might benefit from definitive surgery.20Journal of NeuroInterventional Surgery. Interventional approaches to symptomatic Tarlov cysts: a 15-year institutional experience
What Follow-Up Imaging Shows After Surgery
When patients proceed to surgical treatment, post-operative MRI is the standard tool for assessing whether the intervention worked. The largest surgical series, involving 265 patients who underwent microsurgical cyst sealing, found that on follow-up MRI an average of about 20 months after surgery, cysts had completely disappeared or shrunk in about 94% of patients. The recurrence rate was just under 6%.21Journal of Neurosurgery: Spine. Microsurgical sealing for symptomatic sacral Tarlov cysts: a series of 265 cases
After fenestration procedures, which open the cyst and allow it to drain into the surrounding tissue, post-operative MRI typically shows collapse of the previously fluid-filled cavity.22The Spine Journal. Cold facts on surgical Tarlov cysts One technical report combining sacral laminoplasty with cystic fenestration demonstrated complete cyst removal on post-operative MRI, with no evidence of recurrence at 12 months.23PubMed Central. Sacral laminoplasty and cystic fenestration in the treatment of symptomatic sacral perineural (Tarlov) cysts: Technical case report For surgeons and radiologists alike, the takeaway is that post-operative imaging is not optional. A cyst that looks collapsed on the first follow-up scan but later re-enlarges signals treatment failure and the possible need for revision.
What Tissue Analysis Reveals That Imaging Cannot
Radiology describes the cyst’s shape, size, and behavior with contrast, but it cannot tell you what is happening at the cellular level. When Tarlov cysts are surgically removed and examined under a microscope, the findings are more complex than a simple fluid-filled balloon. Histopathological analysis of resected cyst tissue has shown extensive fibrosis, both fresh and old hemorrhage, and dense infiltration by macrophages, some appearing as multinucleated giant cells. Importantly, neuronal tissue or nerve fibers were not detected in the cyst wall itself in at least one detailed histopathological study, which is somewhat surprising given the classification of these lesions as nerve-root-containing cysts.24Interdisciplinary Neurosurgery. Histopathology of a symptomatic Tarlov cyst – Case report and review of the literature
The evidence of chronic hemorrhage and inflammatory cell infiltration suggests that these cysts are not static structures. They appear to undergo repeated micro-trauma, possibly from the pulsatile movement of cerebrospinal fluid through the one-way valve mechanism, leading to cycles of bleeding and scarring within the wall. This inflammatory picture is invisible on standard MRI but may help explain why some cysts become symptomatic while others do not.
Connective Tissue Disorders and Tarlov Cysts
Radiologists occasionally notice Tarlov cysts in patients with connective tissue disorders, and the association is worth flagging even though it remains poorly understood. Ehlers-Danlos syndrome, a group of inherited conditions affecting collagen, has been linked to meningeal cysts in scattered case reports. One case described an 11-year-old boy with Ehlers-Danlos who developed urinary and bowel incontinence traced to a Tarlov cyst.25PubMed Central. A Case of a Tarlov Cyst in A Pediatric Patient With Ehlers-Danlos Syndrome A broader review of neurological manifestations in Ehlers-Danlos noted that meningeal cysts, including Tarlov cysts, have been reported across several EDS subtypes, though they appear to be rare overall.26PubMed Central. Neurological manifestations of Ehlers-Danlos syndrome(s): A review
The working theory is that weakened connective tissue in the dural sheath may predispose these patients to cyst formation, similar to how dural ectasia (widening of the dural sac) is a recognized feature of Marfan syndrome. For radiologists, the practical implication is straightforward: if you see a Tarlov cyst in a young patient, especially one with a history of joint hypermobility, skin fragility, or other collagen-related findings, it is worth noting the association in the report. The cyst may not be an isolated incidental finding but rather one manifestation of a systemic process. The pediatric case is particularly striking because Tarlov cysts are overwhelmingly reported in adults, and their appearance in a child should always prompt a harder look at the clinical context.
High-Resolution MRI Sequences
Standard T1 and T2 sequences are sufficient for identifying most Tarlov cysts, but high-resolution three-dimensional T2-weighted sequences offer finer anatomical detail that can be clinically useful. These sequences excel at visualizing small structures like individual nerve roots, root sleeves, and the subarachnoid space with enough clarity to distinguish a cyst from a dilated nerve root sleeve or a small meningeal diverticulum.27Feuillets de Radiologie. Applications de l’imagerie T2 3D haute résolution en pathologie rachidienne et médullaire Heavily T2-weighted sequences (sometimes called CISS or FIESTA depending on the scanner manufacturer) produce a myelography-like effect without the need for intrathecal contrast, allowing radiologists to trace the connection between the cyst and the thecal sac non-invasively.
These sequences are not routinely ordered for every spine MRI, and they do not need to be. But when a sacral cystic lesion is ambiguous on standard imaging, or when surgical planning requires precise mapping of which nerve roots enter which cysts, high-resolution 3D sequences can fill in the gaps that conventional slices leave open. They are particularly helpful in patients with multiple cysts, where determining the anatomical relationships between each cyst and its parent nerve root is essential for surgical targeting.