Tarlov cyst (perineural cyst) is a perineural cyst arising at the dorsal root ganglion level, containing CSF (cerebrospinal fluid)-density fluid. The sacral region (S2-3) is the most commonly affected location, with >80% of all Tarlov cysts found here. First described in 1938 by Isadore Tarlov in a cadaveric study. The most important distinguishing feature is identification of the nerve root within the cyst wall — this finding definitively distinguishes Tarlov cyst from arachnoid cyst and other meningeal cysts. The CSF-filled cyst cavity is lined by arachnoid and perineural epithelium. Shows isointense signal to CSF on all MRI sequences with NO enhancement. On CT, bone erosion/widening of sacral foramina (scalloping) is a characteristic finding. Prevalence is high — detected incidentally in ~5-9% of the asymptomatic population. The vast majority are asymptomatic; cysts >1.5 cm may develop radiculopathy, sacral pain, and bladder/bowel dysfunction.
Age Range
20-70
Peak Age
45
Gender
Female predominant
Prevalence
Common
Although the pathophysiology of Tarlov cysts is not fully elucidated, several mechanisms have been proposed. The most accepted theory is that a congenital defect or weakness exists at the junction where the arachnoid membrane merges with the nerve root sheath (nerve root exit point — dorsal root ganglion level). In normal anatomy, the arachnoid membrane shows continuity with the epineurium surrounding the nerve root; a defect at this transition zone allows CSF to seep into the perineural space. Physiological fluctuations in CSF pressure (cough, straining, posture changes, Valsalva) cause progressive cyst enlargement through this defect via ball-valve mechanism: when pressure increases, CSF enters the cyst → when pressure drops, the defect closes → CSF cannot exit → cyst volume incrementally increases. Nerve roots and ganglion cells are found within the cyst wall — this histological feature is the cardinal finding distinguishing Tarlov cyst from arachnoid cyst (no nerve root in wall). As the cyst enlarges, it stretches and compresses nerve roots → neuropathic pain, radiculopathy, sacral dysesthesia. Large cysts create pressure erosion (scalloping) in sacral bones — osteoclastic bone resorption is the result of chronic pressure. Bladder and bowel dysfunction results from S2-4 root compression — these roots provide pelvic organ innervation.
Signature finding of Tarlov cyst: CSF-density cystic lesion at sacral nerve root level + nerve root identified within the cyst wall on axial MRI. Nerve root appears as a thin linear structure within the cyst wall — relatively hypointense against bright T2 CSF background. This finding definitively distinguishes Tarlov cyst from arachnoid cyst at the same location with same signal characteristics (no nerve root in wall). High-resolution axial T2 (≤3 mm slice thickness) or 3D T2 SPACE/CISS sequences are required for nerve root identification.
CSF-isointense hyperintense cystic lesion(s) in the sacral canal/foramina on T2-weighted images. The most critical diagnostic feature is identification of the nerve root within the cyst wall on axial and coronal T2 images — the root is seen as a thin linear structure entering the cyst cavity and extending along the wall. This finding definitively distinguishes Tarlov cyst from arachnoid cyst. The cyst is typically located at the S2-3 nerve root exit and has widened the sacral foramen. Bilateral and multiple cysts are common. Cyst content is homogeneous CSF signal — no septa, debris, or solid component.
Report Sentence
CSF-isointense well-defined cystic lesion at S__ level in the sacral canal with nerve root identified within the cyst wall on axial images; consistent with Tarlov cyst (perineural cyst).
The most characteristic CT finding of Tarlov cyst is bone erosion/widening of sacral foramina (scalloping) and thinning/erosion of the posterior sacral wall. Cyst is at CSF density (0-10 HU). Large cysts may show prominent widening of the sacral canal and loss of internal sacral architecture. Bilateral cysts show symmetric or asymmetric sacral widening. CT is superior to MRI in evaluating the degree of bone erosion and sacral integrity. No enhancement. Thin-section CT (<1 mm) with multiplanar reformats provides detailed evaluation of foramen anatomy and cyst-bone relationship.
Report Sentence
Bilateral bone erosion/widening (scalloping) of sacral foramina (S__-S__) on CT with thinning of the posterior sacral wall; consistent with chronic perineural cyst (Tarlov) pressure.
Tarlov cyst shows CSF-isointense hypointense signal on T1-weighted images. Seen as well-defined, round/oval cystic lesion at sacral nerve root level. On T1, nerve root within the cyst wall may be discernible as a thin iso-mildly hyperintense structure — creating contrast against the hypointense CSF background. No cyst enhancement on contrast-enhanced T1; nerve root at ganglion level may show minimal enhancement (normal ganglion vascularity — not pathological). On fat-sat T1, periforaminal fat tissue is suppressed → cyst boundaries become more visible.
Report Sentence
CSF-isointense hypointense cystic lesion in the sacral canal on T1-weighted images with no enhancement; nerve root is discernible as a thin structure within the cyst wall.
Tarlov cyst shows no diffusion restriction on DWI — ADC values are elevated at CSF-compatible levels. This finding confirms pure CSF content of the cyst. Important in differentiation from nerve sheath tumors like schwannoma: schwannoma may show diffusion restriction in solid component (high cellularity in Antoni A areas). Infected cyst possibility is also excluded with DWI — infected cyst shows DWI restriction and low ADC.
Report Sentence
No diffusion restriction in the cystic lesion on DWI with CSF-compatible ADC values; solid tumoral component and infected cyst are excluded.
Tarlov cyst shows no enhancement on contrast-enhanced MRI. No gadolinium uptake in cyst wall or content. Minimal punctate enhancement may be seen at nerve root ganglion level — this reflects normal dorsal root ganglion vascularity (not pathological; ganglion lacks blood-nerve barrier). Absence of enhancement definitively differentiates from schwannoma (marked homogeneous or heterogeneous enhancement) and neurofibroma (target sign enhancement). Rim enhancement is expected in infected cyst — absent in Tarlov cyst.
Report Sentence
No enhancement in the cystic lesion on contrast-enhanced series, excluding nerve sheath tumors such as schwannoma and neurofibroma; consistent with Tarlov cyst.
Criteria
Incidentally detected cyst causing no clinical symptoms. Usually <1.5 cm. Common finding on lumbar/sacral MRI (5-9% prevalence).
Distinct Features
No treatment required. Reporting: 'incidental perineural cyst, no clinical significance' note sufficient. Follow-up MRI unnecessary — size stability expected. Patient reassurance recommended.
Criteria
Usually >1.5 cm. Cyst causing radiculopathy, sacral pain, perineal dysesthesia, bladder/bowel dysfunction.
Distinct Features
Treatment options: CT-guided percutaneous aspiration ± fibrin glue injection, microsurgical excision, cyst-subarachnoid shunt. Recurrence rate after aspiration is high (50-70%). Surgical excision offers lower recurrence but carries nerve root damage risk.
Criteria
Cysts at multiple sacral levels and/or bilateral. Common presentation (~30-50% of cases are multiple).
Distinct Features
Multiple cysts threaten sacral integrity more. Bone erosion degree proportional to total cyst burden. Surgical planning more complex — intervention at multiple levels may be needed. Prevalence increased in connective tissue disorders like Marfan and Ehlers-Danlos.
Distinguishing Feature
Arachnoid cyst does not contain nerve root in wall — most critical differentiating finding. Arachnoid cyst most commonly thoracic posterior intradural while Tarlov cyst is sacral foraminal perineural. Both show CSF signal and no enhancement. Nerve root-cyst wall relationship on axial high-resolution T2 confirms arachnoid-Tarlov distinction.
Distinguishing Feature
Epidural hematoma is biconvex collection in posterior epidural space with time-dependent T1/T2 signal changes (T1 hyperintense subacute); Tarlov cyst shows stable CSF signal. Hematoma has acute presentation while Tarlov is chronic/incidental. Different locations: hematoma thoracolumbar epidural, Tarlov sacral foraminal.
Distinguishing Feature
Tethered cord is congenital anomaly with low conus + thick/fatty filum; Tarlov cyst is sacral perineural cystic lesion. Different pathologies that may coexist. T1 hyperintensity in tethered cord from fatty filum (suppresses on fat-sat); Tarlov cyst T1 hypointense (CSF signal).
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upThe vast majority (90%+) of Tarlov cysts are asymptomatic incidental findings requiring no treatment — 'perineural cyst, no clinical significance' note is sufficient in reporting. Treatment options for symptomatic cysts (radiculopathy, sacral pain, bladder/bowel dysfunction) are controversial. Percutaneous aspiration ± fibrin glue injection is the least invasive method but recurrence rate is high (50-70%). Microsurgical cyst wall fenestration or partial excision offers lower recurrence (15-30%) but carries nerve root damage risk (root is within cyst wall). Treatment decision requires multidisciplinary approach. Tarlov cyst prevalence is increased in connective tissue disorders like Marfan and Ehlers-Danlos.
The vast majority of Tarlov cysts are asymptomatic incidental findings and require no treatment. For symptomatic cysts (radiculopathy, sacral pain, bladder/bowel dysfunction), options include percutaneous aspiration, fibrin glue injection, or surgical excision. Symptomatic cysts are generally >1.5 cm in diameter. Recurrence rate is high.