Epidemiology and Histology

Tarlov, or perineurial, cysts are one of the most common forms of meningeal cyst. Estimates of the prevalence of meningeal cysts, including Tarlov cysts, in the general population vary, but generally are in the 5% range.⁸ In a study of 500 consecutive patients with back pain undergoing lumbosacral MRI, 5% were found to have one or more meningeal cysts. Among this latter group, the cyst was thought to be the source of the symptoms in 1% of the cases. Tarlov cysts, particularly those that are symptomatic, are more common among women. The reason for this is unclear, and we have postulated that there may be gender-related differences in the fundamental make-up of dura mater or spinal nerve roots that produce this epidemiologic disparity.

Tarlov distinguished perineurial cysts from other meningeal cysts based on several histologic criteria.^1,9,10 He defined them as perineurial dilations that develop between the endoneurium and perineurium, typically of the S2 or 3 nerve roots, just proximal to the junction of the dorsal root ganglion and nerve root (Fig. 115-1). Simply stated, each cyst is a dilated spinal nerve root sheath, and the individual nerve fibers of that root are found running within the cyst cavity or its inner lining. Other meningeal cyst subtypes, such as meningeal diverticula and arachnoid cysts, typically are devoid of nerve root fiber elements.

FIGURE 115-1 This diagram illustrates some of the different types of dural meningeal cysts, including the perineurial (Tarlov) cyst.

Tarlov cysts can be single or multiple, and can develop anywhere along the spine where nerve roots are present. Progressive cyst enlargement can cause significant bony erosion and impingement of adjacent spinal nerve roots, producing corresponding radiculopathies. For example, a Tarlov cyst in the sacral spinal canal arising from the S3 nerve root can cause symptomatic impingement of the ipsilateral S2 nerve root beside it, and of the S4 or S5 nerve root below (Fig. 115-2). A Tarlov cyst can also produce contralateral symptoms if it is large enough to extend across the midline and compress contralateral nerve roots. Additionally, the nerve root fibers running inside a Tarlov cyst often are attenuated and splayed out over the inner wall of the cyst. This neural fiber alteration and stretching also are suspected of causing symptoms.

FIGURE 115-2 Axial (A) and sagittal (B) T2-weighted sacral MRIs reveal a large Tarlov cyst in the S2 region central and rightward within the spinal canal. Its contents have imaging qualities similar to those of cerebrospinal fluid (CSF), and there is widening of the spinal canal around it due to bone erosion. A compressed nerve root is seen adjacent to the cyst (arrow)C, Intraoperatively, the compressed nerve root (upper Penfield dissector) was visualized after retracting the cyst (lower Penfield dissector). The cyst was found to arise from the S3 nerve root, and the compressed nerve root was S2. D, View inside a Tarlov cyst. The nerve root fibers enter the cyst through the ostium and then splay out over the cyst wall.

Tarlov cysts occasionally can be found in combination with other meningeal cysts. For example, patients with connective tissue disorders, such as Marfan syndrome, can have Tarlov cysts and large ectatic dural cysts so extensive that the distal spinal sac extends out into the pelvis (Fig. 115-3).

FIGURE 115-3 This coronal T2-weighted MRI of the sacrum from a patient with Marfan syndrome reveals multiple meningeal cysts. The distal spinal sac continues on as a large ectatic dural cyst eroding rightward into the sacrum. Also, multiple separate Tarlov cysts are seen adjacent to the spinal sac, particularly on the left (arrow).

The pathogenesis of Tarlov cysts remains unclear. Tarlov proposed that cyst formation could be the result of trauma, ischemic degeneration, inflammation, or hemorrhagic infiltration from the subarachnoid space.^1,9,10 Some patients with symptomatic Tarlov cysts report a history of sacral trauma, and evidence of old hemorrhage in the form of hemosiderin deposits and dystrophic calcification within Tarlov cyst walls supports prior trauma as an etiologic factor.^7,11–13 Other reports have suggested that Tarlov cysts result from arachnoidal proliferation or blockage of perineurial fluid flow.^14,15 Nabors et al. support a developmental origin, although an association between Tarlov cysts and spinal dysraphism is not as strong as that with other types of meningeal cysts.¹⁶ Only two patients with symptomatic Tarlov cysts and spina bifida have been reported, and the relationship could have been coincidental.^7,17

Strully et al.^18,19 and Smith²⁰ proposed that Tarlov cysts form as a result of increased CSF hydrostatic pressure. They point out that spinal nerve roots are in communication with the thecal sac, and that there is myelographic evidence that spinal fluid flows within the nerve roots and could produce dilatation due to either higher hydrostatic pressure or inherent, traumatic, or iatrogenic weakness in the nerve root sheath. They also point out that the frequency and size of Tarlov cysts along the spine can be correlated with the rostral-caudal hydrostatic pressure gradient.^17,18 Several reports on patients with Tarlov cysts have documented either a history of straining or coughing or an exacerbation of symptoms by these maneuvers.^7,10,11,18 We also are aware of two cases of Tarlov cysts in patients with pseudotumor cerebri. However, no criteria have been established to determine who might benefit from CSF shunting for Tarlov cysts, and investigations are ongoing.

Diagnosis

The treatment of a symptomatic Tarlov cyst first requires a correct diagnosis. Unfortunately, many patients languish with undiagnosed or untreated symptomatic cysts due to the incorrect “rule” that Tarlov cysts always are asymptomatic, regardless of the presence of blatant compression of adjacent nerves or extensive bone erosion. Such patients often are relegated to an escalation of narcotics, injection procedures, and neuromodulatory medications as they become progressively more symptomatic. It is not uncommon to encounter patients who have developed narcotic dependency after management with extended-release morphine, transdermal fentanyl, or implanted pain pumps before they are finally referred for meningeal cyst evaluation.

Even more unfortunately, we have encountered patients with symptomatic Tarlov cysts that were misdiagnosed with a variety of other ailments and treated unsuccessfully with a variety of procedures, such as hysterectomy, laparoscopic exploration, endometriosis surgery, oophorectomy, appendectomy, surgery for piriformis syndrome, sacroiliac joint fusion with implanted cages, fusion of degenerative discs in the adjacent spine, coccygectomy, and urinary bladder procedures (Fig. 115-4).

FIGURE 115-4 A, Preoperative axial T2-weighted MRI of the sacrum from a patient with a large Tarlov cyst (white arrowhead). A compressed nerve root is just visible to the right of the cyst (black arrowhead). B, This axial CT scan reveals that the patient was misdiagnosed with sacroiliac joint instability. The symptoms did not improve following insertion of a threaded titanium cage to fuse the joint. Symptoms improved only after later treatment of the Tarlov cyst, which can be seen expanding and eroding the ipsilateral spinal canal.

Symptoms

Tarlov cysts can be found anywhere spinal nerve roots are present and can produce corresponding radiculopathic symptoms. In our experience, symptomatic Tarlov cysts are most commonly encountered in the sacral region. The sacral radiculopathy pattern produced can include a multitude of symptoms, including sacral pain and numbness radiating down the backs of the legs to the bottoms of the feet in the S1 or S2 dermatomes; perineal pain; rectal pain; numbness in the S2-5 dermatomes; neurogenic bladder findings such as urgency, frequency, nocturia, and urinary retention with the need to perform a Valsalva or Credé maneuver to initiate voiding (S3-4); bowel dysfunction requiring the use of laxatives (S2-3); dyspareunia in women; and erectile/sexual dysfunction in men. Symptoms often are positional, being exacerbated by sitting or standing and improved by lying down. This finding supports the notion that symptoms are related to variations in hydrostatic cyst pressures. Cyst expansion also can produce adjacent bone erosion, resulting in painful insufficiency fractures.

Patients typically describe a crippling inability to sit, describing the feeling as being like “sitting on a rock,” causing them to constantly shift from hip to hip when seated in a fruitless quest to find a comfortable position. They often are unable to participate in sitting-related activities, such as driving, working seated at a desk, dining out, or going to movies, events, and religious services. Many patients are forced to carry cushions or pillows to sit on wherever they go in an attempt to ameliorate their seated discomfort.

The severe limitation on quality of life experienced by these patients often costs them their employment, results in depression as they find themselves progressively housebound and unable to participate in social activities, and contributes to marital dysfunction and divorce. Making matters worse, some patients may be told by medical professionals that Tarlov cysts are always asymptomatic, despite obvious evidence to the contrary.

Differential Diagnosis

The surgeon should be aware of a host of disease entities that may be comorbid or confused with symptomatic Tarlov cysts.

Other Masses

Schwannomas and neurofibromas can produce a radiculopathic pattern of symptoms similar to those of Tarlov cysts. They also can share imaging characteristics, such as a cystic appearance, lateral location in the spinal canal, and production of bone erosion and foraminal expansion (Fig. 115-5). However, Tarlov cysts do not enhance on MRI following the administration of gadolinium contrast, which is a characteristic typical of schwannomas and neurofibromas. Instead, they have signal characteristics similar to spinal fluid on all sequences, with the possible exception of differing signal when there has been hemorrhage or accumulation of stagnant, more proteinaceous, spinal fluid within a cyst.

FIGURE 115-5 A, This left S1 foramen mass appears hyperintense on T2-weighted imaging. B, Unlike a Tarlov cyst, however, it enhances on T1-weighted imaging following gadolinium administration. At surgery, the lesion was found to be a schwannoma.

We also have encountered other lesions with imaging characteristics similar to those of Tarlov cysts. For example, in one case, a large cystic lesion filling the left S1 foramen had the T2-weighted imaging appearance of a Tarlov cyst, but differed on other sequences (Fig. 115-6). Further workup revealed the lesion to be a large venous angioma within the spinal canal.

FIGURE 115-6 A, The cystic lesion seen centrally and to the left in the spinal canal has imaging characteristics similar to those of a Tarlov cyst on T2-weighted imaging. B, However, on T1-weighted imaging, the lesion is hyperintense to cerebrospinal fluid and has some areas of internal heterogeneity. Further workup with CT myelography, needle biopsy, and angiography revealed the lesion to be a large venous angioma.

Radiography

MRI currently is the gold standard for imaging meningeal cysts.^21,22 Not only is it useful for differentiating them from other lesions, but it also can help distinguish among the different types of meningeal cysts, including Tarlov cysts. For example, Tarlov cysts typically are lateral in the spinal canal and arise from an individual spinal nerve root. Nerve root fibers often are identifiable inside the cyst on T2-weighted images (Fig. 115-7). In contrast, meningeal diverticula are found centrally in the spinal canal, and arise from the tip of the spinal sac, not from an individual nerve root. When they are large, Tarlov cysts can be seen to erode and expand the spinal canal or neural foramina and extend into the retroperitoneal pelvis (Fig. 115-8).

FIGURE 115-7 The nerve root fiber bundle inside this sacral Tarlov cyst is clearly seen on an axial T2-weighted MRI.

FIGURE 115-8 A, Two large intrapelvic Tarlov cysts are seen on this axial T2-weighted MRI (black arrows). Also seen is a separate Tarlov cyst within the sacral spinal canal abutting a nerve root to the right of midline (white arrow). B, On this sagittal image from the same patient, the left intrapelvic cyst can be seen extending out through the S1 neural foramen ventrally into the retroperitoneum.

MRI also is superior for defining anatomic relationships with surrounding structures. For example, a careful review of imaging studies often reveals symptomatic cysts to be blatantly compressing adjacent nerve roots and displacing the spinal sac. Understanding these relationships preoperatively is critical for surgical planning.

CT myelography previously was used preoperatively to distinguish Tarlov cysts from other forms of meningeal cysts based on the premise that Tarlov cysts filled poorly and in a delayed fashion.^1,2,16,19 Other meningeal cysts, such as meningeal diverticula, were believed to fill more rapidly. However, these criteria are not reliable, because the extent of dye penetration into a cyst depends on the degree of its communication with the spinal sac, not on the cyst type. For example, we have encountered Tarlov cysts that communicate quite freely with the spinal sac, and filled readily on CT myelography (Fig. 115-9). Such cysts would have been erroneously categorized as non-Tarlov cysts using the more antiquated radiographic criteria.

FIGURE 115-9 A, Side-by-side Tarlov cysts are present on this axial CT myelogram image (arrowheads). One fills with dye but the other does not, demonstrating that CT myelography is unreliable as a diagnostic tool for this pathology. B, Unlike CT, both Tarlov cysts are clearly seen on T2-weighted MRI, the preferred imaging modality.

Despite the fact that meningeal cysts, particularly Tarlov cysts, can be radiographically impressive, a careful search should be conducted to rule out other pathology that might explain the patient’s symptoms. For example, a complete workup of a sacral Tarlov cyst should include not only a sacral MRI but also an MRI and flexion-extension radiographs of the lumbar spine to evaluate for disc herniations, stenosis, spondylolisthesis, metastases, hemorrhages, and other possible pathologies.

Diagnostic Cyst Aspiration

We usually avoid percutaneous procedures involving meningeal cysts due to the risks involved. However, needle aspiration occasionally is used as a diagnostic tool to determine whether a Tarlov cyst is symptomatic. When conducted with appropriate imaging to confirm correct needle placement, aspiration may temporarily decompress a cyst long enough before it refills for a patient to notice a transient improvement in symptoms. This would imply that the aspirated cyst is at least partially responsible for the patient’s symptoms. Such a cyst could then be a target for treatment.

Treatment

Needle Aspiration and Fibrin Glue Injection

Treatment with percutaneous aspiration followed by fibrin glue injection also has been described. Authors of one report of four patients with symptomatic Tarlov cysts found that percutaneous fibrin glue therapy was effective in alleviating symptoms, although three patients developed postprocedural aseptic meningitis.²³ Blind percutaneous introduction of multiple needles into a Tarlov cyst also increases the probability of causing nerve injury.

In our experience, fibrin glue treatment fails in a large number of patients, and those patients require subsequent surgery. Unfortunately, the introduction of fibrin glue or other substrates into a Tarlov cyst makes subsequent surgical treatment more difficult because it produces scarring or coating. The neural elements within the cyst are then much harder to identify and protect during surgery. We have encountered injured nerve roots intraoperatively in several patients following prior failed fibrin glue treatment (Fig. 115-10). The fibrin glue technique is falling out of favor, both in the United States and abroad, with only a few centers with a significant experience continuing its use.

FIGURE 115-10 This intraoperative photograph reveals damage to the nerve root fibers inside the Tarlov cyst of a patient who underwent attempted treatment with percutaneous needle aspiration and fibrin glue injection. Bruising and posthemorrhagic hemosiderin deposition are seen (arrow). Note that the internal cyst wall has an opaque coating that makes subsequent nerve fiber identification and, therefore, surgery more difficult.

Surgical Treatment

Patient Selection

As with most spinal pathology, the selection of patients for surgical treatment is based on the correlation among symptoms, physical examination, and radiographic findings. Additionally, the size of a Tarlov cyst is an important factor in determining the probability that it is symptomatic. In general, the larger the cyst the more likely it is to produce symptoms. One study found that patients with neurologic deficits that can be radiographically correlated with Tarlov cysts greater than 1.5 cm in diameter enjoyed substantial improvement following surgery.⁷ Furthermore, there was a very strong association between the presence of radicular symptoms and excellent outcome.

However, we also have encountered patients with symptomatic cysts smaller than 1.5 cm. The location of a Tarlov cyst and the extent to which it is compressing adjacent neural structures also are important factors, in addition to its size. For example, an intraforaminal Tarlov cyst in the cervical or thoracic spine arising from one spinal nerve ramus can compress the adjacent ramus of the same nerve root and produce symptoms, even though the cyst is small.

Surgery

The treatment of a symptomatic Tarlov cyst must take into account that the cyst is actually a dilated nerve root. Therefore, simple excision usually is not an option, because it can produce a critical deficit, particularly when in the lumbosacral region. This results in a treatment quandary: how does one eliminate a symptomatic Tarlov cyst without injuring the nerve root fibers inside and producing a neurologic deficit?

In the past, surgical treatments such as decompressive laminectomy, cyst fenestration, and complete cyst excision have fallen from favor due to a lack of success and unacceptable complication rates.^2,3,6,24 More recent surgical techniques have targeted the underlying pathology causing nerve root dilation, that is, the process that allows spinal fluid to accumulate within an affected nerve root.

An initial step in this direction was the description of fenestration and imbrication techniques that involve opening a Tarlov cyst.⁶ The cyst is then reduced in size, either by imbrication or partial wall excision, thereby reconstituting a more normal caliber nerve root, which no longer compresses adjacent structures. However, such techniques do not prevent the continued flow of spinal fluid into the affected nerve root cyst, so they do not eliminate the risk of cyst reexpansion and spinal fluid leakage. Additionally, nerve fascicles in a Tarlov cyst often are found within the cyst wall itself, or so extensively splayed out over the internal surface of the cyst wall that they are not easily seen, even with the aid of an operating microscope. Cyst wall excision to decrease the overall size of a Tarlov cyst therefore increases the risk of producing deficits due to inadvertent nerve root fiber sectioning.

Current surgical techniques are focused on resolving the quandary of how to prevent spinal fluid flow into a symptomatic Tarlov cyst without injuring its nerve root fibers. To this end, we have focused on treatment of the ostium, where spinal fluid and nerve root fibers enter the cyst. We also have made efforts to confine cysts to prevent further compression of adjacent structures (Fig. 115-11).

FIGURE 115-11 A, Two Tarlov cysts abutting nerve roots in the sacral spinal canal are seen on this preoperative axial T2-weighted MRI (arrows). B, Postoperatively, the treated cysts are identified (arrows), as are the decompressed nerve roots just ventral and lateral to each treated cyst.

Intraoperative electrophysiologic monitoring is an integral component of Tarlov cyst surgery. During the process of exposure and cyst dissection, it aids the surgeon in identifying specific nerve roots and gives feedback on the tolerance of nerves to manipulation. Thus, intraoperative monitoring can be used to assess the status of a Tarlov cyst–involved nerve root throughout the process of cyst treatment, and gives the surgeon a baseline to assess nerve function before and after cyst treatment. For example, the nerve root sleeve proximal to a treated cyst can be stimulated following cyst exploration and treatment to determine whether it is still in continuity and conducting as it was before treatment.

Summary

Tarlov cysts are an important clinical entity in the differential diagnosis of spinal radiculopathy, sacral pain syndromes, and sacral spinal insufficiency fractures, particularly in women. The symptoms they produce can be crippling. Although the relationship remains unclear, elevated CSF pressure and trauma may play a role in their pathogenesis. MRI has enhanced our ability to diagnose symptomatic Tarlov cysts and preoperatively assess their anatomic relationships.

The management of symptomatic Tarlov cysts has progressed significantly in the last decade. Trends in the literature have favored surgical treatment in experienced hands to reduce the risk of nerve root injury and spinal fluid leakage. Treatments with percutaneous needle techniques such as aspiration or fibrin glue injection are falling out of favor due to lack of symptomatic improvement and high cyst refilling rates. In our experience, patients with large cysts and corresponding radicular symptoms are more likely to experience substantial relief from surgery. However, patients with smaller cysts also can benefit if focal nerve root compression is identified on preoperative imaging.