Ligamentum flavum cyst: An uncommon but recognizable and surgically correctable category of juxtafacet cyst.

Ligamentum flavum cyst is considered as a category of juxtafacet cyst and is recognized to be rare cause of cord compression and radiculopathy. The pathogenesis of these cysts is not yet fully understood, but it has been proposed that continuous stress to this ligament due to minor repetitive trauma, such as in spondylolisthesis and degenerative disc disease, may lead to the development of cyst. Intraspinal cyst can encroach and displace neural structures that can lead to neurologic symptoms. Although a rare entity, intraspinal cyst should be included in the causative factors of neurogenic claudication or lumbar radiculopathy based on typical magnetic resonance imaging appearance. Timely diagnosis is also imperative in this condition due to a better surgical outcome.

Introduction

Ligamentum flavum cyst is a subtype of juxtafacet cyst [1] and is recognized to be a rare cause of cord and nerve compressions [2]. Cysts of the posterior longitudinal ligament and facet joints are also types of juxtafacet cysts. Moiel et al. first reported on ligamentum flavum cyst in 1967 [3]. Most of the epidural cysts reported in the literature are synovial or ganglion cysts related to the facet joints [4]. On the other side, ligamentum flavum cysts are far less common and less frequently diagnosed preoperatively [5], [6], [7], [8]. In a literature search, we have found that majority of the diagnosis was made intraoperatively by spinal surgeons followed by a histologic tissue diagnosis. However, in this case, we have described the role of the radiologist in the preoperative diagnosis of this condition, which can lead to trouble-free surgical planning. The take-home message in this case report is that magnetic resonance imaging (MRI) of the spine is primarily sought for disc diseases; however, the rest of the structures, including facet joints and the ligamentum flavum, should not be neglected as the cause of radiculopathy.

We have reported a case of ligamentum flavum cyst that we have diagnosed preoperatively on the basis of a unique MRI appearance, which we have later confirmed intraoperatively and histopathologically.

Case report

A 51-year-old diabetic woman presented in the spinal clinic complaining of chronic low back pain since 2009. For the last 2 years, the patient's symptoms aggravated and become more activity related, especially during walking or standing for a long time. Initially, the pain was relieved by rest and analgesics, but with the passage of time, the pain did not improve substantially with conservative therapy. Surgical history revealed cholecystectomy, gastric bypass surgery, and total knee replacement. The patient had a history of type 2 diabetes well controlled on medication with normal HbA1c levels. The general physical examination showed a middle-aged obese (body mass index of 36), mute, and deaf female patient with hyperpigmentation over the facial skin. Blood parameters showed normal serum electrolytes, complete blood count, erythrocyte sedimentation rate, and C-reactive protein. Plain radiographs of the lumbosacral spine showed degenerative changes. On neurologic examination, there was a mild low back tenderness with a hypoactive knee reflex and loss of the right plantar reflex. The patient was advised to undergo an MRI of the spine for the exact etiology of the patient's symptoms. The MRI of the spine demonstrated degenerative changes with a multilevel facet arthropathy more marked at the L4-L5 level. There was an asymmetrically enlarged right ligamentum flavum with intrasubstance cystic lesion (central hyperintensity on T2 and hypointensity on T1) without communication to the facet joint, consistent with a ligamentum flavum cyst. The cyst was compressing and displacing the right-sided cauda equina nerve roots (Fig. 1)

Fig. 1

T1 and T2 sagittal and T2 axial from the lumbar spine shows a thickening of the ligamentum flavum showing a well-defined cystic structure embedded within the ligamentous substance on the right side showing the T2 hyperintense center. The cyst causes an impingement over the right side of the thecal sac and the right lateral recess.

Based on these radiological findings and clinical symptoms, the patient was advised for spinal decompression. Under general anesthesia, a longitudinal midline incision with exposure of the L4-L5 junction was made, and a partial hemilaminectomy on the right L4 lamina was performed. There was moderate thickening of the capsule of the apophyseal joint and the ligamentum flavum. A round, yellowish cyst of about 6-mm diameter filled with a somewhat gelatinous fluid and barely adhering to the dura mater was observed. The cyst compressed the thecal sac and displaced the cauda equina nerve roots. Fortunately, there was no connection found between the cyst and the facet joint. A histologic analysis of the cyst discovered a myxoid and pseudocystic degeneration without synovial lining. Postoperatively, the patient did well and was discharged home with a significant resolution of symptoms.

Discussion

Ligamentum flavum cyst is a form of juxtafacet cyst and its pathogenesis is not yet fully interpreted, but it has been considered that continuous stress to this ligament due to trivial repetitive trauma, such as in spondylolisthesis and degenerative disc disease, may lead to cyst development [9]. Christophis et al. [10] evaluated the cystic origin intraoperatively in 58 cases and suggested that juxtafacet cysts may originate from 3 anatomic structures: the facet joint, the ligamentum flavum, and the posterior longitudinal ligament. Synovial cysts arise from the facet joints and remain outside the ligamentum flavum and characteristically have a synovial lining membrane. Cyst linings are continuous with joint space and histologically represent a pseudostratified columnar epithelium, and cysts are filled with a clear and xanthochromic fluid [11]. The distribution of juxtafacet cysts correlates with mobility and the incidence of degenerative changes, with over 50% occurring at L4-L5 and the remainder mostly found at L5-S1 and L3-L4. These cysts are unusual in the cervical spine and even rarer in the dorsal spine [12]. There are no pertinent clinical symptoms; however, the spinal canal can impinge upon and displace neural structures and can lead to neurologic symptoms. The majority of symptomatic cysts usually present with radiculopathy, such as sciatica in case of lumbar cysts, and can resemble symptoms related to disc herniation [13], [14]. Neuroimaging, especially MRI, plays a vital role in the preoperative diagnosis of this condition. MRI shows that the thickened ligamentum flavum containing the cyst has a high-intensity signal on T2-weighted images and has variable signals on T1-weighted images. MRI diagnoses sometimes proven to be difficult for the diagnosis of ligamentum flavum cyst because of usual MRI images are focused on intervertebral discs. Differential diagnosis on imaging between ligamentum flavum cysts and synovial cysts is useful for surgical planning, as the latter is more difficult to resect, necessitating facet joint exploration [15]. Owing to the lack of data, there are no prospective randomized controlled trials for the management of lumbar juxtafacet cysts [12]. Conservative therapy appears to be less promising, and most conservative therapies have been proven to be temporary and have an unreliable short-term outcome. Surgical removal is the first-choice therapy for better recovery and improvement of the quality of life. The goal of surgical intervention is spinal decompression as well as resection of the cyst and the involved ligament as complete excision at the base of the ligamentous insertion of the cyst assures the lowest rate of recurrence [15].

Conclusion

The origin of ligamentum flavum cyst is not well understood and the occurrence is extremely rare, but should be considered in the differential diagnosis of any neurogenic claudication or lumbar radiculopathy of which disc herniation is the common cause. Due to the availability of better imaging facilities preoperatively including high-resolution MRI, everyone should be aware of this condition as the surgical treatment proves more beneficial and has long-standing aftereffects if performed timely.