Discussion

Intraspinal extradural cysts have occasionally been seen in lumbar spine since MRI was introduced in clinical examination. But those are still rare in the cervical spine. They have been reported as arachnoid, synovial and ganglion cysts. Kao et al1 grouped synovial and ganglion cysts together because they considered that both types of cyst arise at the joint capsule of the facet synovial joint and proposed the term ‘juxtafacet cysts’ to represent both of them. Hatem et al2 used the term ‘intraspinal cervical degenerative cyst’ in the report of three cases of cervical cysts including a case of ligamentum flavum cyst because cysts of the ligamentum flavum, defined as those arising inside this ligament, are differentiated from the juxtafacet cyst. Shima et al3 proposed the term ‘degenerative intraspinal cyst’ as a more accurate description of these cysts in the spinal canal that arise from degenerated spinal structures including not only facet joints, but also ligaments and disc. In the review of the past reports of intraspinal extradural cysts in the cervical spine, five cysts are considered to arise from the ligamentum flavum.2 ,4–6 We use the term ‘degenerative intraspinal cyst’ as we agree with Shima et al for the point that some cysts in the spinal canal arise from degenerated spinal structures including not only facet joints, but also ligaments.

To the best of our knowledge, the total number of degenerative intraspinal cysts that have ever been reported is 67 articles, in 133 patients, including the present case (table 1). Although the cysts occur over a wide age range (8–86 years), they are more commonly encountered in older persons (mean age 67.3 years). The reported case included 65 men, 64 women and unknown 4 cases among the total 133 cases. The level of the cyst was the atlanto–axial junction in 40 cysts (30.1%), C3–C4 in 13 (9.8%), C4–C5 in 10 (7.5%), C5–C6 in 10 (7.5%), C6–C7 in 9 (6.8%) and C7–T1 in 51 (38.3%). Bilateral cysts have been reported in two cases, and both of them were located at C7–T1.7 ,8 A higher incidence of cysts was seen at levels C1–C2 and C7-T1 levels. The degenerative diseases like as disc herniations and canal stenosis usually occur at middle cervical spine and lower lumbar spine. The degenerative intraspinal cysts are often seen at the lower lumbar spine as well as degenerative diseases. But they are not often seen at middle cervical spine. There might be different occurrence mechanism of degenerative intraspinal cysts between cervical and lumbar spine. Krauss et al8 postulate that the C7–T1 level was a common location for juxtafacet cysts because this level is immediately superior to the relatively immobile thoracic spine. In his comment Cooper disagreed with Krauss et al because the C7–T1 level is less mobile than the three superior motion segments and is infrequently the site of disc herniations and spondylosis, which occur most frequently at the sites of most movement. Hatem et al2 mentioned that the presence of the paradoxical motion without discogenic spondylosis at the C7–T1 level and the loss of the mobility in the higher two levels observed in older individuals can explain preferential localisation of these cysts at this level.

These degenerative intraspinal cysts of the cervical spine were usually diagnosed by MRI, which show an oval-shaped cystic lesion at the anterior of the spinal canal at the atlanto–axial junction and posterolaterally in the mid-cervical and lower cervical regions. The signal intensity of the cyst was low to intermediate on T1-weighted images and high on T2-weighted images. Contrast MRI may reveal an enhancing cystic rim. Synovial cysts may also contain gas. MRI is also useful postoperatively to observe the recurrence of the cysts. CT often reveals degenerated facet joints. Calcification or air in the cyst can also be seen in CT.

In the treatment of degenerative intraspinal cysts, most of patients underwent laminectomy and excision of the cysts. Four cases underwent atlanto–axial arthrodesis without excision of the cysts.9–12 One case was reported that spontaneous resolution of a C7–T1 synovial cyst was seen.13 Laminectomy or laminoplasty and excision of the cyst are recommended for cases with cervical spinal canal stenosis. If there is no concomitant canal stenosis, minimal laminectomy with excision of the cyst is enough. There have been no reports of recurrence of cyst in the cervical spine after surgery. Stoodley mentioned that, because recurrence after partial excision is rare, it is preferable to leave cyst remnants attached to the dura than to excise and patch the dura, which carries the risk of cerebrospinal fluid leakage.14 In our case, no recurrence was observed after partial resection of the cyst. We also think it is not necessary to remove the cyst wall adhering to the dura completely. Intraoperative ultrasonography was reported to be very useful in some cases. The border between the cyst and the dura was well visualised by ultrasonography. Intraspinal cysts in the cervical spine are uncommon. When they are diagnosed adequately and treated with appropriate surgery, the clinical outcome is generally good.15–20