Spinal arachnoid cysts are rare pathologies which are asymptomatic in some cases or even missed or undiagnosed in some symptomatic cases, that is why the literature contains a few case reports or small series of such cases [1]. The aim of this study is to introduce our series of primary intradural spinal arachnoid cysts and to evaluate management outcomes.
MRI with different sequences including contrast administration is the gold standard investigation, it can differentiate a spinal arachnoid cyst from a wide variety of cystic spine lesions, and it can also clearly demonstrate the exact location of the cyst, septations or loculations, and degree of cord compression. CT myelography explore spinal arachnoid cysts as filling defects. In some cases, rapid and slow filling cysts may be missed in CT myelography. In case of rapid filling cysts, the cyst merges with the subarachnoid space rapidly, but the dye will be absorbed rapidly from the subarachnoid space and retained within the cyst, so the dye can be demonstrated within the cyst in delayed CT myelography. In case of slowly filling cysts delayed entry of the dye can be demonstrated in delayed CT myelography [9].
Spinal arachnoid cysts have wide range of differential diagnosis, they should be differentiated from neuroenteric cysts which usually occur anterior to the spinal cord in the cervicothoracic region and sometimes associated with septic meningitis, they have thick wall during surgery. Perineural cysts are usually sacral and related to nerve roots. Synovial cysts are usually low lumbar (motion segments) and related to facet joints. Dermoid cysts have reversed MRI signals T1 hyperintense and T2 hypointense. Epidermoid cysts are isointense to CSF but can be differentiated by diffusion-weighted images. Cysticercosis cysts usually have wall calcifications. Inflammatory and neoplastic cysts usually enhance after contrast injection [9].
In this study, we used MRI only in diagnosis and surgical planning for all cases because we have advanced MRI devices (1.5 and 3 Tesla) with high-resolution images. The high sensitivity and specificity of MRI, easy interpretation of images, and being non-invasive maneuver replaced the need for CT myelography which necessitate intrathecal dye injection. Previous similar studies reported that MRI was enough in diagnosis and follow-up of their patients as in our series [10, 12]. On the contrary, Fam et al. reported in their series that MRI was not conclusive in 6 cases out of 22 patients, for whom CT myelography was done. Three patients proved to have intradural spinal arachnoid cysts and one patient had extradural cyst with definite dural defect communicating the cyst to the subarachnoid space [13].
Females presented 1.5:1 ratio in our series which is matching with the previous reports in the literature. Sadek et al. reported female to male ratio 1.8:1 in their series of 17 patients [5] and Fam et al. reported a ratio of 3:1 in their series of 22 patients [13]. The mean age in our series was 40 years; Sadek et al. in their series also reported mean age of 58.2 years [5]. Despite the congenital theory which is supposed to be the origin of spinal arachnoid cysts and expectations of such pathologies at earlier ages, this mean age of patients could be attributed to the long time spent by these cysts to increase in size and longer time to produce symptoms.
The series of Sadek et al. included only primary intradural arachnoid cysts and reported 100% location in the dorsal thoracic region [5], supporting the theory suggesting origin of intradural spinal arachnoid cysts from septum posticum which is more prominent at the same location. We found the predominant location in our series is the dorsal (posterior) thoracic 90% which is matching with the series of Kriss and Kriss 80% [14]; a 3-year-old child had a cervical ventrolateral cyst (Fig. 1) which is against the theory of septicum posticum origin and may be in favor of the theory of origin from abnormal proliferation of arachnoid granulations during the embryonic period leading to formation of diverticula that develop into cysts. Many theories have been postulated to explain the origin of spinal arachnoid cysts but till the moment nothing proved to be the direct cause for spinal arachnoid cyst formation [5].
The approach in our series varied from laminoplasty, hemilaminectomy to formal laminectomy according to cyst location, size, cord compression, and surgeon experience. We depend on the fact that laminectomy does not affect stability of the dorsal spine which is enforced by the rib cage [5, 15, 16, 17] no instrumental fixation was needed in any of our cases.
Clinical outcome of our series revealed complete improvement of pain in five patients and partial improvement in four patients. Complete improvement of motor power in all patients. The only patient who had sensory manifestations showed partial improvement. Daily activity improved in all patients. Eroglu et al. concluded that pain and weakness were the most likely to improve while sensory manifestations were the least likely to improve [1].
The aim of surgery was cord decompression which was achieved in all cases through microsurgical resection in two cases and marsupialization in eight cases; marsupialization was preferred when the inner layer was adherent to the cord making total excision risky. Viswanathan et al. also recommended cyst wall fenestration and partial excision in case of intradural arachnoid cysts to avoid cord manipulation and possible postoperative neurological deficit [10].
One of our cases (Fig. 2) had a recurrent intradural cyst after history of being operated for cystoperitoneal shunt in another center. MRI showed multiloculaed intradural cyst with significant cord compression, X-ray dorsal spine and X-ray abdomen and pelvis showed both spinal and peritoneal ends of cystoperitoneal shunt are in place. The patient was operated for microscopic excision of the outer cyst wall, fenestration of the septa and marsupialization of the inner wall. Shunt tube was left in place because no extra work was needed to leave it in its sound position, and it can decrease the incidence of postoperative CSF leakage from the wound and late cyst recurrence. The patient improved clinically after surgery, and follow-up MRI showed cord decompression.
No post-operative neurological deterioration; CSF leakage or infection had been observed in our series during the follow-up period. No cyst recurrence had been encountered in this series even in marsupialized cysts, Krings et al. reported recurrence in one case where they only closed the dural defect without cyst excision, and this patient was operated again with total resection of the cyst, all symptoms improved after the second operation [9].
Study limitations
The limitations of this study are those inherent to retrospective nature with small populations. We believe that greater number of cases with longer follow-up period will have more reliable results for future research of such rare pathology.