Outcomes of Surgical Treatment of Intraspinal Intradural Lumbar Schwannomas

Background Data: Safe surgical resection of intradural schwannomas in the lumbar region is considered challenging. This is due to the proximity and sometimes the adhesiveness of these lesions to the cauda


INTRODUCTION
Nerve sheath tumors are a group of lesions that arises from the neoplastic differentiation of Schwann cells. They grow as a peripheral appendage to the parent nerve. Conventional schwannoma is the most common type of the major variants of schwannomas in adults. 2 Spinal schwannomas constitute nearly 30% of primary spinal tumors and are among the most common spinal tumors that face spine surgeons. 3,9,26 Spinal schwannomas are found to grow from the sensory (dorsal) nerve root. Multiple schwannomas are associated with neurofibromatosis type 2 (NF-2). 13,22,32 The most common anatomical type of schwannomas is the intradural extramedullary one (49%-83%). 9,25,30 Clinically, schwannomas initially present with nonspecific symptoms such as segmental pain and paresthesia as they are usually slowly growing benign lesions. 17 With further growth of the tumor, they compress the adjacent neural elements causing some degree of paresis. The usual age of presentation ranges from 30 to 50 years. 6,30,32 Total excision is the gold standard treatment in symptomatic patients or in the radiological picture of an enlarging lesion. It usually results in clinical improvement with minimal postoperative morbidity, especially in patients who presented early. 6,14,18,30,32 In this study, the operative findings and clinical outcomes after surgical excision of 20 solitary intradural lumbar schwannomas were evaluated.

PATIENTS AND METHODS
This study included 20 consecutive patients with lumbar intradural schwannomas. Patients' data were retrospectively collected from the patients' medical records after the exclusion of 2 patients due to insufficient medical record data and follow-up. Personal data, diagnosis, and treatment outcomes were kept private, and patients were represented by specific codes. All patients had solitary lumbar intradural schwannomas. They were admitted and underwent operations at the Department of Neurosurgery, Tanta University Hospitals, from January 2016 to January 2019. We included all patients who have undergone operations for intradural solitary schwannoma within the lumbar spine that was verified histopathologically. The exclusion criteria were as follows: those with multiple lesions, those with foraminal and/or extraforaminal extension, those with other pathologies, general contraindications for surgery, and those with incomplete follow-up or data. All patients were evaluated and subjected to clinical history, general and neurological examination, and routine laboratory investigations. Visual Analogue Score (VAS) was used to quantify the degree of pain in every patient preoperatively. We also used the modified McCormick grading system to assess functional status before and after surgery ( Table 1). All patients were evaluated preoperatively with Magnetic Resonance Imaging (MRI). Written consent was obtained from all patients before scheduling surgery. The study was conducted after the approval of our institutional review board All operations were conducted in a prone position under general anesthesia. The targeted spinal level was confirmed using operative fluoroscopy. After skin incision and muscle dissection, central laminectomy was done to minimize the extent of bone removal to prevent postoperative instability. Under the magnification of an intraoperative microscope, the dura mater was incised in the midline over the lesion. Then, the lesion was dissected from the adjacent rootlets. After meticulous dissection from all neural structures, extracapsular en bloc excision of the lesion was carried out along with the division of its parent rootlet. The dura was then closed tightly, followed by the closure of muscle, fascia, and skin in layers with an epidural suction drain inserted for 48 hours. The extent of gross tumor resection was documented.
Postoperative Care: Patients were ambulated and discharged from the hospital after assessment of his/her wound and clinical status. Outpatient clinic visits were scheduled for all patients to assess functional outcomes postoperatively regarding the motor power in both lower limbs and pain relief using the VAS and modified McCormick grading system. We compared the preoperative MRI with the one-month postoperative images to determine the extent of resection.

RESULTS
We operated on 20 patients with intradural lumbar schwannomas, including 11 (55%) females and 9 (45%) males. The mean age at the presentation was 39.45 ± 12.5 (range,  years. Moreover, the mean duration of preoperative complaint was 4.15 ± 2.28 (range, 1-9) months. All patients presented with localized low back pain associated with radicular pain along the lower limbs according to the affected level. Six patients (30%) suffered from either urgency or precipitancy. The mean preoperative VAS of the radicular pain was 8.95 ± 1.1 (range, 7-10). Likely, no preoperative motor deficit was observed in any of our patients.
The neurological evaluation showed a specific dermatomal hypothesis in 7 patients (35%), saddle area hypothesis in 3 patients (15%), diminished knee jerk in 4 patients (20%), and diminished ankle jerk in 3 patients (15%). All schwannomas were solitary and affected lumber levels as follows: 6 cases at L1 (30%), 5 cases at each of L2 and L3 (25% each), and 2 cases at each of L4 and L5 (10% each). MRI study of the lumbar showed the lesions as a single oblong intradural lesion that is isointense in T1WI and hyperintense in T2WI and with homogenous enhancement after intravenous gadolinium injection. None of our patients had any neural foramina widening or spinal deformities. The gross total resection (GTR) of the tumor with the division of the parent rootlet was achieved in 19 patients (95%). MRI study with intravenous gadolinium after one-month follow-up confirmed GTR in all but one patient (Figures 1,2). Subtotal resection (STR) was achieved in one patient (5%) at the level of L1 because of its proximity and adherence to the conus medullaris that made its GTR and preservation of neurological functions relatively difficult ( Figure 3). All lesions were encapsulated, yellowish in color, firm in consistency, and were histopathologically confirmed to be schwannoma. All patients underwent their scheduled surgery with uneventful operative and postoperative course without any reported added deficit. At the time of discharge, 19 patients (95%) showed significant improvement compared to their preoperative status without any new neurological deficits. At one-month followup, the mean VAS improved from 8.95 ± 1.1 (range, 7-10) to 4.3 ± 1.78 (range, 2-9) and at 6-months follow-up to 1.33 ± 1.57 (range, 0-7).

Journal
Six patients with dysuria experienced significant gradual improvement with the complete resolution of their complaints at the end of the follow-up period.
A patient with STR showed unsatisfactory pain relief with a postoperative VAS of 9 and 7 at one-and six-month follow-up postoperatively, respectively. She refused to do further surgical interventions at our service and was transferred to another center. According to the modified McCormick grading system for assessing the functional outcome, the mean preoperative score was 2.1 (range, 1-3) and improved to 1.25 (range, 1-2) and 1.05 (range 1-2) at one-and six-month follow-up postoperatively. Some minor complications were observed, including transient CSF wound leak in one patient (5%) that responded within seven days to diuretics and secondary skin sutures and superficial wound infection in 2 patients (10%) treated within 10 days after administering topical and intravenous fourth-generation cephalosporins.

DISCUSSION
Spinal schwannomas represent about 30% of primary spinal tumors. Sex prevalence showed no significant difference between males (45%) and females (55%) in this study and in other literature. 5,10,11,30 The mean age at presentation in our series was 39.45 years, which agrees with other published data that have reported a mean age range between 30 and 50 years. 18,21,30 All of our patients presented with axial and radicular pain, which again represents the most common initial symptom of patients in other literature. 11,18,28,29 The least common presentation in literature is motor or sensory deficits, which are due to the nerve root irritation by the growing tumor. Moreover, the further growth of the tumor results in compression of the root and/or the cord, causing neurological deficits. In the lumbar region, the motor deficit was rarely the first presentation, which is similar to patients with lumbar canal stenosis in which neurological deficits occur very late. 9,18,31 Our study aimed to assess the clinical outcome after GTR of the solitary intradural lumbar schwannoma with the division of the single parent rootlet. It has been demonstrated that satisfactory pain relief has been reported in patients with the GTR, whereas with STR, a patient had unsatisfactory pain relief. No postoperative neurological deficits were observed in all patients with GTR, despite resection of the parent rootlet. In their study, Safaee et al. 28 have studied 221 cases with spinal nerve sheath tumors, including 74 lumbosacral tumors. They have achieved GTR in 86% of lumbosacral cases. The overall recurrence rate in their study was higher with neurofibroma (17%) than with schwannoma (7%) and in cases with STR (22%) than with GTR (4%). In their study, Satoh et al. 29 have studied 23 patients with solitary schwannomas below the thoracolumbar junction, including 19 intradural and 4 dumbbell tumors. All cases underwent parent rootlet resection, while only 3 of them developed postoperative neurological deficits and those three patients had dumbbell schwannomas. In their study, the mean preoperative modified Japanese Orthopaedic Association (JOA) score had significantly improved from 8.9 to 13.0 postoperatively. The safety of resection of the parent rootlet is explained by the fact that the nerve rootlets descend as the cauda equina within the lumbar spinal canal. The schwannoma mostly arises from a single rootlet that is mostly the sensory dorsal rootlet. 8,22 During the microsurgical excision of the intradural spinal schwannoma, the rootlets could be easily dissected and detached from the well-encapsulated lesion. 29 Another explanation is that the tumor grows slowly; thus, the function of the affected rootlet is carried out and compensated by the regeneration and collateral sprout from the intact nearby healthy rootlets. 4,16,33 This differs from the schwannoma that extends outside the spinal canal through the intervertebral foramen in a dumbbell shape, where the transection of the parent rootlet may carry the risk of injuring the adjacent ventral motor rootlet, increasing the risk of postoperative neurological deficit. 12,29 There is another surgical technique that may minimize the risk of the postoperative deficit is the enucleation leaving the tumor capsule; however, it carries a high risk of recurrence due to the presence of tumor cells of spinal schwannomas near the capsule. 15,22 In their study, Fehlings MG et al. 14  Regarding spinal stability, we did not need to stabilize the spine in any of our cases. This might be due to the use of an intraoperative C-Arm for precise localization of the affected level. This helped us in limiting the craniocaudal extension of the laminectomy. We did not remove more than two laminae in each of our cases. Moreover, we did not violate the articular facets. In the literature, spinal instability and/or kyphosis mostly occurred in cases with extensive laminectomy with violation of the articular facets. Spinal stabilization could be considered in dumbbell-shaped tumors and tumors with extradural extension, with cervicothoracic junction lesions, and with lesions that may require multilevel laminectomies. 18,27 Another option is to perform a laminoplasty instead of laminectomy; however, the authors who compared both techniques have found no significant difference between both methods regarding the outcomes. 24 There were some limitations in our study, such as the small number of patients and the short duration of follow-up. We recommend a longer follow-up period to detect the actual rate of recurrence and late drawbacks of surgery such as spinal instability.

CONCLUSION
Total microsurgical excision of solitary intradural lumbar schwannoma with the resection of the parent rootlet is considered a safe and effective procedure. Accurate localization of the lesion with minimization of the surgical exposure is mandatory for preserving spinal stability and preventing subsequent spinal deformity.