1 Introduction

Schwannomas are some of the most common spinal tumors, accounting for nearly 30% of spinal tumors^[1].Intramedullary schwannomas comprise approximately 0.3% of all medullary tumors and 1.1% of spinal schwannomas^{[2, 3]}. Schwannomas arise from Schwann cells of the neuronal myelin sheath^[1-7]. There are no Schwann cells in intramedullary neural tissues; therefore, the origin of intramedullary schwannomas remains controversial. Few cases of intramedullary schwannomas have been reported since the first case was reported by Penfild et al. in 1932^[4]. Intramedullary schwannomas can occur along the length of the spinal cord, including in the cervical, thoracic, or lumbar cords^[2]. Magnetic resonance imaging (MRI) is an important method for diagnosing intramedullary schwannomas; however, the misdiagnosis rate remains high^{[3, 8, 9]}. Surgical resection is the most common and effective method for treating intramedullary schwannoms^[10]. In the current study, a case of intramedullary schwannoma in the cervical spinal cord is reported, along with its clinical features, radiologic and histopathologic observations, and one-year follow-up MRI results; a literature review of these aspects is also presented.

2 Case report

A 35-year-old woman presented to Beijing Tsinghua Changgung Hospital with a 10-year history of cervical malaise, which rapidly worsened over the 3 months prior to presentation. Her symptoms included right limb paraesthesia and gait deterioration due to imbalance and weakness. Her past medical and family histories did not support a neurofibromatosis diagnosis.A neurologic examination revealed grade 4 right limb muscle strength, as well as right limb pain and numbness. A right Hoffman sign (+) and exaggerated right tendon reflexes were also observed. MRI revealed an 8 mm × 6 mm × 8 mm intramedullary tumor at the C6-C7 vertebrae level. The tumor had clear margins, was isointense on T1-weighted images (Figure 1a), and was slightly hyperintense on T2-weighted images (Figure 1b). Marked homogenous contrast enhancement was also noted (Figures 1c-1e) and there were some signs of swelling around the lesion. After intraoperative O-arm navigation to determine the lesion's exact location, a small explosive incision was made in the spinal cord, between the C6 and C7 vertebrae. The tumor had clear margins and was highly adherent to the surrounding neural tissues. The tumor was completely resected using microsurgical techniques. It was a solid, soft mass, measuring approximately 8 mm × 5 mm × 2 mm. A pathological examination confirmed that the tumor was a schwannoma (Figure 1f). The patient's neurological examination improved postoperatively. One year after surgery, the patient's symptoms were in remission and a post-operative MRI revealed no evidence of tumor recurrence (Figures 2a-2d).

3 Discussion

Intramedullary schwannomas are very rare. Hypotheses regarding their pathogenesis include^{[3, 10-12]}: (1) ectopic Schwann cells originating from the embryonic neural ridge; (2) Schwann cells ensheathing aberrant intramedullary nerve fibers; (3) Schwann cells extending along the branches of the anterior spinal artery; (4) neoplastic growth of Schwann cells into the cord from areas where the posterior nerve roots enter the pia mater; (5) transformation of cells with neuroectodermal origins into Schwann cells; and (6) differentiation of pulripotent mesenchysmal cells into Schwann cells. Ross et al. reviewed 22 cases of intramedullary schwannomas, and reported that the most common location of spinal intramedullary schwannomas is in the cervical cord (50%), followed by the thoracic (22%) and lumbar cords^[2]. López et al. reported a 3:1 male:female ratio for the occurrence of intramedullary schwannomas^[13]. The tumors are usually single lesions, most commonly in the cervical (63%), thoracic, and lumbar (11%) spinal cords^[13]. The average interval between the first symptoms and a diagnosis is 28.2 months (range: 6 months to 20 years)^[13]. Karatay et al. reported a case of intramedullary schwannoma located in the conus medullaris, in conjunction with syringomyelia^[6].

The specific clinical manifestations of intramedullary schwannomas are associated with the tumor's size and location, but they generally exhibit the general characteristics of intramedullary tumors in that they cause upper motor neuron damage. Patients often complain of numbness, pain, paralysis, muscle atrophy, and even dyspnea. Wu et al. reported that somatic and nerve root pain are the chief complaints associated with schwannomas^[14]. MRI is the most useful modality for diagnosing spinal cord tumors, but little research on intramedullary schwannomas has been reported.Our literature review indicated that schwannoma characteristics include: (1) asymmetric, solid nodules or masses, often located in the dorsolateral part of the spinal cord; (2) most lesions are small, spanning no more than three vertebral segments; (3) T1-weighted MRIs generally show an isointense or slightly hypointense lesion, T2-weighted scans show slightly hyperintense lesions, and enhanced T1-weighted scans show homogeneous enhancement, with obviously clear margins; (4) the lesions may be accompanied by cystic changes, necrosis, syringomyelia, localized swelling, and rarely, calcification^{[3, 6-12, 15]}.

Based on the articles reviewed, the most common differential diagnoses associated with intramedullary schwannomas include ependymomas, hemangioblastomas, astrocytomas, and intramedullary tuberculomas^{[1-7, 10-12, 14, 15]}. The preoperative diagnosis of the present case was hemangioblastoma. However, the enhancement of the lesion was less obvious than that typical of a hemangioblastoma, and extensive cystic changes and vascular flow-void effects were not observed. There was also evidence of swelling, which differentiated the present lesion from a hemangioblastoma.

The goal of spinal schwannoma surgical treatment is complete tumor resection. Conti et al. reported a series of 179 patients with intramedullary tumors who were treated operatively^[10]. There were three cases (1 case with neurofibromatosis) of intramedullary schwannomas, and all were totally resected and did not recur^[10]. However, total removal can be difficult due to tumor adherence to the surrounding neural tissues. In these cases, subtotal removal is a wise choice for reducing postoperative complications. Since intramedullary schwannomas are benign, radiotherapy is rarely used following subtotal resection. Formal guidelines for treating recurrent tumors are lacking, but reoperation is currently the most common choice. Lee et al. reported 10 patients with intramedullary schwannomas who underwent surgical treatment and had long follow-up periods^[16]. Gross total resection was performed in 8 cases, whereas subtotal resection was performed in 2^[16]. The mean follow-up period was 75.7 months. The authors did not observe any cases of recurrence or clinical deterioration in these patients^[15].

In conclusion, intramedullary schwannomas are very rare, but should be suspected if MRIs indicate asymmetric, solid nodules or masses, and enhanced T1-weighted scans show obvious homogeneous enhancement of the lesion, clear margins, and localized swelling. The optimal treatment is surgery, aiming for maximal resection, according to the operative situation. The patient prognosis is good, with recurrence or deterioration rarely observed.

Acknowledgements

This project was supported by the Beijing Tsinghua Changgung Hospital Fund (No. 12015C1044). The authors thank Zhenxing Sun and Huifang Zhang for collecting the archived data and Guihuai Wang and Jin Wang for technical assistance.

Conflict of interests

None of the contributing authors have conflicts of interest.