The value of intraoperative ultrasound in the recognition of pseudo-swelling of the spinal cord.

We present the case of a woman who presented with weakness of both legs due to a low grade tumor of the spinal cord. Excision of the tumor was performed and confirmed with intraoperative ultrasound. Prior to dural closure the spinal cord was found to be pushed dorsally with herniation of the cord through the dural defect. Intraoperative ultrasound showed a collection of cerebrospinal fluid in an anterior pocket giving the impression of the cord being swollen. Once cerebrospinal fluid was drained, the cord settled within the thecal space and closure of the dural defect was performed. Surgery for an intramedullary spinal cord tumor can cause a significant amount of swelling and either a duroplasty is required or the dura is left open with meticulous closure of the wound. Ultrasound is helpful to identify pathology anterior to the cord and prevents the potential complications associated with duroplasty or leaving the dura open.

Introduction

Intramedullary spinal cord tumors account for 2–4% of all intrinsic central nervous system tumors. Although some asymptomatic or incidental lesions can be followed up clinically and radiologically, surgery is the first line treatment for most of the patients. Gross total resection of the tumor is the aim, however, it is not always possible, particularly in astrocytomas or higher grade gliomas due to the absence of a well defined plane between normal and abnormal nervous tissue. Occasionally at the end of procedure edema of the spinal cord impedes safe closure of the dura. In this case paper we demonstrate the utility of intraoperative ultrasound imaging during surgery for intramedullary tumors. This modality is particularly helpful for differentiation between real edema of the spinal cord and “pseudoswelling” caused by the herniation of the spinal cord through dural incision. Pseudoswelling caused by such herniation is easily treatable if recognized.

Aloka B-Mode ultrasound with 10 MHz probe is used in our institution intraoperatively for localization of the intramedullary lesion, confirmation of gross total tumor excision and recognition of treatable spinal cord transdural herniation as well as its differentiation from spinal cord swelling.

Case report

We describe the case of a 40-year-old fit and healthy woman who presented with about five months history of progressive paraparesis and a recent right foot drop – McCormick Grade 3 (. She did not report any sensory alterations and reported normal bowel and urinary function. Magnetic resonance imaging of her spine showed an expansion of the conus medullaris with heterogeneous change of signal on T2 but no contrast enhancement. She initially had an open biopsy in another neurosurgical unit that did not confirm any histological abnormality. The size of the abnormality increased in three months as per further follow-up imaging. Based on radiological findings, the impression was of a low grade intramedullary tumor and the patient was offered surgery for debulking/excision of tumor. A revisional surgery with extension of laminectomy and durotomy was performed to expose the spinal cord. The cord was split in the midline in classical fashion to expose a greyish tumor. Initially a good plane separating the tumor from the nervous tissue was seen but progressive loss of tumor margins were noted towards the deeper aspects. Intraoperative specimens were sent for pathological consultation confirmed presence of abnormal tissue but were unable to comment neither regarding the histological differentiation between astrocytoma/ependymoma nor the degree of malignancy. Maximal tumor debulking was achieved and confirmed with intraoperative ultrasound. Assurance of no further neurological deficit was also ascertained through continuous intraoperative neurophysiological monitoring of somatosensory and motor evoked potentials. Just prior to the closure of the dural defect, the spinal cord was found to be pushed dorsally and the edges were seen herniating out through the dural defect. The ultrasound probe was used again at this point and a significant collection of cerebrospinal fluid (CSF) in a pocket anterior to the spinal cord pushing the cord posteriorly was visualized. Once this CSF was drained, the spinal cord settled within the thecal space and the closure of the dural defect was performed without any complications. The rest of the wound was closed in a routine fashion and the patient made a good postoperative recovery with preservation of preoperative mobility and normal sphincter function. The final histological results confirmed an anaplastic ependymoma (WHO grade III). The patient was referred to the oncology service for further treatment.

Left: preoperative MRI image (T2 weighted images) of the spinal cord showing a heterogeneous expansile lesion within the conus medullaris. Right: postoperative MRI image of the same area showing a reduction in the swelling and mass in the area with the tumor cavity being filled with cerebrospinal fluid.

Left column: intraoperative ultrasound and photograph of the spinal cord showing the tumor causing the expansion of the cord. Middle column: pseudo-swelling of spinal cord due to an anterior pocket of trapped cerebrospinal fluid with transdural herniation of the cord post resection of the tumor. The dural margin is very close to the swollen spinal cord preventing a watertight dural closure. Right column: relaxed spinal cord after release of the cerebrospinal fluid trapped in the anterior pocket facilitating dural closure

Discussion

The use of intraoperative ultrasound for neurosurgical procedures was described by Dohrmann and Rubin in 1981 for cranial and in 1982 for spinal pathology(. They described the techniques to ensure identification of maximal normal and abnormal anatomy. Since then its use has been widely popularized and published extensively mainly due to its effectiveness as a real time imaging tool. A higher frequency ultrasound probe is preferable for better quality images.

Surgery for an extensive intramedullary spinal cord tumor can cause a significant amount of swelling and either a duroplasty is required or the dura is left open with meticulous closure of the wound. Ultrasound is very helpful to identify pathology anterior to the cord that could be intradural or extradural (. In our case there was a pocket of CSF anterior to the cord that was pushing it dorsally giving the impression of the cord being swollen.

Such a case of “pseudoswelling” of the spinal cord secondary to anterior CSF collection has not been reported before to best of our knowledge. The intraoperative ultrasound, which is routinely used in our department for localization of intradural pathology, along with intraoperative neurophysiological monitoring (somatosensory and motor evoked potentials) was invaluable. Not only it helped us in identifying the intramedullary tumor prior to opening of the dura, but also prevented neurological deterioration and the potential complications associated with duroplasty or leaving the dura open.

Conclusion

Surgery for an intramedullary spinal cord tumor can cause a significant amount of swelling and either a duroplasty is required or the dura is left open with meticulous closure of the wound. Ultrasound is helpful to identify pathology anterior to the cord and this valuable adjunct prevented the potential complications associated with duroplasty or leaving the dura open.