Syringohydromyelia

      Syringomyelia refers to a cavity in the spinal cord extending lateral to or independent of the central canal. Hydromyelia refers to a dilated central canal of the cord. Most cavities involve both parenchyma and central canal, and the term syringohydromyelia reflects this combined phenomenon. Many use syrinx and syringomyelia as general terms for any cord cyst.

      The clinical presentation is variable depending on the location and extent of the syrinx. The classic picture is segmental weakness and atrophy of the hands and arms, with loss of reflexes and segmental anesthesia. However, typical symptoms are unilateral, confined to the lower extremities, or absent. Accompanying severe pain can be debilitating. The symptoms are unpredictable with periods of waxing and waning and may mimic brainstem compression.

      There are several theories regarding the formation of syringohydromyelia. Gardner's theory suggests that there is a lack of perforation of the foramen of Magendie that forces CSF through the obex into the central canal of the cord. Endnote Williams proposes that there is free flow of CSF upward from the spinal subarachnoid space into the intracranial cisterns, but flow is partially blocked in the reverse direction. Endnote He suggests that this cranial-spinal pressure difference leads to CSF being sucked into the central canal of the cord from the 4th ventricle, creating a syrinx. Williams also proposes that the fluid within the syrinx moves both up and down as a result of changes in epidural venous pressure. This may result in extension of the syrinx cavity. Ball and Dayan, Endnote as well as Aboulker, Endnote believe that the craniospinal pressure gradient is present but reversed in direction from that proposed by Williams. These authors believe that increased CSF pressure in the spinal CSF space results in CSF filtering from the subarachnoid space into the central canal through the spinal cord. Since passage into the IV ventricle is blocked, a syrinx forms.

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      Direct evaluation of the cord and the entire spinal neuraxis with MR provides the most sensitive and specific evaluation of syringohydromyelia. In the largest series by Sherman and his group, Endnote four separate groups of patients were identified based on the appearance of the cavities and any associated anomaly or relevant history. Approximately 41 per cent had syringomyelia associated with tonsillar ectopia (Chiari I malformation), 28 per cent had posttraumatic syrinx, 15 per cent were associated with neoplasm and 15 per cent were idiopathic. When the cavities were analyzed for specific characteristics, a few observations were made. At least one third of patients with nonneoplastic cavities had associated high-signal intensity in the cord contiguous to the cavity or in the parenchyma distal or proximal to the cavity. This high-signal intensity in a simple syrinx is speculated to represent gliosis, edema, demyelination or microcyst malformation. Endnote The presence of edema is supported by the return of the hyperintense parenchyma to normal signal intensity following myelotomy and shunt decompression of the cyst. Endnote

      Enhancement with gadolinium increases the certainty of distinguishing a primary syrinx from a cystic neoplasm. If a cavity is noted within the cord on screening MR sequences, gadolinium should be given and T1-weighted scans acquired in sagittal and axial planes. If no enhancing nodule is seen, a cord neoplasm is unlikely.

      Examination of the entire spinal cord with sagittal images is mandatory. In regions where a cavity is suspected but somewhat equivocal in the sagittal projection, axial scans should be done. Failure to examine the entire spinal neuraxis may result in missing cavities that are separated from the primary or larger cavity. Long segments of normal appearing cord may intervene between a cervical cavity and one located in the conus medullaris. Failure to recognize an additional cavity could result in inappropriate or incomplete shunting and subsequent failure of the patient to improve clinically.  

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