The central canal within the spinal cord normally communicates with the fourth ventricle via the obex, and is barely perceptible if at all on MRI. Hydromyelia is strictly defined as abnormal dilation of the central canal, whereas syringomyelia only refers to dilated cavities within the cord parenchyma separate from the central canal. In practice, these two entities are difficult to distinguish and the combined terms of syringohydromyelia or syrinx are used. Most cases of syringohydromyelia appear to be caused by alterations in CSF flow dynamics, with abnormal pressure gradients being established between different CSF-containing spaces. Causes include Chiari malformations (I and II), hydrocephalus, arachnoiditis, spinal stenosis

Figure 12. Cervical spinal cord syrinx associated with Chiari type I malformation. (sagittal T2-WI)

or tumors. Both Chiari I and II malformations result in impaired flow of CSF through the subarachnoid space at the foramen magnum, with presumed transmission of increased flow and pressure into the central canal of the spinal cord, which then dilates (Fig. 12). Syringohydromyelia can also be seen in the setting of parenchymal loss and my-elomalacia due to prior trauma, infarction, inflammation or hemorrhage. A prominent central canal (which has different clinical implications and should be differentiated from frank hydromy-elia) is present in 25% of patients with tethered cords and may occasionally (-2%) be detected as an incidental finding when children are imaged for unrelated diseases or disorders.

Depending on the cross-sectional region of the cord affected, children with syringohydromyelia may present with symmetric or asymmetric weakness and sensory loss in the upper or lower extremities; scoliosis and ataxia may also be present. The extent of a syrinx is best delineated by sagittal and transaxial T1- and T2-weighted MR images, on which it appears as fluid collections isointense to CSF and located within the cord. The appearance can vary from smooth dilation of the central canal to large eccentric cavities that have a "beaded" appearance because of multiple internal septations. If no obvious explanation for syringohydromyelia is seen on the precontrast images from the patient's first visit (e.g., Chiari II), contrast-enhanced T1-WI images must be performed in order to exclude a spinal-cord tumor.

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