Dorsal arachnoid web with spinal cord compression: variant of an arachnoidcyst? Report of two cases

Spinal arachnoid cysts are diverticula of the subarachnoid space that may c ompress the spinal cord; these lesions are most commonly found in the thora cic spine. Two patients who presented with thoracic myelopathy were noted o n magnetic resonance imaging to have focal indentation of the dorsal thorac ic cord, with syringomyelia inferior to the site of compression. Both patie nts were found at operation to have discrete arachnoid "webs" tenaciously a ttached to the dura mater and pia mater. These webs were not true arachnoid cysts, yet they blocked the flow of cerebrospinal fluid (CSF) and caused f ocal compression of the spinal cord. The mass effect appeared to be the res ult of a pressure gradient created by the obstruction of CSF flow in the do rsal aspect of the subarachnoid space. Both patients responded well to rese ction of the arachnoid web. Arachnoid webs appear to be rare variants of ar achnoid cysts and should be suspected in patients with focal compression of the thoracic spinal cord.