title: Extensive intradural arachnoid cyst of the lumbar spinal canal: case report.
abstract: BACKGROUND
  Spinal intradural arachnoid cysts are rare outpouchings of arachnoid lining occurring mainly in the thoracic and cervical regions. MRI is considered the diagnostic procedure of choice; however, some arachnoid cysts have been reported to elude diagnosis by MRI due to the similar signal intensity of the cyst and the subarachnoid space.
  
  
  METHODS
  We present a case of a 41-year-old woman with an intradural arachnoid cyst of the lumbar spinal canal. Diagnostic studies demonstrated a herniated L4-5 disk, which led to two operations. Despite intractable pain postoperatively, the MRI failed to reveal further focal compression of the nerve root. A myelogram suggested compression of the thecal sac by a dorsal mass lesion. The patient underwent a midline durotomy that revealed two large arachnoid cysts with the nerve roots beneath them pushed ventrally and to the right. Removal of the arachnoid layer resulted in complete fenestration of the cyst and allowed the nerve roots to freely float in the spinal fluid.
  
  
  CONCLUSIONS
  In retrospect, the abnormality could be appreciated on multiple prior scans performed at another institution, demonstrating that in the patient with low back pain, degenerative disc disease is not the only diagnosis to consider.
authors.affiliation: Department of Neurological Surgery, University of California-San Francisco, Moffitt Hospital Room M779, 505 Parnassus Avenue, San Francisco, CA 94143-0112, USA.
authors.forename: Alfredo
authors.initials: A
authors.lastname: Quinones-Hinojosa
authors.forename: Nader
authors.initials: N
authors.lastname: Sanai
authors.forename: Nancy J
authors.initials: NJ
authors.lastname: Fischbein
authors.forename: William S
authors.initials: WS
authors.lastname: Rosenberg
mesh_terms: D000328:Adult; D016080:Arachnoid Cysts / Q000000981:diagnostic imaging / Q000473:pathology* / Q000601:surgery*; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000473:pathology* / Q000601:surgery*; D008279:Magnetic Resonance Imaging; D012720:Severity of Illness Index; D013115:Spinal Canal / Q000000981:diagnostic imaging / Q000473:pathology* / Q000601:surgery*; D013120:Spinal Cord Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000601:surgery*; D014057:Tomography, X-Ray Computed