Percutaneous thecoperitoneal shunt for syringomyelia. Report of three cases. Between January and April, 1990, three consecutive cases of syringomyelia were treated by percutaneous placement of thecoperitoneal shunts. Two of these patients had undergone craniovertebral decompression earlier at other centers and the third was treated primarily by a thecoperitoneal shunt. In each case, the syrinx was associated with Chiari I malformation, although the clinical presentation was due to a myelopathy. All three patients obtained unequivocal benefit from this simple procedure. Postoperative magnetic resonance images showed considerable shrinkage of the cysts corresponding with clinical improvement. 