PATHOPHYSIOLOGY OF SYRINGOMYELIA ASSOCIATED WITH CHIARI I MALFORMATION OF THE CEREBELLAR TONSILS - IMPLICATIONS FOR DIAGNOSIS AND TREATMENT

The mechanisms previously proposed for the progression of syringomyeli a associated with Chiari I malformation of the cerebellar tonsils are controversial, leave many clinical observations unexplained, and under lie the prevalence of different operations currently used as initial t reatment. To explore the mechanism of syringomyelia progression in thi s setting, the authors used anatomical and dynamic (phase-contrast and phase-contrast cine) magnetic resonance (MR) imaging, and intraoperat ive ultrasonography to examine the anatomy and dynamics of movement of the cerebellar tonsils, the wall of the spinal cord surrounding the s yrinx, and the movement of cerebrospinal fluid (CSF) and syrinx fluid at rest, during the respiratory and cardiac cycles, and during Valsalv a maneuver in seven affected patients. In all patients the cerebellar tonsils occluded the subarachnoid space at the level of the foramen ma gnum. Syringomyelia extended from the cervical to the lower thoracic s egment of the spinal cord. No patient had evidence of a patent communi cation between the fourth ventricle and the syrinx on anatomical MR im ages, dynamic MR images, or intraoperative ultrasound studies. Dynamic MR images of three patients revealed abrupt downward movement of the spinal CSF and the syrinx fluid during systole and upward movement dur ing diastole, but limited movement of CSF across the foramen magnum du ring the cardiac cycle. Intraoperative ultrasound studies demonstrated abrupt downward movement of the cerebellar tonsils during systole tha t was synchronous with sudden constriction of the spinal cord and syri nx. Decompression of the foramen magnum was achieved via suboccipital craniectomy, laminectomy of C-1 and C-2, and dural grafting, leaving t he arachnoid intact. Immediately after surgery, the pulsatile downward thrust of the tonsils and constriction of the spinal cord and syrinx disappeared. Syringomyelia resolved within 1 to 6 months after surgery in all patients. Observations by the authors suggest the following pr eviously unrecognized mechanism for progression of syringomyelia assoc iated with occlusion of the subarachnoid space at the foramen magnum. The brain expands as it fills with blood during systole, imparting a s ystolic pressure wave to the intracranial CSF that is accommodated in normal subjects by sudden movement of CSF from the basal cisterns to t he upper portion of the spinal canal. With obstruction to rapid moveme nt of CSF at the foramen magnum, the cerebellar tonsils, which plug th e subarachnoid space posteriorly, move downward with each systolic pul se, acting as a piston on the partially isolated spinal CSF and produc ing a systolic pressure wave in the spinal CSF that acts on the surfac e of the spinal cord. This causes progression of syringomyelia by abru ptly compressing the cord and propelling the fluid in the syrinx longi tudinally with each pulse, and may be responsible for the origin and m aintenance of syringomyelia by the pulsatile pressure waves forcing CS F into the cord through the perivascular and interstitial spaces. Effe ctive treatment occurs when the systolic pressure wave transmitted by the cerebellar tonsils is eliminated by relieving the obstruction to r apid movement of subarachnoid CSF across the foramen magnum. The prese nce of this mechanism can be detected preoperatively on dynamic MR ima ges and during surgery on ultrasound studies by the pulsatile excursio n of the wall of the spinal cord surrounding the syrinx and by its imm ediate disappearance and the expansion of the syrinx during forced ins piration after decompression of the tonsils. Effective treatment is ac hieved with bone and dural decompression of the foramen magnum alone, without entering the arachnoid.