Elucidating the pathophysiology of syringomyelia

Object. Syringomyelia causes progressive myelopathy. Most patients with syr ingomyelia have a Chiari I mal formation of the cerebellar tonsils. Determi nation of the pathophysiological mechanisms underlying the progression of s yringomyelia associated with the Chiari I malformation should improve strat egies to halt progression of myelopathy. Methods. The authors prospectively studied 20 adult patients with both Chia ri I malformation and symptomatic syringomyelia. Testing before surgery inc luded the following: clinical examination; evaluation of anatomy by using T -1-weighted magnetic resonance (MR) imaging; evaluation of the syrinx and c erebrospinal fluid (CSF) velocity and flow by using phase-contrast cine MR imaging; and evaluation of lumbar and cervical subarachnoid pressure at res t, during the Valsalva maneuver, during jugular compression, and following removal of CSF (CSF compliance measurement). During surgery, cardiac-gated ultrasonography and pressure measurements were obtained from the intracrani al, cervical subarachnoid, and lumbar intrathecal spaces and syrinx. Six mo nths after surgery, clinical examinations, MR imaging studies, and CSF pres sure recordings were repeated. Clinical examinations and MR imaging studies were repeated annually. For comparison, 18 healthy volunteers underwent T- 1-weighted MR imaging, cine MR imaging, and cervical and lumbar subarachnoi d pressure testing. Compared with healthy volunteers, before surgery, the patients had decrease d anteroposterior diameters of the ventral and dorsal CSF spaces at the for amen magnum. In patients, CSF velocity at the foramen magnum was increased, but CSF flow was reduced. Transmission of intracranial pressure across the foramen magnum to the spinal subarachnoid space in response to jugular com pression was partially obstructed. Spinal CSF compliance was reduced, where as cervical subarachnoid pressure and pulse pressure were increased. Syrinx fluid flowed inferiorly during systole and superiorly during diastole on c ine MR imaging. At surgery, the cerebellar tonsils abruptly descended durin g systole and ascended during diastole, and the upper pole of the syrinx co ntracted in a manner synchronous with tonsillar descent and with the peak s ystolic cervical subarachnoid pressure wave. Following surgery, the diamete r of the CSF pas sages at the foramen magnum increased compared with preope rative values, and the maximum flow rate of CSF across the foramen magnum d uring systole increased. Transmission of pressure across the foramen magnum to the spinal subarachnoid space in response to jugular compression was no rmal and cervical subarachnoid mean pressure and pulse pressure decreased t o normal. The maximum syrinx diameter decreased on MR imaging in all patien ts. Cine MR imaging documented reduced velocity and flow of the syrinx flui d. Clinical symptoms and signs improved or remained stable in all patients, and the tonsils resumed a normal shape. Conclusions. The progression of syringomyelia associated with Chiari I malf ormation is produced by the action of the cerebellar tonsils, which partial ly occlude the subarachnoid space at the foramen magnum and act as a piston on the partially enclosed spinal subarachnoid space. This creates enlarged cervical subarachnoid pressure waves that compress the spinal cord from wi thout, not from within, and propagate syrinx fluid caudally with each heart beat, which leads to syrinx progression. The disappearance of the abnormal shape and position of the tonsils after simple decompressive extraarachnoid al surgery suggests that the Chiari I malformation of the cerebellar tonsil s is acquired, not congenital. Surgery limited to suboccipital craniectomy, C-l laminectomy, and duraplasty eliminates this mechanism and eliminates s yringomyelia and its progression without the risk of more invasive procedur es.