Syringomyelia secondary to tonsillar herniation caused by posterior fossa tumors.

Background: A number of patients diagnosed as having syringomyelia secondary to tonsillar herniation caused by posterior fossa tumors has recently been described, although the backgrounds of the affected patients were not studied. In order to determine the possible causative contribution of the tonsillar herniation, the present cooperative study was carried out.

Methods: A retrospective study was performed by five cooperating institutes. Magnetic resonance images (MRIs) of patients with posterior fossa tumors were reevaluated with particular attention to the existence of tonsillar herniation and syringomyelia.

Results: The total number of patients with posterior fossa tumors whose MRIs were reinvestigated was 164. In 24 out of the 164 patients, tonsillar herniation was apparent in the sagittal plane of the MRI. Syringomyelia was observed in five of the 24 cases with tonsillar herniation being a consistent associated finding. None of the individuals demonstrated any sensory disturbance directly corresponding to the syrinx formation. All but one of the five patients underwent surgical removal of the tumor, but no one required surgical treatment for the syrinx itself. Disappearance or shrinkage of the syrinx could be confirmed by postoperative MRIs in three of them.

Conclusions: A simple mechanism of blockage of cerebrospinal fluid flow at the foramen magnum may play an important role in syrinx formation, as evidenced by the unexpectedly high incidence of cervical syrinx along with tonsillar herniation caused by posterior fossa tumors shown in the present study.