Spontaneous retraction of cerebellar tonsils after surgery for Arnold-Chiari malformation and posterior fossa cyst.

Magnetic resonance images obtained before and 2 months after surgical decompression for Arnold-Chiari type 1 malformation and an arachnoid cyst revealed retraction of the cerebellar tonsils from the spinal canal into the posterior fossa. This was associated with prominent functional recovery of the patient's ataxia and nystagmus. Such an observation may reflect a benign prognosis. Magnetic resonance imaging is the test of choice for the evaluation of this type of lesion.