Acquired Chiari I malformation (CM1) associated with syringomyelia is an uncommon consequence of intracranial space-occupying or hemodynamic disorders. Arteriovenous malformations (AVMs) are an exceptionally rare cause.

A 14-year-old girl presented with recurrent headaches and progressive right-hand tremor. Magnetic resonance imaging demonstrated a large posterior fossa AVM associated with cerebellar tonsillar descent and extensive C1-T9 syringomyelia. Angiography showed a Spetzler-Martin grade IV AVM involving the vermis and right cerebellar hemisphere, with high-flow fistulous components and prominent superficial and deep venous drainage. Six staged embolization procedures substantially reduced arteriovenous shunting and were followed by marked regression of the syrinx from C1-T9 to C2-T2, without foramen magnum decompression. The residual AVM was subsequently treated by CyberKnife stereotactic radiosurgery (18 Gy). Complete angiographic obliteration was confirmed in 2014. At the latest assessment in February 2025, 14 years after radiosurgery and 11 years after angiographic confirmation of AVM obliteration, follow-up imaging demonstrated complete resolution of syringomyelia, normalization of cerebellar tonsillar position, and no evidence of AVM recurrence.

This 11-year follow-up supports a causal relationship between high-flow posterior fossa AVM, acquired tonsillar descent, and syringomyelia. Treatment of the underlying vascular lesion may reverse these secondary abnormalities without direct Chiari or syrinx surgery.