Secondary Chiari-like formation caused by grafted bone overgrowth following occipitocervical fixation: a rare cause of pediatric quadriplegia.

Secondary Chiari-like formation (SCLF) is an acquired herniation of the cerebellar tonsils through the foramen magnum, typically resulting from a craniospinal pressure gradient or reduced posterior fossa volume. We report a rare case of SCLF caused by massive bone overgrowth after occipitocervical fixation (OCF) in a child. An 8-year-old boy with Klippel-Feil syndrome and basilar invagination developed quadriparesis after OCF. His symptoms worsened following repeat OCF performed due to misdiagnosis. Four months later, the quadriparesis progressed further. Serial imaging demonstrated foramen magnum crowding caused by a hypertrophic graft, along with cerebellar tonsillar herniation. Foramen magnum decompression, performed 10 months after the second surgery, led to rapid motor improvement, allowing independent ambulation within 3 weeks. This case highlights massive graft overgrowth as a unique mechanical etiology of SCLF. Vigilance and careful comparison of serial craniovertebral junction imaging are crucial in pediatric patients with neurological deterioration after OCF.