Acquired Chiari malformation type I after atlanto-occipital dislocation: illustrative case.

backgroundChiari malformation type I (CM-I) is characterized by the descent of the cerebellar tonsils through the foramen magnum. In some patients, this condition can be acquired secondarily due to another condition that alters the dynamics of CSF in the foramen magnum, leading to tonsillar herniation. In this report, the authors describe a case of an acquired CM-I occurring 3 years after a traumatic atlanto-occipital dislocation (AOD). OBSERVATIONS: The authors report the case of a 31-year-old male who initially presented with traumatic AOD associated with a craniocervical hematoma and subsequently developed CM-I. MRI revealed herniation of the cerebellar tonsils through the foramen magnum and extensive cervical spinal cord hyperintensity indicative of a presyrinx state. The patient underwent a suboccipital craniectomy, partial laminectomies of C1 and C2, and duraplasty. After surgery, he exhibited progressive improvement, achieving complete neurological recovery. Postoperative MRI demonstrated complete resolution of the cerebellar descent and the signal changes in the cord. LESSONS: The authors describe the case of a patient presenting with CM-I after experiencing an AOD and a circumferential hematoma in the cervicomedullary junction. Although rare, clinicians should consider CM-I in the differential diagnosis of patients who present with delayed neurological symptoms after sustaining traumatic injury to the craniovertebral region. https://thejns.org/doi/10.3171/CASE25473.