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Chiari malformation is a structural defect where brain tissue extends into the spinal canal due to abnormal skull development. Type I is the most common form in adults, while Types II-IV are more severe and often associated with spina bifida or other congenital anomalies.
The malformation occurs when the posterior fossa is too small or misshapen, forcing the cerebellar tonsils to herniate through the foramen magnum. This creates obstruction of cerebrospinal fluid flow, leading to increased intracranial pressure and potential syringomyelia (fluid-filled cyst in the spinal cord).
Diagnosis requires high clinical suspicion as symptoms can be subtle and non-specific, including occipital headaches worsened by Valsalva maneuvers, neck pain, and neurological deficits. MRI is the gold standard for diagnosis, measuring tonsillar herniation below the foramen magnum, with surgical decompression considered for symptomatic patients with significant herniation.