Chiari 1.5 is a variant of a Chiari 1 (the most common type of Chiari Malformation). A Chiari 1.5 is defined as an “Advanced Form of Chiari 1 Malformation” ^[1] or a more complicated form of CM I with the tonsillar herniation seen in CM I but with the addition of lengthening and inferior displacement of the brainstem and obex being located caudal to the basion-opisthion line [that makes up the foramen magnum.” ^[2]

While many radiologists, and surgeons alike, adhere only to the original four types of Chiari, Chiari 1.5 is now well documented, recognized by the National Institute of Health (NIH), and is quickly becoming an internationally recognized variant.

Why is Chiari 1.5 so important?
Chiari 1.5 is important because it often points to the existence of other pathological factors that should be considered BEFORE decompression surgery is performed, in an attempt to avoid a failed decompression surgery (when the brain fails to become buoyant following decompression and instead falls into the enlarged foramen magnum; ultimately leading to the return of initial symptoms, increased symptoms, and/or further complications).

Diagnosis Requirements: Symptomology; midsagittal MRI indicating at least one herniated tonsil with additional caudal descent of the brainstem through the foramen magnum.

Treatment Options: When a Chiari 1.5, it should be assumed acquired/secondary until all other pathologies are explored and treated BEFORE DECOMPRESSION is considered.

Additional Articles:

• Overview: Chiari Comorbidities & Etiological/Pathological Cofactors
• The Important Questions to Ask Your Neurosurgeon

References:

¹ Kim, In-Kyeong et al. “Chiari 1.5 Malformation : An Advanced Form of Chiari I Malformation.” Journal of Korean Neurosurgical Society vol. 48,4 (2010): 375-9. doi:10.3340/jkns.2010.48.4.375

² “The Chiari Malformations.” edited by R. Shane Tubbs et al., 2nd ed., 9 June 2020.