Chiari Malformations (CM) are a group of congenital conditions where the cerebellar tonsils herniate out of the skull into the spinal canal due. As of the 2022 consensus, there are 4 accepted types (0, 1, 1.5 and 2). Type 1 is the most common with the most prevalent theory being its due to a congenitally small posterior fossa (base of the skull). There are acquired tonsillar herniations that can occur later in life due to other conditions like tethered cord, intracranial hypertension etc.
The herniated brain tissue (cerebellar tonsils) puts pressure on the cerebellum and brainstem. It can also partially or fully block cerebrospinal fluid from its normal flow around the brain and spine resulting in secondary conditions such as Syringomyelia and Intracranial Hypertension. Around 13% of people with Chiari also have Ehlers-Danlos Syndrome (mostly the hypermobile type). It is a common cause of headaches in patients with EDS.
Occipital Headache - caused/worsened by straining
Central Sleep Apnoea
Tingling/Numbness in limbs
The gold standard in imaging for Chiari still remains a supine MRI of the brain (example above) or the craniocervical junction. While upright imaging has been discussed in the online-patient communities it still has not become the tool of choice due to poor quality imaging compared to a 3T supine MRI. This has been highly agreed on at the 2022 Consensus Meeting for Chiari and Syringomyelia through worldwide specialists in this condition. Australia also only has 2 upright scanners which a privately paid only. So please don't feel you need or that upright MRIs are the only option for diagnosis as they have been using supine scans for years and even when the upright was investigated it didn't change things.
Having both EDS and Chiari can impact on the treatment for Chiari Malformation as the decompression surgery used can cause/exacerbate Craniocervical Instability.
More research is needed into why these two are linked and how. As there is such a strong and important link between these two disorders.
MRI of Chiari Malformation
Chiari Awareness Ribbon. The Chiari Awareness month is September
Diagnosis and treatment of Chiari malformation and syringomyelia in adults: international consensus document (2022) https://pubmed.ncbi.nlm.nih.gov/34129128/