Chiari malformation is diagnosed primarily through MRI, which can reveal whether the lower part of the cerebellum (the cerebellar tonsils) has dropped below the base of the skull and into the spinal canal. The traditional diagnostic threshold is a descent of at least 5 millimeters in adults. But reaching that diagnosis often involves more than a single scan, starting with a neurological evaluation and sometimes requiring specialized imaging to assess how the condition is affecting fluid flow around the brain and spinal cord.
The Neurological Evaluation
The diagnostic process typically begins with a clinical assessment. A neurologist or neurosurgeon will review your symptoms, health history, and perform a physical exam that checks coordination, balance, reflexes, sensation, swallowing, and eye movements. These tests target the specific brain structures that get compressed when the cerebellar tonsils push downward: the brainstem, the cerebellum itself, and the upper spinal cord.
The World Federation of Neurosurgical Societies (WFNS) defines “Chiari Syndrome,” the symptomatic version of the malformation, by two key clinical features. The first is a headache, usually at the back of the head, that lasts less than five minutes and is triggered by coughing, straining, or similar actions that briefly raise pressure inside the skull. The second is any sign of brainstem dysfunction (such as abnormal eye movements, difficulty swallowing, or sleep apnea), cerebellar dysfunction (problems with coordination or balance), or spinal cord dysfunction (muscle weakness, numbness, or sensory changes in the arms or hands). Not everyone with a Chiari malformation has all of these symptoms. Some people have none at all.
MRI: The Primary Diagnostic Tool
MRI is the exam of choice for diagnosing Chiari malformation. It provides a detailed view of the brain, the base of the skull, and the upper spinal cord without radiation. The key measurement is how far the cerebellar tonsils extend below the foramen magnum, the opening at the base of the skull where the brain connects to the spinal cord.
For Chiari Type 1 (the most common form diagnosed in adolescents and adults), the traditional cutoff is tonsillar descent greater than 5 mm in patients over 15 years old, and greater than 6 mm in younger patients. However, this number is not absolute. Some people with less than 5 mm of descent still have significant symptoms, while others with more descent have none. One study found that symptoms become increasingly likely as descent exceeds 12 mm. A small study of 147 adults found an asymptomatic Chiari rate of about 2%, supporting the idea that mild tonsillar descent of up to 5 mm can be an incidental, inconsequential finding.
When Chiari is identified on a brain or cervical spine MRI alone, guidelines from the Congress of Neurological Surgeons recommend additional imaging of the full brain and spine. This helps check for related problems like fluid buildup in the brain (hydrocephalus), a fluid-filled cyst in the spinal cord (syrinx), or a tethered spinal cord.
CSF Flow Studies
In some cases, a standard MRI isn’t enough to determine whether the malformation is actually causing problems. A specialized type of MRI called a phase-contrast cine MRI can measure how cerebrospinal fluid (CSF) moves around the base of the skull. Normally, this fluid pulses back and forth with each heartbeat. In someone with a symptomatic Chiari malformation, the descended tonsils can partially or completely block that flow.
This scan is synced to your heartbeat and produces a moving image showing the fluid’s movement. In a normal scan, you’d see fluid flowing freely in front of and behind the brainstem. In a blocked scan, one or both of those flow pathways are absent or severely narrowed. This test helps distinguish a symptomatic Chiari malformation from one that’s present on imaging but isn’t actually causing trouble, a distinction that matters when deciding whether surgery is appropriate. That said, the evidence on whether cine MRI reliably predicts who will benefit from surgery is mixed. Some studies support its use, while others show it may or may not change surgical outcomes.
Conditions That Mimic Chiari
One of the most important parts of the diagnostic process is ruling out conditions that look like Chiari on imaging but have entirely different causes and treatments. The most significant mimic is spontaneous intracranial hypotension (SIH), a condition caused by a leak in the membrane surrounding the spinal cord that allows cerebrospinal fluid to escape. When CSF volume drops, the brain can sag downward, pushing the cerebellar tonsils into a position that looks identical to Chiari Type 1 on MRI.
SIH is considered underdiagnosed. When it’s mistaken for Chiari, patients may undergo skull base surgery that won’t help because the root cause, a spinal fluid leak, goes untreated. The key clinical clue is a positional headache: if your headache gets significantly worse when you’re upright and improves when you lie down, that pattern points more toward a CSF leak than a true Chiari malformation. Doctors can use contrast-enhanced MRI to look for other signs of low CSF pressure, such as thickened membranes around the brain or engorgement of veins inside the skull, to distinguish between the two conditions.
What Else Shows Up on Imaging
About 25% of people diagnosed with Chiari Type 1 also have a syrinx, a fluid-filled cavity that forms inside the spinal cord. A syrinx develops because the blocked CSF flow at the base of the skull forces fluid into the central canal of the cord itself. It can cause additional symptoms like pain, weakness, and numbness in the arms and hands, and in some cases affects bladder function. Finding a syrinx during diagnostic imaging often changes the treatment conversation, since a growing syrinx is one of the clearer indications that the malformation needs surgical attention.
Beyond a syrinx, roughly 35% of Chiari patients have other structural findings on imaging. These include skeletal abnormalities like fused vertebrae in the neck (Klippel-Feil syndrome) or a flattened skull base (platybasia). These associated conditions help paint a fuller picture of how the malformation is affecting the body and what kind of monitoring or treatment may be needed going forward. When Chiari is subtyped as CM1A, it specifically indicates the presence of a syrinx alongside the tonsillar descent.
Incidental Diagnosis
A significant number of Chiari malformations are discovered accidentally, when someone gets a brain MRI for an unrelated reason like migraines, a concussion, or dizziness. When the tonsils are mildly descended and you have no symptoms that fit the Chiari pattern, the finding is generally considered incidental. In these cases, treatment is rarely recommended. Instead, doctors typically suggest periodic follow-up imaging to check whether the descent progresses or a syrinx develops over time. The presence of a Chiari malformation on MRI alone, without corresponding symptoms, does not automatically mean you have a condition that needs to be fixed.