A Chiari malformation occurs when the lower part of the brain, the cerebellar tonsils, is pushed down through the opening at the base of the skull and into the spinal canal, blocking the normal flow of cerebrospinal fluid. This page walks through what a Chiari malformation is, what your symptoms and imaging findings mean, and what to expect from posterior fossa decompression surgery if it is recommended.
Chiari malformation is a structural problem at the base of the skull that crowds the cerebellum and blocks the normal circulation of cerebrospinal fluid.
In a Chiari malformation, the bony space at the back of the skull (the posterior fossa) is too small for the cerebellum it is meant to hold. This crowds the cerebellum and pushes its lower tips, the cerebellar tonsils, down through the foramen magnum, the natural opening where the spinal cord exits the skull. The herniated tonsils then act like a cork in a bottle: cerebrospinal fluid (CSF), which normally moves back and forth between the brain and spine with each heartbeat, can no longer flow smoothly past them.
This blocked, turbulent CSF flow is what produces most Chiari symptoms, and over time it can push fluid into the spinal cord itself, forming a fluid-filled cavity called a syrinx (syringomyelia), which occurs in roughly 65% of people with Chiari I. Symptoms are notoriously variable and do not closely track the size of the herniation on MRI: some people with large herniations have no symptoms at all, while others with only a few millimeters of herniation have significant symptoms. Because of this, and because the symptoms can mimic many other conditions, Chiari is frequently misdiagnosed or diagnosed later than it should be.
Treatment depends entirely on symptom severity and trajectory, not on the herniation measurement alone. Mild or stable cases are often simply monitored. When symptoms are moderate to severe, worsening, or a syrinx is present, posterior fossa decompression surgery, enlarging the space at the back of the skull to relieve the crowding and restore CSF flow, is the standard treatment.
There is no blood test for Chiari. Diagnosis rests on your symptoms, a neurological exam, and MRI, sometimes including a specialized "cine" MRI that shows CSF flow in motion.
The amount of tonsillar herniation seen on a scan does not reliably predict symptoms. Your actual symptoms and how they are changing over time matter more than the millimeters on the MRI.
Chiari can occur alongside syringomyelia, scoliosis, tethered cord syndrome, and connective tissue disorders like Ehlers-Danlos syndrome, which is why evaluation looks beyond the skull alone.
Because Chiari can present in childhood or adulthood and can involve the spine as well as the brain, care is coordinated across neurosurgery, orthopedics, and other specialties over time.
Chiari symptoms come from four overlapping mechanisms: crowding of the cerebellum, compression of the brainstem, blocked CSF flow, and, if a syrinx forms, injury to the spinal cord itself. Select a category to see typical symptoms.
Chiari is classified by how much tissue herniates through the foramen magnum and whether it is associated with other birth defects. Select a type to learn more.
Whether Chiari is watched, treated with medication, or addressed surgically depends on several factors working together, not on any single measurement.
Select a category above, then tap any factor to read more.
If surgery is recommended, here is the typical path from diagnosis through long-term follow-up. Select each step to learn what happens and why.
Outcomes depend on symptom type, how long symptoms have been present, and whether permanent nerve or spinal cord injury has already occurred before treatment.
Exertional headache and neck pain, along with most brainstem signs (swallowing problems, facial pain or numbness, voice changes, tinnitus, eye problems, dizziness), tend to respond well and relatively quickly to decompression. Sleep problems, memory and concentration issues, and spinal cord signs from a syrinx (numbness, tingling, weakness) tend to improve more slowly and may not fully return to normal, particularly if they have been present for a long time before surgery. A syrinx often shrinks and drains on its own once CSF flow is restored, without needing a separate shunt.
General surgical risks: bleeding, infection, blood clots, reactions to anesthesia, and, rarely, stroke or death.
Specific to decompression surgery: cerebrospinal fluid (CSF) leak from the incision, which may need a pressure dressing or, occasionally, surgical repair; pseudomeningocele (a fluid collection under the neck tissues that usually resolves on its own); and, rarely, nerve or brain injury causing permanent disability.
Cerebellar ptosis (excessive settling of the cerebellum from removing too much bone) is a rare but recognized complication of overly aggressive decompression. Headache and neck pain from the incision itself are common in the first several weeks and are expected, not a sign of a poor outcome.
Brown Neurosurgery's Center for Surgical Treatment of the Developing Brain and Spine treats Chiari as one part of a broader picture of how the brain, spinal cord, and skull move and grow together, not as an isolated bone problem.
Our Chiari and tethered cord program, directed by Dr. Petra Klinge, is built around a research question that shapes clinical care directly: how does the brain and spinal cord's attachment to the rigid spine and skull, through structures called the myodural bridges and the filum terminale, contribute to Chiari and to tethered cord syndrome. Our CSF Disorders Laboratory has demonstrated the biomechanical role of these structures using in-vivo ultrasound imaging, work that informs how we evaluate patients whose symptoms don't fit a purely textbook picture.
Because Chiari can affect thinking, mood, and attention, not just headaches and coordination, our program has studied cerebellar cognitive-affective syndrome in both children and adults with Chiari, including how cognitive and emotional symptoms change after decompression surgery. Patients with joint hypermobility or a known connective tissue disorder such as Ehlers-Danlos syndrome receive additional evaluation for craniocervical instability, since our research group has specifically studied the relationship between Chiari, EDS, and spinal cord motion.
Care is delivered through the Center for Surgical Treatment of the Developing Brain and Spine at Rhode Island Hospital and Hasbro Children's Hospital, where a multidisciplinary team, including orthopedic and general surgery, meets every Friday to review diagnoses, cases, and surgical planning together, and follows patients from childhood through adulthood as their needs change.
Chiari care at Brown is led by the Pediatric Neurosurgery division and the Center for Surgical Treatment of the Developing Brain and Spine, following patients from childhood through adulthood.
See full profiles and the complete team on the Brown Neurosurgery website.