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Chiari malformation, Type I

See what's happening at the base of the skull.

In Chiari malformation, the lowest part of the cerebellum sits below the opening where the brain meets the spinal cord. Explore it in 3D, free, in plain language.In Chiari malformation, the lowest part of the cerebellum sits below the opening where the brain meets the spinal cord. Explore it in 3D, free.In Chiari malformation, the lowest part of the cerebellum sits below the opening where the brain meets the spinal cord.

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Fig. 1Midline view of Chiari I. The tonsil (06) extends below the dashed line across the skull opening. The tonsils really sit just to either side of the midline; they're shown here where MRI measures them, at their lowest point. Illustration, not a medical image.Chiari I. The tonsil extends below the dashed line. Tonsils shown at their lowest point, where MRI measures them. Illustration, not a medical image.Still image. Chiari I: the tonsil extends below the dashed line. Tonsils shown at their lowest point, where MRI measures them. Illustration, not a medical image.

What is Chiari malformation?

What it is

The lowest part of the cerebellum, the tonsils, extends below the foramen magnum, the opening at the base of the skull. The cerebellum helps control balance and coordination. It's often linked to a lower back part of the skull that is smaller or shaped differently than usual, which crowds the brain. Less often, it develops later in life for other reasons.The lowest part of the cerebellum, the tonsils, extends below the opening at the base of the skull. It's often linked to a lower back part of the skull that is smaller or shaped differently than usual, which crowds the brain. Less often, it develops later in life for other reasons.

Who has it

Most people are born with the skull differences that lead to it. Some never have symptoms; others first notice them as teens or adults.Most people are born with the skull differences that lead to it. Some never have symptoms; others first notice them as teens or adults.

How it's found

Usually with an MRI scan and a doctor's exam, sometimes by chance during a scan for something else. A doctor looks at the scan together with your symptoms.Usually with an MRI scan and a doctor's exam, sometimes by chance.

What it can feel like

Chiari looks different for everyone, and many people with Type I have no symptoms at all.

The classic sign is a headache at the back of the head that comes on after coughing, sneezing or straining.The classic sign is a headache at the back of the head after coughing, sneezing or straining.

Other symptoms people report

  • Neck painNeck pain
  • Dizziness and trouble with balanceDizziness and trouble with balance
  • Numbness or tingling in the hands and feetNumbness or tingling in hands and feet
  • Weakness or trouble with fine hand movementsWeakness or trouble with fine hand movements
  • Trouble swallowing or speakingTrouble swallowing or speaking
  • Breathing or sleep problems, such as sleep apneaBreathing or sleep problems
  • Ringing in the earsRinging in the ears
  • Blurry or double visionBlurry or double vision
  • A curve in the spine (scoliosis), especially in childrenA curve in the spine (scoliosis)

Many people with Type I have no symptoms at all.

Common misunderstandings

"It's just a bad headache."
Chiari is a structural difference that can affect balance, nerves and the flow of spinal fluid, not only headaches.It can affect balance, nerves and the flow of spinal fluid too.
"The measurement tells the whole story."
Doctors traditionally use about 5 mm below the opening as a cutoff. How far the tonsils extend doesn't always match how someone feels. Some people with a clear descent have no symptoms, and some with less than 5 mm do. Doctors also look at the shape of the tonsils, spinal fluid flow, and whether a syrinx is present.How far the tonsils extend doesn't always match how someone feels. Doctors also look at tonsil shape, fluid flow, and whether a syrinx is present.

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