What Are Low-Lying Cerebellar Tonsils?
Low-lying cerebellar tonsils occur when the bottom portions of your cerebellum (the part of your brain at the back of your skull that controls balance and coordination) drop down through the opening at the base of your skull where your spinal cord connects. 1
Understanding the Normal Anatomy
Think of your skull as having a large opening at its base called the foramen magnum—this is where your brain connects to your spinal cord. 1 Normally, the cerebellar tonsils (two rounded structures at the bottom of your cerebellum) sit comfortably above this opening. 2
- In most people, the cerebellar tonsils rest about 1 mm above the foramen magnum, though they can range from 8 mm above to 5 mm below this opening and still be considered normal. 2
- About 14% of healthy people have tonsils that extend slightly below the foramen magnum without causing any problems. 2
When Does It Become a Problem?
When the cerebellar tonsils descend 3-5 mm or more below the foramen magnum, this is called a Chiari malformation type I, which can cause symptoms by blocking the normal flow of spinal fluid and compressing the brainstem and upper spinal cord. 1
How This Causes Symptoms
The problem occurs through two main mechanisms:
- Blockage of fluid flow: The descended tonsils obstruct the normal circulation of cerebrospinal fluid (the protective fluid surrounding your brain and spinal cord), creating abnormal pressure changes. 1
- Direct compression: The low-lying tonsils can press directly on your brainstem and cranial nerves at the junction where your skull meets your spine. 1
Common Symptoms When It's Problematic
If low-lying cerebellar tonsils are causing symptoms, the most characteristic complaint is:
- Headaches at the back of your head that worsen when you cough, sneeze, strain, or bear down (anything that increases pressure in your head). 1
- Visual problems including abnormal eye movements. 1
- In more severe cases, symptoms can include neck pain, clumsiness, numbness or weakness in the arms or legs, and breathing irregularities. 3
The Key Distinction
Having low-lying cerebellar tonsils on an MRI scan doesn't automatically mean you have a problem—many people have slightly descended tonsils without any symptoms and never need treatment. 4 If your tonsils are less than 2 mm below the foramen magnum and you have no symptoms, this is almost certainly a normal variation that requires no intervention. 2
However, if your tonsils are 5 mm or more below the opening and you're experiencing characteristic symptoms (especially those strain-related headaches), this represents a true Chiari malformation that may benefit from surgical treatment. 1, 4