Low-lying cerebellar tonsils or Chiari I on a brain MRI

What the report words mean

Cerebellar tonsils. The lowest part of the back of the brain, at the bottom of the cerebellum, the part of the brain that controls balance. (MedlinePlus)(NHS Chiari)
Foramen magnum. The large opening at the base of the skull where the brain joins the spinal cord. A report gives how far the tonsils reach below it in millimetres (mm). (Tam 2022)
Chiari I malformation (CM-I). Traditionally, tonsils 5 mm or more below the foramen magnum. A review says this definition is being questioned, but no better one has been agreed and most authors still use it. (Tam 2022) The NHS says it was previously called an Arnold-Chiari malformation. (NHS Chiari)
Low-lying tonsils or tonsillar ectopia. Words often used when the tonsils are only a little below the opening. Some authors use 3 to 5 mm for this range. (Dantas 2020)
Pegged tonsils, impaction or crowding. Words about the shape of the tonsils and whether the fluid space around them is squeezed. A UK review suggests reports comment on these, on the mm of descent and on any cause that can be seen. (Tam 2022)
Syrinx or syringomyelia. A fluid-filled space inside the spinal cord. The NHS says there is a chance of developing one with Chiari I. (NHS Chiari)

How common it is, by age and sex

MRI studies find a Chiari I malformation in about 0.56% to 0.77% of people scanned. In one series of 22,591 people who had a brain and neck MRI, 175 (0.77%) had tonsils more than 5 mm below the foramen magnum. A review puts it at about 0.9% of adults. (Dantas 2020) Another review says more than 1% of people are being diagnosed, and the vast majority are found by chance. (Tam 2022)

In a Norwegian study that scanned 1006 adults aged 50 to 66 from the general population, a Chiari I malformation was found in 2 people (0.2%), one woman and one man. (HUNT MRI 2016) In a US group of 364 people with Chiari I, 76% were women. (Mohd Rosdi 2024)

The tonsils tend to sit higher with age, which a review links to the gradual loss of brain volume over time. (Tam 2022) One study set the limit for normal by age:

Tonsil position below the foramen magnum counted as abnormal, by age (Mikulis, as quoted in Dantas 2020)
Age	Abnormal from
Under 10	6 mm
10 to 30	5 mm
30 to 79	4 mm
80 and over	3 mm

The NHS says Chiari I tends to be present from birth but is normally only found in adulthood, when symptoms develop or when an MRI is done. (NHS Chiari)

When it matters

Many need nothing. The NHS says many people with Chiari I have no symptoms and some are only found after an MRI done for another reason. (NHS Chiari) A review says the vast majority found this way need no treatment. (Tam 2022)
The mm figure is only part of it. How far the tonsils reach does not match symptoms well, and up to 30% of people with marked descent have no symptoms. A review says the mm figure on its own is relatively unhelpful unless it is getting larger over time. (Tam 2022)
Possible symptoms. The NHS lists headaches, usually at the back of the head and brought on by coughing, straining, sneezing or bending over, neck pain, dizziness and balance problems, weakness, numbness or tingling in the arms or legs, vision problems, swallowing problems, hearing changes and feeling sick. (NHS Chiari)
Over time. A review of adults found that people without symptoms tend to do well over time, and those with mild symptoms may improve or stay stable. (Dantas 2020)
Repeat scans and referral. A UK review suggests an MRI of the brain and neck and an assessment by a specialist for any scan labelled Chiari I. (Tam 2022) Another review says there is no agreement on what to do for people without symptoms. (Dantas 2020) Whether another scan or a referral is needed depends on your symptoms and the rest of the report. Your doctor decides.
New or worse symptoms. The NHS says that if you have been diagnosed with a Chiari malformation, contact your doctor if you get new symptoms or your symptoms get worse. (NHS Chiari)
Worry is real. See after an abnormal screening result.

The NHS says to ask for an urgent GP appointment or get help from NHS 111 (your local urgent care line elsewhere) for a headache with vision or eye problems, a headache triggered or made worse by coughing, sneezing, bending down or exercising, or a headache with being sick. It says to call 999 or go to A&E (your local emergency number elsewhere) for a headache with a seizure, numbness or weakness in the body or face, one that started suddenly and is extremely painful, trouble speaking, balancing, walking or remembering, drowsiness or confusion, or loss of vision. (NHS headaches)

Other things a scan may show

The same scan may mention other chance findings, each with its own page: an arachnoid cyst, a pineal cyst, brain shrinkage (atrophy) and white matter changes. A fluid-filled pouch on a nerve root at the base of the spine is covered on Tarlov cyst on a spine MRI.

Who it applies to

Adults told that an MRI of the head or neck, done for something else, shows low-lying cerebellar tonsils or a Chiari I malformation. Children, people already being seen for Chiari or a syrinx, and a scan done because of new weakness, numbness or balance problems are outside this page.

Routine age-based screening

A brain finding on a scan does not replace the routine screenings for your age. See screenings by age and gender.

Where they differ:

Where the line is. Most authors use 5 mm or more below the foramen magnum. (Tam 2022) Others use 3 to 5 mm, 3 mm in children and 5 mm in adults, or limits that fall with age. (Dantas 2020)
How common it is. Clinic MRI series give about 0.56% to 0.77%, (Dantas 2020) a UK review says more than 1% are being diagnosed, (Tam 2022) and a general population study of adults aged 50 to 66 found 0.2%. (HUNT MRI 2016) Studies used different cut-offs, ages and reasons for the scan.
Follow-up. A UK review suggests a brain and neck MRI and a specialist assessment for every scan labelled Chiari I. (Tam 2022) A review of adults says there is no consensus for people without symptoms, (Dantas 2020) and the NHS says you might not need any treatment if you have no symptoms. (NHS Chiari)

Your doctor decides what your result means for you.

Questions people ask

What are low-lying cerebellar tonsils?

The cerebellar tonsils are the lowest part of the back of the brain. On an MRI they normally sit at or just above the foramen magnum, the opening at the base of the skull. A report says they are low-lying when they reach a few millimetres below it. (NHS Chiari) (Tam 2022)

How many millimetres counts as a Chiari I malformation?

The traditional definition is 5 mm or more below the foramen magnum, and most authors still use it. Some use 3 to 5 mm, and one study set lower limits for older ages. There is no single agreed cut-off. (Tam 2022) (Dantas 2020)

How common is it?

MRI studies find a Chiari I malformation in about 0.6% to 0.8% of people scanned, and reviews put it at up to about 1%. In a Norwegian study of 1006 adults aged 50 to 66 it was found in 2 people. (Dantas 2020) (Tam 2022) (HUNT MRI 2016)

Does it cause symptoms?

Many people have no symptoms, and it is often found on an MRI done for another reason. The number of millimetres does not match symptoms well. Your doctor decides what your result means for you. (NHS Chiari) (Tam 2022)

When should I get help for a headache?

The NHS says to ask for an urgent GP appointment or call NHS 111 for a headache made worse by coughing, sneezing, bending down or exercise, and to call 999 or go to A&E for a sudden, extremely painful headache or one with numbness, weakness, confusion or trouble speaking or walking. (NHS headaches)

Low-lying cerebellar tonsils or Chiari I seen by chance on a brain MRI