An unexpected finding during a frightening day

You go to the emergency department because of sudden trouble speaking. The medical team activates a stroke alert and orders brain imaging, such as CT, CT angiography, and CT perfusion. The report says there is no acute stroke, blocked major artery, or other immediate explanation—but an addendum mentions that the cerebellar tonsils extend about 8 millimeters below the opening at the base of the skull, suggesting a Chiari malformation.

This can leave you with two very different questions: What caused today’s emergency? And has this abnormality been in my brain all my life?

In many cases, an incidental Chiari finding is a longstanding structural difference rather than the cause of a sudden event. Still, the meaning depends on symptoms, the quality of the imaging, and whether there are related findings.

An incidental finding is something seen on a test that was ordered for another reason. It may deserve follow-up without being responsible for the emergency that led to the scan.

What is a Chiari malformation?

The cerebellum sits in the lower back part of the skull and helps coordinate movement and balance. Its lowest portions are called the cerebellar tonsils. These are brain structures and are unrelated to the tonsils in the throat.

Below the cerebellum is the foramen magnum, the large opening where the brainstem continues into the spinal canal. In Chiari I malformation, the cerebellar tonsils sit lower than expected and extend through this opening.

The word malformation can sound alarming. Chiari I is not a tumor, and it does not mean that brain tissue is growing uncontrollably. It describes anatomy and how structures fit near the skull base.

What does “8 millimeters below” mean?

Radiologists often estimate tonsil position using a line across the foramen magnum, commonly called the McRae line. They measure how far the lowest part of a cerebellar tonsil extends below that line.

A descent of roughly 5 millimeters or more is often used as an imaging guideline for Chiari I in adults. However, measurement alone does not establish whether the finding is causing symptoms. Some people with substantial descent have no symptoms, while others with less descent may have crowding or disrupted flow of cerebrospinal fluid.

The tonsils may also be uneven, with one side sitting lower than the other. This asymmetry can reflect a person’s anatomy, the angle of the scan, or how the measurement was made. It does not automatically mean that one side is being newly pushed downward.

Why MRI may be discussed

CT can reveal low-lying cerebellar tonsils, but MRI provides a clearer view of the cerebellum, brainstem, spinal cord, and surrounding fluid spaces. If further assessment is appropriate, an MRI may help clinicians evaluate:

The exact position and shape of the cerebellar tonsils
Whether the area around the foramen magnum appears crowded
Whether cerebrospinal fluid flow may be restricted
Whether there is a fluid-filled cavity in the spinal cord, called a syrinx
Whether another condition could explain low tonsil position

In selected situations, a specialized flow study called cine MRI may be considered. It is not necessary for every incidental finding.

Was the Chiari responsible for the stroke alert?

Chiari I usually develops as the skull and brain form, so it may exist for years before being noticed. It is commonly discovered when imaging is performed for an unrelated concern. A scan cannot determine exactly how long it has been present, but an incidental Chiari appearance generally does not represent something that suddenly formed that day.

Sudden speech difficulty, facial drooping, one-sided weakness, or abrupt vision loss should still be evaluated as possible stroke or transient ischemic attack symptoms. Chiari is not a typical explanation for an abrupt, isolated change in speech.

A CT-based stroke evaluation that shows no bleeding, major blocked artery, or obvious damaged brain tissue is reassuring, but it does not make every possible neurological cause disappear. Some small or early strokes are better seen on MRI, and temporary symptoms may require evaluation even when initial imaging is negative. Clinicians interpret the scans together with the examination, timing, and medical history.

Could low-lying tonsils have another cause?

Most Chiari I findings reflect longstanding anatomy. Less commonly, cerebellar tonsils can sit low because of another process that changes pressure inside the skull or spinal canal. Examples include a cerebrospinal fluid leak, hydrocephalus, a mass, or certain pressure disorders.

This distinction is one reason clinicians look beyond a single millimeter measurement. The full scan, symptoms, previous imaging, and any associated brain or spine findings all matter.

Symptoms that may fit Chiari

Many people with an incidental Chiari appearance have no related symptoms. When symptoms do occur, a commonly discussed pattern is pain at the back of the head or upper neck that becomes worse with coughing, sneezing, laughing, straining, or bending.

Other symptoms that may prompt further assessment include:

Persistent pressure or pain at the back of the head
Balance problems, unsteady walking, or poor coordination
Numbness, weakness, or altered sensation in the arms or hands
Difficulty swallowing, frequent choking, or unexplained hoarseness
Double vision, unusual eye movements, or persistent dizziness
Sleep-related breathing problems
Symptoms suggesting spinal cord involvement, including progressive hand clumsiness

These symptoms are not specific to Chiari and can have many other causes. Their pattern, duration, progression, and relationship to coughing or straining are often more informative than the measurement by itself.

Does an incidental Chiari always need treatment?

No. Treatment decisions are not based only on how many millimeters the tonsils extend below the skull. Clinicians also consider whether symptoms are likely related, whether cerebrospinal fluid flow is crowded, and whether a syrinx or neurological changes are present.

People without related symptoms may be monitored or may need no active treatment after appropriate review. Surgery is generally considered for selected patients with significant Chiari-related symptoms, neurological problems, obstructed fluid flow, or associated complications. An imaging phrase such as “suggesting Chiari malformation” does not by itself mean surgery is needed.

When to talk to your doctor

Discuss an incidental Chiari finding with a clinician if you have persistent cough-triggered headaches, swallowing trouble, balance changes, limb weakness or numbness, or questions about whether MRI or specialist review is appropriate. New sudden speech difficulty, facial drooping, severe weakness, or other stroke-like symptoms require emergency assessment rather than being attributed to Chiari. This article provides general education and is not a diagnosis or individual medical advice.

When a Chiari Malformation Appears on an Emergency Scan