What is the Chiari 1 Malformation?

The Chiari 1 malformation (CM-1) was described by Dr. Hans Chiari in research articles published in 1891 and 1894.

CM-1 is an uncommon, complex, neurological deformity present at birth that usually causes symptoms later in life.

CM-I can result in a variety of neurological symptoms that together decrease the quality of life.

The illustration on the right shows the Chiari 1 malformation. The cerebellum is in the back lower compartment of the skull. The lower part of the cerebellum hangs down into the upper part of the spinal canal.

A brief review of anatomy is helpful in understanding the Chiari 1 malformation. The brainstem and cerebellum are located in a compartment within the lower back part of the skull called the posterior fossa. The bottom of the posterior fossa is shaped like a funnel and at the bottom connects through a large opening in the skull, called the foramen magnum, to the spinal canal. In the Chiari 1 malformation, the lower two parts of the cerebellum, called the cerebellar tonsils, hang down through foramen magnum into the spinal canal. If this “hang-down”, known as herniation, is severe, the cerebellar tonsils block flow of spinal fluid between the brain cavity and spinal canal, and can compress the brainstem and upper spinal cord.

The side-view MRI to the left shows the cerebellum, brainstem, and upper part of the spinal cord. The cerebellar tonsils (T) are herniated below the opening at the base of the skull and are putting server pressure on the lower brainstem (B). There is a syrinx (S) in the upper spinal cord.

Together, the brainstem, lower cerebellum, and upper spinal cord contain the largest variety of nerve tracts and nerve clusters (nuclei) in the brain. Thus, the Chiari 1 malformation can result in malfunction of a variety of neurological functions leading to multiple neurological symptoms. Not every person affected has the same mix of functions affected. Thus, symptoms one person with CM-1 complains about may be quite different from that in another person. This variable mix of symptoms can often make it difficult for the provider to diagnose the Chiari malformation.

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What causes the Chiari 1 Malformation?

The exact cause of the Chiari malformation is unknown. It is believed that in many, though not all, the CM-I occurs during early embryonic development of the posterior fossa. In most persons affected, the posterior fossa is smaller than normal and causes crowding of the lower brainstem and a shift of the cerebellar tonsils downward into the former magnum.

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Why do people with the Chiari 1 Malformation develop neurological symptoms?

When we cough, chest pressure increases and causes a brief backup of venous blood in the veins of the brain. This sudden “back up” briefly enlarges the brain. Since the brain is surrounded by spinal fluid, which is mostly water, and since water is not compressible, some of the spinal fluid has to quickly get out of the skull otherwise the pressure inside the skull will increase. Fortunately, some of the spinal fluid is briefly be sent down into the spinal canal, which quickly expands and takes the pressure off.

The cerebellar tonsils in the person with CM-I are stuffed into the large opening between the skull cavity and spinal canal blocking the release of spinal fluid. The brain pressure spikes and the person feels like their head feels like it will “explode” or like it will “pop off.” A more chronic dull headache at the back of the head can also occur in CM-I due to the crowding at the bottom of the posterior fossa. Chiari headaches may radiate to the temples or behind one or both eyes.

In some people the crowding caused by the herniation of the cerebellar tonsils and crowding can cause marked compression of the brainstem making it appears atrophic (shrunken), or elongated (stretched). This downward displacement of the cerebellar tonsils is called the Chiari malformation. Although there is a form of Chiari malformation which occurs in children with meningomyelocele, the Chiari 1 malformation more frequently causes problems in adults.

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What are Common Chiari 1 Malformation Symptoms?

Symptoms from Chiari 1 malformation often develop in a slow and insidious manner. Frequently, the symptoms are vague and progress over a number of years. Many people have symptoms for years before they are properly diagnosed. Some patients develop symptoms after a mild or moderate trauma. Examples include falling off a trampoline or a blow to the back of the head.


The most common symptom is a headache. It usually begins at the back of the head and radiates behind one or both eyes, to the top of the head or near the temples. The headache is often described as a pressure sensation but can be heavy, pounding, sharp or stabbing. For many, the pain varies between these and other sensations.

The headache is often made worse by straining activities (known as valsalva maneuvers) such as coughing, sneezing or even laughing or singing. Some patients report feeling as though "my head will explode" or "my head will pop off." Bending forward or looking up can worsen the headache. When the headache is intense, some patients note dizziness, blurring of vision, nausea or a feeling of being in a "mental fog."

Visual Symptoms

A variety of visual symptoms can occur with Chiari including double vision, decreased peripheral vision, seeing flickers of light ("fireflies"), visual loss, blind spots, photophobia (sensitivity to bright lights), spasm of the eyelids (blepharospasm) and jerking of the eyes (nystagmus).

Dizziness & Auditory Symptoms

Dizziness is usually intermittent, often worse with a headache, and more noticeable when changing positions. A sensation of spinning (vertigo), ringing in the ears (tinnitus), poor balance (disequilibrium) and decrease or loss of hearing may also occur.

Difficulty Swallowing & Hoarseness

Difficulty swallowing (dysphagia), either with liquids or solid foods, or a feeling of "catching" in the throat can occur when attempting to swallow. The progression of dysphagia may be rapid and result in aspiration (inhaling fluid or food products into the lungs). Change in voice character and timber are common complaints. Hoarseness is often noted first by a family member or friend. Some patients report the inability to modulate voice when singing or speaking loudly.


Patients may complain of neck or arm pain. Often the pain is worse with exertion, fatigue, or lifting. The pain may be dull and aching, or shooting and stabbing. It is usually present or worse on one side. Along with pain, some patients experience weakness of the hand or arm, or difficulty with fine movements of the fingers. If syringomyelia is present, the pain may be more widespread.


Tingling, numbness, or “pins and needles” sensation often occur in the same areas as the pain. Numbness can progress over months to years and involve the lower extremities and trunk. These symptoms are usually more noticeable if a person has a syringomyelia (also called syrinx).

Problems Walking

Problems walking may be described as unsteadiness or listing to one side. Patients may report falling or bumping into walls or doors. Infrequently, patients may have spasticity (stiffness and jerkiness) of the legs, which occurs more commonly with syringomyelia.

Respiratory, Heart & Abdominal Symptoms

Effects of Chiari Malformation on the respiratory and cardiovascular systems can cause shortness of breath, chest pain, episodes of rapid heart rate (tachycardia), black out spells, and hypertension. Abdominal symptoms may include nausea, abdominal pain, or vomiting.

General & Cognitive Symptoms

These include poor sleep (insomnia), fatigue and depression. Some note problems with memory, thinking, speech and difficulty with word finding. People may report the feeling of a "brain fog" with difficulty in thinking and concentration. Difficulty in finding the right word may occur.

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When do people with the Chiari 1 Malformation develop symptoms?

Persons with Chiari 1 malformation may develop symptoms in childhood, adolescence, middle age or older. Patients most commonly develop symptoms in their 20's or 30's.

The Chiari 1 malformation occurs more often in women than men. There is no particular ethnic or geographic incidence of the disorder. The Chiari malformation can run in some families, but a specific gene causing the malformation has not been identified.

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What is the treatment for the Chiari 1 Malformation?

The number of people with the Chiari 1 malformation has been estimated to be 1 in 1,200. Although, some studies suggest it may be more common than this. Many people with CM-I have no symptoms. When symptoms are mild, medications for occasional headache, relaxation, and other non-surgical treatment is used. An evaluation by a neurologist for other neurological conditions such as idiopathic intracranial pressure, also called pseudotumor cerebri, is important.

When symptoms from the Chiari 1 malformation do not respond to conservative treatments and become intractable, surgery can be considered. The goal of the surgery, often called a posterior fossa decompression, is to relieve the crowding at the bottom of the posterior fossa and upper cervical spinal canal.

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