Type I Arnold-Chiari Malformation

What Is It?

The Type I Arnold-Chiari malformation is a disease characterized by decent of the cerebellar tonsils below the level of the foramen magnum at the base of the skull. The cerebellum (meaning "little brain") is a part of the hindbrain, sitting just behind and connected to the brain stem, which is important for motor coordination primarily. The most inferior portions are called the cerebellar tonsils. These structures are usually completely within the skull. However, in the Chiari Malformation these structures are lower than normal and are below the level of the foramen magnum, the large hole in the base of the skull through which the brain stem passes and becomes the spinal cord. The Type I malformation is the most simple of the types of Arnold-Chiari malformations. Generally, the decent of the tonsils is the only finding whereas other types can include many other changes.

The decent of the tonsils into this space can crowd the space and compress the structures in the foramen magnum and nearby. The decent can also cause pressure or traction on nerves in the area. All of this can lead to symptoms in many patients.

In some cases of severe malformation, compression of the spinal cord can cause the development of another pathology called a syrinx or syringomyelia. These are fluid filled cavities within the spinal cord which can expand and damage the surrounding spinal cord.

In most cases the cause of Type I Arnold Chiari malformation is unknown. Most are probably congenital, meaning they are present at birth. However, some are thought to be acquired later in life. In particular, things that cause there to be a pressure differential in the cerebrospinal fluid chronically may lead to decent of the cerebellar tonsils. An example would be a lumbo-peritoneal shunt.

What Types of Symptoms Are Typical?

Most Type I Chiari malformations are relatively mild. These can be asymptomatic in some patients. One of the most common symptoms is headache, often described as pressure on the back of the head and top of the neck. This headache is often worsened by coughing or straining.

The decent of the cerebellar tonsils can also compress or stretch nervous system structures in the area, leading to various symptoms. These can include arm and shoulder weakness, soreness or other sensory symptoms, low energy levels, lower cranial nerve dysfunction (impaired gag reflex, facial weakness or numbness, etc.). Most of these more severe neurological symptoms are less common except in very severe cases.

In severe cases in which a syrinx develops in the spinal cord, the sensory and motor symptoms in the arms and legs can be much more severe. It can lead to a spastic paralysis or weakness of the arms and/or legs as well as numbness and tingling.

Because the Chiari malformation is on the midline, usually symptoms are bilateral, meaning they occur in both sides of the body. However, in some cases the symptoms may be more severe on one side or the other.

How Is The Diagnosis Typically Made?

After a patient presents with symptoms typical of Arnold-Chiari malformation and a thorough neurological examination is performed, the malformation is generally confirmed with an imaging study of the brain and cervical spinal cord, most commonly an MRI. The sagittal MRI images are the best for demonstrating the descent of the cerebellar tonsils below the level of the foramen magnum. Special MRI imaging that evaluates the flow of cerebrospinal fluid at the base of the brain may also be used to evaluate the severity of the compression at the foramen magnum.

What Are Some Common Treatments?

If a patient is confirmed to have a significant Type I Arnold-Chiari malformation treatment usually depends on symptoms and the patient's preferences. Asymptomatic patients usually do not require treatment. If symptoms are intolerable, the most common treatment offered by a neurosurgeon is a Chiari decompression (also called a posterior fossa decompression, suboccipital craniectomy or foramen magnum decompression). This treatment aims to open up the foramen magnum to make more room for the descended tonsils. It can be accompanied by enlargement of the dura covering the brain in that area and sometimes resection or shrinking of the descended tonsils. For more information about this procedure see the Chiari decompression page.

Each patient with Type I Arnold-Chiari malformation are different. Each patient should consult their own treating physicians regarding appropriate management options to decide what is best for them.

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Important Note: This site is not intended to offer medical advice. Every patient is different, and only your personal physician can help to counsel you about what is best for your situation. What we offer is general reference information about various disorders and treatments for your education.

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