What is the Chiari I Malformation?
The Chiari I malformation (CM-I) was well described by Dr. Hans Chiari in research articles published in 1891 and 1894. CM-I 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 I 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 I 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, called the foramen magnum, to the spinal canal. In the Chiari I 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 the normal flow of spinal fluid and 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 I malformation can result in malfunction of a variety of neurological functions and thus lead to multiple neurological symptoms. Not every person affected has the same mix of functions affected. Thus, symptoms one person with CM-I complains about may be quite different from that in another person. This variable mix of symptoms often makes it difficult for the examining clinician to come to the correct diagnosis.
What causes the Chiari I 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. 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.
Why do people with the Chiari I 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. This headache 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 I malformation more frequently causes problems in adults.
When do people with the Chiari I Malformation develop symptoms?
Persons with Chiari I malformation may develop symptoms in childhood, adolescence, middle age or older. The most common age for persons with CM-I to develop symptoms is during third or fourth decade of life (in their 20's or 30's).
The Chiari I 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.
What is the treatment for the Chiari I Malformation?
The number of people with the Chiari I malformation has been estimated to be 1 in 1,200. 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 I 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.