Chiari, Part 1 of 6: An Introduction
The Chiari malformation, also known as Arnold-Chiari (a misnomer), is actually a series of increasingly severe abnormalities at the base of the skull, in the region of the foramen magnum, involving displacement of brain tissue into the spinal canal (where it does not belong). There are four divisions, designated as Chiari I through IV. Chiari III and Chiari IV are extremely rare, require much different treatment plans, have a much worse prognosis, and are probably not related to Chiari I and II developmentally. They are not further discussed here.
Chiari I and II are separate maladies, related only by the curious anatomic fact that both are associated with descent of the tonsils below the foramen magnum (see below). The treatments are similar for the two conditions, but prognosis and potential complications vary. Chair I cannot progress to Chiari II and vice versa. They are separate problems.
The foramen magnum is the opening at the base of the skull, through which passes the spinal cord, several blood vessels, and several cranial nerves. In the normal situation, the part of the spinal cord which passes through the opening is called the cervicomedullary junction—the transition between the lower brainstem and the upper cervical spinal cord. The cerebellum, also known as the hindbrain, has much to do with coordination of movement. Located at the back of the head (it is within the skull under the hollow you can feel at the top of the back of your neck—an area known anatomically as the posterior fossa), the cerebellum is usually wholly contained within the posterior fossa and does not descend below (that is, it does not pass through) the foramen magnum. In very many people, the cerebellum does descend through the foramen magnum, but never more than about 5 mm, which is considered within the realm of normal. If it descends greater than 5 mm, then a Chiari I malformation is present.
The lowest portion of the cerebellum, i.e., that portion which is able to pass through the foramen magnum, is called the tonsil. The tonsils are paired structures, one left and one right, and usually both are involved. In Chiari I, they may sometimes descend as much as 18-20 mm below the foramen magnum. In general, descending 6-8 mm is considered a borderline Chiari, greater than 8 mm is likely more significant.
Chiari II malformation is a congenital condition always associated with myelomeningocele (spinal bifida aperta), though it is possible to have myelomeningocele with minimal Chiari II. It is a much broader syndrome than Chiari I, with many associated abnormalities both within the brain and within the spine. Chiari I is never associated with myelomeningocele.
Chiari I is slightly more common in women, by as much as 3:1 in some studies. The incidence of Chiari I may be as high as 1 in 1000 people, though most of these will be asymptomatic. Asymptomatic Chiari I malformation is identified more and more commonly in this age of frequent MRIs.
Disclaimer: Nothing in this or any posting on this website should be construed as a recommendation of medical or surgical treatment in any specific case. The information provided in this blog is for general reference only and should not and cannot replaced evaluation by and discussion with a qualified medical professional.
About the author: Edison McDaniels, MD is a board certified neurosurgeon with over 7,000 neurosurgical operations to his credit. He is a graduate of Stanford University School of Medicine and completed his general surgery internship and neurosurgical residency at the University of Minnesota Hospital and Clinic in Minneapolis, MN. He served as chief resident in neurosurgery at both Hennepin County Medical Center and the University of Minnesota Hospital.
He lives and practices in the American midwest. His hobbies include writing fiction, blogging, and baseball.
Next time: The Chiari I Malformation