The Chiari Malformations 2/6


Chiari, Part 2 of 6: The Chiari I Malformation


Chiari I is very likely an acquired problem, that is, it is not present at birth but develops sometime later. The descent of the tonsils may actually fluctuate by several mm on any given day. Chiari I may also result from increased pressure within the skull itself, perhaps caused by hydrocephalus or tumor. These problems are usually obvious and in such a case the Chiari I will resolve following their treatment and without specific Chiari decompression surgery.

Symptoms associated with Chiari I vary from nothing at all (asymptomatic), to nonspecific headaches, to more ominous clumsiness or tingling in the hands. Occasionally, a person will have trouble with swallowing or gagging when eating, but these are unusual. Idiopathic scoliosis may be caused by Chiari I when advanced syringomyelia is present. Syringomyelia will be discussed in another section of this monograph.

Chiari I is a relatively common diagnosis, though whether or not treatment is necessary in a given situation may be controversial. If there is associated headache, it is usually suboccipital in location (across the back of the head), and worsens with coughing, sneezing, and sometimes straining at stool. The upper neck may be sore.

Headache is very common in the general population and Chiari I is very often asymptomatic. Lots of people have headache, a few people have Chiari I, and occasionally the two groups overlap. This does not mean they are associated and in the absence of other symptoms, the headache should be treated symptomatically (with medications, etc.) without surgical intervention. Very often the headaches will resolve and the Chiari is considered incidental and need not be treated.

To be specific: headache is a very nonspecific symptom and when present with Chiari I does not in and of itself mandate surgical treatment.

Some patients with Chiari I complain of clumsiness in the hands, +/- tingling. On MRI (the definitive imaging study for all forms of Chiari) such patients generally demonstrate crowding at the level of the foramen magnum, as well as decent of the tonsils greater than 5 mm. The degree of crowding is important, as it may indicate a degree of chronic brainstem compression, which can be detrimental over time. In fact, decent of the tonsils without any significant crowding at the level of the foramen magnum is probably not a serious affair.

Pressure on the brainstem is likely responsible for the clumsiness in the hands and, when present, for the trouble swallowing. I have had one teenage patient who was entirely unable to eat or drink without significant gagging over a period of four years. He could not even sip water from a fountain at school. After treating him with a Chairi decompression (described later), these symptoms resolved entirely.

Chronic crowding at the level of the foramen magnum seems to interfere with the normal circulation of CSF at the skull base, creating a differential between the spinal canal and the intracranial space. This differential leads to forces within the spinal cord and brainstem which, over time, can enlarge the normally microscopic central canal. This situation leads to syringomyelia, a potential serious condition in which the function of the spinal cord slowly deteriorates with time.

Note that Chiari I is not associated with spinal abnormalities like tethered cord or myelomeningocele. Nor is hydrocephalus generally present, though it can be and if so should be treated in lieu of the Chiari I. The Chiari I is likely to resolve with effective treatment of the hydrocephalus.

As a rule, Chiari I is not associated with cognitive difficulties. Patients have normal thinking and are not developmentally delayed.

Next time: The Chiari II Malformation


The Chiari Malformations 1/6


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 dispChiari1lacement 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