At minimum, the cerebellar tonsils are below the foramen magnum. On a drawing or radiological image, it looks like the bottom of the cerebellum sort of slid down into the spinal canal by accident.

(You will recall that it is usually a good idea to keep the brain inside the skull!)

Chiari malformation is rarely associated with malformations in other organs.

Chiari malformation is very often associated with abnormalities of the spinal cord

(especially syringomyelia – a fluid-filled & sometimes expanding space in the cord).

Physical exam may show a myelomeningocele or occipital encephalocele.

Brain stem functions can be compromised (life threatening!)

There are three main versions of Chiari malformation, with varying degrees of severity.

Type 1 is not too bad – only the cerebellar tonsils have come through the foramen magnum.

Types 2 & 3 are worse – In type 2, more of the cerebellum has slid into the spinal canal, and the medulla is displaced downward, also, although in some cases it is still above the foramen magnum. Often, a myelomeningocele (that’s when part of the contents of the spinal cord are on the outside of the body, covered by meninges) is also present & partial or complete paralysis occurs below the level of the myelomeningocele.

In type 3, the displacement is severe & the parts that slid down are also outside of the skull or spinal canal, in an encephalocele. It is the most severe and least common of the three main malformations.

Type 2 is classically referred to as “Arnold-Chiari” malformation.

Type 4 is a recent addition – It is cerebellar hypoplasia. The cerebellum is underdeveloped or fails to completely develop. The pathogenesis of type 4 is probably not related to the other types of Chiari malformation. Both types 3 & 4 are usually not compatible with life.

Type 0 (zero) may be added – it is currently a matter of debate. In type 0, the patient has typical Chiari malformation-type symptoms, but no detectable anatomical changes.

Type 1 is often asymptomatic or sometimes doesn’t present until adulthood. The patients usually develop nystagmus, occipital headaches with straining, ataxia, and eventually spastic paresis (weakness) in one or more extremities.

Types 2 and 3 usually present in infancy with cranial nerve palsies (swallowing difficulties and stridor) and respiratory difficulties – due to the unusual pressure on the medulla which contains the “breathing center” of the brain. Paralysis below the level of the myelomeningocele is also common.

Due to the malformations involved, the malformation is very often diagnosed before birth, and if not, is clear at the time of birth. Some patients are affected more severely than others, even within the same type.

Patient with Chiari malformations often develop hydrocephalus. If the patient has hydrocephalus, then they need a ventriculoperitoneal shunt, as usual. Myelomeningoceles & encephaloceles require surgical interventions.

To address the problems of the malformation itself, a posterior fossa “decompression” is done, meaning that the foramen magnum and arch of the atlas (the first vertebral body) is removed, to provide more space. Despite this intervention, some patients continue to worsen (reason unclear).

Treatment for patients with symptomatic Chiari type 1 malformations generally have positive surgical outcomes, but greater functional deficits prior to surgery often means greater residual deficits after surgery.

Think of Arnold (Schwarzenegger) in the Terminator movie, and imagine that his cyborg brain has dropped down into the spinal canal a bit, due to a bad fight (types 1 & 2).

When the fight gets really bad, his back is ripped open, and he ends up with part of his cyborg spinal cord sticking brain sticking out (type 2), or brain sticking out of his neck (type 3).

The problem is that the foramina (plural of foramen) of Luschka and Magendie don’t form properly, and are basically closed. This means that, for most patients, all of the ventricles enlarge, especially the fourth ventricle.