A Chiari malformation is a
structural defect in which a part of the brain at the rear of the skull bulges through
normal opening (foramen magnum) in the skull where it joins the spinal canal. This
pressure on parts of the brain and spinal cord. It can cause mild to severe symptoms.
most cases, the problem is present at birth (congenital).
There are 4 main types of Chiari
malformations: types I, II, III, and IV. In some cases, you may see type IV referred
as cerebellar hypoplasia and not included as one of the Chiari malformation types.
I is the most common. This article is about type II. In this type, two parts of the
brain (the cerebellum and brainstem) bulge through the foramen magnum.
This problem can happen in newborns
who have spina bifida or spinal myelomeningoceles. A myelomeningocele is when part
the spinal cord and meninges grow outside the body instead of inside.
A common problem with type II malformations is too much spinal fluid in the brain
(hydrocephalus). The extra fluid puts more pressure on the brain. The bones of the
skull expand to a larger-than-normal size.