This is the most common form of Chiari malformationAn anatomical abnormality affecting the lowermost part of brain, where it joins the spinal cord, at the top of the neck. Various sub-types are described - see under their individual names. The term hindbrain hernia is sometimes used to incorporate all forms. that is seen in adults, where the tonsils of the cerebellumThis word means the “little brain” and refers to a large part of the brain that sits in lowermost part of the skull, at the back of the head, immediately above the top of the spine. This part of the skull is known as the posterior fossa. protrude through the foramen magnumThe large, ovoid opening at the base of the skull, measuring about 3 x 3.5cm, through which the spinal cord passes, to continue on as the brain stem. but the position of other structures at the base of the brain is normal.