Congenital<unk>  I<unk> , defined as tonsilar herniations of 3 to 5 mm or greater, was previously believed to be in the range of one per 1000 births, but is likely much higher. Women are three times more likely than men to have a congenital<unk> . Type II<unk> s are more prevalent in people of Celtic descent. A study using upright MRI found cerebellar tonsillar ectopia in 23% of adults with headache from motor-vehicle-accident head trauma. Upright MRI was more than twice as sensitive as standard MRI, likely because gravity affects cerebellar position. Cases of congenital<unk>  may be explained by evolutionary and genetic factors. Typically, an infant's brain weighs around 400g at birth and triples to 1100-1400g by age 11. At the same time the cranium triples in volume from 500<unk> 3 to 1500<unk> 3 to accommodate the growing brain. During human evolution, the skull underwent numerous changes to accommodate the growing brain. The evolutionary changes included increased size and shape of the skull, decreased basal angle and basicranial length. These modifications resulted in significant reduction of the size of the posterior fossa in modern humans. In normal adults, the posterior fossa comprises 27% of the total intracranial space, while in adults with<unk>  Type I, it is only 21%. H. neanderthalensis had platycephalic (flattened) skulls. Some cases of<unk>  are associated with platybasia (flattening of the skull base).
Chiari malformation
Glial tumor
Central nervous system cyst
Dandy–Walker malformation
Arachnoid cyst
Subdural hygroma
Cavernous venous malformation
Meningism