Question: When a patient suffers from an Arnold-Chiari type 1 brain compression, how should I choose a diagnosis code? Illinois Subscriber Answer: For type 1 Arnold-Chiari malformations, you’ll report G93.5 (Compression of brain). This code could also be used if the notes indicate the patient suffered compression/herniation of the brain/brain stem. Note: This is only for type 1 Arnold-Chiari malformations. If it is a type 2 malformation, you’d choose a code from the Q07.0- (Arnold-Chiari syndrome) family.