Arnold-Chiari Malformation

"Chiari malformation" is a term sometimes used instead of "syringomyelia." The modern tendency is to refer to conditions by names that describe them anatomically rather than by honoring a person who studied or discovered them. Dr. Hans von Chiari was a pathologist who, in 1891, categorized different congenital malformations in human infants which generated the condition we now call "syringomyelia." Syringomyelia does not have to result from a congenital malformation but this is its most famous cause. Sometimes the term "Arnold-Chiari syndrome" is used to additionally honor a Dr. Arnold who extensively studied one person's case of syringomyelia.4

Chiari malformations (CMs) are structural defects in the cerebellum, the part of the brain that controls balance. When the indented bony space at the lower rear of the skull is smaller than normal, the cerebellum and brain stem can be pushed downward. The resulting pressure on the cerebellum can block the flow of cerebrospinal fluid and cause a range of symptoms including dizziness, muscle weakness, numbness, vision problems, and with balance and coordination. There are three primary types of CM. The most common is Type I, which may not cause symptoms and is often found by accident during an examination for another condition. Type II, also called Arnold-Chiari malformation, is usually accompanied by a myelomeningocele, a form of spina bifida that occurs when the spinal canal and backbone do not close before birth, causing the spinal cord to protrude through an opening in the back. This can cause partial or complete paralysis below the spinal opening. Type III is the most serious form of CM, and causes severe neurological defects. Other conditions sometimes associated with CM include hydrocephalus, syringomyelia, and an abnormal spinal curvature.1

In syringomyelia, there is altered CSF flow due to any of an assortment of problems (congenital malformation, tumor, trauma, etc.). The altered flow leads to cavities of fluid forming in the spinal cord. This creates a swelling within the spinal cord and since the cord is confined within a bony protective "cage", there is no room for the swelling to expand, which creates pain. At the same time, although this condition causes pain it does not necessarily result in euthanasia.

Picture of the brain and cerebellumn of a calf
Brain and cerebellum

Occipital bone hypoplasia and secondary syringomyelia (SM) is a common condition in the Cavalier King Charles Spaniel (CKCS) that is similar to human Chiari type I malformation. The disease appears to be more severe and have an earlier onset with increased inbreeding, especially when breeding from affected dogs. The family tree of idiopathic epilepsy (IE) appears to be a different subset of the CKCS population, although some overlap was observed. Idiopathic epilepsy is more frequent in lines originating from whole-color dogs. Selection for coat color is believed to have influenced the development of both occipital hypoplasia with secondary SM and IE. In addition, breeding guidelines to reduce the incidence of mitral valve disease have placed further pressures on the gene pool.3

Medications may ease certain symptoms, such as pain, but surgery is the only treatment available to correct functional disturbances or halt the progression of damage to the central nervous system. More than one surgery may be needed to treat the condition. Cranial cervical decompression surgery is associated with low mortality and results in clinical improvement in most dogs. However, clinical improvement may not be sustained and some dogs can be expected to deteriorate.2


  1. NINDS Chiari Malformation Information Page. National Institute of Neurological Disorders and Stroke
  2. Rusbridge C. Chiari-like malformation with syringomyelia in the Cavalier King Charles spaniel: long-term outcome after surgical management. Vet Surg. 2007 Jul;36(5):396-405.
  3. Rusbridge C, Knowler SP. Inheritance of occipital bone hypoplasia (Chiari type I malformation) in Cavalier King Charles Spaniels. J Vet Intern Med. 2004 Sep-Oct;18(5):673-8.
  4. Syringomyelia Mar Vista Animal Medical Center

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