Arnold–Chiari malformation is a complex malformation of the caudal brain stem and cerebellum, and typically consists of herniation of cerebellar tissue through the foramen magnum into the cervical spinal canal. It may be associated with spina bifida, hydrocephalus, or meningomyelocele. It is rare in domestic animals, and the cause is unknown. (A more common variation of this disease is recognized in dogs and appears to be inherited, but this has not yet been proved.) In calves, it may be seen with bilateral elongation and extension of the occipital lobes.
Cerebellar hypoplasia has been described in many species. In utero viral infection (bovine viral diarrhea [see Intestinal Diseases in Ruminants: Bovine Viral Diarrhea and Mucosal Disease Complex], bluetongue virus [see Bluetongue], and swine fever virus [see Classical Swine Fever]) during midgestation is the most common cause. Cerebellar lesions may also be seen with bovine fetuses infected with Akabane or Wesselsbron viruses. Clinical or subclinical hydranencephaly and arthrogryposis may accompany the cerebellar disease. The pathologic features include destruction or loss of one or more layers of the cerebellar cortex, particularly the granule and Purkinje cell layers. Prophylactic vaccination of the dam before breeding can prevent the problem. A hereditary cerebellar hypoplasia/dysplasia is seen in Hereford, Shorthorn, Ayrshire, and Angus calves. Cerebellar hypoplasia is present at birth and is nonprogressive, in distinction to the abiotrophies.
Cerebellar abiotrophies have been reported in many species. In abiotrophies, the cerebellar development proceeds normally, and the animal remains unaffected for a period of months or even years before cerebellar neurons begin to die off prematurely. This is in contrast to the cerebellar hypoplasias, in which developing cerebellar germinal cells and neurons are destroyed in utero. In Aberdeen-Angus calves, clinical signs of abiotrophy start early and are accompanied initially by seizures. In Arabian foals and Swedish Gottland ponies, the onset of signs is from birth to 9 mo; in Yorkshire and Large White piglets, 1–3 mo; in Holstein calves, 3–8 mo; and in Merino sheep, 3–6 yr. Most abiotrophies are probably inherited (eg, recessive inheritance for affected Hereford cattle and Welsh Mountain and Corriedale sheep), but toxic etiologies should also be considered. The latter include locoweed, methylmercury, and exposure to organophosphates in utero (see Insecticide and Acaricide (Organic) Toxicity: Organophosphates (Toxicity)). Use of trichlorfon during pregnancy can cause a congenital tremor in piglets due to both cerebellar hypoplasia and hypomyelination.
Hypomyelinogenesis congenita, in which myelination is delayed throughout the CNS, can resemble cerebellar disease due to the severe head and body tremor that usually develops. In contrast to pure cerebellar disease, a persistent fine tremor at rest is usually present as well as a marked intention tremor. Newborn lambs, piglets, and occasionally calves are affected. The condition can be associated with in utero infection by viruses such as the virus of Border disease (see Congenital and Inherited Anomalies: Border Disease) or swine fever virus (see Classical Swine Fever) or exposure to trichlorfon (see Insecticide and Acaricide (Organic) Toxicity: Trichlorfon). Affected lambs are often called “hairy shakers.” The condition is inherited in Saddleback and Landrace pigs, and in Jersey and Shorthorn cattle. Signs are usually nonprogressive or may resolve completely if myelination has only been delayed.
Swayback or enzootic ataxia is largely due to copper deficiency, although there may be a familial predisposition. Hypomyelinogenesis can occur in utero, and cause obtundation, blindness/deafness, falling or lying prostrate, and head tremor in lambs. The condition can be prevented by treating affected ewes in pregnancy. Kids, piglets, and perhaps calves may also be affected.
Cerebellar hypoplasia is seen in kittens after in utero infection with feline panleukopenia virus (see Feline Panleukopenia). The condition is nonprogressive, and affected animals may make suitable pets. Diagnosis can be obtained antemortem using MRI. Concomitant hydrocephalus or hydranencephaly may also be seen. Cerebellar hypoplasia has also been reported in Chow Chows and is seen in association with lissencephaly in Irish Setters and Wirehaired Fox Terriers.
A selective hypoplasia of the cerebellar vermis is also seen in dogs, and when combined with hydrocephalus and cyst-like dilatation of the fourth ventricle, the condition has been termed the Dandy-Walker syndrome, which may have a familial basis. The cerebellar vermis may be partially or completely absent. Clinical signs are typical of a cerebellar disorder and include tremors, ataxia, and hypermetria. Occasionally signs of head tilt and circling may also be present.
Cerebellar abiotrophies have been described in a number of breeds of dogs. In Samoyeds and Beagles, the signs are apparent at the onset of ambulation; in Australian Kelpies, Rough-coated Collies, and Kerry Blue Terriers, the clinical signs are seen in puppies from 4–16 wk of age; and in Brittany Spaniels, Old English Sheepdogs, and Gordon Setters, the signs appear in young or mature adults.
Congenital hypomyelination is seen as a familial/inherited disorder in Springer Spaniels, Chow Chows, Weimaraners, and Bernese Mountain Dogs, usually with signs developing around 2–8 wk of age. In the latter 3 breeds, it is often termed dysmyelination because the clinical signs of whole body tremor usually resolve spontaneously with time. The disorder is rare in cats. The diagnosis can be confirmed using MRI. (see Demyelinating Disorders.)
Caudal occipital malformation syndrome and subsequent syringomyelia has been commonly reported in Cavalier King Charles Spaniels, and less commonly in other small breed dogs. The malformation is comparable to the Chiari type-I malformation described in humans, and includes a congenital malformation of the occipital bone, resulting in a crowded caudal fossa and cerebellar herniation at the foramen magnum. The subsequent disruption of CSF flow results in formation of syringomyelia. A large proportion of asymptomatic Cavalier King Charles Spaniels have the caudal fossa malformation. Although the malformation is present at birth, clinical signs often do not appear until later in life. Clinical signs vary, but commonly include paresthesias (eg, face rubbing, phantom scratching of the back of the head), ataxia, and weakness from syringomyelia. Medical management is often not curative, but can be attempted with gabapentin (10 mg/kg, PO, tid) for paresthesias, omeprazole (0.7 mg/kg, PO, sid) for reduction of CSF production, and analgesics for pain management. Surgical decompression via caudal occipital craniectomy is preferred as definitive treatment; however, recurrence rates of 25–47% have been reported.
Last full review/revision July 2011 by Rebecca A. Packer, MS, DVM, DACVIM (Neurology)