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out of shape: dr. kim smyth explains congenital brain malformations

  • Dr. Kim
  • Posted by Dr. Kim Smyth on
    Staff Veterinarian and Pet Health Writer of Petplan

While the brain is technically one organ, many parts make up one whole brain. Unfortunately, each of these parts can develop abnormally, leading to congenital brain malformations.  Fortunately, congenital brain malformations are relatively rare in cats and dogs, but I will discuss the different types here.


Congenital malformations of the brain occur in utero due to abnormal development of one or more structures of the central nervous system.  Abnormal skull development often accompanies abnormal development of the structures of the brain.  In severe cases, affected animals may die before birth, but milder cases may show clinical signs of a congenital brain malformation shortly after birth.


Four of the more common congenital brain malformations will be described today.



Hydrocephalus, or “water on the brain” as it is commonly called, occurs when an increased accumulation of cerebrospinal fluid (CSF) collects inside the skull.  Generally, the fluid is found inside the ventricles of the brain, where it accumulates for one of the following reasons:

                increased production of CSF

                decreased absorption of CSF

                obstruction to the flow of CSF

Hydrocephalus is the most common congenital brain malformation we see in our veterinary patients.  It is more likely to occur in small breed dogs with large heads, like Chihuahuas, Yorkies, Manchester Terriers, Boston Terriers, Pekingese, and English Bulldogs. Often affected pets will have dome shaped heads and an open fontanelle (soft spot). 


Affected dogs may suffer from gait abnormalities, seizures, and learning disabilities, depending on the severity of disease.  Treatment may or may not be needed and the decision to treat is based on clinical signs.  Oral medications to decrease CSF production may help moderately affected pets while severely affected pets may require surgery. Hydrocephalus is uncommon in cats. 


Intracranial arachnoid cysts

Arachnoid cysts are focal dilated areas just outside the brain.  These cystic areas are filled with CSF and compress the surrounding brain tissue, causing damage and dysfunction.  These cysts can occur in both dogs and cats, but are more common in dogs.  Small breed dogs are more prone than other dogs.  Clinical signs depend on the location in the brain affected by the cyst.  They vary from gait abnormalities to limb paralysis and may include seizures and behavioral problems. 


Arachnoid cysts may be present without clinical signs—often they are found incidentally.  In these cases, they do not need to be treated.  Pets with clinical signs receive treatments varying from anti-seizure medications to surgery, depending on the severity of their signs.



You know from sci-fi movies and the like that our brains are lumpy, bumpy, highly convoluted organs.  In pets with lissencephaly (otherwise known as “smooth brain”), the folds of the brain are significantly reduced or absent.  Affected pets may develop behavior changes, seizures, and aggression during the first year of their life and they may be hard to train.  Lhasa Apsos, Wirehaired Fox Terriers, and Irish Setters are affected more than other breeds.  There is no treatment for lissencephaly.


Caudal occipital malformation syndrome (COMS)

Otherwise known as Chiari-like malformation, COMS is a malformation of the skull which causes overcrowding of part of the brain.  Crowding results in the altered flow of CSF, which can accumulate in the brain (hydrocephalus) or spinal cord.  It is uncommon in cats, but commonly found in Cavalier King Charles Spaniels, Boston Terriers, Chihuahuas, and Yorkies.  Clinical signs include scratching at the neck or shoulder, neck pain, weakness, and paralysis.  There are currently both medical and surgical options for the treatment of COMS. 

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