Blind Puppies – Polymicrogyria (PMG) in Standard Poodles
In 1995, Dr. Tom VanWinkle and his colleagues described a new hereditary disease in standard poodles, polymicrogyria. While the true incidence of the disease is unknown, an increasing number of cases have been reported recently raising concerns that the gene is becoming more common in the breeding population.
What is polymicrogyria?
The normal surface of the brain consists of a series of folds called gyri which gives it the wrinkly appearance that everyone recognizes as a brain. The term gyri is derived from the original Greek word gyros (meaning to turn in a circle) which also gave rise to the words gyrate and gyroscope. In polymicrogyria, the normal folding of the brain during development is disrupted. Instead of forming the normal, large folds, the surface of the brain becomes folded into many (poly-) small (-micro-) folds (-gyria). The brain is not affected uniformly in polymicrogyria. The back of the brain (occipital lobes) are much more severely affected than the front of the brain (frontal lobes).
During the formation of the folds in a normal dog’s brain, the connections between different layers and areas of the brain are also formed, and in polymicrogyria these connections are also disrupted. Thus the affected areas both look and function abnormally. The signs a dog shows with polymicrogyria reflect this uneven distribution of the abnormality in the brain.
What are the symptoms of polymicrogyria?
The most severely affected part of the brain in polymicrogyria, the occipital lobe at the back of the brain, is involved primarily in vision. Thus the most common reported signs of polymicrogyria are vision problems which are often noticeable when the puppy first begins to explore its environment. The mechanics of the eye work. The pupils will constrict and the affected pup will squint and turn away from a bright light. However, the ability of the dog to understand and respond appropriately to what his eye is telling his brain will be disrupted. Thus while they may appear to have some vision, it will not be normal and they may bump into things.
The areas at the front of the brain (frontal lobes) are involved in the sensation of touch and some higher brain functions. Thus affected pups have no trouble knowing when they are touched and may appear to have fairly normal intelligence. Involvement of the other parts of the brain seems to be variable. If the portion of the brain between the front and the back (the parietal lobe) is affected the pup may have coordination problems since this area is involved in movement. The part of the brain that wraps around the side (the temporal lobe) is involved in emotions, learning and memory. If it is affected, the pup may have difficulty with training or with controlling its emotions. This area is also a common site of seizures which can occur in polymicrogyria. Other than seizures, none of these are directly life threatening problems, but they seriously impact the quality of life of the puppy and its relationship with its owner.
Some, but not all, dogs with polymicrogyria also have hydrocephalus (water on the brain). The brain normally has fluid filled spaces within it. In hydrocephalus, these spaces become enlarged with excess fluid. While this most likely reflects another manifestation of the abnormal development of the brain in polymicrogyria, we don’t know why some pups show it and others don’t.
Is this the same as neonatal encephalopathy?
No. Neonatal encephalopathy is a different disease which we recognized in standard poodles about the same time as polymicrogyria was first reported. Though both diseases affect pups at a young age (neonates), neonatal encephalopathy pups do not survive beyond weaning age, whereas dogs with polymicrogyria can live for years. Pups with neonatal encephalopathy are small and weak from birth, whereas pups with polymicrogyria can appear normal early in life. While intractable seizures start around 4-5 weeks of age in neonatal encephalopathy pups, if seizures occur with polymicrogyria, it is later in life.
What else can look like polymicrogyria?
Many diseases can cause blindness, seizures and personality changes. Infections of the brain (encephalitis), low blood sugar, liver shunts, and many others could resemble polymicrogyria, while congenital malformations of the eye could cause blindness without other signs of brain disease. It is important to ensure that some other disease is not causing a puppy’s problems, since some of those can be cured whereas polymicrogyria cannot. Polymicrogyria can be identified on MRI imaging of the brain. If necessary, your veterinarian can refer you to a board certified neurologist who can aid in diagnosing polymicrogyria. A directory to a neurologist near you can be found at www.acvim.org under the “Find a specialist near you” link.
How can I help?
We are currently searching for the gene responsible for this condition. This will permit us to develop a DNA test that can identify carriers of the trait. Breeders will then be able to utilize wise breeding strategies to eliminate the disease and breed healthier dogs.
If you have a litter with a pup you believe might be affected, please contact us. We can help your veterinarian in determining whether or not this is the problem in your pup. In return, we would ask your help in collecting the samples and information necessary to continue searching for the gene responsible for this disease.
Any information provided to us will be kept strictly confidential.
This work is possible through the generous support of the Poodle Club of America Foundation and individual breeders. You can help us move this work forward by contributing to the Neurology Support Fund of the Comparative Neurology Program at the University of Missouri, College of Veterinary Medicine