What is glycogen storage disease?
Also known as glycogenosis or dextrinosis, glycogen storage disease (GSD) is a group of rare genetic disorders characterised by an inability to metabolise glycogen, which is the stored form of sugar in the body. It affects cats, humans, and horses.
- Typically when the cat eats, the body breaks down the food and blood levels sugar rise. The pancreas secretes insulin, and blood containing glucose enters the cells of the liver (known as hepatocytes) and to a lesser degree the heart, brain, skeletal and smooth muscles.
- When the body runs low on glucose, it can draw on those reserves, converting glycogen back into glucose. Only the liver can supply other organs, but stored glycogen in the heart, brain, skeletal and smooth muscles can be used internally.
- This conversion from glucose to glycogen occurs due to an enzyme known as glycogen branching enzyme (GBE) which produces a branching structure. Cats with GSD have are unable to obtain glucose molecules from stored glucogen because they have a defect in or absence of the glycogen branching enzyme. This means an abnormal form of glycogen is stored in the tissues which are unable to be released in the form of glucose molecules when needed. This leads to a build-up of stored glycogen within the tissues of the liver, brain, and muscles while blood sugar levels become dangerously low.
Glycogen builds up in the tissues, especially the liver, heart, and muscles. This leads to an increase in size and eventually scarring (cirrhosis) along with organ dysfunction.
Only type IV affects cats, and it is an inherited condition found in Norwegian Forest cats. It is caused by a recessive gene, which means both parents carry the gene (known as GBE 1), but as they only have one copy, do not have the condition themselves. A copy from both parents is necessary for the disease to occur.
Kittens may die in the womb or at birth from hypoglycemia (low blood sugar levels). Those who do survive will develop normally until they are around 5 to 7 months of age when symptoms develop.
Common symptoms of glycogen storage disease include:
- Persistent fever which is unresponsive to treatment
- Muscle tremors
- Bunny hopping gait
- Muscle weakness eventually turning into atrophy (wasting)
- Muscle paralysis
Your veterinarian will perform a complete physical and neurological examination of your cat and obtain a medical history from you.
- Baseline tests: Biochemical profile, complete blood count, and urinalysis which may reveal low blood glucose and/or elevated liver enzymes.
- Ultrasound: To evaluate the liver, kidneys or heart.
- Echocardiogram: An ultrasound of the heart.
A DNA test can confirm the diagnosis.
Unfortunately, there is no cure for glycogen storage disease. This disease is progressive, and most cats will die or be euthanised by 12-15 months of age.
A DNA test is available to determine if cats carry the defective gene. Cats with the gene should not breed; however, as two genes are necessary for the condition to develop, carrier cats can live a full life.