Fibrous Osteodystrophy (Rubber Jaw Syndrome)
Metabolic problems such as primary hyperparathyroidism, as well as hyperparathyroidism resulting from kidney disease, may cause fibrous osteodystrophy in dogs. This disorder is commonly called rubber jaw syndrome because the jaws become flexible like rubber in the advanced stages of the disease as the bones become too soft.
Excessive production of the parathyroid hormone, which regulates the calcium and phosphorus metabolism in the body, is the cause of primary hyperparathyroidism in dogs. Older dogs are occasionally affected by fibrous osteodystrophy.
Excessive parathyroid hormone circulating in the body for an extended period may steadily leach minerals from the bones. A fibrous connective tissue replaces the lost bone, hence the name fibrous osteodystrophy. The whole skeleton is affected by the disease but the skull bones are the most affected.
Primary hyperparathyroidism causes lameness in dogs as the long bones become deformed or fractured on minimum exertion. Spine too may become fractured under pressure, and this may compress the spinal cord, affecting both sensory and motor functions. The bones in the face may become thickened, damaging the nasal passage and displacing the teeth. The dog may find it difficult to close its mouth as the jaws become thickened non-uniformly. Gums may develop sores that are difficult to heal. On x-rays, the skull bones appear thin and riddled with holes as if they are moth-eaten. In extreme cases, the degeneration of the jawbones makes them as flexible as rubber.
Blood tests may indicate excessive calcium levels in the blood. But they are not conclusive since elevated calcium levels in the blood may result from several other diseases too. Thorough testing, including x-rays, assessment of parathyroid hormone levels and the phosphorous content in the blood, is necessary before a definitive diagnosis is made.
The main focus is on detecting the cause of the excess production of parathyroid hormone and eliminating it. For example, if the overproduction of the hormone is due to a tumor growth in the parathyroid gland, it has to be surgically removed. But such radical intervention results in a sudden dip in the hormone levels in the blood, which can cause a drop in the calcium levels within a day of the surgery. The dog has to be monitored closely during this period and corrective measures should be taken immediately. If the blood levels of calcium continues to remain high or if a relapse occurs after initial improvement, it may indicate either the presence of another tumor or a cancerous growth that has spread to other areas.
Hyperparathyroidism due to Kidney Disease
Kidney disease as well as kidney failure continuing for extended periods may result in high levels of parathyroid hormone in the blood. This condition is more prevalent than primary hyperparathyroidism in dogs. As the kidney disease progresses, phosphates accumulate in the blood, lowering the level of calcium. This acts as trigger for excessive production of parathyroid hormone. The calcitrol form of vitamin D is synthesized by the kidneys. If its production is decreased, it again leads to the elevation of the blood levels of parathyroid hormone.
When hyperparathyroidism is due to kidney disease, the typical symptoms of that disease such as dehydration from vomiting, increased thirst, and frequent urination may be present. Depending on the extent and severity of the kidney disease, the skeletal changes may vary. If the kidney disease has progressed to kidney failure, the fibrous bone formation may be just as severe. Younger dogs are the most affected with abnormal growth and thickening of the facial bones.
Hyperparathyroidism due to kidney disease results in symptoms similar to that of primary hyperparathyroidism too. The whole skeletal system becomes weak, but the jaw bones and the skull bones are the most affected. The jawbones lose calcification and become soft and flexible like rubber. The dog may lose some of its teeth, making chewing of food difficult. It may not be able to close its mouth fully, and it may result in constant drooling. The long bones may be affected by the disease to some extent, resulting in fractures. The dog may develop a stiff gait and lameness too.
Kidney-related hyperparathyroidism is suspected when symptoms of kidney disorders are present in the dog. Lab tests may help in arriving at a diagnosis. They may detect high parathyroid hormone levels in the blood.
Modification of the dog’s diet and supplementation with vitamin D in the form of calcitriol constitute the first line of treatment. There are drugs that help bind the mineral phosphate. If the dog has any form of kidney disease, it has to be treated too. The veterinarian may recommend diets that have low levels of phosphorus. These restrictive diets have to be followed without fail.