There are numerous orthopedic conditions in dogs that are frequently occurring, but certain uncommon disorders like hypertrophic osteopathy are seldom discussed in other veterinary blogs. What is hypertrophic osteopathy in dogs and how is it identified?

Hypertrophic osteopathy (HO) is a paraneoplastic disorder. This means that it occurs secondary to cancer elsewhere in the body. It most often occurs due to a primary lung tumor, but other cancers of the body that causemetastases or “spread of cancer” to the lungs can also cause HO. The exact mechanism of action for hypertrophic osteopathy is unknown, but it is hypothesized that it results from neurologic stimulation coupled with increased blood flow to the bones.

The outer layer of the bones is known as theperiosteum. HO causes aperiostealreaction in the long bones of a dog’s limbs. Periosteal reactions can be the kind that break down bone like with malignant bone tumors such as osteosarcomas, or in the case of HO, reactions can cause excessive bony overgrowth. This overgrowth sometimes occurs in large nodules and can eventually affect all four limbs in these cases.

Eventually, the bony proliferation that is seen with HO can negatively impact blood supply to the bones, and it can also impact the soft tissue that surrounds these areas. HO can cause significant discomfort and pain. Hypertrophic osteopathy mostly occurs in middle-aged to older dogs and should not be confused with hypertrophic osteodystrophy (HOD), a bone disease that causes inflamed growth plates due to fast-growing limbs in young large and giant breed dogs.  

There are multiple types of cancers that have been implicated in cases of hypertrophic osteopathy such as lung, mammary gland, bone, and kidney tumors. Cancer of the gastrointestinal tract can also cause HO. It has also been documented that non-cancerous conditions such as pneumonia and heartworm disease can cause hypertrophic osteopathy1.

Dogs who experience hypertrophic osteopathy may appear lame and begin to limp on the affected leg at first. However, HO can eventually spread to other limbs so it is common for more than one limb to be affected. Limbs may appear swollen and painful, which will make your dog less likely to want to run around and play. Even regular walks may become difficult for him.

You might also notice signs of cancer before your dog develops hypertrophic osteopathy, if at all. Dogs with lung tumors or lung metastases from other cancers may experience coughing, weakness, shortness of breath, or spitting up blood. Chronic vomiting and diarrhea may be a sign of cancer somewhere in the digestive tract, and tumors of the bones, mammary glands, or skin may be noticeable when you pet your pup. If you see a new lump or bump on your dog, bring it to the attention of your veterinarian as soon as possible. Early testing can contribute to early intervention!

Semi-annual check ups with your vet are important, and if your dog has any of the above clinical signs, you may need to schedule an appointment sooner. New lumps and bumps can wait a few days but bleeding and shortness of breath should be evaluated on the same day that they are noticed. Blood work, urine testing, and x-rays are common recommendations for ruling out cancer.

A “negative” or normal test doesn’t necessarily mean that there isn’t cancer elsewhere in the body. However, x-rays are very useful for evaluating your dog’s lungs and limbs. Lung tumors are often single, solitary lesions, and lung metastases can cause grossly abnormal lung patterns. X-rays can also show if the swelling around your dog’s limbs is related to a primary bone tumor or hypertrophic osteopathy.

Treatment will depend on the source of your dog’s illness, and this often means that your dog’s underlying cancer will need to be addressed. Surgery, chemotherapy, and/or radiation will be necessary, but this may be difficult with more advanced stages of cancer. Too often, dogs with HO and underlying cancers are diagnosed too late. This is why early intervention can make all the difference.

Once the cancer or infection has been addressed, hypertrophic osteopathy may reverse on its own. This could take weeks to months, and your dog will need to continue pain medications to keep him comfortable. If HO does not resolve, then the nerve supply to the affected area will need to be cut off. This is known as a unilateral vagotomy, which is performed by a specialty veterinary surgeon.

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Hypertrophic osteopathy is a rare bone disorder that is secondary to an underlying condition such as cancer or metastasis from cancer elsewhere in the body. It causes painful bony overgrowth in the long bones of a dog’s limbs, and the exact cause is unknown. Pain medication is necessary for relief, but ultimately, the underlying disease will need to be treated in order to achieve resolution of clinical signs.

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