The peripheral nervous system of the dog may be affected by various types of tumors, some of them originating in the nerves, while others occur in the nearby tissues but cause damage to the nerves.
Nerve sheath tumors
These tumor growths are commonly found in the nerves running to the forelimbs, resulting in pain and weakness of the affected limb. If the tumor is large enough, it is usually visible as a lump on the limb, but if it is small, the pain and weakness may be mistakenly attributed to an injury to the muscle or bone.
The limb may have loss of muscle mass, and become partially paralyzed. The tumor may spread too, and if it inflicts pressure on the spinal cord, neurologic symptoms may develop in all the limbs. Tumors can develop on the cranial nerves also, especially in the trigeminal nerve controlling the dog’s jaws, causing pain and loss of muscle mass on the affected side. Spread of the tumor to the brain stem, and the resultant compression, may be fatal to the dog. Surgical removal of the tumor in the early stages may help avoid this eventuality, but the growth may come back.
In this neuropathy affecting the peripheral nerves, the tumor growth is not in the nerve tissue, but it damages the nerves. This condition frequently occurs in dogs which have a type of tumor called insulinoma. Other types of tumors can also cause paraneoplastic neuropathy. Partial paralysis of the limbs is the typical symptom. It can affect all the limbs or just one pair. After the onset of the symptoms, the paralysis progresses steadily over the next few weeks. Why this happens is not clear, but the body’s immune system responses triggered by the tumor growth may be responsible for this damaging effect on the nerves. Identification of the tumor, and prompt surgical or medical treatment, may help relieve the symptoms.