Neuromuscular Disorders in Dogs

Neuromuscular Disorders

• The neuromuscular system in dogs has two components: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves that branch out from the spinal cord to the muscles). Individual peripheral nerves control each muscle. When the brain wants to move a muscle, an electrical signal travels down the nerve to the neuromuscular junction---or a gap between the nerve and the muscle. The nerve releases the chemical acetylcholine, which crosses the gap and attaches to receptors on the muscle, causing the muscle to contract. Neuromuscular disorders occur when a disease or infection damages the signal pathway. The signal may not travel correctly from the brain to the nerve (neuropathy, or disease of the nerve), the signal may fail to jump across the neuromuscular junction (junctionopathy, or disease of the neuromuscular junction) or the muscle may not respond (myopathy, or disease of the muscles).

Symptoms

• A dog with a neuromuscular disorder will display signs of muscle weakness and may be unable to stand or exercise. In mild cases, the weakness may sporadic and may occur with exercise. All four legs may be affected or only the hind legs. Before the dog shows weakness, he may walk differently, such as with a stiff gait, short stride and muscle tremors. In severe cases, the dog may be unable to support her weight or hold her head up. If the chest muscles are affected, the dog will have breathing difficulties. In the most severe cases, the dog will become paralyzed.

Diagnosis

• Diagnosis of neuromuscular disorders consists of ruling out other conditions that may be causing the symptoms. Because the limbs are affected, neuromuscular disorders are often confused with orthopedic diseases. Necessary tests include a complete blood count, including tests for hormonal disorders and infectious diseases, chest and abdominal X-rays and chest ultrasound. Other tests include a spinal tap to detect nerve root inflammation and an electromyogram to test the ability of the muscles to respond to stimulation.

Neuromuscular Disorders in Dogs

• Canine myasthenia gravis, which destroys the acetylcholine receptors on the muscles, is the most common neuromuscular disorder in dogs. Another common cause is polymyositis. Lockjaw, or trismus, is a classic symptom of polymyositis, although it may not be experienced by all dogs. All except two species of ticks are capable of causing tick paralysis (toxic neuropathy) in dogs. The dog may die from respiratory paralysis, but recovery can occur within three days after tick removal. Inflammatory neuropathy is a progressive condition, with weakness starting in the hind legs. Most dogs recover fully with treatment, but some may be permanently disabled. Degenerative neuropathies cause loss of both motor and sensory sensations. Progress is gradual and there is no treatment.

Treatment

• Treatment depends on the cause of the neuromuscular disorder. Even with treatment, however, recovery is gradual and the dog may be left with lingering disability.