Disorders of the Peripheral Nerves and Neuromuscular Junction in Dogs

Acquired laryngeal paralysis is common in middle-aged and older dogs, especially in large breeds, such as Labrador Retrievers, Golden Retrievers, and Saint Bernards. In most cases, the cause is unknown, and the disorder is a component of more widespread degeneration of peripheral nerves known as canine chronic axonal degeneration (see below). It can also be caused by an injury or tumor affecting the neck or by thyroid disorders. The condition occurs when nerve degeneration leads to paralysis of one or more cartilages in the larynx ("voice box"). The paralyzed cartilage(s) partially block the airway, causing a voice change, noisy breathing, or a dry cough. In severe cases, the dog may have difficulty breathing, be unwilling or unable to exercise, and the tongue and gums may turn bluish. Some dogs have more general signs of a neurologic disorder, such as weakness and reduced sense of position. Veterinarians diagnose the condition by examining the dog's larynx while the dog is under light anesthesia. Although surgery cannot completely resolve the signs, it can usually relieve the breathing difficulties.

Canine chronic axonal degeneration also affects middle-aged and older dogs. Degeneration of the axons in many peripheral nerves throughout the body results in difficulty walking, incoordination, weakness in all limbs, and muscle wasting. Affected dogs may have laryngeal paralysis as one of the first signs of the disorder. Paralysis of the muscles in the face or difficulty swallowing are also seen. Veterinarians diagnose the condition based on the signs of the disease, electronic tests of nerve speed, and nerve and muscle biopsies. Other diseases with similar signs (such as hypothyroidism) must also be ruled out. There is no specific treatment, and the neurologic signs slowly worsen.

Dancing Doberman disease is a neuromuscular disorder that affects Doberman Pinschers older than 6 months. Initially, dogs repeatedly flex the hip and extend one hind leg while standing. Within several months, most dogs alternately flex and extend both hind legs in a dance-like motion. They often prefer to sit rather than stand. The condition slowly progresses to mild partial paralysis. The front legs are not affected. The cause is unknown. There is no treatment, and signs do not improve. However, this disease usually does not result in severe disability and does not appear to be painful.

Distal denervating disease is a condition that affects multiple peripheral nerves. It is common among dogs in the UK, but has not been reported elsewhere. The cause is unknown. Any age and breed of dog may be affected. The onset of signs varies from a few days to several weeks. There is progressive weakness of all 4 legs, reduced reflexes, and muscle degeneration. Sensory loss is not apparent. Electrodiagnostic testing typically shows loss of nerve control of limb muscles. Peripheral nerve biopsies are usually normal, but examination of nerves within a muscle may be diagnostic. Treatment is supportive, and the prognosis is excellent, with recovery in 4–6 weeks. Relapse has not been reported.

Distal polyneuropathy of Rottweilers is characterized by weakness on one side that slowly progresses to weakness of all 4 legs, reduced reflexes, and muscle loss. Signs can get progressively worse or can wax and wane. Male and female Rottweilers 1–4 years old have been affected. The cause is unknown. Electrodiagnostic testing shows loss of nerve function in muscles toward the end of the limbs and decreased nerve conduction speed. Prognosis is poor, although some dogs may temporarily improve with corticosteroid treatment.

Idiopathic facial paralysis is a common disorder that results in weakness or paralysis of the facial muscles in dogs and cats. Cocker Spaniels, Pembroke Welsh Corgis, Boxers, English Setters, and domestic longhaired cats are at increased risk. There is a sudden onset of an inability to blink one or both eyes, drooping ears, drooping upper lip, and drooling from the corner of the mouth. Facial sensation (controlled by the trigeminal nerve) remains normal. Diagnosis is based on clinical signs and exclusion of other causes of facial paralysis, including ear disease, trauma, and brain stem lesions. The cause is unknown, and there is no specific treatment. Artificial tears often help prevent corneal damage. Partial improvement may occur in a few weeks, but the loss of function often persists.

Acquired myasthenia gravis is an immune-mediated disease of the connections between the muscles and nerves (neuromuscular junction). It is most common in adult German Shepherds, Golden Retrievers, and Labrador Retrievers. Common signs are stiffness (brought on by exercise), tremors, and weakness that improve with rest. Weakness of the face and throat muscles is common, and often there is difficulty swallowing or regurgitation of food after eating. Pneumonia is a frequent complication. Diagnosis requires a blood test. Medications are available for long-term treatment. The outlook for recovery is generally good, but less so for animals that suffer complications. An uncommon form of the disease, called fulminant myasthenia, causes sudden paralysis that quickly leads to respiratory paralysis and death.

Acute idiopathic polyradiculoneuritis causes inflammation of peripheral nerves. Signs often develop 7 to 14 days after a raccoon bite or scratch (leading to the name of Coonhound paralysis); however, not all affected animals have been exposed to raccoons. A similar syndrome can develop in dogs within 1 to 2 weeks of a vaccination. Typically, the hind legs become weak and within 24 to 48 hours the signs progress to partial or full paralysis in all legs and, in some cases, weakness in the face and throat. Occasionally, the front legs are affected first. Typically, muscle wasting is severe within 2 weeks. The dog does not lose its pain perception or bladder and bowel function. There is no effective treatment other than nursing care. Most affected animals begin to improve within 3 weeks, with complete recovery by 2 to 6 months. However, animals with severe signs may not recover completely, and death can occur from respiratory paralysis. Relapses are also seen, especially in hunting dogs that frequently encounter raccoons.

Chronic inflammatory demyelinating polyneuropathy causes gradual weakness of all limbs, with decreased reflexes. The cranial nerves are sometimes also affected. It is fairly common in dogs, but the cause is unknown. Signs can improve with the use of corticosteroids but sometimes return once medications are stopped.

Trigeminal neuritis results in inflammation of and damage to the trigeminal nerve, causing a sudden onset of jaw paralysis. Affected animals cannot close the mouth and have difficulty eating and drinking. Partial paralysis and a loss of sensation in the rest of the face are also possible. The cause is unknown. Signs usually resolve within 3 to 4 weeks. Fluid and nutritional support may be necessary.

Hypothyroid neuropathy can be seen in dogs with a loss of thyroid function (hypothyroidism). Adult dogs, especially of large breeds, are at the most risk. Signs vary widely, and may include partial paralysis, weakened reflexes, loss of paw position sense in all 4 legs, loss of balance, laryngeal paralysis, and regurgitation. In most dogs, more typical signs of a thyroid condition are present, such as obesity and hair loss ( See also heading on page Disorders of the Thyroid Gland in Dogs), but in some cases the neurologic signs are the only signs of illness. Usually, signs resolve within several months of starting thyroid replacement therapy.

Nerve sheath tumors in dogs often arise in the peripheral nerves that extend to the front legs, initially causing weakness and pain in a leg that may be mistaken for a bone or muscle injury. A large tumor may appear as a visible lump. Partial paralysis and muscle wasting eventually develop in the affected leg. If the tumor spreads, it may eventually put pressure against the spinal cord, causing neurologic signs in other legs. Nerve sheath tumors can also form in the cranial nerves, most frequently in the trigeminal nerve. This results in muscle wasting and pain on one side of the jaw. Eventually, the brain stem can become compressed, leading to death. Surgery can be very beneficial at an early stage, but recurrence is common.

In paraneoplastic neuropathy, a cancer outside the nervous system causes damage to nerves. It is most common in dogs with insulinoma but has been associated with a variety of other tumors. This condition is not well understood, but it may be caused by an immune system response to a tumor that indirectly harms the nervous system. Signs typically involve partial paralysis in either 2 or 4 legs that progressively worsens over several weeks. Diagnosis requires identification of the underlying tumor. Signs may improve with successful treatment of the tumor.

Organophosphate poisoning can result from exposure to pesticides, herbicides, or other industrial chemicals. The signs depend on the severity of exposure. The acute form prevents the body’s acetylcholinesterase from working properly. Acetylcholinesterase is an enzyme that is essential for the proper function of connections between neurons, and between nerve and muscle. Signs of severe poisoning can include vomiting, diarrhea, salivation, shortness of breath, muscle tremors and twitching, seizures, or coma.

The intermediate form can cause generalized muscle weakness. Affected animals may not show obvious signs at first, but partial paralysis of the legs and weakness of the neck can develop several days after exposure. The pupils may be dilated. Treatment of acute or intermediate toxicity includes the drug atropine, which blocks the effects of the organophosphate. Other medications are used to relieve the tremors and muscle weakness. Treatment for several weeks may be necessary.

In the delayed form of toxicity, the nerves slowly degenerate. This form is unrelated to the effects on acetylcholinesterase. Signs develop several weeks after exposure and typically involve weakness and loss of motor control in the hind legs. There is no specific treatment.

Tick paralysis is caused by the bite of several species of ticks that results in rapidly progressing paralysis. In Australia, the tick Ixodes holocyclus causes an especially severe form of tick paralysis. Other tick species can induce paralysis on other continents, including North America. Signs begin with partial paralysis in the hind legs that worsens within 24 to 72 hours to total paralysis in all 4 legs. Sensory perception and consciousness remain normal. Difficulty swallowing, facial paralysis, jaw muscle weakness, and respiratory paralysis may develop in severe cases. Treatment consists of removing the tick and applying medication to kill any hidden ticks. For all except Ixodes holocyclus cases in Australia, recovery usually occurs in 1 to 2 days. A serum is available for treatment of Ixodes holocyclus paralysis, but death from respiratory paralysis can occur despite treatment.

Brachial plexus avulsion occurs in dogs due to injury to the spinal nerve roots in the neck and shoulder area that extend nerves into the front legs. In a severe injury, the nerve roots may stretch or tear from their attachment to the spinal cord. Signs vary depending on the severity. If the nerves are completely torn, paralysis of the leg and a loss of sensation and reflexes below the elbow result. The animal puts little or no weight on the leg and drags the paw on the ground. The leg may need to be amputated because of damage from dragging or self-mutilation. Recovery is possible in mild cases in which the nerve roots are bruised but not completely torn.

Peripheral nerve injuries are common in traumatic injuries. The sciatic nerve, which runs from the lower back to the hind legs, may be injured by hip fractures or during surgery to correct a broken leg. Irritants injected in or near the nerve can also cause nerve damage. The leg may be partially paralyzed, or the animal may not be able to flex the knee. The paw and digits cannot flex or extend. There may be loss of sensation below the knee. Injury to the branches of the sciatic nerve in the lower leg, such as the tibial nerve or the peroneal nerve, can result in an inability to extend the paw or flex the digits and reduced sensation over the surface of the foot.

For function to return after nerve connections are lost, the nerve must regenerate from the point of injury all the way to where it ends in the muscle. Nerve tissue regenerates or heals very slowly. Recovery is unlikely if the severed ends of the nerve are widely separated or if scar tissue interferes with healing. Anti-inflammatory drugs have been used to treat traumatic nerve injuries, although there is little evidence of any benefit. Surgery should be performed promptly in cases in which the nerve has been cut. In cases of injury from a fall or a blunt object, surgical exploration and removal of scar tissue may help. Longterm care consists of physical therapy to minimize muscle wasting and to keep the joints moving. Bandages or splints may be necessary to help protect a damaged limb.

Ischemic myoneuropathy is a condition caused by damage to nerves and muscles that can occur when blood flow to them is blocked, usually due to blood clots. It can be seen in dogs with hormonal disorders (hypothyroidism and Cushing disease), kidney disease, heart disease, and cancer. The muscles and nerves in the hind limbs are most commonly affected. Signs include sudden, painful hind limb weakness or paralysis with a loss of several reflexes. Treament includes pain medications, supportive care, and management of the underlying disease that caused the blockage. Medications that prevent clotting may also be used. Improvement can be seen within 2–3 weeks, but it can take up to 6 months for an animal to recover completely. Unfortunately, many animals do not survive the first episode, and those that do have an uncertain longterm outcome because of serious underlying conditions and increased risk for future blood clots.

Also see professional content regarding disorders of the peripheral nerves.