Disorders of the Peripheral Nerves in Cats
Disorders of the peripheral nerves include degenerative diseases, inflammatory diseases, metabolic disorders, cancers, toxic disorders, disorders caused by injury, and blood vessel diseases.
Idiopathic facial paralysis is a common disorder that results in weakness or paralysis of the facial muscles in cats. 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. The cause is unknown. Diagnosis is based on clinical signs and exclusion of other causes of facial paralysis, including ear disease, trauma, and brainstem 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 laryngeal paralysis occasionally occurs in cats. In most cases, the cause is unknown. However, the disorder can also be caused by an injury or tumor affecting the neck or by hypothyroidism. The condition occurs when nerve degeneration leads to paralysis of one or more cartilages in the larynx ("voice box"). The paralyzed cartilage(s) partially blocks the airway, causing a voice change, noisy breathing, or a dry cough. In severe cases, the cat may have difficulty breathing, be unwilling or unable to exercise, and have its tongue and gums turn bluish. Some cats have more general signs of a neurologic disorder, such as weakness and reduced sense of position. Veterinarians diagnose the condition by examining the cat's larynx while the cat is under light anesthesia. Although surgery cannot completely resolve the signs, it can usually relieve the breathing difficulties.
Acquired myasthenia gravis is an immune-mediated disease of the connections between the muscles and nerves (neuromuscular junction). It is uncommon in cats. Common signs are stiffness (brought on by exercise), tremors, and weakness that improves 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. The disorder can be diagnosed with a blood test or by seeing improvement after injection of edrophonium chloride. Medications are available for long-term treatment. The outlook for recovery is generally good, but less so for animals that develop pneumonia. 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. It is rare in cats. 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 cats 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 cat does not lose its pain perception, appetite, 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 cats that frequently encounter raccoons.
Chronic inflammatory demyelinating polyneuropathy is seen in adult cats. The cause is unknown. Partial paralysis slowly spreads to all 4 legs with weakened reflexes. Sometimes, the cranial nerves are also affected. Signs usually improve after treatment with corticosteroids, but relapse may occur when therapy is stopped.
Trigeminal neuritis is common in dogs, but uncommon in cats. The disorder 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.
Diabetic neuropathy is an uncommon complication of diabetes seen more often in cats than in dogs. Signs include weakness, loss of motor control, and muscle wasting. Affected animals often have nerve dysfunction in their lower legs, which results in a flat-footed stance. The condition is thought to be associated with prolonged periods of high blood sugar. Diagnosis requires evidence of diabetes and a nerve biopsy. In some cases, insulin therapy can lead to partial or complete recovery.
Several different types of nerve sheath tumors can be found in animals, but are most common in dogs and cattle. When they occur, they 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 on 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 brainstem 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 has been associated with a variety of 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.
Organophosphates are a common type of pesticide used to protect both animals and plants from destructive insects. Poisoning due to organophosphates occurs in three forms: acute, intermediate, and delayed. Intermediate organophosphate poisoning is especially common in cats, due to exposure to pesticides that contain chlorpyrifos. Often, signs are not obvious initially, but instead cats develop weakness in all 4 legs and an inability to hold up their heads several days after exposure. Cats usually recover after several weeks of drug treatment.
A class of antibiotic drugs used to treat livestock called ionophores can poison cats and other species of animals. For example, in 1995, cat food contaminated with a drug called salinomycin caused sudden neurologic signs in approximately 850 cats. Animals poisoned with ionophores usually recover with supportive treatments and removal of the contaminated food.
Tick paralysis is caused by a tick bite that results in rapidly progressing paralysis. Females of several species of ticks produce a toxin in their saliva that interferes with a cat's nervous system. In Australia, the tick Ixodes holocyclus causes an especially severe form of tick paralysis. Other tick species in other continents can cause tick paralysis in dogs, livestock, and occasionally people. 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 a skin ointment 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 cats 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 bend the knee. The paw and toes 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 or flex the paw or the toes 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, but 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 neuromyopathy is a nerve and muscle disease that occurs due to lack of blood flow to these tissues, usually because of blood clots. It is most common in cats with arterial thromboembolism, a condition that develops secondary to disease of the heart muscle. Blood clots that form within the heart travel through the bloodstream and lodge in the arteries, blocking blood flow to the area. Blockage occurs most commonly at the aorta, resulting in damage to the muscles and nerves in the hind legs. Severely painful partial paralysis develops, and the cat may be unable to flex or extend the leg, lose the knee reflexes, and lose sensation in the lower leg. Diagnosis is based on the history and signs, as well as ultrasound scanning to analyze blood flow to the legs. Treatment includes pain medications and nursing care. Underlying heart disease must also be treated. Neurologic signs can improve within 2 to 3 weeks, but 6 months may be needed for complete recovery. Permanent damage is possible. Health risks persist for many animals because of the underlying disease and high possibility of recurrence. Approximately 60% of cats die or are euthanized early on in the disease.
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