Peripheral Nerve Conditions
The peripheral nervous system (PNS) consists of 3 principle cell types: axonal, glial and supporting stromal tissue arranged in long efferent (motor) and afferent (sensory) fibres relaying information to the CNS. Following injury, the peripheral nervous system (PNS) possesses a regenerative capacity, while regeneration is insufficient in central nervous system diseases. The relatively enhanced regeneration of the PNS is in part attributed to the plasticity of Schwann cells,the major class of PNS glial cells. Schwann cells undergo a remarkable transformation in response to injury, promoting repair.3 Nerve grafting is a widely accepted method for treating peripheral nerve injuries. However, the sources of donor nerves are limited. The shortage of nerve sources for transplantation is a serious problem.
Peripheral nerve disorders are rare in dogs. Most nerve problems are due to physical injury from traffic accidents, falls, and gunshot wounds. Damage to a peripheral nerve causes both loss of sensation and loss of function in the muscles that are served by that nerve. If the nerve to the tail is damaged, for example, there will be temporary or permanent loss of the ability to sense pain in the tail area, to wag the tail, or to lift it. In the legs, one peripheral nerve supplies the muscles that flex, while the other supplies the muscles that extend. Car accidents can damage one or both of these nerves, producing either full paralysis of all the leg muscles or a partial paralysis affecting only a specific group of muscles. Pain-response tests can reveal how much sensation has been lost from the area. For example, if a leg is injured, the dog may have pain sensation on one side of the leg but not on the other. Amputation of an affected limb may be necessary; however, a surgical procedure to transpose muscle attachments and produce a stiff but straight leg is sometimes an alternative.
Paralysis is the loss of controlled movement, which may be caused by the inability of the motor nerves to communicate with one or more muscles. Complete paralysis, affecting all of the muscles, rarely occurs in dogs. Partial paralysis affecting the hind quarters is more common. Both types are mainly caused by spinal cord injury. Paralysis can be either temporary or permanent. Spinal cord trauma from a traffic accident or other injury is the most common cause of paralysis. A slipped intervertebral disk pressing on the spinal cord can also cause partial or complete paralysis. Treatment is with intravenous methylprednisolone. Surgery can also be beneficial.
Causes of Paralysis
Certain tick species found in North America carry a poison in their saliva that affects nerves causing tick paralysis. Initial signs include hind limb weakness which develops about a week after a tick bite. Full paralysis follows in 1 to 3 days. The degree of paralysis will vary with the environmental temperature and the amount of neurotoxin in the tick saliva. The toxin from the Australian Ixodes tick is more potent than that carried by North American ticks. Because the neurotoxin is temperature-sensitive, dogs recover faster when they are kept in cool premises. A dog usually begins to improve within a day after the ticks are removed. Australian tick paralysis, however, can be more severe. Dogs may continue to deteriorate for another couple of days after the ticks are removed, and be given hyperimmune antiserum.
Coon Hound Paralysis is seen most often in hunting dogs 1 to 3 weeks after contact with a raccoon. The cause of this North American disease is unknown. Weakness begins in the hind limbs and progresses forward. Even the most severely affected dogs continue to be able to wag their tails. Spontaneous recovery usually occurs within weeks or months.
Myasthenia gravis is a disorder of neuromuscular transmission characterized by weakness of cranial and skeletal muscles. It occurs in three forms. In about 25% of affected dogs, full paralysis and breathing difficulties occur within 3 days. In another 40%, full paralysis develops more slowly; most of these dogs develop a flaccid esophagus and therefore cannot swallow. The remaining 35% of dogs with myasthenia gravis do not develop full paralysis; the paralysis affects only specific areas, such as the face, pharynx, larynx, or esophagus. The most common diagnostic test involves an injection of edrophonium chloride, a drug that prevents the breakdown of neurotransmitters at receptor sites. The drug temporarily increases muscle strength. A more accurate diagnostic serum antibody test is available, but this is less widely used. Affected dogs are injected with the drug neostigmine. The use of other drugs is controversial. Myasthenia gravis persists for months or years, but spontaneous remissions sometimes occurs.
Botulism is a disease caused by potent protein neurotoxins that are produced by Clostridium botulinum bacteria, found in tainted canned food and carcasses. Paralysis, ascending from the hind limbs, occurs within hours or days of eating tainted food. Mildly affected dogs often recover without treatment. In other cases, antitoxin halts the progression of the paralysis, but it does not reverse any nerve damage that has occurred before treatment.
- Thelma Lee Gross. Skin Diseases of the Dog and Cat
- Bernard F. Feldman, Joseph G. Zinkl, Nemi Chand Jain. Schalm's Veterinary Hematology
- Contribution of Schwann Cells to Remyelination in a Naturally Occurring Canine Model of CNS Neuroinflammation. Kristel Kegler, Ingo Spitzbarth, Ilka Imbschweiler, Konstantin Wewetzer, Wolfgang Baumgärtner, Frauke Seehusen
- Storage and allogeneic transplantation of peripheral nerve using a green tea polyphenol solution in a canine model. Ken Nakayama, Ryosuke Kakinoki, Ryosuke Ikeguchi, Tomoyuki Yamakawa, Soichi Ohta, Satoshi Fujita, Takashi Noguchi, Scott FM Duncan, Suong-Hyu Hyon, Takashi Nakamura