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Peripheral Neuropathy (Polyneuropathies) in Dogs

Polyneuropathy is a nerve disorder that affects multiple peripheral nerves. Unlike the central nervous system, which has the vertebrae of the spine, and the bone of the skull to protect it, the peripheral nerves are more exposed to the elements that enter into the body and come into contact with the body, so they are more susceptible to physical injury and toxic damage. They are spread over the entire body, and are responsible for conscious, coordinated movement (somatic), for automatic physical responses (autonomic), and for the movement of the digestive system (enteric).

Myelin, the white, fatty, lipid material that acts as an insulator coat (also called a sheath) for some nerve fibers, can be lost through a process called demyelination, a condition that causes the myelin to deteriorate, resulting in electrical signals in the nerves being lost, and impairing function. Or, there may be axonal degeneration with secondary demyelination. Axonal degeneration occurs when the actual nerve fibers deteriorate within the myelin sheath.

Symptoms and Types

• Motor and sensorimotor nerve disorders (automatic movement):
  • Weakness or paralysis in all four legs
  • Weak reflexes, or lack of reflexes (automatic physical responses)
  • Weak to no muscle tone
  • Muscle deterioration (atrophy)
  • Muscle tremors, trembling
• Sensory nerve disorders (pain/pleasure nerve receptors):
  • Spatial disorientation (inability to judge the space around oneself)
  • Weakness to loss of consciousness
  • No muscle deterioration
  • No muscle tremors
• Under-active thyroid gland
  • Paralysis of the voice box
  • Paralysis of the throat/esophagus, affects ability to eat and drink
  • Facial paralysis
  • Dizziness, instability
• Dysfunctioning autonomic nervous system (not under conscious control) :
  • Dry nose
  • Dry mouth
  • Dry eyes - low tear production
  • Slow heart beat rate
  • Lack of an anal reflex

 Causes

• Congenital/inherited
  • Dysautonomia: abnormal functioning of the autonomic nervous system, which causes excessive body fluid output, lack of reflexes, and lack of coordination
• Immune-disease
• Metabolic disease
  • Hypothyroidism (under-active thyroid gland)
  • A tumor in the pancreas, the gland that produces insulin
• Infectious
  • Neospora caninum parasite -- affects the hind legs, possibly with paralysis, deteriorates muscles (atrophy), impairs immune system; parasite is transmitted through infected animal meat (i.e., dog eats animal that has the parasite in its body), through contact with animal feces that has the parasite in it, usually another dog's feces, or soil that still has residual infected feces in it; can also be passed from a pregnant animal to its developing fetus through the placenta
  • Coonhound paralysis (polyradiculoneuritis) -- mainly affects hunting dogs that have come into contact with raccoons that carry the infection; affects the four legs, and the muscles that control barking and breathing
• Cancer medications
• Toxins
  • Thallium -- used in rodent poison
  • Organophosphates -- used in fertilizers and pesticides
  • Carbon Tetrachloride -- used in insecticides
  • Lindane -- used for killing weeds, insects, and lice

Diagnosis

Your veterinarian will perform a thorough physical exam on your dog, taking into account the background history of symptoms and possible incidents that might have precipitated this condition. A chemical blood profile, a complete blood count, an electrolyte panel, and a urinalysis will be used for confirming, or for ruling out any underlying diseases. Your veterinarian may also opt to perform additional blood tests, and a spinal tap, to look for specific disorders.

Chest and abdominal x-rays can be crucial for diagnosing visible peripheral polyneuropathies. X-ray and ultrasound imaging can help to rule out (or confirm) cancer, but the most important diagnostic tool for identifying peripheral neuropathies is electrophysiology – measuring the electrical flow of the body's tissues and cells. An analysis of tissue sample (biopsy) from the muscles or peripheral nerves can provide further information about the disease process your dog is experiencing.