Peripheral Neuropathy
Dr. Betsy Dayrell-Hart

Peripheral neuropathy is less commonly diagnosed in dogs and cats than is
spinal cord or brain disease; perhaps it goes unrecognized or is mistaken for
disease of other systems. Peripheral nerve disease can be categorized as a
mononeuropathy (a single nerve is affected) or polyneuropathy (multiple
nerves are affected). Further, problems related to peripheral nerves can
involve sensory, autonomic, or motor segments. In this article,
manifestations of peripheral motor nerve disease will be addressed.

Motor neuropathy is characterized by paresis, flaccidity, depressed or
absent stretch and flexor withdrawal reflexes, and neurogenic muscle atrophy.
Immature animals suffering from peripheral nerve disease frequently develop
abnormal joint and limb deformities and contracture of muscles and tendons,
and abnormal growth. Animals with motor neuropathy are often profoundly
weak, and cannot support weight without assistance, but when held and
supported, may have normal proprioceptive and postural reactions of the
limbs. Sensory abnormality may accompany motor nerve disease, but frequently
sensation remains normal. In response to skin-pinch tests, these animals may
vocalize or otherwise indicate that they perceive pain, but are unable to
withdraw the stimulated limb. When motor nerve disease affects the distal
extremities more than proximal limb or torso muscles, animals often crawl,
keeping joints flexed, and advancing limbs from the shoulder or hip without
extending carpal or tarsal joints.

Motor neuropathy is most often seen affecting one or more limbs, but can a
ffect cranial nerves, eg., III, IV, VI, VII, IX, X, XIII - causing signs of
pupil or eye position abnormality, facial paralysis, voice change, coughing,
gagging or swallowing disorders, or megaesophagus. Dysuria occasionally
results from peripheral nerve disease, and may involve detrusor or sphincter
muscles, or both.

Motor nerve mono - or polyneuropathy can be further characterized
according to location and etiology. For example, the cranial neuropathy most
often seen in small animals is facial nerve paralysis (cranial nerve VII
palsy). Animals with peripheral VII disease (unilateral or bilateral) are
unable to close the eyelid or have a drooping lip, but do not have postural
or proprioceptive deficits, paresis, or altered reflexes in their limbs.
Sialosis and dropping of food when eating are commonly seen. This neuropathy
frequently occurs bilaterally, and often one side of the face will be
affected days or weeks before the other. As recovery occurs, eyelid function
may return to normal, and the formerly drooping lip may become contracted. A
pure cranial nerve VII palsy rarely results in abnormality of the cornea; if
a corneal ulcer is discovered, or if the animal seems to lack corneal
sensation, it should be carefully examined for polyneuropathy. Seventh nerve
palsy is accompanied by lack of tear function only in rare instances. Causes
of facial nerve palsy include endocrinopathy (eg., hypothyroidism, Cushing’s
disease), middle ear disease (infection, tumor, trauma), or inflammation
(viral, bacterial, fungal, protozoal, other) and there is also idiopathic
facial palsy seen in dogs, where no underlying cause is found. In these
dogs, signs often resolve without treatment. If underlying disease is
discovered and treated, nerve function usually returns. Even without return
of nerve function, most animals will adapt to lack of ability to close the
eyelid, and drooling and food-dropping eventually abate. Disorders of
swallowing, gagging, laryngeal movement, and esophageal motility usually are
due to peripheral motor nerve dysfunction without sensory abnormality. The
most common of these neuropathies is laryngeal paralysis (unilateral or
bilateral). Laryngeal neuropathy has been reported as an inherited disease
in several breeds, as well as a result of trauma, endocrinopathy,
intoxication, compression, inflammation, or other disease of the recurrent
laryngeal nerve (a branch of the vagus nerve). Megaesophagus is a common
problem in dogs, but is less often caused by pure peripheral nerve (vagal)
disease than by disease of muscle or of motor end plate (myopathy or
myasthenia gravis).

It should be noted that in all instances, when peripheral disease of the
cranial neves is present, the patient is normal in other respects - alert,
able to walk, and lacking deficits in proprioception, postural reactions or
reflexes. If cranial neuropathy is accompanied by changes in consciousness,
respiratory pattern, heart rate, or sensory function, a central disease of
the brainstem should be considered.

Motor nerve disease affecting limbs can also be categorized as to nerve
distribution and etiology. In general, degenerative, inflammatory, toxic,
endocrine, and paraneoplastic neuropathies affect the animal symmetrically,
and often the hind limbs show signs before forelimbs. For example, botulism,
tick paralysis, diabetic neuropathy, hypothyroid neuropathy, acute canine
idiopathic polyradiculoneuropathy (ACIP or of “Coonhound Paralysis”),
congenital and inherited neuropathy, paraneoplastic neuropathy associated
with insulinoma, lead toxicosis and antineoplastic drug-induced neuropathies
tend to affect both hind limbs, and progress symmetrically. By contrast,
traumatic neuropathy (avulsion, transections from bone fracture or other
injury, injection neuropathy, lymphosarcoma of nerve roots or vertebrae often
affect nerves of only one limb, or of a portion of the limb. When diffuse or
multifocal peripheral nerve disease is present, inflammatory disease
(neosporum caninum, distemper, fungal disease, “allergic” neuropathy) or
diffuse neoplasia (lymphoma, malignant histiocytosis) may be more likely than
the differentials above, although differential diagnoses may include all of
the above.

Electrodiagnostics (EMG, nerve conduction velocity tests, repetitive
stimulations) can help to define whether and which muscles are denervated,
whether some or all nerve fibers are affected, and whether the disease
affects myelin, axons, or both. Further, biopsy of muscle and nerve can
assist in characterizing the disease. Peripheral nerve diseases can be
benign, in that with time and treatment, many animals will recover from them.
This is most true of inflammatory and toxic neuropathies. When diffuse
disease is present, and when it involves respiratory muscles or causes
megaesophagus, pneumonia may complicate recovery. Appropriate monitoring and
treatment of these animals will require intensive nursing care in the
hospital. Recumbent animals are also susceptible to development of decubital
sores and secondary infections of skin, and to urinary tract infections.
Recovery time from peripheral motor neuropathy is variable, but may be
prolonged. Owners of animals with these problems may need encouragement to
continue treatment. Physical therapy may be of great benefit, to help avoid
complications listed above, and in assisting the animal to regain strength.

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