Neuromuscular diseases - challenging diagnostic opportunities

Veterinarians have become increasingly aware of muscle and peripheral nerve disorders in the past decade.

Figure 1: Muscle. Dog. Red O stain. Lipid storage myopathy. (Courtesy of Dr. G. D. Shelton.)

Some diagnoses require physical and neurological examinations, blood and urine analyses, electrodiagnostic tests, radiographic studies, and advanced imaging modalities. However, many neuromuscular diseases have normal or nonspecific serum creatinine kinase and/or EMG changes.

The general practitioner may participate in the diagnosis of many neuromuscular cases. Diagnostic reliability is also dependent on the service of specialized laboratories using current techniques for serological tests, endocrinological assays, and the evaluation of muscle and peripheral nerve biopsies.

Polymyopathy in dogs is generally associated with intermittent weakness and reluctance to move.

Polymyopathy

Muscle atrophy is inconsistent. Pain may be noted on physical examination by palpating various muscle groups. Exercise-associated weakness seen sometimes with polymyopathy may also be present with disorders of the neuromuscular junctions (e.g., myasthenia gravis, myasthenic syndrome, tick paralysis, or botulism). The constellation of muscular and neurological signs represents a real challenge to the clinician. Complementary tests are necessary to evaluate the cause of neuromuscular diseases.

Figure 2: Dog. Normal muscle. (Courtesy of Dr. G. D. Shelton.)

In a recent report (GD Shelton. Proceeding of the 20th ACVIM Forum, 325-327), the author describes the practical approach to the diagnosis and treatment of neuromuscular disorders. Some very useful tests that can be performed in general practices for polymyopathy are the pre- and post-exercise plasma lactates, pyruvates, muscle and nerve biopsies.

In healthy individuals, cellular metabolic energy is generated mainly via the aerobic pathway. In conditions of relative or absolute cellular hypoxia, glycolysis (which does not require oxygen) allows energy production to continue. Glycolysis produces pyruvate and lactate. Defects of muscle oxidative metabolism may result in elevated concentrations of these compounds in tissues and blood. Determination of their plasma levels offers a unique means to evaluate the muscle condition. These assays can be performed by Dr. Diane Shelton's laboratory. Her Web site (http://medicine.ucsd-.edu/vet_neuromuscular) has submission forms and more detailed information.

Muscle and peripheral nerve biopsies should be considered with elevated plasma lactate and/or pyruvate levels and with clinical signs supporting polymyopathy (Shelton's Web site also has detailed information, submission forms, and information on how to handle muscle and nerve biopsies). These must be fresh samples with specific preparation.

In a recent edition of The Veterinary Clinics of North America, Small Animal Practice (SR Platt, GD Shelton, M. Podell, PA Cuddon. Neuromuscular Diseases, January 2002), the authors note that a large variety of acquired myopathies and peripheral neuropathies may be treated adequately.

Myopathies

A significant number of dogs with metabolic myopathies, such as lipid storage myopathy, respond favorably to the long-term administration of L-carnitine (50 mg/kg orally twice daily), coenzyme (1 mg/kg daily), and riboflavin (50 to 100 mg daily) (GD Shelton. Canine Lipid Storage Myopathies. In Kirk's Current Veterinary Therapy XII, Small Animal Practice, 1161-1163, 1995)*.

Figure 1 shows canine lipid storage myopathy, Figure 2 shows normal muscle. Remission of clinical signs is often observed once hormonal disorders (e.g., hypothyroidism, hyperadrenocorticism), inflammatory conditions (e.g. immune-mediated disease, parasitic infestation, tick-transmitted disease), or metabolic myopathies are treated.

*Nutritional supplements are not currently regulated by the government and may not always contain what is indicated on the label (The Wall Street Journal). It is best to encourage the client to use supplements from reputable veterinary or human manufacturers.