NEUROMUSCULAR CASE OF THE MONTH - FEBRUARY 2001

SPECIAL FEATURE:
Rational therapeutics for neuromuscular diseases
Contributed by G. Diane Shelton DVM, PhD
Department of Pathology
University of California, San Diego

Neuromuscular disorders are some of the most challenging cases confronting the veterinary clinician. While several of these disorders are not treatable, there are those that have specific therapies and can result in a good clinical outcome. A careful clinical evaluation and appropriate diagnostic procedures are essential to obtaining an accurate diagnosis and prognosis. Referral to a board certified specialist with expertise in these types of disorders is usually optimal. There is no such thing as a “table top diagnosis” for any of the neuromuscular diseases. With the exception of a few disorders where a simple blood test can provide the diagnosis such as the acetylcholine receptor antibody test for the diagnosis of acquired myasthenia gravis, the most important diagnostic test for these types of disorders is a properly collected and processed muscle biopsy. Evaluation of muscle biopsies can provide an accurate diagnosis in a large percentage of these cases, allow the muscle pathologist to make an accurate prognosis, and suggest specific therapies. The following general points may help the clinician in the evaluation of neuromuscular cases.

• Accurate prognosis depends on a diagnosis
• Identification of a specific therapy depends on a diagnosis
• Premature institution of corticosteroid therapy without an adequate diagnostic work-up may lead to delays in or failure to establish an accurate diagnosis
• Delay or failure to obtain an accurate diagnosis can result in irreversible damage to skeletal muscle in treatable disorders such as masticatory muscle myositis or polymyositis
• Delay or failure to obtain an accurate diagnosis can result in death in cases of myasthenia gravis due to aspiration pneumonia or respiratory failure
• An accurate diagnosis is critical to the development of DNA based testing for genetic diseases of purebred dogs

Remember the Myopathic Effects of Corticosteroids!

• Marked muscle fiber atrophy can result from chronic corticosteroid use (selective atrophy of type 2 fibers, “steroid myopathy”)
• Profound muscle weakness resulting from corticosteroids may worsen existing weakness
• Other side effects of corticosteroid use including hypercorticism

Very Minimum Diagnostics Suggested for Neuromuscular Disorders

• Careful physical and neurological examination localizing the clinical problem to the muscle, nerve, or neuromuscular junction
• Careful history regarding diet, previous drug therapy, travel, and known similar problems in other related animals
• CBC and serum chemistry panel including creatine kinase (CK) and electrolytes
• Evaluation of thyroid status (remember that neuromuscular manifestations of hypothyroidism may be the first clinical sign of hypothyroidism!)
• Acetylcholine receptor antibody titer (acquired myasthenia gravis is a great mimic and can have many clinical presentations)
• Plasma lactate concentration (an increasing number of neuromuscular diseases are being recognized associated with lactic acidemia)

What Can I Do If An Owner Will Only Allow the Very Minimum Diagnostics?

• Warn the owner that a delay in diagnosing the problem and instituting appropriate therapy, if available, may result in irreversible muscle damage
  
  
• Discuss with the owner that some disorders are untreatable and that nothing tried may work. It may be best to know this ahead of time before costly therapeutics are initiated and the owner is disappointed with the outcome
  
  
• A trial course of supplements and vitamins may be instituted. The author recommends L-carnitine (100 mg/kg/d divided BID), coenzyme Q10 (4 mg/kg/day), and supplementation with B vitamins. This therapy should not be harmful to muscle and may help improve muscle strength. Several muscle disorders may be associated with low muscle carnitine concentrations and dramatic improvement may result in some cases. The owner must know that in most cases this is not a cure, but most importantly it cannot hurt. The metabolic benefits of L-carnitine include transportation of fatty acids into the mitochondria for energy generation and the urinary excretion of abnormal metabolites. Coenzyme Q10 functions as a cofactor in several enzyme systems related to energy conversion (electron transport) and is a vital catalyst to energy production at the cellular level. It is also a free radical scavenger.
  
  
• Passive range-of-motion (ROM) exercises. The disability associated with muscular weakness may be greatly aggravated by contractures. Passive ROM exercises positively influence reinnervation and the sprouting process in denervated muscle and may be of therapeutic value.
  

For illustration only. The author has no financial interest relating to sale of these products

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