Q&A: Appropriate Tests in Neuromuscular Diseases

Neuromuscular disease is a broad term that encompasses many diseases and ailments that impair the functioning of the muscles, either directly, as pathologies of the voluntary muscle, or indirectly, as pathologies of nerves or neuromuscular junctions.

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In this article:
Neuromuscular diseases
MCQ exams: clinical scenario
MCQ exams: answer
MCQ exams: explanation

Neuromuscular diseases

Neuromuscular diseases are those that affect the muscles and their direct nervous system control; problems with central nervous control can cause either spasticity or some degree of paralysis (from both lower and upper motor neurone disorders), depending on the location and the nature of the problem. Some examples of central disorders include cerebrovascular accident, Parkinson's disease, multiple sclerosis, Huntington's disease and Creutzfeldt–Jakob disease. Spinal muscular atrophies are disorders of lower motor neurone while amyotrophic lateral sclerosis is a mixed upper and lower motor neurone condition.

Neuromuscular disease can be caused by autoimmune disorders, genetic/hereditary disorders and some forms of the collagen disorder Ehlers–Danlos Syndrome, exposure to environmental chemicals and poisoning which includes heavy metal poisoning. The failure of the electrical insulation surrounding nerves, the myelin, is seen in certain deficiency diseases, such as the failure of the body's system for absorbing vitamin B-12.

Diseases of the motor end plate include myasthenia gravis, a form of muscle weakness due to antibodies against acetylcholine receptor, and its related condition Lambert-Eaton myasthenic syndrome (LEMS). Tetanus and botulism are bacterial infections in which bacterial toxins cause increased or decreased muscle tone, respectively. Muscular dystrophies, including Duchenne's and Becker's, are a large group of diseases, many of them hereditary or resulting from genetic mutations, where the muscle integrity is disrupted, they lead to progressive loss of strength and decreased life span.

Further causes of neuromuscular diseases are :

Inflammatory muscle disorders

• Polymyalgia rheumatica (or "muscle rheumatism") is an inflammatory condition that mainly occurs in the elderly; it is associated with giant-cell arteritis (often responds to prednisolone).
• Polymyositis is an autoimmune condition in which the muscle is affected.
• Rhabdomyolysis is the breakdown of muscular tissue due to any cause.

Tumors

• Smooth muscle: leiomyoma (benign)
• Striated muscle: rhabdomyoma (benign)

MCQ exams: clinical scenario

A 22 year old woman complains of increasing fatigue and double vision. She has recently had a barium swallow examination because of dysphagia, but the test was normal.

On examination her reflexes, sensation and coordination are normal. She is noted to have difficulty speaking clearly. She has breathing problems which appear to improve with edrophonium chloride injections.

Which of the following tests would be most useful?

a) HLA B27
b) anti-cholinesterase antibody
c) rheumatoid factor
d) Antinuclear antibody
e) Anti-DNA

MCQ exams: answer

The correct answer is B.
Anti-cholinesterase antibody testing is the most useful.

MCQ exams: explanation

The typical features of Myasthenia Gravis (MG) are fatigability and weakness of skeletal muscles without reflex, sensory, or coordination abnormalities. Many cases begin with involvement of ocular muscles such as diplopia and ptosis. Weakness may remain confined to ocular muscles for long periods or involve bulbar muscles, which control chewing, swallowing, or articulation. The disorder remains confined to ocular muscles in up to 15 percent of patients, but becomes generalized in the majority.

Examination of neck extensors and flexors is often more sensitive in demonstrating generalized disease than the motor exam of other muscle groups.

Improvement of weakness after edrophonium chloride (Tensilon) injection is a useful sign, although false-negative and false-positive tests do occur. In recent years, however, the detection of anti-cholinesterase antibody in the serum, using a commercially available immunoprecipitation test, has significantly changed the evaluation of MG patients with a sensitivity of over 70%.

The usual AChR antibody test measures the binding of antibody to AChR labeled with radioactive alpha-bungarotoxin. The toxin itself is attached irreversibly to the ACh binding site of AChR. The antibody binding test result is positive in nearly all adults with moderately severe or severe MG, in 80% with mild generalized MG, and in 50% with ocular MG, but in only 25% of those in remission. The test is less reliable in juvenile than in adult MG. In a few patients only antibodies that block the binding of ACh to AChR can be detected.

The antibody titer correlates only loosely with disease severity, but in individual patients a >50% decrease in titer for more than 12 months is nearly always associated with sustained clinical improvement.

Striated muscle antibodies also occur in MG patients. Their role remains unknown, but they often are associated with thymoma.

Reference(s)
1). MedlinePlus: Neuromuscular Disorders. www.nlm.nih.gov. Retrieved 24-04-2016.
2). Hill, M (2003). The Neuromuscular Junction Disorders. Journal of Neurology, Neurosurgery & Psychiatry. 74 (90002): ii32–ii37. doi:10.1136/jnnp.74.suppl_2.ii32. PMC 1765619. PMID 12754327.
3). Kraker, Jessica; a. Zivkovic, Sasa (2011). Autoimmune Neuromuscular Disorders. Current Neuropharmacology. 9 (3): 400–8. doi:10.2174/157015911796558000. PMC 3151594. PMID 22379454.
4). Swash, Michael; Schwartz, Martin S. (2013-03-14). Neuromuscular Diseases: A Practical Approach to Diagnosis and Management. Springer Science & Business Media. p. 86,196. ISBN 9781447138341.