Differentials
Lambert-Eaton myasthenic syndrome (LEMS)
SIGNS / SYMPTOMS
Prominent proximal leg and arm weakness accompanied by diminished muscle stretch reflexes, which improve with brief exercise.
Autonomic symptoms such as dry mouth, constipation, and impotence are present in some patients.
Eye muscles are occasionally involved at presentation.
Respiratory failure is uncommon.
Associated with small cell lung cancer in approximately 50% to 60% of patients.[85][86]
INVESTIGATIONS
Negative anti-acetylcholine receptor (anti-AChR) antibody test in virtually all patients.
Antibodies against voltage-gated calcium channels present in more than 90% of patients.
Low cyclic adenosine monophosphate (cAMP) that increases with rapid stimulation or exercise is also a useful finding.
Botulism
SIGNS / SYMPTOMS
Symptoms similar to MG but there may be hypotension, bradycardia, diarrhea followed by constipation, and urinary retention. Blurred vision is also common.
INVESTIGATIONS
Repetitive nerve stimulation (RNS) at high rates (≥20 Hz) may reveal a small increment in the motor response.
Penicillamine-induced myasthenia gravis
SIGNS / SYMPTOMS
Symptoms similar to MG can occur after weeks or months of treatment. They may remit on withdrawal of the drug; recovery may be slow or incomplete.
INVESTIGATIONS
Anti-acetylcholine receptor (anti-AChR) antibody test: the properties of the AChR in these patients differ from those in patients with idiopathic MG in terms of specificity and affinity.
Primary myopathies
SIGNS / SYMPTOMS
Gradually progressive muscle weakness that is not fatigable.
These include autoimmune inflammatory myopathies or heritable myopathies as in mitochondrial diseases, such as progressive external ophthalmoplegia, or oculopharyngeal muscular dystrophy (OPMD).
Positive family history in some myopathies.
INVESTIGATIONS
Elevated serum levels of creatine kinase (and aldolase) are often observed.
Muscle biopsy may reveal the diagnosis. Ragged red fibers, accumulation of abnormal-looking mitochondria in subsarcolemmal space, and COX-negative muscle fibers suggest mitochondrial diseases.[87]
Mutation analysis of a short GCG repeat expansion in PABPN1 gene is diagnostic of OPMD.[88]
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