Differentials
Botulism
SIGNS / SYMPTOMS
Subacute onset of generalised, descending weakness with diplopia, ptosis, dysarthria, dysphagia, and respiratory failure. Prominent autonomic dysfunction with dilated pupils, hypotension, bradycardia, diarrhoea followed by constipation, and urinary retention. Lambert-Eaton myasthenic syndrome (LEMS) is differentiated by the subacute onset and by identifying exposure to botulinum toxin.
Careful clinical examination is essential.
INVESTIGATIONS
May be distinguished from LEMS by post-exercise or post-tetanic facilitation that may last for minutes. Mouse bioassay to confirm botulinum toxin intoxication is available, although results may not be available for several days.
Myasthenia gravis
SIGNS / SYMPTOMS
Suggestive features include prominent ocular weakness that spares the pupils, and normal tendon reflexes.[38]
Autonomic dysfunction is not caused by myasthenia gravis and is an important distinction between Lambert-Eaton myasthenic syndrome (LEMS) and myasthenia gravis.
INVESTIGATIONS
Abnormal post-exercise or tetanic facilitation (>100%) usually differentiates LEMS from myasthenia gravis.
The presence of acetylcholine receptor (AChR) or MuSK antibodies strongly suggests myasthenia gravis, although up to 13% of patients with LEMS have AChR antibodies, and myasthenia gravis/LEMS overlap syndromes do occur rarely.[33]
Myopathy
SIGNS / SYMPTOMS
Progressive weakness without autonomic features may be seen in mitochondrial, inflammatory, and some late-onset muscular dystrophies. Fluctuation in strength or facilitation with exercise is not noted and reflexes are preserved. Family history is frequently positive in mitochondrial and late-onset muscular dystrophies.
INVESTIGATIONS
Elevations in serum creatine kinase suggest a myopathic process, although they may be normal. EMG differentiates a myopathic process, whereas muscle biopsy is confirmatory.
Chronic inflammatory demyelinating neuropathy
SIGNS / SYMPTOMS
Progressive weakness with loss of tendon reflexes; autonomic symptoms may be present. Sensory examination is typically abnormal. Careful clinical examination typically differentiates Lambert-Eaton myasthenic syndrome from inflammatory neuropathy.
INVESTIGATIONS
Nerve conduction studies confirm a peripheral neuropathic process.
Absence of voltage-gated calcium-channel antibodies.
Guillain-Barre syndrome (GBS)
SIGNS / SYMPTOMS
Can be difficult to differentiate because of similar clinical characteristics.
In GBS, patients may have a history of influenza-like or respiratory illness or gastroenteritis before onset of neurological symptoms; no prior history of malignancy or auto-immune disease.
Classic presentation is a progressive symmetrical muscle weakness affecting lower extremities before upper extremities, and proximal muscles before distal muscles, accompanied by paraesthesias in the feet and hands.
INVESTIGATIONS
Nerve conduction studies show a patchy demyelination, slowing of motor nerve conduction velocities, prolonged distal and F-wave latencies, and dispersed response.
Absence of voltage-gated calcium-channel antibodies.
Presence of subtype-specific antiganglioside antibodies.
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