Lambert-Eaton myasthenic syndrome

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Lambert-Eaton myasthenic syndrome

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LAMBERT–EATON MYASTHENIC SYNDROME (LEMS) osms.it/lambert-eaton-myasthenic PATHOLOGY & CAUSES ▪ Rare autoimmune disorder ▫ Autoantibodies inhibit presynaptic calcium channels on motor neurons → reduced acetylcholine release in neuromuscular junction ▪ Muscle weakness ▫ Improves temporarily after repeated muscle use (no signiﬁcant muscle atrophy) ▪ Mostly affects somatic nervous system, can also affect autonomic nervous system’s parasympathetic part ▪ Middle-aged adults (most cases) CAUSES Type II hypersensitivity reaction ▪ B cells produce antibodies that target, block voltage-gated calcium channels located presynaptically on motor neurons → only few unbound channels available to open, allow calcium in → ↓ calcium within neuron (insufﬁcient to trigger acetylcholine release) → ↓ acetylcholine release in neuromuscular junction → attached muscle ﬁber does not contract ▪ Repeated stimulation by brain’s electrical impulses → enough calcium might get through remaining unbound calcium channels → acetylcholine release → muscle contraction RISK FACTORS ▪ Malignancy ▫ Strong small-cell lung cancer association; stimulus for antibody production is same calcium channel expression in neoplastic cells ▫ Other associated malignancies include 662 OSMOSIS.ORG lymphoproliferative disorders (e.g., Hodgkin’s lymphoma) ▪ Autoimmune diseases ▫ Hashimoto’s thyroiditis, diabetes mellitus type 1, vitiligo COMPLICATIONS ▪ Respiratory muscle involvement → respiratory failure ▪ Underlying malignancy → can lead to death SIGNS & SYMPTOMS ▪ Progressive, symmetrical proximal muscle weakness (e.g., shoulders, hips, thighs) → difﬁculty climbing stairs/standing when seated ▫ Paraneoplastic LEMS: more rapidly progressive course ▪ Warming-up phenomenon ▫ Repeated muscle use → weakness temporarily relieved ▪ Reﬂex strength ↓ ▫ Muscle activation → reﬂex recovery/ improvement ▪ Small minority ▫ Ocular, oropharyngeal muscle involvement ▪ Advanced stages ▫ Possible respiratory muscles involvement → respiratory failure (myasthenic crisis) ▪ Autonomic symptoms ▫ Dry mouth (most common), constipation, blurry vision, erectile dysfunction, urinary problems, syncope

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Lambert-Eaton myasthenic syndrome

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A 60-year-old man presents to the office because of complaints of a persistent cough, shortness of breath, and weakness. These symptoms have been ongoing for the past six months, and he says that his muscle weakness now seems to be worsening. He reports that the weakness seems to be at its worst when he is sitting down and resting, and improves when he gets up and moves around for awhile. He has a 50-pack-year history of cigarette smoking and currently smokes two packs per day. He has a medical history of hypertension and chronic obstructive pulmonary disease (COPD), and is irregularly-compliant with follow-up office visits. A chest radiograph shows flattening of the diaphragm and elongation of the mediastinum, and there is an irregularly shaped 2 cm nodule near the hilum on the right. Which of the following is the most likely diagnosis?

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