[Atypical paraneoplastic myasthenic syndrome: Lambert-Eaton syndrome or myasthenia?]

We report a case of paraneoplastic myasthenic syndrome with clinical features suggesting Lambert Eaton syndrome but without the electromyographic elements required for diagnosis. Anti-calcium channel antibodies were also lacking. The electromyogram evidenced a block and the Tensilon test was positive. The efficacy of anticholinesterases argued in favor of myasthenia but anti-acetylcholine receptor antibodies were negative. The block was more of a mixed nature, involving both presynaptic transmission as in Lambert Eaton syndrome and post-synaptic transmission as in paraneoplastic myasthenia. The primary tumor was identified as a small-cell neuroendocrine lung carcinoma on mediastinal biopsies obtained directly on CT-scan guided puncture of a mediastinal node. Thoracotomy was thus avoided. The Lambert Eaton syndrome is a paraneoplastic manifestation of small-cell lung cancer in 50% of the cases unlike generalized myasthenia which apparently is never associated with small-cell lung cancer. A mixed paraneoplastic neuro-muscle junction disorder with aspects of each can be exceptionally observed.