To describe a patient with Lambert-Eaton myasthenic syndrome (LEMS) with anti-voltage gated calcium channel (VGCC) antibodies who, in the absence of malignancy, developed dermatomyositis (DM) with anti-transcription intermediary factor 1-gamma (TIF1-gamma) antibodies.

LEMS is a presynaptic neuromuscular junction disorder characterized by reduced acetylcholine release secondary to auto-antibodies to VGCC, which are found in >85% of cases. Dermatomyositis is an inflammatory myopathy associated with various autoantibodies >60% of cases. The anti-TIF1-gamma antibody shows specificity for DM with high prevalence in cancer associated myositis. Independently, LEMS and DM are rare and can be paraneoplastic conditions. Only a few cases of comorbid LEMS and DM have been reported.

A case report with analysis of clinical data and review of the literature.

A 60-year-old female was diagnosed with LEMS in 2003 with symptoms of progressive proximal limb weakness and fatigue. EMG/NCS was consistent with LEMS with >400% increment during high frequency stimulation. Autoantibodies against P/Q-type VGCC were detected with level of 523 pmol/L. She responded well to pyridostigmine and 3,4-diaminopyridine. In 2016, the patient developed a rash (face, shoulders, elbows and hands), arthralgias and myalgias. Her CK level was normal. EMG/NCS redemonstrated post-exercise increment, but had new findings of diffuse myopathy involving limbs and paraspinal muscles. A muscle biopsy demonstrated multifocal perimysial inflammation and perifascicular atrophy consistent with dermatomyositis. Myositis panel testing showed a weakly positive anti-U1RNP antibody and positive anti-TIF1-gamma (called anti-P155/140) antibody. Her clinical symptoms improved after treatment with prednisone and methotrexate. Malignancy screening has been negative.

This represents the first reported case of anti-VGCC and anti-TIF1-gamma positive LEMS with associated DM. Concurrent LEMS and DM is a very rare syndrome and usually paraneoplastic. A high clinical suspicion, along with appropriate antibody testing, electrodiagnostic studies and muscle biopsy can improve diagnostic accuracy and guide appropriate treatment of concurrent autoimmune neuromuscular conditions.