Case Presentation: We present a case of a 65 year old female with a recent diagnosis of small cell lung cancer (SCLC) who had completed four cycles of carboplatin and etoposide. She was subsequently initiated on maintenance program cell death ligands (PD-L1) inhibitor, atezolizumab every three weeks since January 2019. Seven months after initiation, she presented with a month of lower extremity weakness progressing to her upper extremities, slurred speech, truncal ataxia and diplopia. Autoimmune, endocrine and metabolic workup were negative. Facilitation was present on examination. Electromyelogram did show increment and voltage gated calcium channel was elevated at 405, all consistent with Lambert Eaton Syndrome (LEMS). P/Q calcium channel and VGKC antibodies were positive on the paraneoplastic panel. However, repeat CT scans from October to November showed an interval decrease in size of the pulmonary parenchymal and subpleural nodules from SCLC suggesting an improvement in the underlying malignancy.

Discussion: LEMS can be both a paraneoplastic syndrome of SCLC and an adverse effect of immunotherapy. Common immunotherapy causing LEMS are PDL1 inhibitors such as nivolumab, ipilimumab and atezolizumab. There have only been a few case reports describing an association with immunotherapy for SCLC causing LEMS and there is no diagnostic test available to differential a paraneoplastic cause from an immunotherapy related cause. Inadequate testing poses a dilemma in the treatment plan. Patient was started on pyridostigmine 60mg every six hours and IVIG daily for five days which she tolerated well and had some improvement in her symptoms. Given the concern for immunotherapy related LEMS, 1.5mg/kg of prednisone was also started. She was discharged with an ongoing treatment plan of Firdaspe, IVIG and steroids with her outpatient neurologist.

Conclusions: LEMS should be considered as potential neurological adverse event in patients receiving immunotherapy. This case highlights the diagnostic dilemma that patients with SCLC on immunotherapy may present. Differentiating between paraneoplastic and immunotherapy related LEMS would help better target treatment options with either treatment of the underlying SCLC for paraneoplastic LEMS or discontinuation immunotherapy and initiation of steroids for immunotherapy related LEMS.