Atezolizumab-induced cerebellar ataxia in a patient with metastatic small cell lung cancer

Introduction The use of immune checkpoint inhibitors (ICIs), which have an important role in the treatment of malignant tumors, is increasing. Although rarely observed, neurological immune-related adverse events (irAEs) associated with ICIs result in high morbidity and mortality. Small cell lung cancer (SCLC) is a common cause of neurological paraneoplastic syndromes (PNSs). The differentiation between PNSs and neurological irAEs is important in patients using ICIs. Cerebellar ataxia caused by atezolizumab is a rare irAE. Case report In this context, we present a 66-year-old man with SCLC who developed immune-mediated cerebellar ataxia after three cycles of atezolizumab, a programmed cell death ligand-1 inhibitor. The admission brain and spinal gadolinium-based contrast-enhanced magnetic resonance imaging (MRI) supported the preliminary diagnosis and indicated leptomeningeal involvement. However, the blood tests and a lumbar puncture did not reveal any structural, biochemical, paraneoplastic, or infectious cause. Management and outcome of high-dose steroid treatment resulted in an improvement in the radiological involvement, as evidenced both clinically and on follow-up whole spine MRI. Therefore, the immunotherapy was discontinued. The patient was discharged on day 20 without neurological sequelae. Discussion In light of this, we present this case to emphasize the differential diagnosis of neurological irAEs originating from ICIs, which require rapid diagnosis and treatment, and clinically similar PNSs and radiologically similar leptomeningeal involvement, in the case of SCLC.