Anti-LGI1 autoimmune encephalitis in a patient with rheumatoid arthritis and MGUS

Background: Anti-leucine-rich glioma inactivated 1 limbic encephalitis (anti-LGI1 LE) is one of the most frequent autoimmune encephalitis, commonly coexisting with other autoimmune diseases. Rheumatoid arthritis (RA) and monoclonal gammopathy of unknown significance (MGUS) are commonly associated with autoimmune phenomena. However, neither RA nor MGUS have been described in the literature to date as coexisting with anti-LGI1 LE. Case description: We present the case of anti-LGI1 LE in a male patient with rheumatoid arthritis, who was also found to have an MGUS. The patient was initially treated with corticosteroids and IV immunoglobulin. After a mild relapse, his treatment was complemented with rituximab, resulting in complete regression of the disease symptoms. Conclusions: Our report provides evidence for the coexistence of anti-LGI1 LE with RA and/or MGUS, thus extending the differential diagnosis of patients suffering with these disease entities that present with neuropsychiatric symptoms suggestive of encephalitis. Moreover, this case raises challenges on the management of the coexistence of these diseases, given the lack of therapeutic guidelines and their potential interaction on a pathophysiological and a clinical level.