Case Report: Acute polymorphic psychosis and NMDA-R IgG antibodies in serum: a follow-up case study.

Introduction: Anti-N-methyl-D-aspartate receptor (NMDA-R) encephalitis is a neuropsychiatric disorder with additional psychiatric features caused by NMDA-R immunoglobulin G (IgG) antibodies in cerebrospinal fluid (CSF). This report presents the follow-up of a patient in whom we assumed mild NMDA-R encephalitis in the first psychotic episode. Case study: A patient with a prior episode of an acute polymorphic psychotic syndrome relapsed five and a half years later following a severe COVID-19 infection. Serum NMDA-R antibodies were again detected with a titer of max. 1:320 using fixed-cell-based assays, but conventional magnetic resonance imaging (MRI), electroencephalography (EEG), and CSF findings were largely normal. NMDA-R antibody levels in serum decreased to 1:80 after approximately one month without immunotherapy. [ Discussion: The international consensus criteria for a probable/definite diagnosis of NMDA-R encephalitis or autoimmune psychosis were not fulfilled, especially as no NMDA-R IgG antibodies were identified in CSF using different antibody assays and EEG/CSF routine findings were inconspicuous. NMDA-R encephalitis was therefore not diagnosed (as initially suspected). Independent of the NMDA-R IgG antibodies, there were possible signs of an autoimmune process. For a better understanding of similar patients, multimodal diagnostic approaches including complementary antibody tests could be promising.