The first reported case of Balamuthia amoebic encephalitis initially presenting as anti-NMDAR encephalitis.

Balamuthia mandrillaris (B. mandrillaris) is a highly pathogenic free-living amoeba that can invade the central nervous system and cause Balamuthia amoebic encephalitis (BAE). BAE is associated with an extremely high mortality rate, and no treatment regimen with well-established efficacy is currently available. We report a case of BAE in a 63-year-old man who was initially diagnosed with anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis. Because the patient initially exhibited no overt signs of infection and metagenomic next-generation sequencing (mNGS) failed to identify any pathogens in the cerebrospinal fluid (CSF), the presence of frequent seizures and anti-NMDAR antibodies in the CSF initially led the treating clinicians to a diagnosis of anti-NMDAR encephalitis. As the disease progressed, multiple intracranial lesions developed. Repeat mNGS subsequently detected B. mandrillaris in the CSF, thereby establishing the diagnosis of BAE. Despite combination antimicrobial therapy, the patient's condition failed to improve, and he ultimately died. This case underscores the importance of maintaining a high index of suspicion for amoebic infection in patients with encephalitis, as B. mandrillaris infection may elicit autoimmune responses early in the disease course and mimic autoimmune or other noninfectious forms of encephalitis.