Balamuthia mandrillaris amoebic encephalitis clinically mimicking myasthenia gravis exacerbation: a diagnostic challenge

Balamuthia mandrillaris is a rare cause of granulomatous amoebic encephalitis (GAE) with a high mortality rate. Here, we describe the diagnostic challenges in a 70-year-old immunocompromised man with myasthenia gravis (MG). The patient presented with vision loss, diplopia, generalized weakness and respiratory compromise, initially attributed to MG exacerbation. Neuroimaging revealed multiple ring-enhancing brain lesions concerning for infection. A comprehensive cerebrospinal fluid (CSF) and serologic workup, including bacterial and fungal cultures, pan-viral PCR panel, toxoplasma PCR/serology, tuberculosis PCR and prion testing, did not reveal a causative pathogen. During the 5-day hospitalization, prophylactic trimethoprim-sulphamethoxazole (TMP-SMX) was initiated because of the patient’s long-term corticosteroid therapy and lymphopenia, to prevent P. jirovecii pneumonia. Despite persistent neurological symptoms, he remained haemodynamically stable, maintained respiratory function and showed improvement in strength with supportive therapy. He was discharged with close outpatient follow-up and continuing TMP-SMX. Five days later, he re-presented with fever and altered mental status. The imaging at this time shows the progression of intracranial lesions, and he underwent a brain biopsy which demonstrated chronic encephalitis with structures suspicious for amoebae, and finally, the reference laboratory PCR confirmed B. mandrillaris . The patient died the same day the diagnosis was established. This case highlights the difficulty of distinguishing GAE from MG relapse, the limited yield of CSF studies and the critical role of early brain biopsy and rapid PCR in diagnosing Balamuthia infection.