Anti-AMPA receptor limbic encephalitis as the initial manifestation of metastatic breast cancer.

Anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor limbic encephalitis is a rare neurological disorder often associated with malignancies. We present the case of a patient in her forties who initially presented with symptoms misattributed to migraine, later progressing to fever, confusion, declining consciousness and focal seizures. MRI findings were consistent with encephalitis, and cerebrospinal fluid (CSF) analysis revealed lymphocytic pleocytosis with negative infectious workup. Despite initial empiric treatment for infectious encephalitis, her condition deteriorated. CT of the chest, abdomen and pelvis showed suspicious nodules in the left breast; biopsy confirmed the presence of metastatic invasive ductal carcinoma. She made a marked improvement following plasma exchange. Subsequently, the CSF autoimmune encephalitis panel returned positive for AMPA receptor antibodies, confirming the diagnosis of paraneoplastic encephalitis. This case underscores the need for early consideration of autoimmune and paraneoplastic causes in atypical or refractory encephalitis presentations, even prior to the availability of confirmatory antibody testing.