Anti-N-Methyl-D-Aspartate Receptor Encephalitis (ANMDARE) in a Patient With Hashimoto's Thyroiditis.

Although encephalitis is more commonly caused by various infections, other etiologies that may rarely cause encephalitis must garner the attention of medical practitioners. In the realm of immune-mediated etiologies, anti-N-methyl-D-aspartate receptor encephalitis (ANMDARE) is the most common. It usually presents in a typical fashion with psychiatric symptoms followed by abnormal movements such as orofacial-lingual dyskinesia, tremor, dystonia, bradykinesia, ballism, or choreoathetosis occurring at or within the first month of onset, often affecting women and having a high correlation with ovarian teratomas. Our case report describes a 59-year-old Hispanic male with Hashimoto thyroiditis who presented with rapid cognitive decline. The diagnosis was confirmed with positive detection of NMDA receptor antibodies in the patient's cerebrospinal fluid following a lumbar puncture. The patient was treated with the first-line therapy of intravenous (IV) immunoglobulins and corticosteroids with temporary relief of symptoms. Due to the rare occurrence and possible atypical presentation of ANMDARE, this case illustrates the importance of maintaining a high index of clinical suspicion when diagnosing a patient with an unknown cause of cognitive dysfunction, especially when considering various differentials based on the patient's history.