Autoimmune encephalitis (AE) prevalence has increased in recent years due to accessibility to antibody testing, with proportionate increasing rates of misdiagnosis. Cases of misdiagnosis often fail to meet diagnostic criteria. Sensitivity and specificity depend on the diagnostic category (possible, probable, or definite). In a large real-world cohort, data suggests possible autoimmune encephalitis has sensitivity of 83% with specificity of 27%, with specificity increasing to 99% and 98% in probable and definite cases, respectively when antibody and infectious disease testing are incorporated [5].  

In our case, basic diagnostic criteria was met, without seropositive antibody testing, yielding a working diagnosis of seronegative AE. Our patient presented with subacute onset of stereotyped temporal lobe seizures, memory loss, and psychiatric changes. MRI initially with asymmetric mesial temporal lobe hyperintensity and FLAIR nonsuppression. After months of escalation in immunotherapy, surveillance MRI noted diffuse glioma of the left mesial temporal lobe, definitively diagnosed via brain biopsy.  

Less than 10% of misdiagnosed AE represents cerebral neoplasms. Meanwhile, there is clear overlap in treatment, with steroids' role in symptomatic management of edema associated with CNS neoplasms, resulting in misleading responsiveness to treatment.  

This case sheds light on a rare but notable misdiagnosis of cerebral neoplasm as AE, and exemplifies how diagnostic criteria is more sensitive than specific, especially without confirmed antibodies. With increased prevalence of AE, we should remain wary of potential differential diagnoses that exist and may even meet diagnostic criteria. This exemplifies the level of nuance and problem solving involved in diagnosing and treating patients with complex neurological diseases, such as AE.