We report a case of a middle-aged woman with a diffuse, nonenhancing, progressively atrophic T2-hyperintense lesion involving the left frontotemporal lobes and insula found to be synchronous high-grade sinonasal neuroendocrine carcinoma (SNEC) after initial endonasal resection. In 2014, a 47-year old woman underwent resection of a left-sided high-grade ethmoidal neuroendocrine carcinoma after presentation with weight gain and increased levels of serum and urine cortisol. Concurrent with the initial presentation, she was noted to have a nonenhancing, hyperintense signal change on T2-weighted images on the left frontotemporal lobes and insula thought to be paraneoplastic. Moreover, low titer antibodies to voltage-gated potassium channels were present, raising concern for limbic encephalitis. However, the patient was asymptomatic. A little more than a year after initial presentation, she noted excessive fatigue, daytime somnolence, and cognitive decline. Imaging revealed a gradually progressive, nonenhancing, T2-hyperintense signal abnormality with progressive atrophy in the left anteroinferior frontal lobe, anteromedial temporal lobe, insula bilateral cingulate gyri, and bilateral thalami. Given the progressive nature of the abnormality, stereotactic biopsy was performed, which confirmed the lesion to be metastatic, infiltrative SNEC. In summary, this is a rare case of a synchronous presentation of a high-grade SNEC with an unusual appearance that diffusely infiltrated the brain, likely directly involving the left olfactory nerve and spreading along olfactory projections. This case draws physicians' attention to the possibility that although paraneoplastic syndromes are most likely benign, dissemination of the primary cancer is a diagnostic possibility.