A 72-year-old woman presented to the hospital with subacute cognitive and behavioral changes. She received dexamethasone and acyclovir before transferring to a second hospital where she deteriorated clinically to the point of requiring full mechanical ventilatory support. Positive anti-NMDA antibodies in the serum and cerebrospinal fluid confirmed autoimmune encephalitis. A CT of the chest, abdomen, pelvis, abdominal ultrasound, and chest and abdominal MRI showed no evidence of malignancy. Following treatment with intravenous (IV) methylprednisolone and plasmapheresis (PLEX), she slowly recovered and returned to baseline living at home. Eight months later, she was hospitalized for a relapse of anti-NMDA encephalitis manifesting as subacute cognitive and behavioral changes. She received IV steroids, PLEX, and two doses of 1000 mg IV rituximab with improvement. A follow-up PET/CT ten months later demonstrated a new soft tissue mass in the right subscapular space. Biopsy of the right shoulder mass confirmed a desmoid type fibromatosis. Patient received 56 Gy radiation treatment to the right shoulder with clinical improvement. She has continued on maintenance rituximab for encephalitis and remains neurologically stable. 

Our findings suggest that anti-NMDAR encephalitis may be associated with more rare cancer forms including desmoid tumors. Initial malignancy screening in anti-NMDAR encephalitis may not detect an underlying cancer and ongoing paraneoplastic surveillance should be considered, particularly in older patients with relapsing disease. With timely recognition and treatment, patients may achieve substantial improvement.