A Devastating Neurological Disorder: Anti-Dipeptidyl-Peptidase-Like Protein 6 (DPPX) Encephalitis Causing Rapidly Progressive Dementia.

Rapidly progressive dementia (RPD) is caused by a heterogeneous group of neurological disorders, and the prototype is Creutzfeldt-Jakob disease (CJD). However, treatable causes including autoimmune encephalitis are often underrecognized and undertreated. A 72-year-old female patient was admitted with a 10-month history of rapidly progressive cognitive decline, visual hallucinations, paranoid behavior, diarrhea, and an 18-kg unintentional weight loss. On the physical exam, she was only oriented to the person and demonstrated an exaggerated startle response with diffuse rigidity. The initial clinical suspicion included CJD versus autoimmune encephalitis. Comprehensive laboratory testing, thyroid peroxidase, thyroglobulin antibodies, and autoimmune encephalitis panel were negative. The EEG showed mild to moderate diffuse slowing without any epileptiform abnormalities. An MRI brain revealed mild hippocampal atrophy. CSF testing revealed mild lymphocytic pleocytosis; RT-QuIC analysis and 14-3-3 protein were negative. There was no clinical improvement after treatment with IV steroids and IVIG. Repeated autoimmune encephalitis panel testing performed on a research basis was positive for dipeptidyl-peptidase-like protein 6 (DPPX) antibodies in serum and CSF. Unfortunately, our patient passed away before additional treatment could be attempted. Anti-DPPX encephalitis is a rare autoimmune disorder and an unrecognized cause of RPD. Early diagnosis and rapid escalation of treatment are imperative to avoid devastating neurological consequences.