Neuropsychiatric regression without explanation makes up the symptom cluster known as Down syndrome regression disorder. For a study, researchers assessed the prevalence of neurodiagnostic abnormalities in people with Down syndrome regression disorder. They explored if these abnormalities showed how well treatment was working. A multi-center, retrospective case-control study was conducted. Patients had to have 4 of the 5 symptom groups (cognitive decline, expressive language impairment, sleep disturbance, loss of capacity to conduct daily tasks, and a new mobility condition) with the subacute onset and have no other known cause for their symptoms. Although they had higher risks of autoimmune disease (P=0.02, 95% CI 1.04-1.75), those with Down syndrome regression disorder were equivalent to a sample of people who solely had Down syndrome. EEG (n = 19, 26%), neuroimaging (n = 16, 22%), and CSF (n = 9) all revealed abnormalities indicative of neurodiagnostic conditions. About 5 cases of pleocytosis, 9 cases of high total protein, 7 cases of elevated IgG index, and 2 cases of oligoclonal bands were noted. Neurodiagnostic abnormalities were more likely found in tests conducted within 2 years of the onset of symptoms (P=0.01, 95% CI 1.64-37.06). Immunotherapy was approximately 4 times more likely to have a therapeutic effect in patients with neurodiagnostic abnormalities than in patients without abnormalities (OR 4.11, 95% CI 1.88-9.02). IVIg was beneficial in 14 of 17 (82%) patients in those with normal neurodiagnostic testing (n = 43), even though other immunotherapies were all universally ineffective. According to the study, Down syndrome regression disorder patients had a novel presence of neurodiagnostic testing abnormalities, which lent support to the idea that this symptom cluster might have a neurologic and neuroimmunologic cause.