For a study, researchers sought to determine the prevalence of neurodiagnostic abnormalities in people with Down syndrome regression disorder and whether these abnormalities were indicative of responses to therapeutic intervention. A multi-center, retrospective case-control study was carried out. Patients had to have a subacute onset, and 4 of the 5 symptom groups were present (cognitive decline, expressive language, sleep derangement, loss of ability to perform activities of daily living, and a new movement disorder) with no other explanation for their symptoms. Individuals with Down syndrome regression disorder had similar rates of autoimmune disease as individuals with only Down syndrome (P=0.02, 95% CI 1.04–1.75). EEG (n=19,26%), neuroimaging (n=16, 22%), and CSF (n=9, 17%) revealed neurodiagnostic abnormalities. Pleocytosis was noted in 5 cases, as was an increase in total protein in 9, an increase in IgG index in 7, and oligoclonal bands in 2. Testing within 2 years of the onset of symptoms increased the likelihood of neurodiagnostic abnormalities (P=0.01, 95% CI 1.64–37.06). Immunotherapy was nearly 4 times more likely to have a therapeutic effect in people with neurodiagnostic abnormalities than in people without (OR 4.11, 95% CI 1.88–9.02). IVIg was effective in 14 of 17 (82%) patients with normal neurodiagnostic studies (n=43), while other immunotherapies were uniformly ineffective. The study reported the presence of novel neurodiagnostic testing abnormalities in individuals with Down syndrome regression disorder, lending credence to the possibility that the symptom cluster had a neurologic and neuroimmunology etiology.