AB, a 20-year-old woman with Down syndrome (DS) and moderate intellectual disability, was referred to a Down syndrome clinic for loss of skills. Her medical history was signifcant for atrial septal defect repair in infancy and Graves’ disease in remission for the last four years of medications. She presented with a one-year history of slowing in her general movements and with behavioral regression. Te family noted that her symptoms became more noticeable when she changed day programs from a less structured to a more demanding environment.Te change was implemented because she had been so successful the prior year. She previously spoke in full sentences, but now her speech was sof and limited to 1 or 2 words at a time. She had tremors that had increased signifcantly over a few months. Tese motor symptoms impacted her ability to complete activities of daily living. She had occasional bowel or bladder accidents if she was not able to get to a bathroom in time. Te patient exhibited some symptoms of depression, including decreased interest in activities and somber afect. Te decline in function did not appear to afect her comprehension as she was still able to follow conversations and directions as before, albeit more slowly. Neurological fndings were signifcant for paucity of expressive speech, hypophonia, decreased facial expression, slow gait, tremors, and cogwheel rigidity. Her motor fndings were more pronounced on the lef side. Extensive testing including complete blood count, celiac panel, thyroid panel, vitamin B12 level, HIV screen, complete metabolic panel, sedimentation rate, urine HVA/VMA, ANA, paraneoplastic panel, and myasthenia gravis antibodies was unremarkable. Her EEG showed mild difuse slowing consistent with her diagnosis of DS. An MRI of her brain showed cystic expansion of the cavum septum pellucidum and vergae, measuring 7.5 cm in greatest transverse dimension.Te lateral ventricles were enlarged and obstructed at the level of the foramina of Monro with no obvious periventricular interstitial edema (See Figure 1). Follow-up images three months, one year, and two years later remained stable. A CT scan of her brain did not show calcifcations. Developmental regression related to depression was initially suspected, leading to mental health interventions. Her psychiatrist started citalopram, which improved her afect and level of interest, but not her motor slowing. Te neurologist initiated carbidopa-levodopa 25 mg–۱۰۰ mg three times daily for symptomatic treatment of extrapyramidal features. Tis led to an immediate and dramatic response with return to baseline motor function. Te patient had no side efects of treatment and sustained the response for a fairly long period on the same dose of carbidopa-levodopa. Follow-up examination eighteen months afer initiation of therapy showed recurrence of cogwheel rigidity and increase in tremors, requiring doubling of the dose of carbidopalevodopa and giving it four times a day.