Tehran University of Medical Sciences Case Reports in Clinical Practice 2538-2683 11 3 2026 09 28 116 121 Afshin Samaei Neuromuscular Rehabilitation Research Center, Semnan University of Medical Sciences, Semnan, Iran. Amir Torkaman Department of Neurology, Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran. Maryam Ezzedin Clinical Research Development Unit, Kowsar Educational, Research and Therapeutic Hospital, Semnan University of Medical Sciences, Semnan, Iran. Elham Dirandeh Department of Neurology, Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran. AND Clinical Research Development Unit, Kowsar Educational, Research and Therapeutic Hospital, Semnan University of Medical Sciences, Semnan, Iran. 2026 05 09 2026 08 09 Moyamoya syndrome (MMS) is a rare, progressive occlusive vasculopathy characterized by stenosis of the distal internal carotid arteries and the formation of collateral vessels resembling a “puff of smoke” on angiography. Patients with Down syndrome (DS) have a significantly higher risk of MMS; however, diagnosis is often delayed due to underlying intellectual disability. We report the case of a 21-year-old male with Down syndrome who presented with recurrent ischemic strokes. Initial magnetic resonance imaging (MRI) revealed scattered infarcts in both hemispheres. Magnetic resonance angiography (MRA) and digital subtraction angiography (DSA) demonstrated bilateral terminal internal carotid artery stenosis with compensatory anastomoses from the external carotid arteries, confirming the diagnosis of moyamoya syndrome. Due to adequate collateral circulation, the patient was managed conservatively with dual antiplatelet therapy rather than surgical revascularization. Unfortunately, the clinical course was complicated by Stevens–Johnson syndrome following antiseizure medication and subsequently severe pneumonia with bilateral pleural effusion, which ultimately led to the patient’s death. While confirming previous findings on the diagnostic challenges of MMS in DS, this report emphasizes that systemic complications pose a serious threat to these patients. https://crcp.tums.ac.ir/index.php/crcp/article/view/1186 https://crcp.tums.ac.ir/index.php/crcp/article/download/1186/791