MUC5AC and autosomal dominant mitochondrial myopathy with exercise intolerance: Intimomedial mucoid degeneration (IMMD) is a rare vascular disorder characterised by the deposition of mucin in the intima and media, which leads to elastic tissue degeneration and aneurysm formation of the arterial wall.1-4 Although the condition was initially thought to involve only the aorta, subsequent publications have reported IMMD to affect the major branches of the aorta, as well as smaller vessels such as the coronary and brachial arteries.1,5,6-8 The aneurysms in IMMD usually have a saccular or fusiform morphology and cause symptoms related to the location of the aneurysm.6,8