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The ratio of the width of the larger A1<\/jats:sub> segment to the smaller A1<\/jats:sub> segment was calculated. Patients with an A1<\/jats:sub> ratio greater than 2 were categorized as having A1<\/jats:sub> segment hypoplasia. The relationship of A1<\/jats:sub> segment hypoplasia to both patient and aneurysm characteristics was then assessed.<\/jats:p><\/jats:sec>RESULTS<\/jats:title>Of 204 patients that presented with an ACoA aneurysm, 34 (16.7%) were found to have a hypoplastic A1<\/jats:sub>. Patients with A1<\/jats:sub> segment hypoplasia were less likely to have a history of smoking (44.1% vs 62.9%, p = 0.0410). ACoA aneurysms occurring in the setting of a hypoplastic A1<\/jats:sub> were also found to have a larger maximum diameter (mean 7.7 vs 6.0 mm, p = 0.0084). When considered as a continuous variable, increasing A1<\/jats:sub> ratio was associated with decreasing aneurysm dome-to-neck ratio (p = 0.0289). There was no significant difference in the prevalence of A1<\/jats:sub> segment hypoplasia between ruptured and unruptured aneurysms (18.9% vs 10.7%; p = 0.1605).<\/jats:p><\/jats:sec>CONCLUSIONS<\/jats:title>Our results suggest that a hypoplastic A1<\/jats:sub> may affect the morphology of ACoA aneurysms. In addition, the relative lack of traditional risk factors for aneurysm formation in patients with A1<\/jats:sub> segment hypoplasia argues for the importance of hemodynamic factors in the formation of ACoA aneurysms in this anatomical setting.<\/jats:p><\/jats:sec>","DOI":"10.3171\/2016.7.jns16736","type":"journal-article","created":{"date-parts":[[2016,9,30]],"date-time":"2016-09-30T13:59:52Z","timestamp":1475243992000},"page":"89-95","source":"Crossref","is-referenced-by-count":41,"title":["Relationship of A1 segment hypoplasia to anterior communicating artery aneurysm morphology and risk factors for aneurysm formation"],"prefix":"10.3171","volume":"127","author":[{"given":"Lorenzo","family":"Rinaldo","sequence":"first","affiliation":[{"name":"Departments of Neurosurgery,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Brandon A.","family":"McCutcheon","sequence":"additional","affiliation":[{"name":"Departments of Neurosurgery,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Meghan E.","family":"Murphy","sequence":"additional","affiliation":[{"name":"Departments of Neurosurgery,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mohamad","family":"Bydon","sequence":"additional","affiliation":[{"name":"Departments of Neurosurgery,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Alejandro A.","family":"Rabinstein","sequence":"additional","affiliation":[{"name":"Neurocritical Care, and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Giuseppe","family":"Lanzino","sequence":"additional","affiliation":[{"name":"Departments of Neurosurgery,"},{"name":"Neurointerventional Radiology, Mayo Clinic, Rochester, Minnesota"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1754","reference":[{"key":"b1-jns16736","series-title":"AJNR Am J Neuroradiol","doi-asserted-by":"publisher","first-page":"1513","DOI":"10.3174\/ajnr.A1633","article-title":"Difficult aneurysms for endovascular treatment: overwide or undertall?","volume":"30","author":"Brinjikji W,","year":"2009","unstructured":"Brinjikji W, Cloft HJ, Kallmes DF: Difficult aneurysms for endovascular treatment: overwide or undertall?. 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