▁are ▁formed ▁during ▁the ▁development al ▁process ▁during ▁early ▁f etal ▁development . ▁ ▁Ost ium ▁sec und um ▁ ▁The ▁ost ium ▁sec und um ▁at rial ▁sept al ▁defect ▁is ▁the ▁most ▁common ▁type ▁of ▁at rial ▁sept al ▁defect ▁and ▁compr ises ▁ 6 – 1 0 % ▁of ▁all ▁con gen ital ▁heart ▁dise ases . ▁ ▁The ▁sec und um ▁at rial ▁sept al ▁defect ▁usually ▁ar ises ▁from ▁an ▁en larg ed ▁for amen ▁ov ale , ▁in ade qu ate ▁growth ▁of ▁the ▁sept um ▁sec und um , ▁or ▁excess ive ▁absor ption ▁of ▁the ▁sept um ▁prim um . ▁About ▁ 1 0 ▁to ▁ 2 0 % ▁of ▁individuals ▁with ▁ost ium ▁sec und um ▁AS D s ▁also ▁have ▁mit ral ▁val ve ▁pro lapse . ▁ ▁An ▁ost ium ▁sec und um ▁AS D ▁accompanied ▁by ▁an ▁acquired ▁mit ral ▁val ve ▁st en osis ▁is ▁called ▁L ut emb acher ' s ▁synd rome . ▁ ▁Natural ▁history ▁Most ▁individuals ▁with ▁an ▁un correct ed ▁sec und um ▁AS D ▁do ▁not ▁have ▁significant ▁sympt oms ▁through ▁early ▁ad ul th ood . ▁ ▁More ▁than ▁ 7 0 % ▁develop ▁sympt oms ▁by ▁about ▁ 4 0 ▁years ▁of ▁age . ▁ ▁Sym pt oms ▁are ▁typically ▁decre ased ▁exercise ▁toler ance , ▁easy ▁fat ig ability , ▁pal pit ations , ▁and ▁syn cope . ▁ ▁Comp lications ▁of ▁an ▁un correct ed ▁sec und um ▁AS D