Discussion

Congenital ductus arteriosus aneurysm is a saccular or fusiform dilatation of a persistent ductus arteriosus. These aneurysms likely develop in the third trimester perhaps due to abnormal intimal cushion formation or elastin expression. Ductal aneurysm is most commonly diagnosed in children under 2 months of age,^{Reference Dyamenahalli, Smallhorn and Geva1,Reference Lund, Hansen, Brocks, Jensen and Jacobsen2} with only a few cases reported in adulthood.^{Reference Mitchell, Seifert, Miller, Jamieson and Shumway3} Earlier reports on incidence of ductal aneurysms included only patients with incidental diagnosis during work up of abnormal chest radiography or during thoracotomy performed for other pathologies with a reported incidence of 0.8%. A prospective study in 2002 by Jan et al, evaluating 548 term newborns with screening echocardiography, concluded a much higher incidence of 8.8%.^{Reference Jan, Hwang, Fu, Chai and Chi4} A retrospective study by Dyamenahalli et al, evaluating 200 consecutive third-trimester fetal ultrasounds, suggested an incidence of aneurysmal ductus arteriosus of 1.5%.^{Reference Dyamenahalli, Smallhorn and Geva1}

Ductal aneurysm has been described in the literature as a possible normal variant occurring as an isolated anomaly, but it may also be associated with connective tissue disorders and genetic syndromes. In the literature, there are several reports of aneurysmal ductus in association with genetic syndromes such as Marfan,^{Reference Crisfield5} Ehler-Danlos,^{Reference Chang, Chang and Sheih6} Smith–Lemli–Opitz, Trisomy 13, and Trisomy 18.^{Reference Dyamenahalli, Smallhorn and Geva1}

Mutations in smooth muscle contractile elements, including alpha smooth muscle actin encoded by ACTA2 gene and smooth muscle myosin encoded by MYH11 gene, have been implicated in ductus arteriosus aneurysm. The proposed mechanism for persistence and aneurysmal dilatation of ductus arteriosus is failure of the ductus to constrict after birth. ACTA2 mutation has been recently identified as a major cause of familial thoracic aortic aneurysm and ductus arteriosus aneurysm.^{Reference Logeswaran, Friedburg and Hofmann7} Several MYH11 mutations have been implicated in adults with thoracic aortic aneurysms and patent ductus arteriosus.^{Reference Zhu, Vranckx and Van Kien8} A dominant causal variant in MYH11 associated with smooth muscle dysfunction has been reported recently in families with chronic intestinal pseudoobstruction.^{Reference Dong, Baldwin and Choi9} The MYH11 variant in our patient is a novel mutation not previously described. This mutation could be a pathogenic one in our patient due to the existence of two concomitant smooth muscles disorders: ductal aneurysm and intestinal atresia. The authors believe that the patient will likely need close follow up to monitor for potential late development of thoracic aortic aneurysm.

The pathogenesis of congenital ductal aneurysm remains ill defined, although several theories have been postulated.^{Reference Logeswaran, Friedburg and Hofmann7} These include (a) delayed closure of the aortic end of the ductus, resulting in exposure of the ductal wall to systemic arterial pressure, (b) necrosis and mucoid degeneration of the media of the ductus, resulting in weakness of the wall, (c) congenital weakening of the ductal wall from defective elastin, resulting in abnormal intimal cushion formation, and (d) intrauterine constriction of the ductus close to the pulmonary end with post-stenotic dilation. In our patient, the likely mechanism of aneurysmal ductus is the failure to constrict after birth from defective smooth muscle myosin.

A majority of ductal aneurysms spontaneously regress without complications.^{Reference Jan, Hwang, Fu, Chai and Chi4} Spontaneous resolution of the ductal aneurysm is possible by one of two mechanisms: most due to progressive constriction and the remaining from thrombus formation.^{Reference Logeswaran, Friedburg and Hofmann7} However, severe and life-threatening complications have been reported. These include thromboembolic extension into the main pulmonary artery or the aorta, spontaneous rupture, erosion or compression of adjacent structures, and infection. Based on a review of literature by Lund et al, the complication rate is more than 30% in children less than 2 months and it increases to 60% beyond 2 months of age.^{Reference Lund, Hansen, Brocks, Jensen and Jacobsen2} However, these complication rates are based on autopsy findings, and therefore the true risk may be lower.

Close follow-up with echocardiogram is recommended to assess for spontaneous resolution of ductal aneurysms. It has been suggested that surgical resection of a large ductal aneurysm may be considered if there is (a) patent ductus with aneurysm beyond the neonatal period; (b) associated connective tissue disease; (c) evidence of thrombus extension into adjacent vessels or thromboembolism; or (d) significant compression of adjacent vessels.^{Reference Hornberger10} Currently, there are no clear guidelines regarding indications for use of anticoagulation in ductal aneurysm, and this has to be decided on a case-by-case basis. Close follow-up is recommended in patients who had large ductal aneurysms, if there is an underlying genetic disorder. In our patient, although large ductal aneurysm has spontaneously resolved by thrombus formation, without any complications, the presence of a novel MYH11 mutation warrants closer follow-up in the future, as there is a potential risk of development of thoracic aortic aneurysm and dissection.