MANUSCRIPT TEXT:
A four year old boy was diagnosed with hypertrophic cardiomyopathy with moderate degree of obstruction of the left ventricular outflow tract secondary to asymmetric septal hypertrophy and no features of congestive heart failure during infancy. He was also found to have myxomatous mitral valve, systolic anterior motion of anterior mitral leaflet and moderate mitral regurgitation (Fig 1, Video 1). In view of phenotypic features suggestive of Noonan syndrome (Fig 3), he underwent genetic evaluation with target gene sequencing and was detected to have a novel heterozygous mutation in exon 13 of LZTR 1 (Leucine- zipper-like transcriptional regulator 1) gene1. Echocardiogram on follow up showed increase in left ventricular outflow tract obstruction and appearance of an aneurysm in the membranous portion of interventricular septum (Fig 3 and 4, Video 2 and 3). There were no features of right ventricular inflow or outflow obstruction, tricuspid regurgitation or aortic regurgitation. Membranous septal aneurysms in children are usually seen in association with ventricular septal defects. The appearance of the membranous septal aneurysm in this boy may be attributed to the direction of jet arising from the point of contact of the anterior mitral leaflet with the hypertrophied basal septum and hitting this part of the interventricular septum. It is an unusual mechanism for formation of membranous septal aneurysm, given its absence in the earlier echocardiograms and absence of any left to right shunt across the aneurysm. For membranous septal aneurysms to become clinically important, they have to interfere with the closure mechanism of the cardiac valves, cause a mass effect, or cause flow obstruction2. Histopathology of resected aneurysms have shown almost total loss of elastic fibres and extensive accumulation of mucopolysaccharides3. This patient has been initiated on beta blockers due to severe left ventricular outflow tract obstruction and is planned for septal myomectomy, mitral valve repair and membranous septal aneurysm resection due to the fear of thrombus formation and rupture of the aneurysm.