Subaortic Membrane Resection and Septal Myectomy

Left ventricular outflow tract obstruction (LVOTO) caused by subaortic membrane (SAM) can result in increased left ventricle (LV) systolic pressure and wall stress and LV hypertrophy. This video presents the case of a nine-year-old male with severe subvalvular stenosis caused by a discrete SAM and interventricular septum hypertrophy. The surgical technique described by Yacoub et. al. was utilized including excision of subaortic membrane, mobilization of the left and right fibrous trigons, and extended septal myectomy (1).

SAM is a common cause of subaortic stenosis that can lead to LVOTO, LV hypertrophy, increase in end diastolic pressure, oxygen demand and supply mismatch, and functional mitral regurgitation (MR). Treatment is achieved by excision of the membrane, mobilization of the trigons, and extended septal myectomy. The functional MR is improved as a byproduct of adequate sub valvular resection.

In the presence of SAM, fibrous proliferation occurs on top of the left and right fibrous trigons due to the high turbulent velocity jet. As a result, the trigons become fused with the anterior mitral valve leaflet (AMVL), hindering its systolic anterior excursion. This not only increases the dynamic obstructive gradient across the LVOT, but also induces functional MR.

Furthermore, the AMVL is also being actively sucked inside the LVOT because of the Venturi effect caused by the high velocity turbulent jet passing through the narrow LVOT. Hence, the mobilization of the fibrous trigones is essential for improving LVOT obstruction, AMVL systolic motion, and MR.
Although an adjuvant septal myectomy is currently still controversial, it is sometimes considered fundamental because it helps to relieve the subvalvular LVOTO and decrease the incidence of recurrence of the SAM (2).

The Patient

The patient is a nine-year-old boy from Egypt with shortness of breath (NYHA II) and an ejection systolic murmur at the upper right parasternal border radiating to the carotid arteries. A transthoracic echocardiogram showed subvalvular stenosis caused by a discrete SAM with a peak pressure gradient across the LVOT of 120 mmHg, concentric LV hypertrophy, moderate MR, and mild aortic regurgitation.

The Surgery

The patient underwent subaortic membrane resection via a median sternotomy. Cardiopulmonary bypass was initiated and the heart was arrested using antegrade cold blood cardioplegia. An oblique aortotomy was done to improve exposure of the aortic valve and subvalvular components. The surgical technique described by Yacoub et. al. was employed, including the steps of inspection, excision of sub-aortic membrane, mobilization of the left and right fibrous trigons, and extended septal myectomy (1).

The procedure was completed successfully without any complications and the patient had an uneventful recovery. A postoperative echocardiography showed no residual LVOT obstruction and a peak pressure gradient of 20 mmHg. Anterior systolic motion of the MV was abolished and the MR improved dramatically as a byproduct.

In addition to SAM resection for treating the fixed LVOTO, mobilization of the fibrous trigons during the procedure was essential for addressing the dynamic LVOTO caused by the splinted AMVL in systole. Extended septal myectomy provides additional relief of LVOTO and decreases the incidence of recurrence.

References

1. Yacoub M, Onuzo O, Riedel B, Radley-Smith R, Hanley FL. Mobilization of the left and right fibrous trigones for relief of severe left ventricular outflow obstruction. J Thorac Cardiovasc Surg. 1999;117(1):126-133. doi:10.1016/S0022-5223(99)70477-0
2. Hirata Y, Chen JM, Quaegebeur JM, Mosca RS. The role of enucleation with or without septal myectomy for discrete subaortic stenosis. J Thorac Cardiovasc Surg. 2009;137(5):1168-1172. doi:10.1016/j.jtcvs.2008.11.039

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