Discussion
For patients with drug-resistant, symptomatic HOCM, mechanical relief in the form of septal reduction therapy is effective to alleviate symptoms and potentially improve prognosis.3 Surgical myectomy remains the gold standard when performed in high-volume centers with good procedural success and low mortality. However, surgical myectomy is morbid and is best performed by experienced operators. For patients with co-morbidities, many centers will choose ASA as first line septal reduction therapy.4, 9 But due to the constraints of septal arterial anatomy, the alcohol-induced infarction may be insufficient and inaccurate, or even too large, resulting in significant and potentially morbid myocardial infarction. In series of multiple studies, up to a third of patients have an unsatisfactory outcome. In addition to inaccurate ablation, 5–15% of patients cannot receive ASA due to an inability to locate an appropriate septal vessel.
Endocardial septal ablation by RF has been recently developed to alleviate LVOT obstruction in some centers. After 6 months to one year follow up, preliminary results demonstrated a > 50% reduction in the resting gradient.13 In our study, the use of the CARTOSOUND system and ICE enabled more accurate delivery of RF energy. Nineteen of 20 patients underwent RF ablation successfully. The mean reduction of gradient is 47% after six months follow up, similar with prior reports. No serious complications, including vascular complications or permanent AVB occurred intra-procedurally or during follow up.9In spite of the small number of case series identified from the literature, the preliminary results suggest RF ablation as an alternative treatment in some patients, especially in those who are unsuitable for ASA or surgical myectomy.
This study also demonstrated that the hemodynamic effects of the procedure are variable among patients. For example, a gradient reduction of only 3% was observed in patient No.7 (Table 2 and Figure 5A). We further assessed factors that could influence the impact of this procedure in reduction of LVOT gradient (Figure 6). We found that the outcome of RF septal ablation is superior in patients with limited basal septal hypertrophy, shorter AML and a normally positioned PM.
In patients with HOCM, increased LV wall thickness is most commonly located in the anterior free wall, basal septum and posterior portion of septum.17 The asymmetric septal hypertrophy narrows the LVOT, partially obstructing blood flow and the SAM of the mitral apparatus towards the hypertrophied septum lead to LVOT obstruction.3 Septal reduction therapy, such as surgical myectomy usually preferentially targets basal segment of the IVS to abolish Venturi forces, which are considered the major cause of SAM. For RF ablation, the lesion is mainly located in SAM-septal contact area, a similar target of myectomy. However, in some patients with diffuse septal hypertrophy, in addition to Venturi forces, overall disturbed blood flow also contributes to SAM.18, 19The anterior-directed flow caused by mid-septal thickening can overlap with the posterior surfaces of the AML and push it to hypertrophied septum which results in obstruction. In this study, the patterns of interventricular sepal hypertrophy of 16 patients are recorded and analysed. Besides the basal septum, the hypertrophied region can extend into mid or apical septum in whom the reduction of gradients is lower than others (Table 2 and Figure 6), which emphasized the role of mid-septal hypertrophy. Ablation of the SAM-contact area is accurate and effective for patients with limited hypertrophy in basal septum but might be insufficient for patients with mid-septal hypertrophy. Extended myectomy has been designed and applied to redirect flow away from the mitral valve20 and extended ablation targeting mid-septal segments need to be evaluated for patients in needs in the future.
In addition to left ventricular hypertrophy, structural abnormalities of the mitral valve and sub-mitral apparatus, such as leaflet elongation and anterior displacement of the papillary muscles also contribute to SAM pathophysiology.21 The average length of AML is longer in HOCM patients (34 mm vs. 24 mm). For those with an extraordinarily elongated AML (> 30 mm), the residual portion often extends past the point of coaptation. Without the constraint of the LV–left atrium pressure difference, it can freely move with LV flow, even at low velocity. Late diastolic and early systolic flow then pushes the protruding leaflets into apposition with the septum.18, 19, 22 Meanwhile, the anterior displacement of PM exacerbates the magnitude of obstruction. There is closer proximity between the DPM and the hypertrophied septum especially the mid-septum and even contact each other at end of systole (Figure 3). This malformation positions mitral leaflets anteriorly into the flow stream cause a crucial overlap of the inflow and outflow portions of the LV that predisposes to SAM. Sherrid et al. indicated that a simple septal reduction procedure was not enough if AML > 30 mm and (or) DPM exist and resect - plicate - release (RPR) operation was required to alleviate LVOT gradients.15, 23 Consistent with this observation, the reduction of gradient in patients with elongated AML or DPM is significantly lower. (Figure 6) In our approach, RF energy was applied to create a localized reduction in contractility responsible for the dynamic obstruction. There was a non-significant reduction of the septal thickness after ablation. The structural abnormalities of mitral valve and papillary muscle are nearly impossible to rectify by RF ablation. Therefore, for patients with AMLs > 30 mm and (or) DPM, surgery with resect-plicate-release operation should be preferred.
For septal myectomy and ASA, the risk of permanent pacemaker implantation during follow up varied between 2.4–12.5% and 1.7–22.0%, respectively.4 In this study, the His and left bundle were annotated in the CARTO map before ablation in order to avoid injury to the conduction system and no permanent AVB occurred during follow up. After 6 months follow up, LBBB/RBBB persisted in two patients and the QRS duration was significantly prolonged in the overall cohort. In cases where the LBB courses directly through the SAM area, we intended to preserve the function of LBB which may affect the outcome of ablation. As for patient with prior RBBB, LBB preservation is more necessary. Patient No. 20 with a history of RBBB due to previous ASA developed into transient AVB several seconds after RF septal ablation. Methylprednisolone was administrated and AV conduction recovered soon after. Considering the high incidence of AVB, further ablation was aborted. In addition to the risk of AVB, tissue edema after RF ablation in SAM area may aggravate the LVOT obstruction transiently.14 Myocardial edema is well recognized in CMR studies following RFA in other procedures.24 In this study, paradoxical increase of LVOT gradient after procedure was observed in 8 patients. Methylprednisolone was administered for the following three days after ablation and edematous LVOT obstruction alleviated.