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.