MV Surgery as Adjunct to Surgical Myectomy for Obstructive HCM

Less Is More Than Enough

Evolving Surgical Approaches

Dynamic LVOTO is caused by narrowing of the LVOT due to hypertrophy of the basal interventricular septum in combination with systolic anterior motion (SAM) of the mitral valve (MV) leaflets and leaflet septal contact. The distortion of mitral leaflet coaptation caused by SAM leads to significant mitral regurgitation (MR), which is quantitatively related to the degree of obstruction (1). The extended Morrow myectomy is the procedure most commonly performed in contemporary practice to surgically relieve dynamic LVOTO (1–4). This procedure is performed through an aortotomy in a relatively blind fashion with resection of septal muscle about 1 cm beyond the site of SAM septal contact, leaving 8 to 10 mm of residual septal muscle. Historically, MV replacement was proposed as an alternative to septal myectomy because it eliminates LVOTO and MR as well as the risk of ventricular septal perforation that may result from overly aggressive myectomy. However, it is rarely performed alone in contemporary practice given the long-term complications inherent to prosthetic valves and anticoagulation therapy. Isolated myectomy results in remarkable clinical improvement with surgical mortality rates of <1% when performed at experienced centers (4).

Etiology of MR in Obstructive HCM

Transesophageal echocardiography, cardiac magnetic resonance imaging, and direct pathologic inspection have demonstrated that primary structural abnormalities of the MV apparatus are common among HCM patients independent of the hypertrophy pattern. These abnormalities include elongated mitral leaflets with increased surface area, abnormal chordal attachments, and papillary muscle structural abnormalities and malposition (1,3,5). These abnormalities in the MV apparatus predispose the MV leaflets to being drawn into the LVOT by hydrodynamic drag forces along with a Venturi effect resulting in SAM (Figure 1). In the vast majority of cases, MR is SAM dependent because malcoaptation of the MV leaflets in midsystole causes an interleaflet gap, which typically results in eccentric posteriorly directed MR (1,5).

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Figure 1

Mechanism of Dynamic LV Outflow Obstruction and Associated MR

LV = left ventricle; LVOT = left ventricular outflow tract; MV = mitral valve; MR = mitral regurgitation; SAM = systolic anterior motion.

Extensive myectomy results in enlargement of the LVOT area and redirection of forward flow with loss of the drag and Venturi effects on the MV. Previous surgical studies from highly experienced centers including ours have shown that, in the absence of iatrogenic MV injury, pre-existing MV prolapse, or chordal rupture, MV surgery in addition to myectomy is not needed as long as the myectomy is adequate (Table 1). Some centers have suggested MV intervention (e.g., anterior MV leaflet plication, extension or retention plasty, and reconstruction of the subannular mitral apparatus) whenever SAM significantly contributes to the LVOT pressure gradient and/or produces significant MR (6–8). Studies from the Cleveland Clinic and Italy have shown that in a subset of patients with relatively mild septal hypertrophy and significant LVOTO, individualized MV repair techniques (e.g., plication and shortening of the A2 scallop of the MV, reorientation of papillary muscles, and transaortic secondary chordae cutting) with or without concomitant myectomy eliminate LVOTO and avoid MV replacement (9,10).

From 11% to 20% of patients with obstructive HCM demonstrate structural MV disease causing significant MR. These include degenerative abnormalities (MV prolapse and chordal rupture), restrictive abnormalities (traumatic fibrosis of the anterior MV leaflet from repeated septal contact), papillary abnormalities (abnormal insertion of the papillary muscle head directly into the anterior leaflet, accessory papillary muscles), and significant mitral annular calcification. These patients frequently require concomitant MV surgery, with MV repair being the preferred procedure.

In this issue of the Journal, Hong et al. (11) describe the largest series to date consisting of nearly 2,000 patients with obstructive HCM undergoing septal myectomy at the Mayo Clinic. More than 90% of these patients underwent isolated extended myectomy, eliminating significant MR (grade ≥3) in >98% of patients. Concomitant MV intervention was performed in only 174 patients (8.7%). Of these, 99 (57%) were known to have associated intrinsic MV abnormalities requiring MV surgery before myectomy. Of the remaining 75 patients, 33 were found on either intraoperative transesophageal echocardiogram or direct valve inspection to have structural MV abnormalities necessitating MV intervention. An additional 12 patients underwent MV surgery for iatrogenic injury to the MV apparatus during surgery. The remaining 30 patients underwent MV surgery for various indications, such as persistent SAM and anterior mitral leaflet redundancy. In only about 25% of patients requiring MV intervention was the MV replaced, whereas in >75% MV repair was feasible. The reported 10-year survival rate was higher with MV repair than with MV replacement.

The study by Hong et al. (11) confirms findings of previous investigations. MR in HCM is mostly SAM dependent and may be abolished in the vast majority of patients with extended myectomy alone. Residual MR that is more than mild usually results from incomplete extension of myectomy toward the apex or in the minority of patients from iatrogenic MV damage (<1% in the current study). The study by Hong et al. also demonstrates that if performed at HCM dedicated centers, MV repair rather than replacement is feasible in the majority of patients and carries a better prognosis.

In most cases, intrinsic MV abnormalities requiring MV surgery were detected pre-myectomy. However, in a substantial number of patients these abnormalities were only revealed intraoperatively. Therefore, it is prudent that surgeons performing myectomy have special expertise in MV repair and anticipate the need for possible MV intervention.

The study by Hong et al. (11) also confirms the safety of multiple cardiopulmonary bypass periods during myectomy. In 58 patients, the decision to perform MV intervention was made after the adequacy of myectomy was evaluated with intraoperative transesophageal echocardiography. This has been proven to be safe with no significant difference in late survival. This approach has most likely also contributed to the low rate of MV interventions.

Hong et al. (11) acknowledge that, like all retrospective studies, their study has limitations. However, most troubling is the lack of data on the patients lost to follow-up. Moreover, it can be seen from the Kaplan-Meier curves that only about 50% of patients who underwent concomitant MV surgery were followed for >2 years. Information on the causes of late deaths was incomplete. Additionally, no information on the anterior MV leaflet and/or area and the septal thickness were given. Thus, one cannot learn about the relationship between these parameters and concomitant MV surgery and outcomes.

The impressive results presented by Hong et al. (11) are based on the experience of an expert center. As with other surgical procedures, results of myectomy are highly dependent on experience (4). Indeed, the postoperative mortality rate in the Nationwide Inpatient Sample was alarmingly high at 5.9% compared with only 0.4% at HCM dedicated centers (12).

In contrast to the Mayo Clinic approach, other well-experienced groups, such as the Cleveland Clinic, have performed MV interventions as part of their surgical approach, with excellent results and no increase in adverse outcomes. Perhaps this approach is best for patients with minimal hypertrophy when myectomy alone might not be sufficient. Additionally, the more conservative approach taken by Hong et al. (11) resulted in a lower rate of MV replacement.

The study by Hong et al. (11) demonstrates that concomitant MV intervention, which increases the complexity of an already technically demanding operation, can effectively be reserved for either significant intrinsic MV abnormalities causing MR independent of SAM or iatrogenic damage to the valve. Furthermore, the study highlights that adequate extended myectomy alone is the only treatment required for the vast majority of patients with obstructive HCM to effectively relieve LVOTO and significant MR. It also highlights the need to be prepared to perform MV intervention when necessary and to have adequate preoperative and intraoperative assessment of MV anatomy and degree of residual MR after myectomy. As in many things in life, the study shows that doing less is often more than enough.