# Hypertrophic Cardiomyopathy: Septal Myectomy

## Introduction

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disorder, affecting approximately 1 in 500 individuals. It is characterized by asymmetric left ventricular hypertrophy in the absence of hemodynamic loading conditions. When associated with significant left ventricular outflow tract (LVOT) obstruction, surgical septal myectomy remains the gold standard treatment for patients refractory to medical therapy.

## Pathophysiology and Clinical Presentation

### Genetics and Structural Changes

HCM follows autosomal dominant inheritance with variable penetrance, most commonly caused by mutations in sarcomeric protein genes, particularly MYH7 and MYBPC3. Pathological hallmarks include myocyte disarray, interstitial fibrosis, and small vessel coronary disease. Asymmetric septal hypertrophy with a septum-to-free wall ratio exceeding 1.3:1 is the classic morphology. Hypertrophy may involve the basal septum, mid-ventricle, or apex, each with differing clinical implications.

### LVOT Obstruction

Dynamic obstruction occurs in approximately 70% of HCM patients at rest or with provocation. The mechanism involves systolic anterior motion (SAM) of the mitral valve leaflet contacting the hypertrophied septum. SAM causes both LVOT obstruction and secondary mitral regurgitation with a posteriorly directed jet. Obstruction is worsened by decreased preload (Valsalva, dehydration), decreased afterload, and increased contractility. A resting LVOT gradient of 30 mmHg or greater is clinically significant, and 50 mmHg or greater is the threshold for intervention.

### Clinical Manifestations

Dyspnea on exertion is the most common symptom, accompanied by angina, syncope, presyncope, and palpitations. The risk of sudden cardiac death (SCD) is 0.5-1% per year, higher in young patients and athletes. Heart failure with preserved ejection fraction may progress to end-stage HCM with reduced EF in 5-10% of patients. Atrial fibrillation is common, occurring in 20-25% of patients, and carries a high thromboembolic risk.

![Echocardiographic image demonstrating SAM and LVOT obstruction in HCM](images/hcm-sam-echo.png)

## Diagnostic Evaluation

### Imaging

Transthoracic echocardiography is the first-line study, assessing septal thickness, LVOT gradient at rest and with Valsalva, SAM, and mitral regurgitation severity. Cardiac MRI is the gold standard for morphological assessment, and late gadolinium enhancement quantifies fibrosis burden, which is prognostic for SCD and arrhythmia. Transesophageal echocardiography is essential intraoperatively to assess adequacy of myectomy and residual mitral regurgitation.

### Hemodynamic Assessment

Provocative maneuvers including Valsalva, standing, amyl nitrite, and exercise are used to unmask latent obstruction. Exercise echocardiography is indicated for patients with symptoms but a resting gradient below 50 mmHg. Invasive hemodynamics with cardiac catheterization may be used in selected cases, with the Brockenbrough-Braunwald-Morrow sign (post-PVC pulse pressure decline) being a classic finding.

## Medical Management

First-line therapy consists of non-vasodilating beta-blockers such as metoprolol or nadolol to reduce heart rate and contractility. Verapamil or diltiazem serves as second-line therapy for patients intolerant of beta-blockers. Disopyramide is a negative inotrope with class IA antiarrhythmic properties that serves as a useful adjunct for obstruction. Mavacamten, a cardiac myosin inhibitor, is FDA-approved for obstructive HCM and reduces LVOT gradient and SAM. Vasodilators, high-dose diuretics, digoxin, and inotropes should be avoided as they worsen obstruction.

## Surgical Septal Myectomy

### Indications

Surgery is indicated for severe symptoms (NYHA class III-IV) despite maximally tolerated medical therapy, with a resting or provocable LVOT gradient of 50 mmHg or greater, septal thickness of 15 mm or greater at the site of planned resection, and significant SAM-related mitral regurgitation contributing to symptoms.

### Surgical Technique (Morrow Procedure)

The procedure is performed via median sternotomy with cardiopulmonary bypass and aortic cross-clamping. An aortotomy provides exposure to the LVOT and interventricular septum. The extended septal myectomy involves a trough-shaped resection of the basal septum extending beyond the point of mitral-septal contact toward the mid-ventricle. The depth of resection is approximately 50% of septal thickness at the point of maximal hypertrophy. The width extends from the nadir of the right coronary cusp to the commissure between the right and left cusps. The length is extended to or beyond the bases of the papillary muscles, typically 5-7 cm.

### Adjunctive Procedures

Mitral valve repair or replacement is performed if intrinsic leaflet pathology is present beyond SAM-related mitral regurgitation. Anomalous papillary muscle resection or mobilization addresses muscular contributions to obstruction. Abnormal chordal attachments to the septum or free wall should be divided. A concurrent maze procedure is performed for patients with atrial fibrillation. ICD implantation for SCD risk in selected patients is considered separately from the myectomy decision.

![Surgical illustration of extended septal myectomy through transaortic approach](images/septal-myectomy-technique.png)

## Outcomes and Complications

### Results of Myectomy

Operative mortality at experienced centers is less than 1%, with a strong volume-outcome relationship. LVOT gradient reduction to below 10 mmHg is achieved in more than 95% of patients. Mitral regurgitation improves significantly or resolves in more than 80% without direct mitral valve intervention. Long-term survival approaches that of the age-matched general population. Symptom relief to NYHA class I-II is achieved in more than 90% of patients at 5 years.

### Complications

Complete heart block requiring permanent pacemaker occurs in 2-5% of cases, with higher rates in repeat operations. Iatrogenic ventricular septal defect occurs in less than 1%. Aortic regurgitation from injury to the aortic valve is rare with proper technique. Residual obstruction requiring reoperation occurs in less than 2% at experienced centers.

## Septal Myectomy vs. Alcohol Septal Ablation

| Feature | Septal Myectomy | Alcohol Septal Ablation (ASA) |
|---------|----------------|-------------------------------|
| Approach | Surgical (transaortic via sternotomy) | Percutaneous (catheter-based ethanol injection) |
| Mechanism | Direct muscle resection | Chemical infarction of basal septum |
| Operative/procedural mortality | <1% (experienced centers) | 1-2% |
| Complete heart block requiring PPM | 2-5% | 10-20% |
| Residual obstruction | <2% | 10-20% (may require repeat) |
| Preferred patients | Younger; thick septum >25 mm; concomitant pathology; abnormal papillary anatomy | Elderly; comorbidities; unfavorable surgical anatomy |
| Concomitant procedures | MV repair, maze, ICD, anomalous muscle resection | None |
| Long-term durability | Excellent (decades of follow-up) | Good but less long-term data |

Alcohol septal ablation (ASA) is a percutaneous alternative involving ethanol injection into the first septal perforator artery. Myectomy is preferred for younger patients, severely thick septa exceeding 25 mm, concomitant cardiac pathology requiring surgery, and abnormal papillary or subvalvular anatomy. ASA may be appropriate for elderly patients with comorbidities, unfavorable surgical anatomy, or patient preference. Myectomy has lower rates of residual obstruction, heart block (compared to ASA's 10-20%), and need for reintervention. Both procedures should be performed at experienced HCM centers with minimum case volume thresholds.

![Comparison of outcomes between septal myectomy and alcohol septal ablation](images/myectomy-vs-asa-outcomes.png)

## Key Clinical Pearls

Extended myectomy beyond the point of mitral-septal contact is critical, as inadequate resection is the most common cause of residual obstruction. SAM-related mitral regurgitation typically resolves after adequate myectomy without need for mitral valve intervention. The volume-outcome relationship is well established, and referral to experienced HCM centers performing 20 or more myectomies per year dramatically reduces operative mortality. Cardiac MRI with late gadolinium enhancement is essential for SCD risk stratification and surgical planning. Mavacamten may reduce the need for myectomy in some patients, but long-term data and its role relative to surgery are still being defined.

## References

1. Ommen SR, Mital S, Burke MA, et al. 2020 AHA/ACC Guideline for the Diagnosis and Treatment of Patients With Hypertrophic Cardiomyopathy. *Circulation*. 2020;142(25):e558-e631.
2. Desai MY, Bhonsale A, Smedira NG, et al. Predictors of Long-Term Outcomes in Symptomatic Hypertrophic Obstructive Cardiomyopathy Patients Undergoing Surgical Relief of Left Ventricular Outflow Tract Obstruction. *Circulation*. 2013;128(3):209-216.
3. Olivotto I, Oreziak A, Baez-Escudero JL, et al. Mavacamten for Treatment of Symptomatic Obstructive Hypertrophic Cardiomyopathy (EXPLORER-HCM): A Randomised, Double-Blind, Placebo-Controlled, Phase 3 Trial. *Lancet*. 2020;396(10253):759-769.
4. Maron BJ, Dearani JA, Ommen SR, et al. Low Operative Mortality Achieved With Surgical Septal Myectomy. *J Am Coll Cardiol*. 2015;66(11):1307-1308.