Asymmetric hypertrophic cardiomyopathy

Asymmetric hypertrophic cardiomyopathy is the most common morphological variant or phenotype of hypertrophic cardiomyopathy (HCM).

Epidemiology

Approximately 60-70% of cases with hypertrophic cardiomyopathy make up for the asymmetric phenotype ^1-3.

Associations

Asymmetric hypertrophic cardiomyopathy might be associated with the following ^1-4:

• dynamic left ventricular outflow obstruction (in up to 30%) – gradient ≥30mmHg ^1-4
• systolic anterior motion (SAM) of the anterior mitral leaflet
• mitral leaflet abnormalities or mitral regurgitation (in up to 20%)
• left atrial enlargement
• myocardial fibrosis

Clinical presentation

The clinical presentation of asymmetric hypertrophic cardiomyopathy does not differ significantly from other types of hypertrophic cardiomyopathy. Patients might present with a performance deficit, dyspnea, angina, presyncope, syncope on exertion or might be entirely asymptomatic ².

Complications

Complications of asymmetric hypertrophic cardiomyopathy include ^2-4:

• diastolic dysfunction
• myocardial ischemia
• ventricular arrhythmias
• sudden cardiac death

Pathology

Asymmetric hypertrophic cardiomyopathy features an asymmetric disproportionally increased myocardial wall thickness. The interventricular septum or more precisely the anteroseptal segments of the myocardium are most frequently affected but other segments might also be involved ^1,2.  A wall thickness of ≥15 mm or z-score ≥2 and a ratio of >1.5 compared to non-hypertrophied normal segments is considered diagnostic ¹.

Subtypes

The following subtypes of asymmetric septal hypertrophic cardiomyopathy can be distinguished ¹:

• basal septum hypertrophic cardiomyopathy
• diffuse septum hypertrophic cardiomyopathy
• mid septum hypertrophic cardiomyopathy
• free-wall hypertrophic cardiomyopathy

Microscopic appearance

Microscopically hypertrophic cardiomyopathy is characterized by the following features ^2,5:

• cardiac myocyte hypertrophy
• cardiac myocyte disarray
• interstitial or replacement fibrosis
• bizarre enlarged nuclei with nuclear hyperchromasia and pleomorphism

Radiographic features

Asymmetric septal hypertrophic cardiomyopathy has been defined by the following imaging criteria ^1-4:

• basal septal thickness ≥15 mm in adults
• a wall-thickness ratio of the septum versus the inferior wall >1.5 
• in children z-score≥2

Echocardiography

Echocardiography can visualize asymmetric hypertrophy and can provide additional information on cardiac function including cardiac strain.

MRI

Cardiac MRI can demonstrate asymmetric hypertrophy and can assess cardiac function including cardiac volumes and cardiac strain. It can provide additional prognostic information concerning associated myocardial fibrosis and potential differential diagnosis.

• cine imaging
  • focal or diffuse asymmetric wall thickening (e.g. sigmoid shaped interventricular septum)
  • increased mass and increased left ventricular ejection fraction
• cardiac strain imaging: decreased global longitudinal strain 
• cardiac tissue characterization
  • T1 mapping: increased native T1 values
  • ECV: increased
• IRGE/PSIR:
  • non-ischemic focal mesocardial, fan-shaped or patchy late gadolinium enhancement
  • often visible at the right ventricular insertion sites (hinge points)
  • indicating replacement fibrosis or myocardial scarring

Radiology report

The radiological report should include a description of the following:

• location and extent of hypertrophic wall segments
• cardiac volumes and measurements including left ventricular mass
• cardiac wall motion abnormalities
• signs of diastolic dysfunction
• presence of myocardial crypts
• signs of myocardial fibrosis, myocardial scar or focal replacement fibrosis
• mitral annular plane systolic excursion (MAPSE)
• left ventricular outflow obstruction
• systolic anterior movement (SAM) of the mitral valve

Treatment and prognosis

Besides medical treatment patients with asymmetric hypertrophic cardiomyopathy might benefit from interventions as myomectomy or septal ablation especially if in association with left ventricular obstruction and higher gradients (e.g. ≥50 mmHg) ^3,4. Patients with a history of cardiac arrest, significant arrhythmias or first-degree family history of sudden cardiac death, as well as massive hypertrophy (myocardial wall thickness ≥30 mm), should be considered for an implantable cardioverter-defibrillator ³.

Differential diagnosis

Conditions that might mimic the clinical presentation or imaging appearance of asymmetric hypertrophic cardiomyopathy include ^3,4,6:

• concentric hypertrophic cardiomyopathy
• midventricular hypertrophic cardiomyopathy
• apical hypertrophic cardiomyopathy (Yamaguchi syndrome)
• mass-like hypertrophic cardiomyopathy
• hypertensive heart disease
• cardiac amyloidosis
• cardiac sarcoidosis
• Anderson-Fabry disease or Danon disease
• athlete heart syndrome