Interventricular septal bulge​ (also known as a sigmoid septum) is a common finding in imaging studies in the elderly population and refers to an isolated thickened basal septum resulting in a sigmoid configuration.

Although it is unclear whether this entity is part of the normal aging process or lays within the phenotypic spectrum of hypertrophic cardiomyopathy (HCM), on many occasions both pathologies pose differential diagnostic problems, with potential implications for management and prognosis.

Epidemiology

Its prevalence increases with age, reaching 10% in individuals ≥70 years. There is no sex preference, in contrast with hypertrophic cardiomyopathy which shows a female predominance ^1,2.

It is believed that the cardiac aging process includes shortening of the long axis and decreased aortoseptal angle, which can contribute to the sigmoid-shaped septum ^3,4.

Clinical features

There is no conclusive association between hypertension and septal bulge. Due to the high prevalence of hypertension in the elderly, a history of it is of scarce help in suggesting or ruling out the diagnosis ¹. 

Left ventricular hypertrophy signs on ECG are more frequent in patients with hypertrophic cardiomyopathy (65%) than those with a septal bulge (12%). However, ECG lacks specificity for the diagnosis owing to the same findings can be seen due to hypertension or aortic stenosis ^3,5.

Hypertrophic cardiomyopathy displays an extensive array of genetic mutations. Tests are gathered in commercial panels that can be positive in 44% of those with hypertrophic cardiomyopathy ^ref. 

However, older patients with basal septal hypertrophic cardiomyopathy appear to be less commonly associated with a detectable mutation than other morphologic variants (23%) ^ref. This prevalence of detectable mutations is even lower in septal bulge (8%) ^ref.

Clinical features that point  against a septal bulge​ or sigmoid septum: 

• family history of HCM or amyloidosis or positive genotype

• hypertrophic changes in ECG in the absence of an increased afterload (hypertension, aortic stenosis, others)

Radiographic features

There are no validated diagnostic criteria and one may suggest the diagnosis of septal bulge based on a combination of image and clinical features.

CT/MRI

The main imaging feature is the presence of asymmetric basal septal hypertrophy in an elderly patient (diastolic thickness 12-15 mm). However, clinical and technical difficulties may rise doubts, especially among the 14-15 mm grey region.  

Imaging features pointing against a septal bulge​ or sigmoid septum: 

• interventricular septum diastolic thickness ≥15 mm or basal septal thickness/midventricular septal thickness ratio ≥1.5

• a systolic anterior motion of the mitral valve and/or left ventricular outflow tract obstruction

On MRI

• late gadolinium enhancement: representing fibrosis

• prolonged T1 relaxation time

Differential diagnosis

• hypertrophic cardiomyopathy (HCM)

• cardiac amyloidosis

• hypertension

• aortic stenosis