A Chiari network refers to a filamentous, weblike structure in the right atrium that results from incomplete resorption of the embryonic sinus venosus. It is an uncommon anatomical variant.

Epidemiology

Prevalence estimates for the general population vary widely, ranging from 2%-10% of randomly selected, asymptomatic patients ².

Clinical presentation

While thought to be of no innate clinical significance, the presence of a Chiari network is associated with the following ³:

• tricuspid regurgitation

• infectious endocarditis

• embolic transient ischaemic attack and/or stroke

  • requires presence of a patent foramen ovale

Pathology

Resorption of the right valve of the embryologic sinus venosus occurs during the normal development of the right atrium, forming the Eustachian valve and the crista terminalis. Incomplete resorption, however, may result in either the persistence of a prominent Eustachian valve or a Chiari network, which shares the same point of anatomical attachment but differs in appearance. 

Radiographic features

Echocardiography

A Chiari network may be visualised with both transthoracic (TTE) or transoesophageal (TEE) approaches, most often an incidental finding when the former is utilised to investigate an unrelated pathology.

Transthoracic echocardiography

Multiple transthoracic windows and views should be used to adequately assess the right atrium, including the apical 4-chamber view, parasternal right ventricular inflow tract view, and a basally angulated parasternal short axis view. The following grey scale features are characteristic of a Chiari network;

• reticulated network of hyperechoic membranous structures located within the right atrium

  • typically originates in the most posterior extent of the right atrium (RA), extending a variable distance medially and inferiorly

  • may be attached to various other structures within the RA, including the interatrial septum

• freely mobile throughout systole and diastole

  • absence of diastolic migration into the right ventricle (RV) or the right ventricular outflow tract (RVOT) 

• most often tethered to the orifice of the inferior vena cava, around which it may appear to rotate ¹

CT

Chiari network can sometimes be seen as a lacelike filling defect but can be challenging to diagnose with confidence due to heterogeneity of contrast enhancement within the right atrium. Identification of its attachment to the inferior vena cava ostium can aid in establishing a diagnosis.

History and etymology

Originally described in 1897 by the Austrian pathologist Hans Chiari (1851-1916) ⁴.