Filigreed Network of Venous Valves (Chiari)

The "Filigreed Network of Venous Valves (Chiari)," more commonly referred to as the Chiari network, is a congenital anatomical structure found within the right atrium.

• Anatomy and Formation of the Chiari Network

  • The Chiari network is described as a fine, lattice-like or serpentine fenestrated membrane located on the floor of the right atrium.

  • It is a fenestrated, filamentous remnant of the right valve of the sinus venosus (specifically the valve of the inferior vena cava or the eustachian valve). It is a thin, mobile, web-like or net-like structure seen in the right atrium, often near the entrance of the inferior vena cava (IVC) or coronary sinus. 

  • Embryology: Incomplete regression of the right sinus venosus valve, particularly the eustachian and thebesian valves. It develops from the incomplete regression of structures from the right sinus horn (sinus venosus) during embryonic development. 

  • Prevalence: Seen in up to 2–4% of the population on echocardiography. 

  • This network often extends from the Thebesian or Eustachian valve, attaching to the right atrial wall or directly to the atrial wall itself. It can also be seen emanating from the coronary sinus within the right atrium.

  • A prominent redundant Chiari network can be observed in various echocardiographic views, including the tricuspid inflow view and apical four-chamber view. Echocardiography is a known method for its recognition.

• Clinical Significance of the Chiari Network

  • Generally, the Chiari network itself holds no clinical significance and is a variant of normal.

  • However, it is known to be associated with hypoplasia of the right-sided cardiac structures.

  • It is also referenced in the context of systemic venous valve anomalies and cardiac malpositions.

  • Usually benign and incidental. Occasionally associated with atrial arrhythmias, catheter entrapment, or paradoxical embolism if coexistent with a patent foramen ovale (PFO). Important to distinguish from thrombus or endocarditis vegetation.

• Relationship to Cor Triatriatum Dexter

  • In cor triatriatum dextrum, the Eustachian membrane can be very thickened and effectively divide the right atrium into two distinct chambers, potentially causing inflow obstruction to the right atrium and tricuspid valve. The cor triatriatum is a thick membrane that may cause partial or complete separation between the smooth (sinus venarum) and trabeculated (appendage) portions of the right atrium.

• Associated Venous Valves (Eustachian and Thebesian)

  • Other "venous valves" that are fibromuscular webs located at the openings of major veins into the right atrium, and these are directly linked to the Chiari network:

    • Eustachian Valve: This valve is situated at the opening of the inferior caval vein into the right atrium. A fibrous structure called the tendon of Todaro runs from the union of this valve into the septum between the coronary sinus and the oval fossa.

    • Thebesian Valve: This valve is present within the coronary sinus and is not readily imaged in most normal hearts.

  • These valves, along with the Chiari network, are part of the right atrial venous valve system.

  • Additionally, thin, valve-like membranes may partly cover the orifices where the left hepatic vein joins the ductus venosus at its entry into the inferior vena cava, and where the right hepatic vein enters the inferior vena cava separately. While their precise function is unknown, it's conjectured they may facilitate directional flow of venous streams. The vestibular spine is involved in the formation of the inferior zone of apposition of these venous valves that guard the systemic venous sinus.

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