These are multicystic tumours of the atrioventricular node which originate during embryogenesis of heart.

Their histogenesis is debatable and various suggestions are:

1.  Derived from the invaginations of epicardial mesothelium within atrioventricular canal.

2.  Derived from displaced endodermal foregut tissue (suggested from positive immunostaining for CEA and keratin). Thus pathogenetically , these lesions are similar to bronchogenic cysts.

Clinical presentation :  These may occur at any age and partial or complete atrioventricular heart block is the dominant feature. 

Often sudden death may be the first sign of this tumor.

It may almost completely replace the atrioventricular node which may lead to intra-atrial conduction defect, paroxysmal atrial arrhythmia and spontaneous intermittent pre-excitation (through multiple left-sided accessory pathways).

Gross features:  There is thickening of the atrial septum with or without elevated nodules in the region of atrioventricular node.

Microscopic features:  Polycystic lesion may replace the atrioventricular node.

It is composed of cysts, ducts and solid nests of cells.

Cavities are lined by single or multilayered cuboidal cells. The cells are PAS (diastase resistant) and alcian blue positive.

Some cell nests resemble squamous or transitional epithelium.