Discussion
Pankaj Kaul, Kalyana Javangula and Shahme A Farook, Massive benign pericardial cyst presenting with simultaneous superior vena cava and middle lobe syndromes
Journal of Cardiothoracic Surgery 2008
Primary mediastinal cysts constitute approximately one fifth of all mediastinal masses. The cysts may originate from pleura or pericardium, tracheobronchial tree, gastrointestinal tract, neurogenic tissue, thymus gland or lymphoid tissue. Benign teratomas may present as epidermoid cysts, dermoid cysts or cystic teratomas [1]. Mediastinal cystic masses may also result from specific or non-specific infections or parasitic infestations like Echinococcus [2].
Anterior mediastinal cysts most commonly are pleuropericardial, thymic, teratomatous or cystic hygromas.
Pleuropericardial cysts are benign mesothelial cysts that arise as a result of persistence of one of the mesenchymal lacunae that normally fuse to form the pericardial sac [3], or, as suggested by Lillie [4], due to the failure of an embryological ventral diverticulum to fuse. Alternatively, they may be believed to arise from the infolding of the advancing edge of the pleura during its embryological development. These cysts are unilocular, contain clear watery fluid, present typically in anterior cardiophrenic angle, more often on right side than left. Microscopically, the wall has a single layer of mesothelial cells resting on a loose stroma of connective tissue.
True thymic cysts are thin walled, unilocular and contain normal thymic tissue within their walls and arise from third branchial pouch. Microscopically, the wall is lined by low cuboidal epithelium. However, malignant degeneration within a thymoma may result in a cystic thymoma, with a residual mass projecting into the cavity of the cyst from the wall.
Typically, lymphangiomas arise from neck and extend into mediastinum. They contain chyle and are classified according to the size of the spaces into cystic hygromas or cavernous lymphangiomas. Cystic hygromas are multiloculated, and a mediastinal hygroma is almost always an extension of a cervical hygroma. However, rarely, a uniloculated primary anterior mediastinal lymphogenous cyst containing yellow or brown fluid may be found [5].
Teratodermoids are classified generically as benign germ cell tumours. They are further divided into three categories: epidermoid cysts which are lined by simple squamous cell epithelium, dermoid cysts which have squamous epithelial lining containing elements of skin appendages like hair and sebaceous glands and teratomas which may be solid or cystic and contain identifiable cellular elements of two or three germinal layers [1].
