In any case, local disruption of thymic tissue can induce the formation or growth of cysts. In the current series, they mainly encompassed thymic cysts of congenital origin, and we excluded the cystic masses of neoplastic origin such as cystic thymomas or degeneration after surgery. Of note, Suster and Rosai reported 18 cases of multiocular thymic cysts, in addition to uniocular cysts.
We experienced two cases of meningocele in the posterior mediastinum. Although the etiology of intrathoracic meningocele is unknown, it is frequently associated with von Recklinghausen disease (neurofibromatosis type 1). Due to its rarity, in-trathoracic meningocele is seldom considered in the differential diagnosis of a mediastinal cyst. A meningocele may be defined as a saccular protrusion of the spinal meninges through an intravertebral fora-men. As reviewed by Erkulvrawatr et al, 20% of the cases were associated with neurologic impairments such as weakness, sensory disorder, hyperactive reflexes, and occasional headache, probably due to low cerebrospinal fluid pressure.
We also encountered an extremely rare case of a thoracic duct cyst in 1976; pathologically, the cyst was unilocular with an associated connection to the thoracic duct. As Tsuch-iya et al later described another case, we made a preoperative diagnosis of thoracic duct cyst by lym-phangiogram. They can have a distinct connection with the thoracic duct. Regarding pathogenesis, thoracic duct cysts occur secondary to a congenital weakness and degenerative wall changes, which result in aneurysm-like dilatation and subsequent cyst formation.
The clinical characteristics in mediastinal cysts are outlined in Table 3. Overall, 38 patients (36.2%) with mediastinal cysts were symptomatic.