ABSTRACT
Schistosomes may reach the CNS once worms have matured, paired and laid eggs. In order for symptomatic CNS involvement to occur, eggs must reach the CNS through retrograde venous flow in the Batson vertebral epidural venous plexus which connects the portal venous system and the venae cavae to the spinal cord and cerebral veins (Batson 1940; Pittella 1997; Ferrari 2004; CarodArtal 2008). This route permits migration of the adult worms to sites close to the CNS followed by in situ oviposition or massive embolization of eggs from the mesenteric-pelvic system (Batson 1940; Pittella 1997; Ferrari 2004; Carod-Artal 2008). The small round eggs of S. japonicum may
travel all the way to the brain but the eggs of S. mansoni and S. haematobium, which are slightly larger, are retained within the lower spinal cord (Ferrari 2004). Asymptomatic CNS involvement, as seen in advanced hepatosplenic and cardiopulmonary schistosomiasis, may result from the above-mentioned venous mechanism of invasion or via arterial invasion. During arterial migration the eggs may pass into the arterial circulation through pulmonary arteriovenous shunts or through portopulmonary anastomoses via the azygos vein, which shunts form as a consequence of portal hypertension (Batson 1940; Pittella 1997; Ferrari 2004; Carod-Artal 2008). The lack of symptoms is attributed to the sparse distribution of the eggs and to the scant periovular inflammatory reaction observed in this severe chronic form of infection (Pittella 1997; Ferrari 2004).
