[Overdrainage in the treatment of hydrocephalus]

In infants or children, hydrocephalus is usually the consequence of an obstacle in the cerebro-spinal fluid (CSF) pathways and is most frequently treated by the insertion of a ventriculo-peritoneal shunt (rarely ventriculo-atrial). The CSF flow through such a shunt is equal to the ratio of the difference (DP) between the CSF input and output pressures over the valve resistance. When the child is in the upright position, a DP increase occurs due to the height of the hydrostatic column between the inlet and the shunt outlet. Thus if the shunt drains correctly in the prone position, it overdrains in the upright position. As the CSF flow through the shunt in standing patients is higher than the CSF secretion, the excess fluid will be taken out of the ventricles, resulting in pericerebral collections, slit ventricles or post-shunt craniostenoses. Many different techniques have been proposed to reduce overdrainage. The Orbis-Sigma shunt was designed in the "Service des Enfants-Malades" precisely for this purpose. In the upright position this shunt becomes a flow regulator and thus reduces overdrainage. Its use has decreased by 2-fold the number of mechanical complications related to overdrainage.