Neuroimaging in Pediatric Hydrocephalus

Abstract

Hydrocephalus may be defined as an expansion of the CSF compartment of the head at the expense of the brain. Common in children, it is classically recognized from the triad of a macrocephaly, a ventriculomegaly with effacement of the pericerebral spaces, and typically, a cause. It may be obstructive, and the obstruction may sit either along the ventricular pathways or in the cisterns; it may be nonobstructive, when no obstruction can be demonstrated. In obstructive hydrocephalus, the main pathogenetic factor is the loss of compliance, that is, the loss of the ability of the system to buffer the force of the systolic pressure wave. In nonobstructive hydrocephalus, the main pathogenetic factor seems to be a secretion-absorption mismatch. Depending on the clinical course, hydrocephalus may be acute, progressively acute, or chronic.

CSF is secreted by the choroid plexuses and absorbed passively by the dural absorption sites; simultaneously, water can be actively transported by AQP4 channels in and out the parenchyma across the ependymal and subpial surfaces, as well as to the capillaries. Because of the systolic inflow of blood into the skull cavity at each cardiac stroke, the CSF is submitted to oscillatory movements, the force of which need to be buffered not to do injury to the parenchyma. Based on this, hydrocephalus can be considered the single mechanical consequence of multiple processes which may affect secretion, absorption, transport, and movements of the CSF, because of either a loss of compliance, or of a secretion-absorption mismatch, or both. Mechanically it affects the brain by compressing the vascular bed, which results in parenchymal ischemic changes. The CSF-parenchyma molecular exchanges also may be affected by the CSF flow alterations and by the deleterious effect of hydrocephalus on the ependyma and the subependymal progenitor zone.

Hydrocephalus must be evaluated, and as much as possible, followed by using MR imaging (ultrasonography however is precious in infants). Beyond the classic triad of macrocephaly, ventriculomegaly, and a cause, imaging of hydrocephalus should investigate the associated brain lesions possibly due to hydrocephalus itself, or to its causal pathology. This implies that hydrocephalus constitutes different diseases in the fetus, the neonate, the children, and the adolescents, because the causal diseases, the brain injury related to them, and the response of the parenchyma are all age-dependent. Postoperatively, imaging aims at assessing the mechanical efficacy of the CSF diversion, as well as at identifying early and late complications.