What is Hydrocephalus?

Hydrocephalus is a condition of increased volume of CSF in the ventricles resulting in ventricular enlargement. Depending on the underlying pathogenetic mechanism, hydrocephalus is classified as obstructive or communicating.

In obstructive hydrocephalus a focal obstacle lies on CSF pathways along the ventricular system. Depending on the site of obstruction (Monro's foramina, the mesencephalic aqueduct, or Magendie's and Luschka's foramina) hydrocephalus can be confined to one or both lateral ventricles, to the lateral and third ventricles (triventricular hydrocephalus) or may be extended to all four ventricles (tetraventricular hydrocephalus). The fourth ventricle may also be "trapped" and focally enlarge. The most frequent conditions responsible for obstructive hydrocephalus are tumours, intra- or extra axial, particularly in the posterior fossa, infections, aqueduct stenosis and intraparenchymal haematomas.

Communicating hydrocephalus (always tetraventricular) may be due either to a lack of resorption or to an increased production of CSF. A lack of resorption is observed following infectious/inflammatory conditions or subarachnoid haemorrhage. An overproduction of CSF is only seen in choroid plexus tumours.

The physiopathological consequences of hydrocephalus vary according to the rapidity with which the condition occur. A sudden raise of intracranial pressure is seen in acute forms of hydrocephalus which precipitate a life-threatening condition requiring immediate therapy. If hydrocephalus develops insidiously, a balance is reached between the volume rise and the compliant ventricular system leading to conditions with a less severe increase in pressure. Normal pressure hydrocephalus (NPH) is believed to represent a condition of "steady state". In infants, as cranial sutures are still unfused, the compliance is extended to the whole head, and macrocephaly with split sutures and an enlarged and bulging fontanelle is seen as a consequence.

A sudden increase of intracranial pressure must be suspected in the presence of headache and vomiting associated with variable impairment of consciousness and may be confirmed by the observation of papilloedema at ophthalmoscopy. In chronic hydrocephalus the symptomatology may be rather subtle and the suspicion of hydrocephalus may even be raised on the basis of characteristic modifications of the cranial vault bones as appreciated on standard skull radiograph.

Normal pressure hydrocephalus presents with a different clinical picture.

Tomographic imaging provides a prompt and exaustive tool in the diagnosis of hydrocephalus and in the identification of its cause.

Ventricular enlargement may be well depicted both on CT and on MR (Fig.1) and must be differentiated from ventricular enlargment related to atrophy. Hydrocephalic ventricular enlargement is associated with disappearance of the subarachnoid spaces, particularly at the cranial convexity. Another associated imaging finding is periventricular CSF accumulation ("transependymal migration") which is visible on CT as an hypodense and on MR as a T2PD hyperintense rim on the ventricular lining. Periventricular CSF accumulation is related to the increased pressure and thr duration of hydrocephalus. In the chronic hydrocephalus (such as in normal pressure hydrocephalus), the differentiation between hydrocephalic ventricular enlargement with transependymal migration and atrophic ventricular enlargement associated with periventricular degenerative white matter lesion may be subtle. Hyperdynamic aqueductal systolic CSF flow, as depicted by flow artefact on conventional MR images, or precisely measured on cine dynamic phase-contrast techniques, has been described as a test that may be valuable in ascertaning the diagnosis.

Imaging Examples

Hydrocephalus, Fig.1 (a)
Hydrocephalus, Fig.1 (b)
Hydrocephalus, Fig.1 (c)

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