Classification and terminology
Communicating hydrocephalus (extraventricular obstructive) occurs when cerebrospinal fluid resorption is impaired. Causes include hemorrhages, infections with proteinaceous exudate formation, venous hypertension, and tumor dissemination. CT and MRI findings are variable, ranging from completely normal to marked enlargement of the ventricles and subarachnoid spaces.
Non-communicating hydrocephalus (intraventricular obstructive) is associated with a physical block within the ventricular system itself. The most vulnerable areas are the narrowest segments of the outflow pathways: the foramina of Monro, the cerebral aqueduct (aqueductus cerebri), and the outflow foramina of the fourth ventricle.
It is important to distinguish true hydrocephalus from ventriculomegaly, which is compensatory ventricular enlargement secondary to parenchymal atrophy or primary brain hypoplasia. In such cases, the term "hydrocephalus" is clinically inappropriate.
Radiological signs of intraventricular obstruction
Key CT and MRI markers indicating acute or decompensated non-communicating hydrocephalus:
- Disproportionate enlargement of the temporal horns of the lateral ventricles.
- Downward bulging of the floor of the third ventricle and enlargement of its anterior and posterior recesses.
- Transependymal edema (periventricular interstitial CSF seepage).
- Effacement of the basal cisterns, major sulci, and subarachnoid spaces of the convexital surface.
Topographic diagnosis: Regional ventricular enlargement precisely indicates the level of the obstruction. For example, in cerebral aqueduct stenosis, the lateral and third ventricles are significantly enlarged, while the fourth ventricle remains normal in size.
Note: Classical radiographic signs (erosion of the dorsum sellae, "beaten copper" skull) are considered unreliable in modern diagnostic radiology.
Diagnostic features in young children
In children under 2 years of age, the ventricles and subarachnoid spaces normally appear wider. This physiological norm should not be confused with hydrocephalus or cortical atrophy.
Sometimes benign enlargement of the subarachnoid spaces occurs because skull growth outpaces brain development. To differentiate it from pathological subdural collections on MRI, two criteria are evaluated: the absence of mass effect on the adjacent brain parenchyma and the presence of traversing bridging veins (crossing veins) within the enlarged space. The condition typically resolves spontaneously by age 2.
In infants with open fontanelles, hydrocephalus manifests as progressive macrocephaly (crossing percentile lines), bulging fontanelles, thinning of the cranial vault, and suture diastasis. Because the skull is pliable, signs of increased intracranial pressure differ from those in older children and adults.
Evaluation of CSF shunt systems
If a ventriculoperitoneal (VP) or other shunt is present, the radiologist must evaluate its adequacy.
- Evaluation of dilatation: Current ventricular dimensions must be compared with baseline or previous imaging studies of the patient.
- Cause of dysfunction: Obstruction most commonly occurs due to ingrowth of choroid plexus tissue or glial scarring.
- System integrity: Checked via scout view (scout view) on head CT or on a plain radiograph covering the pathway from the skull to the abdominal cavity. Look for disconnection of tubing fragments and calcification at the fracture ends.
Important: The physician must know the exact type of implanted shunt to avoid mistaking normal radiolucent valve components for a physical fracture of the system.
How to remember the vulnerable bottlenecks of the ventricular system: moving along the CSF flow from top to bottom — foramina of Monro (from lateral to third ventricles), cerebral aqueduct (from third to fourth ventricles), outlets of the fourth ventricle (into the subarachnoid space).
Class and exam questions
Why is spinal MRI recommended for unexplained communicating hydrocephalus?
Occult spinal cord tumors can cause hydrocephalus by secreting large amounts of proteinaceous exudate or via leptomeningeal dissemination, which disrupts normal CSF absorption.
Does CSF hyperproduction indicate choroid plexus papilloma?
Papillomas can indeed hyperproduce CSF, but this is an extremely rare mechanism. Much more often, these tumors cause hydrocephalus due to mass effect, hemorrhage, or protein debris.
How to differentiate a subdural effusion from a subdural empyema on MRI?
Sterile effusions (a frequent complication of meningitis) exhibit signal characteristics similar to CSF and do not show contrast enhancement. Empyemas contain high protein content, appear hyperintense on T2-weighted images, and demonstrate marked leptomeningeal and pachymeningeal contrast enhancement.
What radiological signs are typical for shaken baby syndrome (SBS)?
Key signs: subdural/subarachnoid hemorrhages, shear injuries at the gray-white matter junction, and massive diffuse brain edema (predominantly frontoparietal/occipital) with loss of structural differentiation.
What else to study on this topic
- Intraventricular mass lesions (meningioma, colloid cyst, central neurocytoma) and their MRI features.
- Complications of neuroinfections: ventriculitis, ependymitis, and the formation of loculated CSF collections.
- Specific CSF circulation disorders in Chiari II malformation.
- Venous sinus thrombosis as a cause of increased intracranial pressure in children.
- Pathogenesis and radiological outcomes of hypoxic-ischemic injury in severe cerebral edema.
Master the entire topic in the "Diagnostic Radiology" course
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