Definition. Enlargement of the subarachnoid spaces over the frontal poles and their cortical sulci with rapid increase of the head circumference [9–11]. Also worth noting is the absence of radiological or clinical features of increased ICP such as periventricular lucency or a tense anterior fontanel [12].

Radiological findings . Normal or mildly dilated ventricles are noted with enlarged basal cisterns and widening of the interhemispheric fissure. CT demonstrates extra-axial fluid collections that usually share the density of CSF, while MRI or cranial U/S demonstrates veins coursing through the fluid collection from the surface to the inner table of the skull (known as the cortical vein sign).

Presentation . Typically infants are asymptomatic and present with an enlarging head that starts to appear in the first year of life and usually compensates by 12–18 months without treatment [12]. Frontal bossing may be seen and, occasionally, slight motor developmental delay is noted. This is likely due to the large head causing neuromuscular issues with balance and walking rather than an intrinsic effect upon brain development.

Etiology . The pathophysiology is not fully understood, but a defect in CSF absorption has been postulated secondary to delayed maturation of the arachnoid villi [13, 14]. An association of widened SAS has also been reported in some children with craniosynostosis [15]. The fact that it may follow intraventricular or subarachnoid hemorrhage [16, 17] or superior vena cava obstruction [18] also suggests a common distal transient defect in CSF absorption.

Management . This etiology is sometimes referred to as “benign external hydrocephalus” because it’s commonly viewed as a self-limiting condition of infancy that resolves spontaneously during childhood [19, 20]; therefore, conservative follow-up is mandated. As pediatricians and pediatric neurosurgeons, it is crucial to emphasize to the parents that children with this diagnosis have normal neurologic development [10]. Shunting may be required in rare instances of manifestations of increased ICP, bulging fontanel, considerable macrocrania, or frontal bossing, but this is a rare necessity in this etiology [17, 21]. A few reports exist of the use of acetazolamide or even transient mannitol treatment with good outcomes [22], but this is not common practice.

Differential diagnosis : The etiology to distinguish from benign subdural collections of infancy is symptomatic chronic extra-axial fluid collections.

Definition. Fluid collections over the frontal convexities associated with prominent cortical sulci and interhemispheric fissure, with normal or slightly enlarged ventricles, suggest benign subdural collections of infancy [23].

Various terms are used to describe this condition: “chronic subdural hematoma” “effusions,” “hygromas,” and “extra-axial fluid collections” [10, 23].

Etiology. In a study of 103 patients, 36 % were believed to be due to head trauma (including non-accidental head trauma), 22 % were post-meningitic, and 19 % post-shunting [24]. Other reported causes include hypoxic brain damage and coagulation defects [25].

Management. Serial subdural taps (to exclude infection and to help drainage), Burr hole drainage, and subdural shunting may all be considered [26].

Definition. Total or near total absence of the cerebral tissue to be replaced by CSF and covered by intact meninges and cranial vault (post-neurulation defect) [27, 28].

Etiology. Bilateral ICA occlusion causing territorial infarcts and subsequent loss of the cerebral tissue supplied by the ICAs [29]. Perinatal infections also implicated and association with maternal smoking has been described [30].

Differential diagnosis. One must consider the diagnosis of maximal hydrocephalus, in which there is progressive dilatation of CSF spaces with a minimal amount of cortical tissue. This tissue can actually demonstrate a remarkable degree of re-expansion with shunting. Children with maximal hydrocephalus may just show subtle neurodevelopmental delay in contrast to hydranencephaly patients who are profoundly abnormal neurologically [28].

Uniloculated hydrocephalus is dilatation of one segment of the ventricular system due to obstruction at the foramen of Monroe causing unilateral ventricular dilatation. Alternatively, obstruction at the cerebral aqueduct and fourth ventricular outlet may cause dilatation of the fourth ventricle [31].

Various forms of isolated ventricular compartments have been described [32, 33]:

Etiology. Compartmentalization of the ventricular system increases following infection (meningitis, ventriculitis, and shunt infection) or intraventricular hemorrhage .

Pathophysiology. Formation of septations from microglial membranes extending through denuded ependyma from underlying glial tufts or post-inflammatory debris obstructing the ventricular foramina is believed to result in the development of loculations [34, 35].

Treatment. Most children will require multiple procedures, often utilizing a combination of endoscopic membrane fenestrations with shunts. Endoscopic techniques are used to communicate the loculated regions together to allow for simplified shunting [36–38].

Definition. DWM is defined as agenesis of the cerebellar vermis with cystic dilatation of the fourth ventricle and hydrocephalus. Other associated pathological changes include elevation of the transverse sinus, enlargement of the posterior fossa, and occlusion of the foramina of Luschka and Magendie [7, 39, 40]. The condition can be detected antenatally by fetal sonography as early as the 14 week of gestation [41].

Radiological findings. Due to variability of pathological features, a set of radiological abnormalities have been proposed [42, 43]. Hydrocephalus is not always present especially at birth but it occurs in 75–80 % of cases by 3 months of age.

Other findings of DWM include:

Hydrocephalus was thought initially to result from occlusion of the fourth ventricular outlet foramina; however, other causes are believed to include maldevelopment of the subarachnoid space or aqueductal stenosis from upward herniation of the posterior fossa cyst. A contribution of venous anatomy has also been suggested as a cause due to abnormal location of the torcula.

Dandy–Walker variant is a DWM differentiated by a slightly less abnormal cerebellar vermis, a smaller cystic cavity, and a posterior fossa that is not markedly dilated [44–47]. Recently it has been considered as a separate entity from the DWM and given the name of “vermian-cerebellar hypoplasia.”

Persistent Blake’s pouch cyst is a persistence of a congenital dorsal appendage of the fourth ventricle, which then forms a posterior fossa cyst widely communicating with the fourth ventricle.

Mega cisterna magna is an enlarged cisterna magna combined with vermian dysgenesis but with vermian tissue remaining between a normal fourth ventricle and the cistern [47].

Posterior fossa arachnoid cyst /retrocerebellar arachnoid cysts are true cysts that do not communicate with the fourth ventricle and are not accompanied by cerebellar hypoplasia. The radiological appearance may give the impression of communication because the cyst compresses the cerebellar tissue.

This is a term that generates considerable debate and is often referred to as “slit ventricle syndrome” since the condition is usually described with shunted children who develop manifestations of increased intracranial pressure without ventriculomegaly [54, 55].

Children may be classified within several different groups:

Normal pressure hydrocephalus is a well-described clinical and radiographic entity usually diagnosed in elderly adults with ventriculomegaly, but only normal or upper edge of normal ICP is less commonly found in pediatric patients [61]. Another commonly used term is arrested hydrocephalus, which refers to the condition of non-shunted persistent ventriculomegaly with normal ICP and no obvious clinical manifestations. These children also pose a management challenge but usually only require observation.

Definition. A fairly high risk of ventricular dilatation (7–29 %) may occur within variable periods after trauma ranging from 2 weeks to several years [62].

Pathophysiology. Obstruction due to intraventricular hemorrhage , small clots, or contusions, compressing the narrow ventricular passages, or a decrease in CSF absorption due to subarachnoid hemorrhage, infections, or early surgical procedures resulting in significant subarachnoid scarring are among the proximal causes of posttraumatic hydrocephalus [63].

Clinical presentation. The acute presentation is with overt manifestations of increased ICP. These may be headache, bradycardia, or changes in the neurologic exam, typically recognized within the intensive care setting. It should also be suspected in cases of CSF leakage or increasing CSF collections following a trauma-related craniotomy. In the chronic setting, which can be days to weeks post-injury, this entity may present with a more insidious progressive cognitive decline, behavioral changes, or failure to attain functional improvement after the initial head trauma [64–66].

Various modalities can be used to detect and follow hydrocephalus, depending on the age of the patient and the acuity of the presentation. Ultrasound is the modality of choice for neonates with open fontanels as it visualizes the supratentorial compartment with excellent resolution. It is widely utilized in neonates with IVH for diagnosis and surveillance imaging because it is safe, routinely available and cost-effective. It cannot always visualize the entire intracranial space but is an excellent first choice for very young children, particularly in NICU and outpatient settings. CT and MRI, discussed in significant detail in Chaps. 18 and 20, offer far more detailed evaluations of the entire ventricular system and a better visualization of possible underlying etiologies. Relative risks and benefits of these tests are also fully covered in other dedicated portions of this textbook. With respect to the radiologic definition of hydrocephalus, there is no single radiological parameter that can be totally relied upon, but some criteria do exist: