An increase in the rate of formation or decrease in the rate of absorption would be expected to cause accumulation of CSF and increased ICP but the only examples of overproduction of CSF are papillomas of the choroid plexus, and even in this circumstance, there is usually an associated ventricular obstruction, either of the third or fourth ventricle or of one lateral ventricle. Sometimes in these cases there is both a generalized dilatation of the ventricular system and basal cisterns (possibly because of increased CSF volume), and an asymmetrical enlargement of the lateral ventricles caused by obstruction of one foramen of Monro.
Syndromes of Hydrocephalus
Ventricular enlargement gives rise to various syndromes depending on the age of the patient and the rapidity of evolution. The type that occurs early in life, before fusion of the cranial sutures, leads to enlargement of the head. If the cranial sutures have fused and the accumulation of CSF is very gradual, the head remains normal in size and the ventricular enlargement may remain asymptomatic or later in life cause gait, urinary and cognitive difficulties. A special form of the latter is arrested or compensated hydrocephalus of late adult life that is one of the causes of normal-pressure hydrocephalus that is addressed in Chap. 6 and in the following text.
The cranial bones fuse by the end of the third year; for the head to enlarge, the tension hydrocephalus must develop before this time. It may begin in utero, but usually happens in the first few months of life. Even up to 5 years of age (and very rarely beyond this time), a marked increase of ICP, particularly if it evolves rapidly, may separate the newly formed sutures (diastasis). Hydrocephalus, even of mild degree, also molds the shape of the skull in early life, and in radiographs, the inner table is unevenly thinned, an appearance referred to as “beaten silver,” or as convolutional or digital markings. The frontal skull regions are unusually prominent (bossed) and the skull tends to be brachiocephalic, except in the Dandy-Walker syndrome where, because of bossing of the occiput from enlargement of the posterior fossa, the head is dolichocephalic. With marked enlargement of the skull, the face looks relatively small and pinched, and the skin over the cranial bones is tight and thin, revealing prominent distended veins. The usual causes of this disorder are (1) intraventricular matrix hemorrhages in premature infants, (2) fetal and neonatal infections, (3) type II Chiari malformation, (4) aqueductal atresia and stenosis, and (5) the Dandy-Walker syndrome.
In congenital hydrocephalus, the head usually enlarges rapidly and soon surpasses the 97th percentile. The anterior and posterior fontanels are tense even when the child is in the upright position. The infant is fretful, feeds poorly, and may vomit frequently. With continued enlargement of the brain, torpor sets in and the infant appears languid, uninterested in his surroundings, and unable to sustain activity. Later, the upper eyelids are retracted, and the eyes tend to turn down; there is paralysis of upward gaze, and the sclerae above the irises are visible. This is the “setting-sun sign” that has been incorrectly attributed to downward pressure of the frontal lobes on the roofs of the orbits. The fact that it disappears on shunting the lateral and third ventricles indicates that it is caused by hydrocephalic pressure on the mesencephalic tegmentum. Gradually, if left untreated, the infant adopts a posture of flexed arms and flexed or extended legs. Signs of corticospinal tract damage are usually elicitable. Movements are feeble, and sometimes the arms are tremulous. There is no papilledema, but the optic discs become atrophic, pale and vision is reduced.