This term expresses a massive accumulation of fluid inside the skull. The fluid is, in the great majority of cases, in the ventricles, more especially the lateral ventricles, but in certain congenital cases it is in the subdural space. Hence the terms Internal and External hydrocephalus. The latter condition scarcely occurs" unless there be a congenital defect in the formation of the brain, such as perforation, or absence of the corpus callosum, so that the ventricles communicate with the subdural space.
External hydrocephalus is sometimes used as equivalent to cedema of the membranes, but this is an incorrect use of the term, which really expresses a considerable collection in a cavity.
Apart from the form ex vacuo already considered in the preceding page, hydrocephalus as met with in the adult is mostly related to a definite disturbance of the circulation in the brain. The walls of the ventricles do not probably have much to do with the supply of the cerebro-spinal fluid to the ventricles. The choroid plexus which consists of a convolution of arteries, capillaries, veins, and lymphatics, is the chief agent in this function. It is important to note that the arteries reach the plexus by a totally different route from that by which the veins and lymphatics emerge. The arteries are branches of the middle cerebral and reach the plexus near its anterior extremity in the descending cornu of the ventricle. On the other hand the veins of the plexus pass backwards and emerge from the brain through the great transverse fissure. The veins gather themselves together into the Veins of Galen, one of which is connected with each lateral ventricle. The two veins of Galen running side by side pass backwards between the corpus callosum and the corpora quadrigemina to open into the straight sinus. It will be observed that from their different anatomical relations the arteries and veins are not liable to be obstructed simultaneously, and that if the arteries remain dilated whilst the veins are obstructed the resulting passive hyperemia may be aggravated.
Obstruction of the veins of the choroid plexus or of the veins of Galen so as to produce passive hyperemia seems the principal mode of causation of acquired hydrocephalus. Rapid accumulation is brought about mainly by pressure from without, and chiefly by pressure on the veins of Galen. This is liable to be the case in tumours of the cerebellum (as in Fig. 339). The cerebellum being placed in a space limited by the tentorium lying above it, any increase in bulk exercises considerable pressure inside this limited space. But the veins of Galen pass into the tentorium so as to end in the straight sinus, and thus are liable to pressure. Tumours of the choroid plexus, especially at its posterior part, may similarly obstruct the veins and induce hydrocephalus. Thrombosis of the veins of Galen is another cause of hydrocephalus, but an unusual one. In most cases the thrombus originates in the sinuses, and grows into the veins of Galen, but Newman has recorded a case in which these veins alone seemed to be the seat of thrombosis.
Fig. 339. - Great dilatation of the lateral ventricles (which have been laid open) from tumour of the cerebellum. (From a coloured picture by Dr. A. Macphail).
In Tubercular meningitis hydrocephalus is nearly constant, but it is lather difficult of explanation. The disease is characterized by an inflammatory exudation at the base of the brain, and it might be supposed that the fluid accumulated in the ventricles would be inflammatory. But, although tubercles are sometimes found on the choroid plexus, yet the exudation in the ventricles is rarely inflammatory in character. It consists of a clear pellucid fluid with little albumen, and of a low specific gravity (about 1010), presenting thus the characters rather of a transudation than an inflammatory exudation. The explanation is probably to be found in part in the exudation outside the ventricles. This is generally abundant where the velum interpositum emerges from the ventricles in front of the cerebellum, and here, besides filling up the lymph spaces, it is liable to cause pressure on the veins of Galen as they pass through the comparatively narrow isthmus. This is the more likely to produce hyperemia and exudation from the fact already mentioned, that the arteries reach the plexus by a different route, and are not so liable to pressure.
It has been pointed out in a previous section that various malformations affecting brain and cranium are due to dropsy of the cerebro-spinal canal during early foetal life. A similar dropsy may occur at later periods of intrauterine life, or even after birth. The condition may be designated chronic hydrocephalus, which, it will be understood, is often congenital, and may by the enlargement of the head produce serious difficulty in delivery. No proper explanation of the dropsy has been offered. It is said that chronic hydrocephalus often goes along with rickets, and it has been suggested that as a rickety skull is more yielding than a normal one, too little pressure is exercised on the brain, and the accumulation of fluid is allowed. This is not, however, a sufficient explanation, especially of the severe congenital cases. The probability is that the disease depends on a congenital defect in the apparatus for the secretion and absorption of the cerebro-spinal fluid.
The dropsy may be general, affecting all the ventricles, but the fourth is often but little dilated, the fluid being mainly in the lateral and third ventricles. Sometimes the lateral ventricles are alone distended, the foramen of Monro being closed. Sometimes the ventricle of the septum lucidum is obviously distended as it lies between the two lateral ventricles. As the accumulation increases the convolutions are flattened out and they may be completely unfolded, the brain forming a smooth globe over the greatly distended ventricles. The distension may be so great as to cause thinning of the covering brain substance to an extraordinary degree, leaving little between the pia mater and the fluid. The surface of the ventricles (ependyma) is usually somewhat thickened, giving a kind of leathery membranous lining to the cavity, sometimes with granular elevations, especially on the surface of the corpus striatum and fourth ventricle. There may even be rounded prominences consisting of grey brain substance (Virchow). The fluid is clear and pellucid, and has a low specific gravity (1001 to 1009).
With this great dilatation of the ventricles the head is greatly enlarged, and as the bones are more yielding in some parts than others, an alteration in shape occurs. The fontanelles and sutures are widened and their closure greatly delayed. The frontal bone is pushed forward so that the forehead rises perpendicularly or overhangs the eyebrows; the parietals bulge laterally, and the occipital bones are pushed backwards. The head in this way becomes greatly increased in circumference, while it is usually much flattened at the vertex. The bones of the face, even though they are of normal size, look dwarfed beside the enlarged cranium, and the face has a pinched look. The eyeballs are rendered prominent by the pressure on the roof of the orbit, and enlarged veins are generally seen beneath the thin skin of the head.
Although there is this great thinning of the brain substance, it is remarkable how the functions may be retained. A child in this condition may remain very intelligent, and when recovery occurs may pass through life with no permanent defect in the functions of the brain, but in the majority of eases there is defective mental development ranging from total idiocy, with perhaps epilepsy, to the lesser degrees of imbecility or of mental enfeeblement. The yielding of the skull seems to prevent any such excess of pressure as to seriously damage the brain, which has a remarkable power of accommodating itself to alterations in position of its parts. If recovery take place, the skull to some extent collapses, the fontanelles and sutures close, often with the formation of additional centres of ossification, forming Wormian bones in the sutures. But the fontanelles are late in closing, and the cranium retains somewhat of the hydrocephalic shape during life.
In some very rare cases the corpus callosum and its pia-arachnoid have given way, the fluid coming thus to the surface and filling the cavity of the dura mater, thus forming a Hydrocephalus externus. In that case the hemispheres are folded aside, and the central parts of the brain are exposed, the brain being as a whole pressed down towards the base. But this scarcely occurs except in congenital cases, and is not consistent with the prolongation of life.
A Partial dropsy of the ventricles is of occasional occurrence, either as a congenital or acquired condition. One lateral ventricle may be distended, or even one horn. There may also be a dropsy of the third ventricle alone, or of the ventricle of the septum lucidum. These partial dropsies will cause displacements according to their situation.