Thrombosis of the Cerebral Veins and Sinuses.
It has for centuries been known that coagulation of the blood might take place in the sinuses in a way different from the ordinary post-mortem clots, but this was looked upon rather as an anatomical curiosity than as a fact of practical importance and clinical significance, and it is chiefly among observers of the present century that we find a growing knowledge of the conditions under which it occurs and the symptoms to which it gives rise.
Thrombi in the cerebral sinuses are not essentially different from those formed elsewhere, and the reader is referred to the account given in the article on General Pathology for a history of their formation, growth, appearances, and transformations. For our purposes it is sufficient to recall that they may be white, red, or striated, either partly or wholly obstructing, and that they may become degenerated and partly or wholly washed away. The most important distinction of all, however, is that into two classes, of which the first consists of those which are simply depositions of fibrin in a comparatively healthy vessel, and the second of those which are dependent on a phlebitis.
In order that a thrombus may form it is necessary that there should be, in the first place, a special condition of the walls of the veins—not necessarily, however, inflammation, though this is one of the most frequent and probably the most active form; second, a slackening of the blood-current; and, third, perhaps a peculiar state of the blood, though this latter is not certain. A thrombus tends strongly to grow, and when already formed furnishes a most favorable point for the deposition of more fibrin.
The cerebral veins furnish a very suitable place for the coagulation of the blood for several reasons: they are roomy in proportion to the amount of blood they carry; they are tortuous and abundantly anastomosing, so that the current of blood is almost reversed at some points, and can easily stagnate; the veins of the diploë are held open by their bony walls, and the sinuses by their stiff membranous ones, so that they cannot collapse and thus limit the extension of a thrombus once formed.
The sinuses most frequently affected, though none are free from the liability, are the cavernous, superior longitudinal, and lateral.
The results of thrombosis of the sinuses and veins are not equivalent to those of a similar process in the arteries, and they may be said in a general way to be more diffused, as might be expected from the much greater freedom of anastomosis. Limited softening is rarely a consequence of occlusion even of a considerable number of veins, but it has been observed. A large area of softening of one hemisphere, not involving the temporal and occipital lobes, has been seen with thrombosis of the parietal veins58 (the internal capsule and ganglia were not affected).
58 Gaz. des Hôp., 1880, 1066.
Passive congestion in the brain, as elsewhere, although apparently entirely incompatible with the normal function, seems to be able to sustain a low form of structural integrity.
Bleeding may take place from the congested veins behind the obstruction, constituting a distinct form of cerebral hemorrhage which does not depend upon an arteritis, although if miliary aneurisms were present the occurrence of thrombosis would undoubtedly tend to their rupture. The writer, however, is not aware of such a coincidence having been actually observed. Hemorrhages are usually diffuse, composed of or accompanied by a number of small effusions, and situated on or near the surface of the brain or distinctly meningeal. Punctiform hemorrhages are exceedingly common.