This disease, as the name implies, depends on the presence in the meninges of the bacillus of tubercle. The tubercle bacillus is readily detected in the affected structures, lying in the membranes but more especially in the walls of the arteries. The bacillus reaches the membranes by several different paths, of which three may be more definitely distinguished. In the first place, the disease is, in many cases, part of a general tuberculosis, and is associated with the usual lesions in other organs, but the meningitis leads to such pronounced symptoms that it often monopolizes the attention of the clinical observer. The primary seat of the affection is frequently in the bronchial, cervical, or mesenteric glands, the tuberculosis extending to the veins, and so to the blood. There may, however, be an extension by the blood without the occurrence of a general tuberculosis. The primary seat may be in the lungs, and the extension to the membranes of the brain may be the only secondary extension. In the second place the infection may be derive^ from a local tuberculosis of the brain, which takes the form of a tubercular tumour (see p. 678). When such a lesion originating in the brain substance reaches the surface of the brain, there may be an infection of the membranes with a resulting tubercular meningitis. In the third place a tuberculosis of the bones of the skull or vertebral column may extend to the membranes. The tuberculosis is mostly in the basilar portions of the occipital or sphenoid bones, or the bodies of the upper cervical vertebrae. In a case observed by the author, for example, a tuberculosis of the basilar bone and the first cervical vertebra had extended to the pituitary body and thence to the soft membranes.
The meninges are affected by tuberculosis more frequently in children than in adults; it thus seems as if the soft membranes in children were more adapted to the growth of the bacillus. But tubercular meningitis is more frequent in the adult than is usually supposed, and many obscure head cases are found post mortem to be cases of general tuberculosis with, it may be, a very limited meningitis.
The virus reaching the meninges lodges in the lymph spaces of the membranes, and produces inflammation and tubercles in the walls of the finer arteries of the pia mater, and in the subarachnoid space. These conditions are peculiarly localized in the basal parts of the brain, although extending sometimes to the lateral aspects and to the spinal cord. It may be that this is related to the fact that the arteries are distributed from the base, or, more probably, that the cerebro-spinal fluid stagnates here more than elsewhere, and allows of the growth of the microbe.
The Appearance of the brain post mortem is somewhat as follows. On removing the dura mater, the cerebral hemispheres are generally seen to be fuller than usual, and the surface of the arachnoid is somewhat dry and glazed. This is due to the pressure from the ventricles, which are distended with fluid, usually clear serum. The amount of fluid is sometimes very great, and this prominent feature gave rise to the name Acute hydrocephalus applied to this disease (see p. 691). In the neighbourhood of the lateral ventricles, and especially posteriorly, the brain substance is soft and almost diffluent (white softening). On exposing the base of the brain, the appearances of inflammation in the membranes are to be looked for. These are often somewhat obscure, and in appearance trivial. In the slighter cases they consist merely of a turbidity or opacity over the pons and optic chiasma. The subarachnoid space is occupied by a serous exudation with some pus, giving often a greenish colour. But usually the exudation is more abundant and covers the basal structures, extending to the surface of the cerebellum, and up the fissures of Sylvius, where it often reaches the lateral aspects of the hemispheres. The concentration of the exudation at the base is of great importance, and the covering-in of the optic chiasma is often the most significant and sometimes the only prominent sign of the existence of the disease.
The affection usually extends to the Meninges of the spinal cord, although here it may be even more insignificant in appearance than in the brain. There may be little more than an undue redness with a granular appearance on the surface of the cord.
The evidences of inflammation are much more prominent to the naked eye than the tubercles. These are often distinctly visible only on microscopic examination, as they are so much buried in the exudation and attached to such small arteries. On opening up the fissure of' Sylvius, however, where the exudation generally glues the opposed surfaces of the brain together, it is usually possible to see the little white tubercles, often no larger than pins' heads. If a piece of the soft membrane be removed, however, even from a part where the exudation is very abundant, and examined with a low power under the microscope, the tubercles are readily seen as spindle-shaped prominences on the small arteries (see Fig. 346).
Fig. 346. - Arteries of the pia mater in a case of tubercular meningitis. The frequent spindle-shaped swellings indicate the position of the tubercles, x 16.
There is not in this form of tuberculosis a distinct formation of typical tubercles with giant-cells and epithelioid cells, the nodules described as tubercles being composed of aggregations of round cells. The walls of the arteries are often specially affected, so that they present localized prominences (see Fig. 346) as if the agent had acted at certain spots. The swellings are mainly due to accumulation of cells in the adventitia (Fig. 347), but the intima is very commonly also the seat of an inflammatory infiltration as shown in the figure. Besides this there are tubercles in the walls of veins and in the tissue apart from the vessels, and there is a general infiltration of the membranes with multitudes of leucocytes (see Fig. 347). The inflammatory infiltration extends along the nutrient vessels into the substance of the brain and spinal cord, as well as into the cerebral nerves. The superficial parts of the brain substance in particular are hyperaemic, and not infrequently the seat of small haemorrhages.
Fig. 347. - Tubercula meningitis, a, an artery with infiltration chiefly of external coat, but also of internal, which shows villous projections. The connective tissue around is also infiltrated, and so are the walls of the nutrient arteries passing into the brain substance on either side, b, b. x 30.
Tubercular meningitis like other forms of tuberculosis may undergo healing. This has been asserted on clinical grounds, but has also been confirmed by post-mortem examination (Futterer). The membranes are found thickened and contain shrunken and calcified remains of miliary tubercles.
The White softening of the parts around the ventricles has given rise to some discussion as to its cause. It may extend somewhat deeply into the brain substance, involving fornix, septum lucidum, corpus callosum, and even the thalamus opticus and corpus striatum, and it is sometimes so extreme as to reduce the tissue to the consistence of thick cream. It is clear from the comparative absence of symptoms that this extreme softening does not exist during life. Probably the fluid in the ventricles macerates and loosens out the brain tissue without interrupting the functions, and after death a more pronounced softening occurs. The condition is not an inflammatory one, and it occurs mainly where the fluid gravitates, namely, in the brain substance around the posterior parts of the ventricles.
Neisser, Die Entzund. der seros. Haute des Geh.u. Ruckenm., 1845; Huguenin, in Zjemssen's Handb., xi., 1. Epidemic cerebrospinal meningitis - Meissnek, Schmidt's Jahrb., 129 and 136; Klebs, Virch. Arch., xxxiv.; Willich, (Pneumonia and mening.) D. med. Wochenschr., 1875, No. 23; Ficket, (do.) Annal. de la soc. med. chir. de Liege, 1880; Gowers, Dis. of nerv. syst., 1893 and 1899 (literature fully); Frew, Glasg. Med. Jour., 1884, xxii., p. 21; Eberth, D. Arch. f. klin. Med., xiii.; Naowerk, do., xix.; Cornil et Babes, Les bacteries, 1886, p. 446; Flexner and Barker, Amer. Journ. of Med. Sc, 1894. Tubercular meningitis - Fdtterer, Abriss. der path. Anat., p. 166; Hektoen, Journ. Exper. Path., hi., 21.