This term may be applied to conditions in which there is a general irritation of the brain, presumably by an irritant circulating in the blood. It may be held to include a considerable number of varied conditions. As the irritant, being in the blood, will attack all parts equally, we may expect to find evidences of irritation in the spinal cord as well as in the brain, but as in other cases of morbid poisons, there will be a selective character.
(a) In Acute febrile diseases it may be presumed that the brain is irritated by poisons in the blood. According to Popoff, this is evidenced in the case of typhoid fever by the presence of abundant leucocytes. These are stated to be particularly abundant around the vessels, evidently spreading out from these. The leucocytes were frequently found in the spaces around the ganglion cells (pericellular lymph spaces), and even, in some cases, in the substance of the ganglion cells.
Middleton has pointed out that in delirium tremens, tubercular meningitis, uraemia, fracture of the skull with injury to the brain, erysipelas, etc., the brain substance is over-run with leucocytes, the appearances being somewhat similar to those to be described as occurring in hydrophobia. In all cases where irritation of the nervous centres is evidenced during life by Delirium or otherwise, these signs of irritation may be looked for in the brain after death. In some cases of erysipelas, diphtheria, and septic inflammations, micrococci have been detected forming centres of irritation with minute softenings.
The causation of this disease is obscure, but as it has the general characters of an acute inflammation, followed by a chronic and atrophic stage, it may be supposed that it is due to an irritant of some kind acting on the brain and spinal cord generally.
The earlier periods of the disease are characterized by evidences of abnormal cerebral activity, such as mania, which is sometimes very furious. In this acute period the brain is swollen and hyperaemic, and there are multiple foci in which collections of leucocytes and capillary haemorrhages are Visible.
In the chronic periods, which most cases have reached before death, the appearances are those of a chronic inflammation of the brain and its membranes, along with degeneration and atrophy of the brain substance.
The disease has thus very much the characters of a meningitis and interstitial cerebritis with loss of the proper nervous tissue as a consequence, a cirrhosis of the brain.
The appearances are prominently those of Atrophy of the brain. On removing the calvarium the dura mater is commonly found wrinkled and the soft membranes cedematous, evidently from shrinking of the brain substance. The oedema of the membranes is most visible in the sulci, which gape and are filled up with cedematous connective tissue. The fluid is chiefly over the parietal and occipital lobes, perhaps from gravitation. There is also distension of the ventricles, frequently very great, so that the brain substance lying between the fluid in the ventricles and the cedematous membranes is greatly shrunken. The surface of the ventricles is beset with little prominent granulations which are often very marked, especially in the floor of the fourth ventricle where the appearance has been likened to the tongue of a cat (langue de chat). The pia mater is adherent to the surface of the convolutions, so that on attempting to remove it portions of brain substance come away. These adhesions may be taken as indications of the inflammatory nature of the disease, and their locality affords some evidence of the localization of the lesions. Crichton Browne states that they occur mainly, in the anterior three-fourths of the brain, affecting the frontal lobe chiefly in its anterior and posterior thirds, and the parietal in all its convolutions.
The brain as a whole is greatly reduced in weight. Taking the normal weight of the brain, including membranes, etc., as fifty ounces for the adult male, and forty-five for the female, the weight in general paralysis often falls to thirty-five ounces. The loss of weight does not affect the brain uniformly; it is mainly the cerebral hemispheres that are affected, the basal ganglia and peduncular parts being much less so, and the cerebellum not at all.
The membranes show irregular thickenings. The Pia mater presents milky opacities or more obvious patches of thickening. The Dura mater is so frequently affected that at one time it was supposed to be the primary seat of disease. It presents patches of thickening, opacities, and even flat bony developments, and it is commonly adherent to the calvarium. Not infrequently there are extravasations of blood of smaller or larger size accompanied by appearances to be described afterwards as Pachymeningitis chronica haemorrhagica. The Calvarium itself sometimes presents thickening, which is usually diffuse, and may be chiefly due to the shrinking of the brain; sometimes there are also local prominences or actual exostoses. Sometimes the diploe is converted into dense bone, so that the calvarium is much heavier than normal - so-called sclerosis of the bone.
These appearances are due to inflammation and shrinking of the brain substance, the dropsy of the ventricles and cedema of the membranes occurring in order to fill up the space formerly occupied by the brain, the cranium being a cavity of fixed size and the dropsy being ex vacuo.
The appearances under the microscope are chiefly those of diffuse sclerosis and atrophy of the brain substance. We have already seen that there is sclerosis of the posterior columns of the cord, and this may even be an early condition. In the brain there are often traces of the earlier acute condition in the form of little clumps of pigment around the vessels, the remains of former extravasations. But the appearances are more those of degeneration. The ganglion cells are shrunken and frequently pigmented; the blood-vessels show sometimes fatty degeneration, sometimes a homogeneous glancing appearance (colloid), and they are frequently obstructed by glancing masses which are partly calcareous and partly colloid. These appearances are not to be regarded as in any way peculiar to this disease, they are simply evidences of the profound atrophy of the brain, and may occur to a limited extent in old persons where there has been only a senile atrophy.