Abscess of the brain being, like abscess elsewhere, a septic process, it arises by the implantation of pyogenic microbes in the brain substance. The disease is a somewhat frequent.

The abscess is usually single, but occasionally there is more than one.

Causation And Mode Of Infection

The brain being enclosed in several envelopes, one of which is a bony case, the pyogenic microbes can only find access to it either by lesions of the enveloping structures, or by means of the arteries. The latter is a very unusual mode of infection, but it has been met with, especially in cases of disease of the lungs characterized by suppurating cavities. In two cases in which thrush was coincident, the abscesses in the brain contained growths of the oidium albicans. In the great majority of cases it is by the extension of septic processes from the bones of the skull that abscesses of the brain are brought about.

Given a septic inflammation in any part of the cranium, there are several ways in which the infection may spread inwards. There may be a direct extension from the bone to the dura mater. In that case a septic pachymeningitis is the result. Pus may collect inside the dura mater and may be limited by adhesions, so as to lead to a subdural abscess, perhaps with erosion of the brain. Or, without any considerable accumulation of pus between the dura mater and the bone, the septic process may advance into the brain. There is usually in this case adhesion of the dura mater to the brain and some softening of the latter.

Again infection frequently travels by the veins and venous sinuses. A septic inflammation of a cranial bone may directly involve .the wall of a vein or sinus. Thrombosis is thus induced, and as there are septic agents present, a thrombo-phlebitis is the result. The extension may be directly to one of the sinuses of the dura mater. On the other hand the extension may be first to one of the veins of the diploe and thence to the sinus. The veins of the diploe open partly into veins outside the skull and partly into those inside. In the latter case thrombo-phlebitis is liable to extend to the intra-cranial venous sinuses. As the most frequent seat of the original disease is the temporal bone, the extension, whether directly or by the veins of the diploe, is usually to the sigmoid and lateral sinuses. In this connection it is also of consequence that the posterior temporal vein of the diploe opens into the lateral sinus. On the other hand, it is to be remembered that the intra-cranial sinuses receive blood from the cerebral veins, and there is thus a communication between the bones and the cerebral substance. The thrombosis which accompanies the process is liable to block the sinuses, and the blood may in consequence even partially regurgitate into the veins of the brain, carrying septic agents with it, or there may be a propagation of the septic thrombosis into these veins.

In this mode of extension there is liable to be a septic Lepto-meningitis from the infection spreading to the pia-arachnoid. Abscess of the brain may therefore be associated with that disease, or either abscess or lepto-meningitis may occur alone.

A more unusual and possibly more doubtful mode of extension is along the perivascular lymphatics. The infective process may possibly penetrate along the outside of the veins without producing a septic thrombosis of the veins themselves.

The primary septic inflammation of the bones has, in the majority of cases, its origin in suppurative disease of the middle ear (otitis media). It is very seldom that an acute otitis media, even though it be suppurative, leads to septic inflammation inside the skull. It is usually the chronic cases, in which the tympanic membrane has been lost and there has been, it may be for years, a discharge from the external meatus. The suppuration in the middle ear affects the mucous membrane, which is here closely connected with the bone and acts as a periosteum. Hence the bone is liable to be affected, becoming carious or even undergoing necrosis. The tympanic cavity and mastoid cells both send blood into the sinuses of the dura mater by means of veins which pass through the bone; those of the tympanum passing into the petrosal sinus and those of the mastoid cells into the lateral sinus.

The abscesses arising from ear disease are usually situated either in the temporo-sphenoidal lobe, or in the cerebellum. In the latter case the extension has occurred for the most part by the sinuses and veins.

Injuries to the head sometimes give rise to abscess of the brain by extension of the septic process. Compound fractures with freely open wounds have seldom this result. It is mostly punctured fractures where infective matter is carried deeply inwards. There may even be a septic extension without fracture where the bones of the skull have been exposed in a wound and dirt ingrained into them.

Disease of the nose and orbit are rare causes of cerebral abscess, and it is only when the bones are affected, as in some cases of syphilis, that extension will occur. Septic inflammations of the skin of the face and of the scalp, more particularly erysipelas, sometimes lead to septic leptomeningitis, but seldom to abscess of the brain. The extension here occurs by the veins, a thrombo-phlebitis extending, it may be, from the orbit whose veins have communications with the cavernous sinus inside the skull.

Formation And Character Of The Abscess

The abscess begins presumably with a small softening which goes on to suppuration. The tally formed abscess contains a thick pus, usually greenish in colour, and in the case of ear disease generally exhaling a pungent putrid odour. In many cases the abscess is really a chronic one and the pus rpuscles are disintegrated. It is bounded usually by a distinct wall formed of granulation tissue, sometimes partly developed into connective tissue. This frequently forms a definite separable membrane, which separates the abscess from the brain tissue, the latter being often softened in the immediate vicinity. The membrane takes some time to form, being rarely distinct till the third week and not fully formed for two months or even longer. When once formed and encapsuled the abscess may remain long stationary, but it usually enlarges gradually, and may finally burst into the lateral ventricles or on to the surface of the brain.

The abscess causes enlargement of the part affected, and the convolutions over it are flattened and softened. According to Ballance some 40 per cent, of all cases of abscess of the brain are secondary to middle ear disease and are, with few exceptions, located either in the lateral lobe of the cerebellum or less frequently in the temporo-sphenoidal lobe.