Fig. 253. Exploration for a Cerebellar Abscess. A behind, and C in front of the lateral sinus; B, Lateral sinus.
Opening of a cerebellar abscess. The cerebellum may be explored from two different points, either in front or behind the lateral sinus. The posterior route is adopted if the abscess is superficial in the outer portion of the lateral lobe, usually the result of lateral sinus thrombosis or disease of the posterior mastoid cells. The anterior route is indicated if it is thought that the abscess is deeply placed in the anterior inferior portion of the cerebellum, that is, in those cases in which it is apparently a complication of labyrinthine suppuration, or the result of disease of the inner wall of the antrum and mastoid cavities (Fig. 253).
(a) Behind the lateral sinus. After exposure of the lateral sinus the bone is removed either by means of the gouge and mallet or by bone-forceps, until a considerable area of the dura mater is exposed behind and below the curve of the sinus (Fig. 253). The dura mater is then incised as already described.
The cerebellum is explored by thrusting the instrument inward for about an inch. As a rule the abscess is found at once. If it be not discovered at the first attempt, the instrument should be directed forwards, upwards, and inwards towards the posterior surface of the petrous bone. Care, however, must be taken that it is not pushed in too far, otherwise it may pierce the anterior upper margin of the cerebellum, and if an abscess be present, the meninges may thus become infected. If the surgeon has exposed the dura mater by trephining, it is necessary to push the exploratory instrument at least two inches inwards and forwards in order to reach an abscess situated in the anterior inferior portion of the cerebellum. In such cases it is by no means difficult to miss a small abscess, and further, drainage is frequently incomplete when an abscess is discovered. For this reason, if the cerebellum be explored first behind the lateral sinus and no abscess is discovered, it should further be explored by the anterior route in front of the lateral sinus. If the cerebellar abscess be secondary to lateral sinus thrombosis, and if there be no doubt as to the diagnosis, the inner wall of the sinus should be made as aseptic as possible, and the dura mater forming it incised freely; the cerebellum being thus explored through the site of infection.
(b) In front of the lateral sinus. The lateral sinus is first exposed ([Fig. 253]). The triangular area of bone situated in front of it, between it and the semicircular canals, and forming the inner boundary of the antrum and mastoid cavities, is now removed with the gouge and mallet or with a suitable pair of forceps. If it be certain that internal-ear suppuration exists, or if the operation be secondary to opening of the labyrinth, the posterior wall of the petrous bone may be removed until the internal auditory meatus is almost reached. If, however, the labyrinth be intact, care must be taken not to chisel away too much bone for fear of encroaching on the posterior semicircular canal. On exposure of the dura mater an extra-dural abscess may be met with, usually the result of internal-ear suppuration. Even if no pus be seen, it is always a wise precaution, if internal-ear suppuration coexists, to separate the dura mater from the posterior wall of the petrous bone by means of an elevator in order to prevent any deeply situated extra-dural abscess being missed. After the dura mater has been exposed sufficiently it is opened by a crucial incision. In this region absence of increased tension within the brain and lack of bulging outwards of the cerebellar tissue do not necessarily imply the absence of an abscess; the cerebellum to all appearances may appear normal and flaccid, although a small abscess may be present.
The cerebellum is explored in various directions to a distance of not more than one inch. After the pus has been evacuated a tube is inserted as described above. In the majority of cases this method is far superior to opening the cerebellum behind the lateral sinus, especially as it is now recognized that the chief cause of cerebellar abscess is internal-ear suppuration.
After-treatment. This is similar to that of any ordinary abscess, but care must be taken that free drainage is maintained. The main part of the mastoid wound is lightly plugged with gauze, the tube inserted into the brain abscess being brought flush with the surface of the skin. The gauze filling the wound cavity should be arranged around the tube so that it rests comfortably within the wound and is not kinked. If the drainage tube be in its proper position, pus should be seen to ooze out of it.
Although the mastoid cavity itself need not be dressed daily, if necessary the outer dressings may be removed twice a day, in order to see that drainage of the abscess is continuous. After the first two or three days, the tube is gradually shortened. If the abscess be a recent one and not encapsuled, it becomes rapidly obliterated by pressure of the surrounding brain tissue, so that the tube may be forcibly ejected within a few days. On the other hand, if the abscess has existed for a considerable period and is bounded by a thick wall, which may be extremely resistant, the purulent discharge may continue for many days and necessitate the continuance of drainage. Generally speaking, the tube may be shortened every second or third day, and can usually be dispensed with by the end of the second week, if not before. It is, however, very necessary that the tube should not be withdrawn until it is certain that the abscess cavity has been obliterated completely.
The general treatment of the case in no way differs from that already described for the mastoid operation in which the wound has been left open posteriorly.
Complications. (i) On turning back the flaps of the dura mater, a hernia, consisting of friable congested brain tissue, may occur at once. This is extremely rare as a result of a simple abscess of the brain, but is significant of encephalitis frequently associated with meningitis (see [p. 436]). If an abscess be suspected, the brain should be explored as already described. If, however, no abscess be discovered, the treatment consists in removal of more bone and further incision of the dura mater, in order to permit of free drainage and to relieve tension.