From these and other statistics it may be regarded as generally accepted that the majority of brain abscesses are secondary to chronic disease of neighbouring bone, and more especially middle ear disease. This being the case, it might naturally be inferred--on anatomical grounds--that the temporo-sphenoidal lobe of the cerebrum and the cerebellum are more liable to infection than any other part of the brain. From 100 cases treated at my hospital the temporo-sphenoidal lobe and cerebellum were involved in the proportion of about 2 to 1. Körner,[52] reporting on another 100 cases, gives the following data:--
Abscess of the cerebrum 62 cases „ „ cerebellum 32 „ „ in both situations 6 „
Hunter Tod,[53] reporting on 100 cases treated at the London Hospital, found that in children under 10 years of age temporo-sphenoidal abscess occurred in 87 per cent. cases, and cerebellar in 13 per cent., whereas in adults cerebral abscess occurred in 65 per cent. and cerebellar in 35 per cent. The development of temporo-sphenoidal and cerebellar abscesses in the same case was observed in 5 per cent. cases.
=Pathology.= Insomuch as chronic middle ear disease forms the main predisposing factor in the development of abscess of the brain, the pathology of brain abscess in general may be considered by discussing the main features peculiar to otitic abscess in particular.
As the result of chronic middle ear disease, the mucous lining of the middle ear and its accessory cavities becomes destroyed, the antrum filled with cholesteatomata, and the middle and external ears with granulations. The discharge of pus, previously free, is obstructed, partial or complete blockage occurring. The destruction of the mucous lining allows of invasion of the surrounding bone, the veins become thrombosed and filled with bacteria, and the cancellous spaces blocked with granulations. Further erosion of the bone results, both in the upward direction towards the tegmen tympani and in the backward towards the lateral sinus groove and cerebellum. The veins of the tegmen communicate freely with those of the temporo-sphenoidal lobe, whilst those ramifying in the mastoid region either communicate with the lateral sinus itself or with the anterior cerebellar venous system. Infection may therefore spread (1) _upwards_ to the temporo-sphenoidal lobe, or (2) _backwards_ to the lateral sinus and cerebellum. In the former case, meningitis or temporo-sphenoidal abscess develops: in the latter instance, meningitis, lateral sinus thrombosis, or cerebellar abscess.