The extra-dural space between the posterior aspect of the petrous bone and the lateral sinus region is first inspected, and, in the event of the discovery of a collection of pus, this is gently wiped away and further exploration postponed till the occasion should prove the necessity.
When the indications are of such a nature as to demand _exploration of the cerebellum_, the mastoid region is first cleansed and packed with gauze, after which the dura mater is incised either in front or behind the sinus according to the probable situation of the abscess cavity. After the evacuation of the abscess a drainage tube is stitched in position in the manner previously indicated.
Difficulties and dangers attendant on the process of trephining for brain abscess.
_Failure to find the abscess_ is usually dependent on one of the following causes:--
(_a_) The abscess may be missed if the localizing features are misleading, if the abscess be small, and if the exploring instrument be passed in a faulty direction or to an insufficient depth.
(_b_) The abscess may be traversed but not tapped if unsuitable exploring instruments be used--an aspirating syringe, for instance, through which the thick pus will not pass. Even when suitable instruments are used, the attempt to evacuate the abscess may fail if the operator does not carry out what may be called a system of progressive exploration, that is to say, if he does not periodically open the blades of the evacuator (see p. 266).
(_c_) The abscess may be encountered but not penetrated if the surgeon has to deal with a chronic abscess, the wall of which is merely pushed aside by the advancing instrument (see chronic abscess of the brain, p. 268).
_Complications_ arising during and after the evacuation of the abscess are as follows:--
1. Leaking into the pia-arachnoid and the development of general meningeal infection.
2. Perforation of the ventricular spaces.
5. Osteomyelitis of the neighbouring diploic tissue.
_General meningeal infection_ will bring about a fatal result. Fortunately, the general rise of intracranial pressure and the formation of adhesions between the brain and the overlying membranes tend to prevent its development.
_Perforation of the ventricular spaces_ may occur under two conditions: (1) when the abscess is associated with internal hydrocephalus, and (2) when the exploring instrument is passed too far.