In the case of the temporo-sphenoidal lobe, it is necessary to extend the incision behind the auricle vertically upwards for an inch or more ([Fig. 252]); whereas if the cerebellum has to be explored, an incision is carried backwards at right angles to the post-aural incision, just below its mid-point ([Fig. 253]). In the former case, on reflecting the soft tissues from the underlying bone, the squamous portion of the temporal bone, immediately above the zygomatic ridge, will be exposed; in the latter, the base of the skull behind and below the mastoid process and lateral sinus will be laid bare.
Fig. 251. Topography of the Auditory Region of the Skull. A, Point of trephining for a temporo-sphenoidal abscess; B, For a cerebellar abscess; C, Dotted line marking a portion of the lateral sinus.
1. Trephining. The trephine used should be three-quarters of an inch to one inch in diameter according as the patient is a child or an adult. Either the hand trephine or Macewen’s improved pattern mounted with a guard may be used. If available, the trephine may be worked by a motor, but in this case it should be remembered that the bone will be pierced more quickly than by the hand instrument.
Trephining for a temporo-sphenoidal abscess. The object of the operation is to expose the lowest portion of the middle fossa just above the roof of the antrum and tympanic cavity. The trephine, therefore, should be placed so that it is situated just above the suprameatal spine, its lowest margin being slightly above the zygomatic ridge (Fig. 251). After the disk of bone has been removed the exploration of the abscess is then carried out.
Trephining for a cerebellar abscess. The point at which the bone is trephined must be behind and below the curve formed by the transverse and sigmoid portion of the lateral sinus; that is, behind the mastoid process and below Reid’s base-line.
If the mastoid operation has not been performed, the centre pin of the trephine should be placed at a point 1¼ to 1½ inches behind the centre of the external auditory meatus, and an inch below Reid’s base-line (Fig. 251). If, however, the mastoid has already been opened and the lateral sinus exposed, the trephine should be placed so that its anterior border is just behind the sinus and its upper border well below Reid’s base-line.
2. After performing the mastoid operation. If this has been done already, the wound is reopened, and cleansed by filling it with hydrogen peroxide. After gently curetting away any granulations the wound cavity is irrigated and then packed in order to dry it. Under good illumination, careful inspection is made to see if a fistula or a tract of diseased bone extends in any direction. Whether the middle or posterior fossa should first be explored depends not only on the clinical symptoms but also on the condition found on opening the mastoid cavity.
Fig. 252. Exploration for a Temporo-sphenoidal Abscess. A, Above the tegmen tympani; B, Through the tegmen tympani. Occasionally these methods are combined; the bone between the openings being also removed.