Pontine Angle Tumours

With the aid of encephalography it is as a rule possible, provided a correct technique is used, to diagnose even small tumours in the pontine angle and also to differentiate between intraand extra-axial tumours. The differential diagnosis is mainly based on the appearance of the cisterns. An extra-axial process usually bulges into the pontocerebellar and medullary cisterns from the lateral side, whereas an intra-axial process compresses them from above and from the medial side. A small acoustic neurinoma may be almost completely outlined by gas in the pontocerebellar cistern. If the tumour is larger it displaces the brain stem towards the opposite side. Because of that the cisterns of the tumour side become widened, whereas the corresponding cisterns of the opposite side are compressed. The displacement of the brain stem also results in a typical displacement and rotation of the fourth ventricle. The rotation may give an impression of a distortion of the fourth ventricle, which may lead to a false diagnosis of an intra-axial tumour. In some cases, however, an extra-axial tumour bulges into the brain stem instead of displacing it. An asymmertically located pontine tumour or a tumour of the fourth ventricle growing through the foramen of Luschka may on the other hand extend into and more or less completely compress the cistern, which accordingly cannot be gas-filled at the examination. In these cases it is sometimes difficult even at operation to find out whether the tumour is