Radiology of the Massa Intermedia

Abstract

AGRAY commissure referred to as the massa intermedia links the two thalami across the third ventricle. Such a massa can usually be seen clearly at the time of pneumography and appears as a round or oval shadow in lateral view. This shadow varies in size from case to case and may at times be so extensive as to simulate the filling defect caused by a third ventricle tumor. Until the publication of a recent paper by Davie and Baldwin (3), the radiological appearance of the massa intermedia had been given only fleeting attention in standard works (1, 2, 4–8). This paper presents a study of the size, site, and configuration of the massa intermedia on the basis of air ventriculography. Inferences about the actual extent of the structure could then be offered. Cases and Procedure Eight hundred and sixty patients were studied: 476 males and 384 females suffering from movement disorders as parkinsonism, dystonia musculorum deformans, intention tremor, or torticollis. None of the patients gave any evidence suggestive of a third ventricle tumor. The mean age was fifty-five years with a range of seven to seventy-two years. Ventriculography was performed to determine the target area prior to stereotactic thalamic surgery. Both ventriculography and thalamic surgery were carried out on all patients. Injection and reinjection of air varied between 15 and 50 cc, with the volume of injected air being determined only by adequate visualization of the ventricular system. Lateral and anteroposterior films based on 12-foot radiography were obtained with a 500 Fluorodex Westinghouse x-ray instrument using a 50 Dynamax tube and a 500 mA generator. Roentgenographic Findings Neither size nor site of the massa intermedia could be assessed reliably from anteroposterior roentgenograms. In fact, for the entire series adequate visualization of this structure was obtained in only 9 frontal projections, leading to exclusive reliance on lateral projections. On the basis of these lateral projections, variability in size, site, and configuration characterized the radiological shadow of the massa intermedia. The observations could be divided into the following categories: (a) absence of massa intermedia, (b) presence of massa intermedia with poorly defined borders, and (c) presence of massa intermedia with adequate outline of radiological shadow. Absence of a shadow corresponding to the mass intermedia was noted in 146 cases or 16.9 per cent. A shadow with poorly defined borders (Fig. 1) suggested the presence of the massa intermedia in 61 cases or 7.1 per cent. Here the shadow of the massa blended with the outline of the lateral walls and roof of the third ventricle. As a result, the size, site, and configuration of this commissure could not be assessed. The massa intermedia could be distinguished clearly with sharply delineated borders (Fig. 2) in 653 cases or 75.9 per cent.