CONGENITAL DILATATIONS OF THE SEPTUM PELLUCIDUM.

Abstract

Many papers have been published describing the various aspects of dilatations of the septum pellucidum. Septal dilatation may be caused either by a cyst which has no communication with the ventricles or by a cavum with openings into the third and lateral ventricles. The purpose of this report is threefold: first, to review the incidence of congenital dilatations on the basis of a group of routine autopsy specimens and to correlate the findings with the incidence of cavum septi pellucidi in a series of normal pneumoencephalo-grams and ventriculograms; second, to review the clinical aspects of communicating cysts of the septum pellucidum in a number of cases from the National Hospital, London, England; third, to report two new cases of cysts of the septum pellucidum with a roentgenographic follow-up showing a change to a communicating cyst with subsequent collapse of the cavum. Anatomical Review The septum pellucidum, which separates the lateral ventricles, appears in the mid-sagittal section of a normal brain as a triangular structure, with the base of the triangle directed toward the inferior aspect of the corpus callosum. The septum consists of two thin, vertically positioned glial layers which are in apposition, and, if separated, form a cavum of varying size (Fig. 1). The inferior, anterior, and superior boundaries of the septum are, respectively, the rostral lamina, and rostrum and trunk of the corpus callosum. The posterior border is represented by the ascending pillars of the fornix. The true width of the septum pellucidum can be accurately evaluated only in the living subject, by roentgenographic air studies of the brain. The shrinkage and distortion seen in the autopsy specimen, although a negligible factor, will not be encountered in the roentgen examination. Only the brow-up anteroposterior pneumoenceph-alogram will indicate the width and height of the septum pellucidum, which is demonstrated as a thin opaque line separating the air-filled radiolucent anterior horns. The length of the septum cannot be determined because the lateral air study does not demonstrate its position. Review of the Literature The appearance of the cavum septi pellucidi in the pneumoencephalogram was first described in 1930 (1). The roentgenographic features of a cyst of the septum pellucidum were reported one year later and it was indicated that cysts may produce neurologic symptoms, whereas in the presence of a cavum the clinical examination is negative (2). Additional cases of septum pellucidum dilatations caused by cysts were subsequently reported, stressing, to a lesser and greater extent, anatomy (3, 4), incidence (5–7), clinical features and treatment (8–12), and roentgenology (13, 14).