J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.12.1.39 on 1 February 1949. Downloaded from J. Neurol. Neurosurg. Psychiat., 1949, 12, 39. DEVELOPMENTAL DEFECTS OF THE CISTERNA MAGNA AND DURA MATER BY E. GRAEME ROBERTSON From the Department ofNeurology and Neurosurgery, Royal Melbourne Hospital, Melbourne, Australia INDEX the cistern in specimens, for without floating the Page arachnoid away from the brain it is difficult to INTRODUCTION.. .. .. .. .. .. 39 delimit the periphery of the cistern. It is only NORMAL ANATOMY OF THE CISTERNA MAGNA .. 39 when gas in the cistern clearly delimits it that these VARIATIONS IN THE CISTERNA MAGNA: RECOGNITION DURING ENCEPHALOGRAPHY .. .. .. 40 -variations can be recognized. ALLIED ABNORMALITIES OF CISTERNA MAGNA, TEN- Normal Anatomy of the Cisterna Magna TORIUM, AND FALX CEREBRI .. .. .. 49 AccouNT OF COINCIDENT CYST IN VERMIS .. .. 50 As the arachnoid membrane passes upwards from SUMMARY .. .. .. .. .. .. 51 the spinal canal through the foramen magnum to enclose the contents of the posterior cranial fossa Introduction it becomes more extensive. Anteriorly the sub- Cerebral dqfects of developmental origin, apart arachnoid space suffers no interruption in front of the medulla and pons. Thus, during encepha- from those involving the nervous parenchyma, are Protected by copyright. to be found chiefly in the neighbourhood of the lography, gas is 'able to pass upwards' to reach the roof of the-third ventricle. The complex develop- interpeduncular cisterns and thence the cerebral mental evolution of the structures in this region subarachnoid space (Fig. 1). Posteriorly and may predispose to imperfect development, whether laterally, however, the arachnoid membrane is the cause be some variety of damage which impairs closely applied to the pia mater on the infeior' development, or. some genetically transmitted surface of the cerebellum. Thus, in this dir,tlon, factor. the upward passage of gas is blocked (Figs. 1 and The suprapineal recess varies considerably in 13a). 'The attachment over the vermis -in the size, and in ten of five hundred cases it was found median plane posteriorly is usually at a higher to be much larger than usual. The two leaves level than the attachment to the laterally placed which fuse to form the septum pellucidum some- hemispheres. In front, the close apposition of pia times remain separated and a fluid-filled cavity and arachnoid ceases at the ventral border of the separates the lateral ventricles. Sometimes one or cerebellum, a wide subarachnoid space which is botlh walls of this cavity are fenestrated, thus continuous with that over the anterior surfaces of allowing communication between the lateral ven-' the medulla and pons being found in the cerebello- tricle and the cavum septi pellucidi. The arach- pontine angles. The term is to the portion noid-lined space in the velum interpositum is cisterna magna* applied- http://jnnp.bmj.com/ obliterated during normal development, but some- of the wide subarachnoid space below the inferior times persists into adult life. Finally, the corpus surface of the cerebellum, behind the medulla, and callosum and fornix may be absent or may cease above the dura and arachnoid over the lower and to develop at an early stage. medial 'part of the occipital bone. The foramen The cisterna magna,also is subject to considerable of Magendie lies^ at the apex of the anterior part of variation in size and shape, due to causes active the cistern, at a higher level than the lower surface during development. The shape and extent of the of the vermis. The normal disposition in this cisterna magna are important in the performance region is responsible for the filling of the ventricles on October 2, 2021 by guest. of encephalography, and the recognition of ab- when gas is introduced with the head in a flexed n6rmality is important if ventricular filling is to be * In the original description, the cisterna magna cerebello- medullaris includes that part of the subarachnoid space anterior secured in the patient in whom an unusual state and lateral to the medulla. However, it facilitates description, and exists. The lack of recognition of the anomalies to is common usage, to apply the term cisterna magna only to that part behind the medulla. The anterior part may well be called the be described is due to the difficulty of delineating medullary cistern. 39 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.12.1.39 on 1 February 1949. Downloaded from 40 E. GRAEME ROBERTSON position. The gas rises behind the cervical spinal but ventricular filling may be secured at an earlier cord and then through the dorsal part ofthe foramen stage by dorsiflexing the head. The cistern is thus magnum to the cisterna magna. It passes to the brought to a lower level than the foramen of -upper part of the cistern, and when thegas-fluid Magendie, and gas is decanted into the fourth level sinks below the lowest part of the vermis gas ventricle. will flow forwards and upwards, between the cere- In some cases the lateral limits of the cistern are bellar tonsils, to the foramen of Magendie and thus unduly wide and are placed at a higher level than into the fourth ventricle. Gas in this position is the foramen of Magendie. Gas entering dorsally seen clearly in lateral views (Fig. 1). In axial will not be directed to the foramen of Magendie, views (films taken with the central ray directed, but will flow forwards at a higher level to the with varying inclination, along the median plane cerebello-pontine angles and thus into the pontine of the skull) it is seen as a dense, narrow shadow in cisterns. This is a commonest cause of failure to the mid-line, widening below into the shadow of fill the ventricles during encephalography. Unless the remainder of the cisterna magna and above into a special manoeuvre is employed, gas will not then that of the fourth ventricle. Some gas rises directly enter the ventricles during encephalography. The to the anterior part of the cistern, anterior to the head is fully flexed during periods of introduction sill provided by the vermis, and thus to the foramen ofgas and slowly dorsiflexed between such injections. of Magendie, the amount so doing increasing as In such full flexion the foramen of Magendie is the head becomes more erect, higher-than the point on each side at which the cisterna magna decants into the lateral recesses of Variations in the Cisterna Magna: the medullary and pontine cisterns. Recognition during Encephalography Secondly, the pia and arachnoid may not fuse The extent of the cistern over the inferior surface over the inferior surface of the cerebellum, hence of the cerebellum varies considerably. In the the usual barrier at the upper level of the cisternaProtected by copyright. majority of cases the cistern extends about 2-5 cm. magna does not exist (Figs. 4 and 13c). Gas above the foramen magnum along the inner table entering dorsally passes cranially around the of the occipital bone, reaching a point midway cerebellum to the cisterna vent magne cerebri and between the posterior lip of the foramen magnum thus to the cerebral subarachnoid space. In normal and the, internal occipital protuberance (Fig. 1). cases the subarachnoid space over the superior The depth (that is, the axial extent) is usually 5 mm. surface of the cerebellum may fill with gas from the The cistern is-sometimes small, sometimes consider- cisterna vene magnx, but no gas passes from there ably larger. Only when the cistern is large is it well down to the inferior surface of the cerebellum at a seen in axial views. Owing to its position over the lower level (Fig. 5). When the cisterna magna is lower part of the occipital table, the lateral extent not closed above, gas reaches the superior surface of the shadow is best seen in occipito-vertical views of the cerebellum without passing along the ventral taken with the head well flexed before gas has and ambient cisterns (Fig. 4). In this case, the gas enteg;ed the ventricles or anterior subarachnoid is not directed to the foramen of Magendie, hence spaces. The width is very variable. Sometimes ventricular filling is difficult to secure. If the head the shadow is about 2 cm. wide, sometimes much be kept erect, instead of flexed, some gas rises broader. The dense median shadow of the inferior directly to the foramen of Magendie and enters the occipital crest often divides the shadow into two ventricles. http://jnnp.bmj.com/ more or less symmetrical halves (Fig. 2). The anomaly which is the subject of this paper The cistern was found to extend as high as the was first seen four years ago. The cisterna magna internal occipital protuberance four times in one was found to extend above the inion, almost to the hundred cases (Fig. 3). In these cases the depth apex of the occipital bone (Fig. 6a). Thus, its apex and the width of the cistern were usually larger was considerably above the usual level of the than normal. Such a cistern causes a dense shadow tentorium. In addition to extending unusually during encephalography, owing to the large volume high, the cistern was also very wide in its lower of gas which it contains. This may lead to con- part (Fig. 6b). Fifteen c.cm. of gas were introduced on October 2, 2021 by guest. fusion in interpretation of encephalograms unless without the appearance of ventricular shadows. the significance of the shadow is recognized. Such By dorsiflexing the head the gas could be decanted a large cistern may, by its capacity, interfere with through the foramen of Magendie into the ventricu- the early filling of the ventricles.