[Cisterna megamagna. Dimensions and relations to cerebral atrophy. Clinical and x-ray computed tomographic demonstration].

Polaroids of 9,400 consecutive unselected computed tomography (CT) scans are reviewed and 31 cases with Mega Cisterna Magna (MCM) are detected. The incidence is 0.33% (31/9,400). Clinical and radiological data relating to these 31 cases shows that none of the patients presented neither any clinical sign of a posterior fossa space occupying lesion nor any radiological image such as hydrocephalus or forward displacement of the fourth ventricle. The dimensions of the cistern ranged as follows: width between 22.2-92.5 mm, depth between 22.2-37.0 mm, height between 26.0-39.0 mm. The authors propose 20 mm as the limit value of width and depth of a normal cisterna magna and 26 mm for its maximal height, these dimensions being the most frequently encountered ones of these 31 patients. The correlation between the magnitude of the cistern (the sum of the width and depth of the cistern: w + d) and the severity of cerebral atrophy (the sum of the maximum and minimum widths of the anterior horns of the lateral ventricles which is the most reliable atrophy index defined by Hirashima: a + b) is illustrated in figure 5. The authors conclude that Mega Cisterna Magna which has a constant incidence in routine CT examinations is predominantly a consequence of cerebral atrophy and does not have any specific clinical pathological correlate and does not require any form of treatment.