Megacisterna Magna

Dandy-Walker complex, cerebellar hypoplasia

A cisterna magna with a depth of more than 10 mm , in the absence of cerebellar anomalies.

An estimate of about 1:8000 births has been recently suggested.²

This condition is usually diagnosed with imaging techniques and pathologic data are limited. It has been suggested that the enlargement of the cisterna magna may be secondary to a distension of the Blake’s pouch that however does not displace superiorly the cerebellar vermis, as it happens in the Dandy-Walker complex.³

When the diagnosis is made in fetal life, an association with trisomy 18 has been reported.¹ This however may represent the consequence of cerebellar hypoplasia that is frequent in these cases rather than a primary enlargement. Other cerebral anomalies may be present. The association with bilateral periventricular nodular heterotopia, a condition with X-linked dominant trasmissione is well established, although precise risk figures are not available.^4-6

Megacisterna magna was originally described in postnatal patients using purely subjective criteria. In the obstetric literature the term has been used to indicate cases with a cisterna magna depth in excess of 3 SDs above the mean, or 10 mm.^{7, 8} The definition is now well established.⁹ However, there is most likely a discrepancy between the classic postnatal radiologic definition of the condition (that identifies a rare finding) and the obstetric one (that implies a prevalence of around 1%.⁸ Indeed, most fetuses with a prenatal diagnosis of megacisterna magna are found to be normal at the postnatal investigation.¹⁰ The value of the obstetric definition appears to be the identification of a generic risk factor for associated anomalies.¹⁰ Magnetic resonance may be of value to diagnose additional cerebral malformations such as heterotopia.

This condition should be differentiated from the Dandy-Walker complex (which is characterized by an open fourth ventricle), cerebellar hypoplasia (small cerebellum) and posterior fossa arachnoid cysts (mass effect with asymmetric distortion of the cerebellum).

In the absence of associated anomalies the prognosis is excellent and neurologic complications are rarely encountered. Indeed, magecisterna magna is frequently an incidental finding in radiologic examinations performed after birth.¹¹ Overall, the outcome of 84 infants diagnosed in utero to have isolated mega cisterna magna has been reported and 81 of these had normal development at least in the first months of life.^{2, 10, 12-14} These data may be biased by the limited follow-up.In one of the best postnatal study, it was indeed suggested that these infants may be prone to mild developmental delay in later life, which is compatible with adult data.^{11, 15} Fetuses with mega cisterna magna associated with abnormal cerebral findings were find to have neurologic sequelae in about one third of cases. However, these were rarely severe.¹⁵ Bilateral periventricular nodular heterotopia, which may be associated with mega cisterna magna, affects predominantely female, that have a normal intelligence but tend tot develop epilepsy in the second decade of life. Affecetd males are rarely encountered but tend to have a more severe outcome.^4-6

Other than the need for a detailed ultrasound scan, no modification of standard obstetric management is indicated. The only aneuploidy found in excess in this cases is trisomy 18, which is virtually excluded by a detailed second trimester scan.

1. Nyberg DA, Mahony BS, Hegge FN, Hickok D, Luthy DA, Kapur R. Enlarged cisterna magna and the Dandy-Walker malformation: factors associated with chromosome abnormalities. Obstet Gynecol. 1991;77(3):436-42.

2. Long A, Moran P, Robson S. Outcome of fetal cerebral posterior fossa anomalies. Prenat Diagn. 2006;26(8):707-10. Epub 2006/06/10.

3. Robinson AJ, Goldstein R. The cisterna magna septa: vestigial remnants of Blake's pouch and a potential new marker for normal development of the rhombencephalon. J Ultrasound Med. 2007;26(1):83-95. Epub 2006/12/22.

4. Bargallo N, Puerto B, De Juan C, Martinez-Crespo JM, Lourdes Olondo M. Hereditary subependymal heterotopia associated with mega cisterna magna: antenatal diagnosis with magnetic resonance imaging. Ultrasound Obstet Gynecol. 2002;20(1):86-9. Epub 2002/07/09.

5. Battaglia G, Granata T, Farina L, D'Incerti L, Franceschetti S, Avanzini G. Periventricular nodular heterotopia: epileptogenic findings. Epilepsia. 1997;38(11):1173-82. Epub 1998/05/14.

6. Oda T, Nagai Y, Fujimoto S, Sobajima H, Kobayashi M, Togari H, Wada Y. Hereditary nodular heterotopia accompanied by mega cisterna magna. Am J Med Genet. 1993;47(2):268-71. Epub 1993/08/15.

7. Filly RA, Cardoza JD, Goldstein RB, Barkovich AJ. Detection of fetal central nervous system anomalies: a practical level of effort for a routine sonogram. Radiology. 1989;172(2):403-8. Epub 1989/08/01.

8. Mahony BS, Callen PW, Filly RA, Hoddick WK. The fetal cisterna magna. Radiology. 1984;153(3):773-6.

9. ISUOG. Sonographic examination of the fetal central nervous system: guidelines for performing the 'basic examination' and the 'fetal neurosonogram'. Ultrasound Obstet Gynecol. 2007;29(1):109-16.

10. Gandolfi Colleoni G, Contro E, Carletti A, Ghi T, Campobasso G, Rembouskos G, Volpe G, Pilu G, Volpe P. Prenatal diagnosis and outcome of fetal posterior fossa fluid collections. Ultrasound Obstet Gynecol. 2012, in press. Epub 2011/12/17.

11. Zimmer EZ, Lowenstein L, Bronshtein M, Goldsher D, Aharon-Peretz J. Clinical significance of isolated mega cisterna magna. Arch Gynecol Obstet. 2007;276(5):487-90. Epub 2007/04/25.

12. Dror R, Malinger G, Ben-Sira L, Lev D, Pick CG, Lerman-Sagie T. Developmental outcome of children with enlargement of the cisterna magna identified in utero. J Child Neurol. 2009;24(12):1486-92. Epub 2009/02/26.

13. Forzano F, Mansour S, Ierullo A, Homfray T, Thilaganathan B. Posterior fossa malformation in fetuses: a report of 56 further cases and a review of the literature. Prenat Diagn. 2007;27(6):495-501. Epub 2007/03/21.

14. Haimovici JA, Doubilet PM, Benson CB, Frates MC. Clinical significance of isolated enlargement of the cisterna magna (> 10 mm) on prenatal sonography. J Ultrasound Med. 1997;16(11):731-4; quiz 5-6. Epub 1997/11/14.
15. Bolduc ME, Limperopoulos C. Neurodevelopmental outcomes in children with cerebellar malformations: a systematic review. Dev Med Child Neurol. 2009;51(4):256-67. Epub 2009/02/05.

Pilu G, Malinger G, Buyukkurt S: Mega cisterna magna. Visual Encyclopedia of Ultrasound in Obstetrics and Gynecology, www.isuog.org, May 2013

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