1	Review of Embryology and Fetal MR Findings in Chiari and Dandy-Walker Malformations. (eSE 104) Kevin Auerbach, M.D. Phuong Vinh, M.D. Kailash Amruthur, M.D. Kevin Irish, M.D.
2	Introduction Fetal MR is an emerging imaging modality which is gaining popularity due to its excellent anatomic detail. At our institution MR is used to delineate and further evaluate findings on level 3 ultrasound. As prenatal intervention becomes more accessible Fetal MR will play a greater role in the workup of fetal abnormalities.
3	Introduction, cont. The evaluation of suspected posterior fossa abnormalities, notably Dandy Walker and Chiari malformations, is one of the more common neurologic indications for Fetal MR at our institution. MR allows for differentiation between these entities and common normal variants which can simulate them.
4	Embryology The posterior fossa contains many important structures including the brainstem(midbrain, pons, and medulla) as well as the cerebellum and the associated CSF spaces(4th ventricle, foramina of Luschka and Magendie, and the aqueduct of Sylvius). The embryological development of these structures begins at about 3 weeks of gestation and is complete at around 20 months after birth.
5	Embryology 3wks- Primary brain vesicles form 3-5wks- the neural tube bends resulting in the cranial, cervical, and pontine flexures. The rhomboencephalon subdivides into 8 rhombomeres. 6 wks- cerebellar buldge arises at rhombic lips and the cerebellum develops from the fusion of the alar plates of the 1^st rhombomere.
6	Embryology The pontine flexure seperates the metencephalon(future pons and cerebellum) from the myelencephalon(future medulla oblongata), thus forming the 4^th ventricle, the roof of which becomes the cerebellum.
7	Embryology 6-7weeks- the flocculonodular lobe and dentate nuclei of the cerebellum form. The remainder of the cerebellum develops in a rostro-caudal manner. Growth of the vermis occurs at the 3^rd month of gestation and becomes fully foliated by 4mths gestation while the hemispheres continue to develop till 20mths of life. The medial(Magendie) outflow tract forms at 8wks and the lateral(Luschka) slightly later.
8	Fetal MR While we previously used SSFSE imaging for antenatal MR, we now almost exclusively use multiplanar FIESTA imaging oriented to the fetal brain. This sequence is both T1 & T2 weighted with high T2 contrast and uses very short TR and TE resulting in short acquisition times which limits motion artifacts, but still offers excellent spatial resolution.
9	Fetal MR Here is a sagital FIESTA sequence image of the fetal brain. This is the equivalent to the appearance of the sagital T1 weighted image which is routinely used in the evaluation of midline structures on the post natal MR.
10	Normal 19 wk Gestation 4^th Ventricle Cerebellar vermis Craniocervical Junction Torcula Cisterna magna
11	"Click on each image to..." Click on each image to see a video scrolling through the normal appearance of a 23 week fetal brain.
12	Normal at 30 Wks
13	Normal at 34 Wks
14	CASE
15	DWM 24wks
16	Dandy Walker Malformation ~ 1 in 5000 live births. Most cases are sporatic, but there are infrequent familial cases associated with syndromes(Meckel-Gruber and Walker-Warburg syndromes) as well as many chromosomal anomalies. There is a reported association with isotretinoin use during pregnancy. Most frequently there is hydrocephalus, but cerebellar signs and mental retardation are less common with some references reporting normal intelligence in up to 75-80% of individuals, however this is likely in the absence of associated brain abnormalities. Presumed etiology is delayed or blocked foramen of Magendie or persistence/thickening of the membranacea superior which forms the roof of the 4^th ventricle.
17	CASE
18	DWV 34wks
19	Dandy Walker Variant It is unclear if this is a separate entity or just a slightly milder form of the “true” malformation.
20	CASE
21	Megacisterna Magna 38 wks
22	CASE
23	Megacisterna Magna 36 wks
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25	Chiari Malformations Congenital hindbrain abnormalities which result in abnormal structural relationships between the cerebellum, brainstem/upper cervical cord, and the skull base. Classified as I-IV. Etiology is not well understood and although classified together may not be related to a single cause. Types III and IV are rare and typically incompatible with life.
26	CASE
27	Chiari II
28	19 weeks SLIUP
29	Chiari II Incidence is approximately 1 per 1000, more frequent in females. The exact etiology is unknown with multiple suggested etiologies, possibly related to inadequate growth of the posterior fossa secondary to loss of CSF from the inevitably present Myelomeningocele, or may be the result of primary connective tissue abnormality resulting in a small posterior fossa. This serious abnormality has immediate mortality rates quoted as up to 15% in the first years of life with hindbrain dysfunction as the major cause of mortality.
30	CASE
31	Chiari III 30 wks
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33	Sent for evaluation of suspected cerebellar vermis hypoplasia on ultrasound.
34	Summary While antenatal MR can provide valuable information about multiple organ systems, in this presentation we showed the findings/appearance of the posterior fossa abnormalities which we are most frequently asked to evaluate with MR. The ability to distinguish these malformations from the more “benign” abnormalities which may cause a similar appearance on antenatal ultrasound allows for more informed parents and obstetricians when planning births and arranging for neonatal care.
35	Conclusion With its excellent anatomic detail the use of antenatal MR will continue to grow in the future. As prenatal interventional techniques continue to be developed, MR may play a greater role in the workup of fetal abnormalities and possibly be used in the selection of subgroups of patients in whom antenatal intervention is of optimum benefit.
36	References Parisi, Melissa A.; Dobyns, William B. 2003. Human malformations of the midbrain and hindbrain: review and proposed classification scheme. Molecular Genetics and Metabolism 80:36-53. Niesen, Charles E. 2002. Malformations of the posterior fossa: Current perspectives. Seminars in Pediatric Neurology 9(4):320-334. Cai, Christopher; Oakes, Jerry. 1997. Hindbrain Herniation Syndromes: Chiari Malformations(I and II). Seminars in Pediatric Neurology 4(3):167-178 Golden, Jeffrey A.; Harding, Brian N. 2004. Developmental Neuropathology, Switzerland, ISN Neuropath Press. Stevenson KL. 2004. Chiari Type II malformation: past, present, and future. Neurosurgical Focus 16(2):E5.