By Osborn, Salzman, Jhaveri

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Left) Sagittal T1WI MR shows normal 4th ventricle position and appearance. The fastigium st is in normal position, helping to distinguish from Chiari 2 malformation. There is inferior displacement of the ectopic cerebellar tonsils ſt through the foramen magnum with odontoid process retroflexion and clivus foreshortening. (Right) Axial T2WI MR confirms inferior displacement of ectopic cerebellar tonsils ſt through the foramen magnum, producing foramen magnum crowding. 9% pediatric patient groups ○ Asymptomatic CM1 discovered incidentally on imaging relatively common; perhaps best described as cerebellar tonsillar ectopia) Natural History & Prognosis • Natural history not clearly understood ○ Many patients asymptomatic, CM1 discovered incidentally ○ Increasing ectopia → ↑ risk of syrinx • Children respond better to treatment than adults DIAGNOSTIC CHECKLIST Consider • Degree of tonsillar correlates with clinical severity • Unless tonsils > 5 mm and pointed ± "crowded posterior fossa" probably not clinically significant Image Interpretation Pearls • Don't use 5 mm cut-off point alone to diagnosis CM1 (with pathological and clinical implications) SELECTED REFERENCES 1.

Ultrasound Obstet Gynecol. ePub, 2015 De Cock J et al: A newborn with neurocutaneous melanocytosis and DandyWalker malformation. Pediatr Neurol. 50(3):276-8, 2014 Barkovich AJ et al: A developmental and genetic classification for midbrainhindbrain malformations. Brain. 132(Pt 12):3199-230, 2009 Dandy-Walker Continuum Pathology-Based Diagnoses: Congenital Malformations (Left) Sagittal T2WI MR in a patient with DWM demonstrates hydrocephalus, a large posterior fossa, cephalad rotation of a small, incompletely lobulated vermis with a very shallow fastigial crease ﬉, and a very thin cyst wall ﬈.

Left) Sagittal MRV demonstrates torcularlambdoid inversion. The transverse sinuses st angle upward toward the torcular ﬇ as the cyst has prevented normal fetal torcular descent. Note the persistent fetal occipital sinus ſt. (Right) Coronal T2WI MR demonstrates a huge, fluidfilled posterior fossa. Again, notice that the transverse sinuses ﬉ are angled upward toward the torcular herophili ﬈. 26 ○ Fastigial recess, primary fissure, PF/brainstem normal • MCM ○ Enlarged pericerebellar cisterns communicate with basal subarachnoid spaces • Occipital bone may appear scalloped/remodeled with all DWS types (including MCM) • Routine MR imaging (thin sagittal views crucial) PATHOLOGY • Most severe to mildest: DWM with 4th ventriculocele → classic DWM → HVR → BPC → MCM • Numerous syndromes associated with DWS CLINICAL ISSUES • Marked heterogeneity in genetic, clinical findings • DWM: 80% diagnosed by 1 year Dandy-Walker Continuum Abbreviations • Dandy-Walker (DW) spectrum (DWS), DW complex (DWC), "classic" DW malformation (DWM) • Hypoplastic vermis with rotation (HVR), formerly DW variant (DWV) • Persistent Blake pouch cyst (BPC), mega cisterna magna (MCM) Definitions • DWS represents heterogeneous spectrum of cystic posterior fossa (PF) malformations IMAGING General Features • Best diagnostic clue ○ DWM: Large PF with small, counterclockwise (CCW) rotated vermis ○ HVR, BPC: Failure of "closure" of 4th ventricle • Location ○ Posterior fossa • Size ○ Variable • Morphology ○ DWS (from most to least severe) – 4th ventriculocele (10-15% of cases) □ DWM with large 4th ventricle erodes occipital bone → "encephalocele" – "Classic" DWM □ Cystic dilatation of 4th ventricle → enlarged PF, superiorly rotated hypoplastic vermis □ Torcular-lambdoid inversion: Cyst mechanically hinders normal fetal caudal migration of torcular – HVR (formerly DW variant) □ Variable vermian hypoplasia, no or small cyst, normal sized PF/brainstem, "keyhole" vallecula – BPC □ "Open" 4th ventricle communicates with infravermian cyst, normal fastigial recess, and primary fissure – MCM □ Enlarged retrocerebellar cistern communicates with foramen magnum and basal subarachnoid spaces □ Cistern crossed by falx cerebelli, tiny veins □ Normal vermis/4th ventricle Radiographic Findings • Radiography ○ Enlarged calvaria, particularly posterior fossa ○ DWM: Lambdoid-torcular inversion (transverse sinus grooves elevated above lambda) – Sinuses are originally above lambda in fetus; cyst mechanically hinders descent CT Findings • NECT ○ DWM: Large posterior fossa – Variable-sized cyst communicates with 4th ventricle – Torcular-lambdoid inversion (torcular above lambdoid suture) ○ Occipital bone may appear scalloped, remodeled with all DWS types, including MCM MR Findings • T1WI ○ Sagittal DWM – Floor of 4th ventricle present – 4th ventricle opens dorsally to variable-sized CSF cyst – Cyst wall difficult to discern – Vermian remnant (± fastigium, fissures) rotated CCW, over cyst – ± remnant fused to tentorium – Elevated torcular with high/steeply sloping tentorium (classic) ○ Sagittal HVR – Smaller PF ± cyst – 4th ventricle "open" with partial CCW rotation vermis, presence of fastigium, fissures variable ○ Sagittal BPC – Rotated but normal-appearing vermis – Free communication of 4th ventricle with prominent inferior CSF space – Basal cisterns compressed posteriorly or effaced ○ Sagittal MCM – Normal vermis (not rotated/hypoplastic) – 4th ventricle is "closed" • T2WI ○ Associated anomalies – Cortical dysplasia, heterotopias, myelination delays (syndromic DWS) • FLAIR ○ ± very slight signal difference between cyst, CSF ○ ± compressed basal cisterns • DWI ○ Very slight restriction may be seen if reduced fluid motion • MRV ○ Elevated torcular herophili (DWM) Pathology-Based Diagnoses: Congenital Malformations TERMINOLOGY Ultrasonographic Findings • Identical to MR; can diagnose in fetus Nonvascular Interventions • Cisternography delineates cyst wall Imaging Recommendations • Best imaging tool ○ MR best characterizes severity, associated anomalies • Protocol advice ○ Routine MR imaging (thin sagittal views crucial) DIFFERENTIAL DIAGNOSIS Dandy-Walker Spectrum • "In-between" cases common Posterior Fossa Arachnoid Cyst (AC) • Location: Retrocerebellar, supravermian, or in cerebellopontine angle • Included in DW spectrum by some authors 27 Pathology-Based Diagnoses: Congenital Malformations Dandy-Walker Continuum • Normal 4th ventricle compressed or displaced • AC not traversed by falx cerebelli, tiny veins • ACs lined by arachnoid cells/collagen Molar Tooth Deformity (Joubert) • Prototype = Joubert anomaly • Episodic hyperpnea, oculomotor apraxia, retinal dystrophy, ± renal cysts, hepatic fibrosis • Split vermis, bat wing 4th ventricle, mesencephalon is shaped like molar tooth Isolated 4th Ventricle • Inferior 4th ventricle "closed" vs.

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