Posterior Fossa Cyst/Fluid Collection



Posterior Fossa Cyst/Fluid Collection


Karen Y. Oh, MD



DIFFERENTIAL DIAGNOSIS


Common



  • Incorrect Scan Plane


  • Mega Cisterna Magna


  • Dandy-Walker Continuum: Classic


  • Dandy-Walker Continuum: Variant


  • Arachnoid Cyst


Less Common



  • Cerebellar Hypoplasia


  • Vein of Galen Malformation


  • Arteriovenous Fistula


ESSENTIAL INFORMATION


Key Differential Diagnosis Issues



  • Review normal anatomy of posterior fossa structures



    • Cisterna magna should measure ≤ 10mm


    • Assess cerebellar size



      • Diameter should measure ≈ gestational weeks in mm in second trimester


      • Normative charts exist for more accurate third trimester measurements


    • Cerebellum should be bilobed and symmetric


  • Identify if vermis hypoplastic/absent



    • Hypoplasia/absent in Dandy-Walker continuum


    • Beware of oblique scan plane



      • May simulate vermian defect


    • Later in gestation check sagittal plane



      • Vermis is midline echogenic posterior fossa structure between cerebellar hemispheres


  • Doppler is critical



    • “Cyst” may actually be a vascular malformation


    • May not have significant flow if thrombosis present



      • May see clot


    • MR will show typical flow void from high velocity flow



      • Thrombosis will have variable signal based on evolution of blood products within clot


Helpful Clues for Common Diagnoses



  • Incorrect Scan Plane



    • An overly coronal image gives false impression of mega cisterna magna or even Dandy-Walker malformation


  • Mega Cisterna Magna



    • Measure space between cerebellum and inner skull



      • Normal linear echogenic dural folds traverse space


    • Mega cisterna magna if > 10 mm


    • Cerebellum and vermis normally formed


    • Complete detailed anatomy scan



      • Carefully evaluate brain parenchyma


      • Can be associated with chromosomal abnormalities (trisomy 18)


    • May not have clinical significance in isolation


  • Dandy-Walker Continuum: Classic



    • Agenesis or severe hypoplasia of vermis


    • Communication of 4th ventricle with large posterior fossa cyst/cisterna magna


    • Elevated torcular herophili (confluence of sinuses)


    • Do not diagnose in first trimester



      • Normal rhombencephalon is cystic at that time


    • 70-90% have additional anatomic abnormalities



      • Chromosomal abnormalities in 50%


      • Other CNS findings include callosal dysgenesis, encephalocele, polymicrogyria, heterotopias, holoprosencephaly


    • May develop hydrocephalus antenatal



      • More common to have postnatal hydrocephalus


  • Dandy-Walker Continuum: Variant



    • Partial agenesis/hypoplasia of inferior vermis



      • Vermis not always completely formed until 17 weeks gestation


    • Torcular herophili in normal location, unlike Classic DW


    • Avoid over-diagnosis



      • Beware of oblique scan plane


      • If early in gestation, follow until 18 weeks to allow for complete development of vermis


    • May be associated with other CNS and extra-cranial abnormalities



      • Similar to Classic DW in frequency


  • Arachnoid Cyst



    • Anechoic extra-axial cyst



      • Contains cerebral spinal fluid



      • Most common over cerebral convexities in fetus


      • 1/3 in posterior fossa


      • If in posterior fossa, normal dural folds will be displaced


    • Size is variable



      • Need to monitor growth


      • If rapid growth → obstructive hydrocephalus


    • Can have scalloping of inner skull


    • If supratentorial and midline look for agenesis of corpus callosum


    • Mostly sporadic but occasionally seen with syndromes



      • Consider amniocentesis


Helpful Clues for Less Common Diagnoses



  • Cerebellar Hypoplasia



    • Cerebellum present but small for gestational age creating enlarged cisterna magna


    • Differentiate from atrophy of cerebellar hemispheres



      • Cerebellum small and cerebellar fissures enlarged if atrophy


      • Hypoplasia implies small cerebellum but fissures normal in proportion with folia


    • Can be isolated or associated with chromosomal or structural abnormalities



      • Trisomy 18, 13


      • Pontocerebellar hypoplasia (rare)


      • X-linked cerebellar hypoplasia (rare)


  • Vein of Galen Malformation



    • Elongated midline cystic structure



      • In cistern of velum interpositum and quadrigeminal plate cistern


      • Extends posteriorly to occipital region


      • Actually the medial prosencephalic vein of Markowski


    • Doppler shows turbulent flow



      • High velocity, low resistance arterial flow


    • May present with hydrops in third trimester, due to high output heart failure


    • Rarely may thrombose



      • Look for hypoechoic intraluminal clot


      • Can also see calcifications in clot


  • Arteriovenous Fistula

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Aug 10, 2016 | Posted by in OBSTETRICS | Comments Off on Posterior Fossa Cyst/Fluid Collection

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