Absent Calvarium



Absent Calvarium


Anne Kennedy, MD



DIFFERENTIAL DIAGNOSIS


Common



  • Exencephaly, Anencephaly


  • Acrania


Less Common



  • Amniotic Band Syndrome


  • Skeletal Dysplasia, Mimic



    • Osteogenesis Imperfecta


    • Hypophosphatasia


    • Achondrogenesis


Rare but Important



  • Twin Reversed Arterial Perfusion (TRAP)


ESSENTIAL INFORMATION


Key Differential Diagnosis Issues



  • Is it a single or multiple gestation?



    • TRAP only possible in monochorionic twin or higher order multiple


  • Is there a head?



    • If not, consider TRAP sequence in a twin pregnancy


    • Possible bands in singleton or a twin


  • Is the cranial vault present?



    • If absent, differential diagnosis is between amniotic bands and anencephaly


    • If present, then differential diagnosis is between the mimics



      • Measure all long bones


      • Look for fractures


      • Assess compressibility of cranial vault


  • Is there formed brain?



    • In anencephaly, there is a gelatinous mass of angiomatous stroma in the skull base


    • In exencephaly and acrania, a fully formed brain may be seen particularly early in pregnancy before repeated episodes of trauma destroy it


  • Is the maternal serum alpha fetoprotein (msAFP) elevated?



    • Increased msAFP associated with open neural tube defect


  • Is the fetus mobile within the uterine cavity?



    • If fetus seems tethered at one location, look carefully for amniotic bands



      • Change maternal position to try to float fetus away from uterine wall


      • Look for liner echoes attaching fetus or externalized organs to uterine wall


Helpful Clues for Common Diagnoses



  • Exencephaly, Anencephaly



    • Brain present but not contained within skull vault



      • Brain contour lobulated (“Mickey Mouse”) or spiked (“Bart Simpson”)


    • In first trimester, may present with short crown rump length


    • Brain subject to trauma from fetal movement, abrasion against uterine wall, etc.



      • By third trimester may no longer see recognizable brain due to liquefaction


      • Look for echogenic debris in amniotic fluid


    • Orbits prominent → “frog eye” appearance


    • Often associated with cervical/lumbar spine defects


    • Polyhydramnios common


  • Acrania



    • Absent calvarium above orbits


    • Occiput ossifies in cartilage; therefore, skull base intact


    • Meninges may be present



      • May provide protective barrier from erosion


Helpful Clues for Less Common Diagnoses



  • Amniotic Band Syndrome



    • Multiple defects


    • Look for amputation/constriction defects in extremities


    • Look for asymmetric facial cleft


    • Look for missing orbit in association with “anencephalic” appearance


    • Key to establishing the diagnosis is demonstration of bands



      • Fine linear structures in amniotic fluid


      • Band often “tethers” fetus to uterine wall


      • Change maternal position to float fetus away from uterine wall


  • Skeletal Dysplasia, Mimic



    • Calvarium is present but poorly ossified


    • Brain is seen “too easily”


    • Brain morphology is normal



      • Even though ossification is abnormal the brain is protected from erosion/trauma


    • Calvarium can be deformed by transducer pressure


    • Osteogenesis Imperfecta



      • Multiple fractures


      • Beaded ribs



      • Short extremity bones


      • “Thick” bones


    • Hypophosphatasia



      • Thin, bowed long bones


    • Achondrogenesis



      • Type I associated with poor mineralization of skull and completely unossified spine


      • Fractures of ribs but not long bones seen in type Ia


      • Spine ossification may be seen in pedicles in type Ib


      • Type II has normal ossification of the skull but deficient ossification of spine


Helpful Clues for Rare Diagnoses



  • Twin Reversed Arterial Perfusion (TRAP)



    • Multiple gestation



      • Monochorionic placentation


    • Cranium often completely absent


    • Upper extremities often poorly developed


    • Even in well-developed TRAP fetus the brain is abnormal, skull vault may be present but surrounded by marked soft tissue edema


    • Hallmark of this diagnosis is reversed flow in the umbilical artery



      • Flow is away from placenta toward the TRAP fetus


Other Essential Information

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Aug 10, 2016 | Posted by in OBSTETRICS | Comments Off on Absent Calvarium

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