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Neurology aNd Special SeNSeS  ` neurology—embryology  Neurology aNd Special SeNSeS  ` neurology—embryology  SecTioN iii  491




                  Neural tube defects    Neuropores fail to fuse (4th week) Ž persistent connection between amniotic cavity and spinal
                                          canal. Associated with maternal diabetes and folate deficiency.  α-fetoprotein (AFP) in amniotic
                                          fluid and maternal serum (except spina bifida occulta = normal AFP).  acetylcholinesterase
                                          (AChE) in amniotic fluid is a helpful confirmatory test.
                   Spina bifida occulta  Failure of caudal neuropore to close, but no herniation. Usually seen at lower vertebral levels. Dura
                                          is intact. Associated with tuft of hair or skin dimple at level of bony defect.
                   Meningocele           Meninges (but no neural tissue) herniate through bony defect.
                   Myelomeningocele      Meninges and neural tissue (eg, cauda equina) herniate through bony defect.
                   Myeloschisis          Also called rachischisis. Exposed, unfused neural tissue without skin/meningeal covering.
                   Anencephaly           Failure of rostral neuropore to close Ž no forebrain, open calvarium. Clinical findings:
                                          polyhydramnios (no swallowing center in brain).

                                                            +/− Tuft of hair         Skin defect/thinning   Skin thin or absent
                       Skin                                 +/− Skin dimple
                                                                                       Subarachnoid
                                                                                          space
                                       Dura
                                       Leptomeninges



                       Spinal             Transverse
                        cord              process




                              Normal                                               Spina bifida occulta                                            Meningocele                                          Myelomeningocele


                  Holoprosencephaly      Failure of the embryonic forebrain (prosencephalon) to separate into 2 cerebral hemispheres;
                                          usually occurs during weeks 5–6. May be related to mutations in sonic hedgehog signaling
                   A
                                          pathway. Associated with other midline defects including cleft lip/palate (moderate form) and
                           ★              cyclopia (severe form).  risk for pituitary dysfunction (eg, diabetes insipidus). Can be seen with
                                          Patau syndrome (trisomy 13).
                       Monoventricle     MRI reveals monoventricle  A  and fusion of basal ganglia (star in  A ).







                  Lissencephaly          Failure of neuronal migration resulting in a “smooth brain” that lacks sulci and gyri. May be
                                          associated with microcephaly, ventriculomegaly.



























          FAS1_2019_12-Neurol.indd   491                                                                                11/8/19   7:39 AM
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