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Hemimegalencephaly
Luke L. Linscott, MD
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KEY FACTS

  • Terminology

    • Imaging

      • Pathology

        • Clinical Issues

          • Diagnostic Checklist

            TERMINOLOGY

            • Definitions

              • Megalencephaly: Brain size & weight > 2 standard deviations above age-related normals
              • Hemimegalencephaly (HME): Hamartomatous overgrowth of part vs. entire hemisphere
                • Classic HME (or unilateral megalencephaly): Holohemispheric involvement
                • Partial HME (or focal megalencephaly): Portion of hemisphere involved
                  • Difficult to distinguish from focal cortical dysplasia
                • Total HME: Involvement of unilateral cerebral hemisphere, cerebellum, & brainstem (rare)

            IMAGING

            • General Features

              • CT Findings

                • MR Findings

                  • Ultrasonographic Findings

                    • Nuclear Medicine Findings

                      • Imaging Recommendations

                        DIFFERENTIAL DIAGNOSIS

                          PATHOLOGY

                          • General Features

                            • Gross Pathologic & Surgical Features

                              • Microscopic Features

                                CLINICAL ISSUES

                                • Presentation

                                  • Demographics

                                    • Natural History & Prognosis

                                      • Treatment

                                        DIAGNOSTIC CHECKLIST

                                        • Consider

                                          • Image Interpretation Pearls

                                            • Reporting Tips

                                              Selected References

                                              1. Crino PB: Focal Cortical Dysplasia. Semin Neurol. 35(3):201-8, 2015
                                              2. Re TJ et al: Magnetic resonance fiber tracking in a neonate with hemimegalencephaly. J Neuroimaging. 25(5): 844-7, 2015
                                              3. Kamiya K et al: Accelerated myelination along fiber tracts in patients with hemimegalencephaly. J Neuroradiol. 41(3):202-10, 2014
                                              4. Mirzaa GM et al: Megalencephaly and hemimegalencephaly: breakthroughs in molecular etiology. Am J Med Genet C Semin Med Genet. 166C(2):156-72, 2014
                                              5. Wang DD et al: Unilateral holohemispheric central nervous system lesions associated with medically refractory epilepsy in the pediatric population: a retrospective series of hemimegalencephaly and Rasmussen's encephalitis. J Neurosurg Pediatr. 1-12, 2014
                                              6. Williams F et al: The diagnosis of hemimegalencephaly using in utero MRI. Clin Radiol. 69(6):e291-7, 2014
                                              7. Honda R et al: Long-term developmental outcome after early hemispherotomy for hemimegalencephaly in infants with epileptic encephalopathy. Epilepsy Behav. 29(1):30-5, 2013
                                              8. Poduri A et al: Somatic mutation, genomic variation, and neurological disease. Science. 341(6141):1237758, 2013
                                              9. Barkovich A et al: Pediatric Neuroimaging. 5th ed. Philadelphia: Lippincott Williams & Wilkins. p. 407-409, 2012
                                              10. Lee JH et al: De novo somatic mutations in components of the PI3K-AKT3-mTOR pathway cause hemimegalencephaly. Nat Genet. 44(8):941-5, 2012
                                              11. Rivière JB et al: De novo germline and postzygotic mutations in AKT3, PIK3R2 and PIK3CA cause a spectrum of related megalencephaly syndromes. Nat Genet. 44(8):934-40, 2012
                                              12. Soufflet C et al: The nonmalformed hemisphere is secondarily impaired in young children with hemimegalencephaly: a pre- and postsurgery study with SPECT and EEG. Epilepsia. 45(11):1375-82, 2004
                                              13. Flores-Sarnat L et al: Hemimegalencephaly: part 2. Neuropathology suggests a disorder of cellular lineage. J Child Neurol. 18(11):776-85, 2003
                                              14. Flores-Sarnat L: Hemimegalencephaly: part 1. Genetic, clinical, and imaging aspects. J Child Neurol. 17(5):373-84; discussion 384, 2002
                                              15. Ishibashi H et al: Somatosensory evoked magnetic fields in hemimegalencephaly. Neurol Res. 24(5):459-62, 2002
                                              16. Di Rocco F et al: Hemimegalencephaly involving the cerebellum. Pediatr Neurosurg. 35(5):274-6, 2001
                                              17. Arai Y et al: A comparison of cell phenotypes in hemimegalencephaly and tuberous sclerosis. Acta Neuropathol. 98(4):407-13, 1999
                                              18. Hanefeld F et al: Hemimegalencephaly: localized proton magnetic resonance spectroscopy in vivo. Epilepsia. 36(12):1215-24, 1995
                                              Related Anatomy
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                                              Related Differential Diagnoses
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                                              References
                                              Tables

                                              Tables

                                              KEY FACTS

                                              • Terminology

                                                • Imaging

                                                  • Pathology

                                                    • Clinical Issues

                                                      • Diagnostic Checklist

                                                        TERMINOLOGY

                                                        • Definitions

                                                          • Megalencephaly: Brain size & weight > 2 standard deviations above age-related normals
                                                          • Hemimegalencephaly (HME): Hamartomatous overgrowth of part vs. entire hemisphere
                                                            • Classic HME (or unilateral megalencephaly): Holohemispheric involvement
                                                            • Partial HME (or focal megalencephaly): Portion of hemisphere involved
                                                              • Difficult to distinguish from focal cortical dysplasia
                                                            • Total HME: Involvement of unilateral cerebral hemisphere, cerebellum, & brainstem (rare)

                                                        IMAGING

                                                        • General Features

                                                          • CT Findings

                                                            • MR Findings

                                                              • Ultrasonographic Findings

                                                                • Nuclear Medicine Findings

                                                                  • Imaging Recommendations

                                                                    DIFFERENTIAL DIAGNOSIS

                                                                      PATHOLOGY

                                                                      • General Features

                                                                        • Gross Pathologic & Surgical Features

                                                                          • Microscopic Features

                                                                            CLINICAL ISSUES

                                                                            • Presentation

                                                                              • Demographics

                                                                                • Natural History & Prognosis

                                                                                  • Treatment

                                                                                    DIAGNOSTIC CHECKLIST

                                                                                    • Consider

                                                                                      • Image Interpretation Pearls

                                                                                        • Reporting Tips

                                                                                          Selected References

                                                                                          1. Crino PB: Focal Cortical Dysplasia. Semin Neurol. 35(3):201-8, 2015
                                                                                          2. Re TJ et al: Magnetic resonance fiber tracking in a neonate with hemimegalencephaly. J Neuroimaging. 25(5): 844-7, 2015
                                                                                          3. Kamiya K et al: Accelerated myelination along fiber tracts in patients with hemimegalencephaly. J Neuroradiol. 41(3):202-10, 2014
                                                                                          4. Mirzaa GM et al: Megalencephaly and hemimegalencephaly: breakthroughs in molecular etiology. Am J Med Genet C Semin Med Genet. 166C(2):156-72, 2014
                                                                                          5. Wang DD et al: Unilateral holohemispheric central nervous system lesions associated with medically refractory epilepsy in the pediatric population: a retrospective series of hemimegalencephaly and Rasmussen's encephalitis. J Neurosurg Pediatr. 1-12, 2014
                                                                                          6. Williams F et al: The diagnosis of hemimegalencephaly using in utero MRI. Clin Radiol. 69(6):e291-7, 2014
                                                                                          7. Honda R et al: Long-term developmental outcome after early hemispherotomy for hemimegalencephaly in infants with epileptic encephalopathy. Epilepsy Behav. 29(1):30-5, 2013
                                                                                          8. Poduri A et al: Somatic mutation, genomic variation, and neurological disease. Science. 341(6141):1237758, 2013
                                                                                          9. Barkovich A et al: Pediatric Neuroimaging. 5th ed. Philadelphia: Lippincott Williams & Wilkins. p. 407-409, 2012
                                                                                          10. Lee JH et al: De novo somatic mutations in components of the PI3K-AKT3-mTOR pathway cause hemimegalencephaly. Nat Genet. 44(8):941-5, 2012
                                                                                          11. Rivière JB et al: De novo germline and postzygotic mutations in AKT3, PIK3R2 and PIK3CA cause a spectrum of related megalencephaly syndromes. Nat Genet. 44(8):934-40, 2012
                                                                                          12. Soufflet C et al: The nonmalformed hemisphere is secondarily impaired in young children with hemimegalencephaly: a pre- and postsurgery study with SPECT and EEG. Epilepsia. 45(11):1375-82, 2004
                                                                                          13. Flores-Sarnat L et al: Hemimegalencephaly: part 2. Neuropathology suggests a disorder of cellular lineage. J Child Neurol. 18(11):776-85, 2003
                                                                                          14. Flores-Sarnat L: Hemimegalencephaly: part 1. Genetic, clinical, and imaging aspects. J Child Neurol. 17(5):373-84; discussion 384, 2002
                                                                                          15. Ishibashi H et al: Somatosensory evoked magnetic fields in hemimegalencephaly. Neurol Res. 24(5):459-62, 2002
                                                                                          16. Di Rocco F et al: Hemimegalencephaly involving the cerebellum. Pediatr Neurosurg. 35(5):274-6, 2001
                                                                                          17. Arai Y et al: A comparison of cell phenotypes in hemimegalencephaly and tuberous sclerosis. Acta Neuropathol. 98(4):407-13, 1999
                                                                                          18. Hanefeld F et al: Hemimegalencephaly: localized proton magnetic resonance spectroscopy in vivo. Epilepsia. 36(12):1215-24, 1995