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Hemimegalencephaly
Usha D. Nagaraj, MD
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KEY FACTS

  • Terminology

    • Imaging

      • Pathology

        • Clinical Issues

          • Diagnostic Checklist

            TERMINOLOGY

            • Definitions

              • Hemimegalencephaly (HMEG): Nonneoplastic overgrowth of part or entire hemisphere
                • Classic HMEG (or unilateral megalencephaly): Holohemispheric involvement
                • Partial HMEG (or focal megalencephaly): Portion of hemisphere involved
              • Dysplastic megalencephaly (DMEG): Macrocephaly with significant portions of both hemispheres involved
              • Same pathologic & genetic spectrum as focal cortical dysplasia type II & tuberous sclerosis complex (TSC)

            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. Jeon TY et al: Structural MRI and tract-based spatial statistical analysis of diffusion tensor imaging in children with hemimegalencephaly. Neuroradiology. 62(11):1467-74, 2020
                                              2. Reghunath A et al: A journey through formation and malformations of the neo-cortex. Childs Nerv Syst. 36(1):27-38, 2020
                                              3. Barkovich AJ et al: Pediatric Neuroimaging. 6th ed. Wolters Kluwer. 448-51, 2019
                                              4. Xu Q et al: mTOR inhibitors as a new therapeutic strategy in treatment resistant epilepsy in hemimegalencephaly: a case report. J Child Neurol. 34(3):132-8, 2019
                                              5. Shrot S et al: Dysplasia and overgrowth: magnetic resonance imaging of pediatric brain abnormalities secondary to alterations in the mechanistic target of rapamycin pathway. Neuroradiology. 60(2):137-50, 2018
                                              6. Alcantara D et al: Mutations of AKT3 are associated with a wide spectrum of developmental disorders including extreme megalencephaly. Brain. 140(10):2610-22, 2017
                                              7. Ikeda KM et al: Evolution of epilepsy in hemimegalencephaly from infancy to adulthood: case report and review of the literature. Epilepsy Behav Case Rep. 7:45-8, 2017
                                              8. Calvo-Garcia MA et al: Imaging evaluation of fetal vascular anomalies. Pediatr Radiol. 45(8):1218-29, 2015
                                              9. Crino PB: Focal cortical dysplasia. Semin Neurol. 35(3):201-8, 2015
                                              10. Keppler-Noreuil KM et al: PIK3CA-related overgrowth spectrum (PROS): diagnostic and testing eligibility criteria, differential diagnosis, and evaluation. Am J Med Genet A. 167A(2):287-95, 2015
                                              11. Re TJ et al: Magnetic resonance fiber tracking in a neonate with hemimegalencephaly. J Neuroimaging. 25(5): 844-7, 2015
                                              12. Sarnat HB et al: Infantile tauopathies: hemimegalencephaly; tuberous sclerosis complex; focal cortical dysplasia 2; ganglioglioma. Brain Dev. 37(6):553-62, 2015
                                              13. Kamiya K et al: Accelerated myelination along fiber tracts in patients with hemimegalencephaly. J Neuroradiol. 41(3):202-10, 2014
                                              14. Keppler-Noreuil KM et al: Clinical delineation and natural history of the PIK3CA-related overgrowth spectrum. Am J Med Genet A. 164A(7):1713-33, 2014
                                              15. Mirzaa GM et al: Megalencephaly and hemimegalencephaly: breakthroughs in molecular etiology. Am J Med Genet C Semin Med Genet. 166C(2):156-72, 2014
                                              16. 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
                                              17. Williams F et al: The diagnosis of hemimegalencephaly using in utero MRI. Clin Radiol. 69(6):e291-7, 2014
                                              18. 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
                                              19. Poduri A et al: Somatic mutation, genomic variation, and neurological disease. Science. 341(6141):1237758, 2013
                                              20. Barkovich AJ et al: A developmental and genetic classification for malformations of cortical development: update 2012. Brain. 135(Pt 5):1348-69, 2012
                                              21. 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
                                              22. 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
                                              23. 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
                                              24. Flores-Sarnat L et al: Hemimegalencephaly: part 2. Neuropathology suggests a disorder of cellular lineage. J Child Neurol. 18(11):776-85, 2003
                                              25. Flores-Sarnat L: Hemimegalencephaly: part 1. Genetic, clinical, and imaging aspects. J Child Neurol. 17(5):373-84; discussion 384, 2002
                                              26. Ishibashi H et al: Somatosensory evoked magnetic fields in hemimegalencephaly. Neurol Res. 24(5):459-62, 2002
                                              27. Di Rocco F et al: Hemimegalencephaly involving the cerebellum. Pediatr Neurosurg. 35(5):274-6, 2001
                                              28. Arai Y et al: A comparison of cell phenotypes in hemimegalencephaly and tuberous sclerosis. Acta Neuropathol. 98(4):407-13, 1999
                                              29. 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

                                                          • Hemimegalencephaly (HMEG): Nonneoplastic overgrowth of part or entire hemisphere
                                                            • Classic HMEG (or unilateral megalencephaly): Holohemispheric involvement
                                                            • Partial HMEG (or focal megalencephaly): Portion of hemisphere involved
                                                          • Dysplastic megalencephaly (DMEG): Macrocephaly with significant portions of both hemispheres involved
                                                          • Same pathologic & genetic spectrum as focal cortical dysplasia type II & tuberous sclerosis complex (TSC)

                                                        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. Jeon TY et al: Structural MRI and tract-based spatial statistical analysis of diffusion tensor imaging in children with hemimegalencephaly. Neuroradiology. 62(11):1467-74, 2020
                                                                                          2. Reghunath A et al: A journey through formation and malformations of the neo-cortex. Childs Nerv Syst. 36(1):27-38, 2020
                                                                                          3. Barkovich AJ et al: Pediatric Neuroimaging. 6th ed. Wolters Kluwer. 448-51, 2019
                                                                                          4. Xu Q et al: mTOR inhibitors as a new therapeutic strategy in treatment resistant epilepsy in hemimegalencephaly: a case report. J Child Neurol. 34(3):132-8, 2019
                                                                                          5. Shrot S et al: Dysplasia and overgrowth: magnetic resonance imaging of pediatric brain abnormalities secondary to alterations in the mechanistic target of rapamycin pathway. Neuroradiology. 60(2):137-50, 2018
                                                                                          6. Alcantara D et al: Mutations of AKT3 are associated with a wide spectrum of developmental disorders including extreme megalencephaly. Brain. 140(10):2610-22, 2017
                                                                                          7. Ikeda KM et al: Evolution of epilepsy in hemimegalencephaly from infancy to adulthood: case report and review of the literature. Epilepsy Behav Case Rep. 7:45-8, 2017
                                                                                          8. Calvo-Garcia MA et al: Imaging evaluation of fetal vascular anomalies. Pediatr Radiol. 45(8):1218-29, 2015
                                                                                          9. Crino PB: Focal cortical dysplasia. Semin Neurol. 35(3):201-8, 2015
                                                                                          10. Keppler-Noreuil KM et al: PIK3CA-related overgrowth spectrum (PROS): diagnostic and testing eligibility criteria, differential diagnosis, and evaluation. Am J Med Genet A. 167A(2):287-95, 2015
                                                                                          11. Re TJ et al: Magnetic resonance fiber tracking in a neonate with hemimegalencephaly. J Neuroimaging. 25(5): 844-7, 2015
                                                                                          12. Sarnat HB et al: Infantile tauopathies: hemimegalencephaly; tuberous sclerosis complex; focal cortical dysplasia 2; ganglioglioma. Brain Dev. 37(6):553-62, 2015
                                                                                          13. Kamiya K et al: Accelerated myelination along fiber tracts in patients with hemimegalencephaly. J Neuroradiol. 41(3):202-10, 2014
                                                                                          14. Keppler-Noreuil KM et al: Clinical delineation and natural history of the PIK3CA-related overgrowth spectrum. Am J Med Genet A. 164A(7):1713-33, 2014
                                                                                          15. Mirzaa GM et al: Megalencephaly and hemimegalencephaly: breakthroughs in molecular etiology. Am J Med Genet C Semin Med Genet. 166C(2):156-72, 2014
                                                                                          16. 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
                                                                                          17. Williams F et al: The diagnosis of hemimegalencephaly using in utero MRI. Clin Radiol. 69(6):e291-7, 2014
                                                                                          18. 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
                                                                                          19. Poduri A et al: Somatic mutation, genomic variation, and neurological disease. Science. 341(6141):1237758, 2013
                                                                                          20. Barkovich AJ et al: A developmental and genetic classification for malformations of cortical development: update 2012. Brain. 135(Pt 5):1348-69, 2012
                                                                                          21. 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
                                                                                          22. 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
                                                                                          23. 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
                                                                                          24. Flores-Sarnat L et al: Hemimegalencephaly: part 2. Neuropathology suggests a disorder of cellular lineage. J Child Neurol. 18(11):776-85, 2003
                                                                                          25. Flores-Sarnat L: Hemimegalencephaly: part 1. Genetic, clinical, and imaging aspects. J Child Neurol. 17(5):373-84; discussion 384, 2002
                                                                                          26. Ishibashi H et al: Somatosensory evoked magnetic fields in hemimegalencephaly. Neurol Res. 24(5):459-62, 2002
                                                                                          27. Di Rocco F et al: Hemimegalencephaly involving the cerebellum. Pediatr Neurosurg. 35(5):274-6, 2001
                                                                                          28. Arai Y et al: A comparison of cell phenotypes in hemimegalencephaly and tuberous sclerosis. Acta Neuropathol. 98(4):407-13, 1999
                                                                                          29. Hanefeld F et al: Hemimegalencephaly: localized proton magnetic resonance spectroscopy in vivo. Epilepsia. 36(12):1215-24, 1995