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  • Updated 08.20.2019
  • Released 04.11.1995
  • Expires For CME 08.20.2022



This article includes discussion of hemimegalencephaly, hemimacrocephaly, unilateral macrocephaly, and unilateral megalencephaly. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.


Hemimegalencephaly is a rare central nervous system disorder of neuronal cell lineage, proliferation, maturation, and migration characterized by in utero enlargement of all or most of 1 cerebral hemisphere of the developing baby. The clinical hallmark is early onset intractable focal epilepsy with associated hemiparesis and developmental delays. Hemimegalencephaly may occur in the context of a defined syndrome, emphasizing the importance of comprehensive history and examinations being performed. Occasionally, unilateral cerebral enlargement may also involve brainstem and cerebellum, as well as rare hypertrophy of the face or body. Intractable epilepsy is virtually universally refractory to antiepileptic medications, and cerebral hemispherectomy is almost always the treatment of choice. As this update demonstrates, hemimegalencephaly is likely a heterogeneous group of disorders with distinct pathophysiologic characteristics resulting in a shared clinical presentation. Outcomes are variable, are dependent on degree of neuronal abnormalities, and are often dependent on surgical intervention. Initial studies investigating the genetic and histologic footprint of hemimegalencephaly are promising, but further studies are required to better understand the underlying mechanisms of the disease. Currently, pharmacologic, surgical, and developmental interventions remain at the forefront of long-term treatment strategies.

Key points

• Hemimegalencephaly is likely a diverse group of disorders with the common feature of enlargement and abnormal cellular structure of one cerebral hemisphere.

• Patients can present from birth to young adulthood.

• Common presenting symptoms include hemiparesis, intractable epilepsy, and developmental delay.

• Hemimegalencephaly may be associated with neurocutaneous syndromes.

• Early surgical intervention for intractable epilepsy may lead to improved epileptic and developmental prognosis.

Historical note and terminology

First described by Sims in 1835, hemimegalencephaly is a rare central nervous system disorder of neuronal proliferation and migration characterized by congenital enlargement of all or most of 1 cerebral hemisphere (120; 43; 76; 20; 128; 44; 86; 80; 06). Clinically, hemiparesis, developmental delays, and intractable seizures are characteristic. In some cases, unilateral cerebral enlargement may also involve brainstem and cerebellum, creating the appearance that 2 brains of different sizes had been joined in the midline (57).

Patients with hemimegalencephaly are divided into 2 groups, 1 in which hypertrophy is localized to the CNS and the other in which hypertrophy may involve the face or other parts of the body (36; 83; 129). Most early descriptions of hemimegalencephaly were of patients with both brain and somatic hemihypertrophy (138; 58; 111). Thus, comprehensive history and examination remain the foundation of evaluation of any patient with apparent hemimegalencephaly. Though rare, misdiagnosis of hemimegalencephaly may occur in the context of a contralateral small hemisphere (hemimicrencephaly) secondary to ischemia, heterotopias, or other neuronal migrational abnormalities (104; 114; 118).

Epilepsy, which develops in most children with hemimegalencephaly, is typically refractory to medical management (107; 94), and resective epilepsy surgery (hemispherectomy) remains the treatment of choice in most cases (131; 119; 39; 78; 114; 77; 66; 137). Anatomic hemispherectomy (AH) was first introduced by Dandy in 1928 for the treatment of malignant gliomas and was expanded for the treatment of seizures by McKenzie in 1938 (09). This was initially deemed a last resort for patients with “catastrophic epilepsy,” an uncommon subset of seizure disorders marked by uncontrollable seizures and severe cognitive deficits. Later, Krynauw employed anatomic hemispherectomy in cases of intractable seizures in patients with unilateral CNS abnormalities (71). Although seizure reduction was noted in the first iterations of the anatomic hemispherectomy procedure, complications such as hemorrhage, hemosiderosis, and hydrocephalus minimized its utility for the treatment of epilepsy. Today, modern variants, including functional hemispherectomy, first introduced by Rassmusen in 1978, have minimized complications while still achieving comparable seizure outcomes (127; 99; 39).

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