Kimon Bekelis MD (Dr. Bekelis of Dartmouth-Hitchcock Medical Center has no relevant financial relationships to disclose.)
Robert J Singer MD (Dr. Singer of Dartmouth-Hitchcock Medical Center/Geisel School of Medicine at Dartmouth has no relevant financial relationships to disclose.)
Originally released April 11, 1995; last updated September 12, 2016; expires September 12, 2019

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 a shared clinical presentation. Outcomes are variable, depending much on success of the surgical procedure, as well as possible less detectable neuronal abnormalities in the contralateral hemisphere. Developmental intervention and therapy continue to be essential components of long-term management.

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 (Sims 1835; Feld et al 1955; Laurence 1964; Bignami et al 1968; Townsend et al 1975; Fitz et al 1978; Mikhael and Mattar 1978; Manz et al 1979; Antonelli et al 2004). 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 (Hallervorden 1923).

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 (Dekaban and Sakuragawa 1977; Matsubara et al 1983; Trounce et al 1991). Most early descriptions of hemimegalencephaly were of patients with both brain and somatic hemihypertrophy (Webster 1908; Hallervorden 1926; Rugel 1946). 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 (Raghavendra et al 2006; Salamon et al 2006; Sato et al 2007).

Epilepsy, which develops in most children with hemimegalencephaly, is typically refractory to medical management (Robain et al 1988; Paladin et al 1989) and resective epilepsy surgery (hemispherectomy) remains the treatment of choice (Vigevano et al 1989; Shimizu 2005; Di Rocco et al 2006; Luders and Schuele 2006; Salamon et al 2006; Loddenkemper et al 2008; Kiehna et al 2016; Wang et al 2016). Cerebral hemispherectomy was introduced more than 50 years ago to treat intractable seizures in patients with unilateral CNS abnormalities (Krynauw 1950). However, its regular use in children with hemimegalencephaly has come after procedural modifications to minimize complications of hemorrhage, hemosiderosis, and hydrocephalus (Tinuper et al 1988; Peacock et al 1991; Di Rocco et al 2006). Techniques for cerebral hemispherotomy and corpus callosotomy have also been described and may be effective in palliating severe epilepsy (Kwan et al 2008; Marras et al 2010).

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