Megalencephaly

Megalencephaly
Classification and external resources
Specialty	medical genetics
ICD-10	Q04.5
ICD-9-CM	742.4
OMIM	155350
DiseasesDB	22519
MeSH	D058627
Orphanet	2477
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Megalencephaly (or macrencephaly, abbreviated MEG) is a growth development disorder in which the brain is abnormally large. It is characterized by a brain with an average weight that is 2.5 standard deviations above the mean of the general population. Megalencephaly is usually diagnosed at birth and is confirmed with an MRI. A mutation in the PI3K-AKT pathway is believed to be the primary cause of brain proliferation and ultimately the root cause of megalencephaly. This mutation has produced a classification of brain overdevelopment that consists of two syndromes including, megalencephaly-capillary malformation (MCAP) and megalencephaly-polydactyly-polymicrogyria-hydrocephalus (MPPH). Approximately 1 out of 50 children (2%) are said to have the characteristics of megalencephaly in the general population. There are several neuropsychiatric disorders linked with megalencephaly, however, studies have shown that autism is the most prevalent association with the malformation of MEG. Although no treatment currently exists for megalencephaly, management methods are focused at reducing deficits linked with autism. Most recent research is targeted at creating inhibitors to reduce the mutational pathway that causes megalencephaly.

Macrocephaly is a term used to refer to a person who has an abnormally large head. The circumference of the head must be above the 95th percentile or at least 2.5 standard deviations from the mean of normal weight and gender groups in the United States. A person with macrocephaly does not necessarily indicate that megalencephaly is also present. Large skulls usually exhibit no neurodevelopment conditions at all, meaning most individuals with macrocephaly are healthy.

Hemimegalencephaly is an extremely rare form of macrocephaly and is characterized by uneven development of brain hemispheres (one-half of brain is larger than other). The syndrome can be presented by itself or in association with phakomatosis or hemigigantism. Additionally, hemimegalencephaly will frequently cause severe epilepsy, focal neuro-logical deficits, macrocrania, and mild to severe mental retardation.

Megalencephaly-capillary (MCAP) is one of the two major syndromes of megalencephaly. Typically, MCAP and MPPH can be distinguished by somatic features. MCAP includes many characteristics that are observed at birth including: cutaneous vascular malformations, especially capillary malformations of the face and cutis marmorata, polydactyly, connective tissue dysplasia, and focal or segmental body overgrowth. Furthermore, MCAP can occasionally be linked with asymmetric brain overgrowth (hemimegalencephaly) as well as segmental overgrowth of the body (hemihypertrophy).

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