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"Megalencephaly"

jcjohnson303 or Jacob Johnson

Megalencephaly (or Macrencephaly abbreviated MEG) is a growth development disorder in which the brain is abnormally large. It is characterized by a brain that has an average weight greater than 2.5 standard deviations. 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. This mutation has produced a classification of brain over-development that is comprised 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. There are several neuropsychiatric disorders linked with megalencephaly, however studies have shown that autism is the most prevalent association with the malformation. 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.

Classification

Macrocephaly

Macrocephaly is a term used to define a person who has an abnormally large head. The circumference of the head must be above the the 95th percentile or at least 2.5 standard deviations from the mean of normal weight and gender groups in the United States.

Hemimegalencephaly

Hemimegalencephaly is an extremely rare form of macrocephaly that 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 frequently will cause severe epilepsy, focal neuro-logical deficits, macrocrania, and mild to severe mental retardation.

MCAP

Megalencephaly-capillary (MCAP) is one of the two major syndromes of megalencephaly. Typically MCAP and MPPH can be distinguished by somatic fatures. 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 occassionally be linked with asymmetric brain overgrowth (hemimegalencephaly) as well as segmental overgrowth of the body (hemihypertrophy).

MPPH

Megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH) is one of the two major syndromes contributing to megalencephaly. Typically MCAP and MPPH can be distinguished by somatic fatures. In differentiation to MCAP, MPPH lacks Lacks consistent somatic features other than postaxial polydactly. Furthermore, brain and body development is normally symmetric in the majority of patients that appear to have MPPH symptoms.

Causes

Although still very little is known as to the direct cause of megalencephaly, recent studies have begun to provide early indications of possible sources for its formation.

PI3K-AKT

Recent studies have shown that mutations in phosphoinositide 3-kinase (PI3K) and AKT (also known as protein kinase B) pathway have been identified in MCAP and MPPH. These mutations have been shown to cause a gain of function an activation of the PI3K-AKT pathway. This cellular pathway is critical in the regulation of diverse cell functions including, cell growth, proliferation, metabolism, survival, apoptosis, angiogenesis, tumorigenesis and most importantly in regards to megalencephaly, brain development. AKT is an crucial signaling molecule part of the PI3K pathway and is also involved in many cellular functions. These include, brain development, synaptic plasticity and neurodevelopment. Loss of function in AKT can cause microcephaly in humans while inactivation of the pathway can cause hemimegalencephaly. There are also several cancers linked to mutations in the AKT pathway including melanoma and lung cancer

Pur-alpha

Pur-alpha (purα) is a sequence-specific singl-stranded DNA and RNA-binding protein. Studies have shown that the protein is primarily active during early development and is believed to have a role in brain enlargement. Although the exact function is still controversial, it is believed that pur-alpha is responsible for neuronal proliferation as well as maturation of dendrites. Thus, pur-alpha is also considered a potential root cause of megalencephaly and brain overgrowth.

Pathophysiology

One impact of megalencephaly is the complete lack of motor development. One medical study was assessed three patients presenting megalcenphaly who showed severely impaired motor and speech development as well as distinct facial abnormalities including skull bossing, a low nasal bridge and large eyes.

Management

Diagnoses

Diagnoses of megalencephaly have changed over the years, however the development of more advanced equipment, physicians have been able to confirm the disorder with better accuracy. A physical exam is performed first when characteristics of megalencephaly have appeared, usually at birth or during early child development. A physician will take head measurements in order to determine the circumference. This is known as the head circumference. Then a family background will be recorded in order to determine if there was a history of megalencephaly in the family.

A neurological exam will then be performed using the technology of an MRI machine to confirm the diagnosis of megalencephaly. These imaging tests give detailed information regarding brain size, volume asymmetry and other irregular developments linked with MCAP, MPPH and hemimegalencephaly.

There is also a strong correlation of Epilepsy and megalencephaly and can aid doctors in their diagnosis.

If a diagnosis of megalencephaly is confirmed, the child is referred to a specialist who focuses on managing the symptoms and improving the lifestyle.

Treatment

There is currently no specific treatment for megalencephaly, however periodic head measurements may be assessed to determine the rate of brain growth. Those that develop neurological disorders may be prescribed anti-epileptic drugs for seizures. Studies have shown that reducing epilepsy can increase cell apoptosis and reduce the proliferation of neurons that leads to brain overgrowth

Prevention

Since there are very few treatment methods focused on managing megalencephaly, future research is targeted at inhibiting mutation of pathway. This process could cause problems as understanding the underlying mechanism of the mutations is a poorly understood topic. The genetic coding that initiates a single mutation is sporadic and patterns are hard to detect in many cases. Even thought very little research has been done on create inhibitors of the PI3K-AKT pathway, several pharmaceutical companies have begun to focus their interests in designing a prevention method for this purpose.

Prognosis

The prognosis of megalencephaly depends heavily on the underlying cause and assosciated neurological disorders. Because the majority of megalencephaly cases are linked with autism, the prognosis is equivalent to the corresponding condition. Since, hemimegalencephaly is associated with severe seizures, hemiparesis and mental retardation and therefore has a poor prognosis. In most cases, those diagnosed with this type of megalencephaly usually do not survive through adulthood.

Future Research

Future research is targeted at further understanding mutations and how they lead to MCAP and MPPH. The majority of studies of megalencephaly have included mice who present brain abnormalities and overgrowth. The next step is to move to clinical trials involving humans in order determine the exact genetic mutation causing sequences. Additionally, scientists and pharmaceutical companies have began to show interest in mutation inhibition and designing preventative methods eliminate the underlying cause of megalencephaly altogether.

Epidemiology

Approximately one out of every 50 (2%) children in the general population are said to have megalencephaly.. Additionally, it is said that megalencephaly affects 3-4 times more than females. Those individuals that are classified with macrocephaly, or general head overgrowth, are said to have megalencephaly at a rate of 10-30% of the time.

Autism

There is an extremely high association between megalencephaly and autism Approximately 20% of autistic children have megalencephaly, with megalencephaly being the most common physical characteristic of autism. Patients that present both megalencephaly and autistic characteristics usually also show signs of hyperactivity as a major symptom. However, there is no definitive evidence that autism is a primary cause/result of megalencephaly. This is because megalencephaly has also been describe in other neurodevelopmental disorders such as schizophrenia or Alzheimers.

Since most children with megalencephaly also have autism, the majority of treatment is focused on managing the signs and issues caused by autism.

History

It is believed that megalencephaly was discovered in 1972. Prior to diagnoses using an MRI to confirm brain overgrowth, cases of megalencephaly were diagnosed by autopsy in which the physical brain was measured and weighed.

References

Jacob Johnson (talk) 18:04, 22 September 2013 (UTC)