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What is Microcephaly and explain in detail?
Microcephaly (from New Latin microcephalia, from Ancient Greek μικρός mikrós "small" and κεφαλή kephalé "head") is a medical condition involving a smaller-than-normal head. Microcephaly may be present at birth or it may develop in the first few years of life. Since brain growth is correlated with head growth, people with this disorder often have an intellectual disability, poor motor function, poor speech, abnormal facial features, seizures and dwarfism.The disorder is caused by a disruption to the genetic processes that form the brain early in pregnancy, though the cause is not identified in most cases. Many genetic syndromes can result in microcephaly, including chromosomal and single-gene conditions, though almost always in combination with other symptoms. Mutations that result solely in microcephaly (primary microcephaly) exist but are less common. External toxins to the embryo, such as alcohol during pregnancy or vertically transmitted infections, can also result in microcephaly. Microcephaly serves as an important neurological indication or warning sign, but no uniformity exists in its definition. It is usually defined as a head circumference (HC) more than two standard deviations below the mean for age and sex. Some academics advocate defining it as head circumference more than three standard deviations below the mean for the age and sex.There is no specific treatment that returns the head size to normal. In general, life expectancy for individuals with microcephaly is reduced, and the prognosis for normal brain function is poor. Occasional cases develop normal intelligence and grow normally (apart from persistently small head circumference). It is reported that in the United States, microcephaly occurs in 2 to 12 babies per 10,000 births.

Signs and symptoms
There are a variety of symptoms that can occur in children. Infants with microcephaly are born with either a normal or reduced head size. Subsequently, the head fails to grow, while the face continues to develop at a normal rate, producing a child with a small head and a receding forehead, and a loose, often wrinkled scalp. As the child grows older, the smallness of the skull becomes more obvious, although the entire body also is often underweight and dwarfed.Severely impaired intellectual development is common, but disturbances in motor functions may not appear until later in life. Affected newborns generally have striking neurological defects and seizures. Development of motor functions and speech may be delayed. Hyperactivity and intellectual disability are common occurrences, although the degree of each varies. Convulsions may also occur. Motor ability varies, ranging from clumsiness in some to spastic quadriplegia in others.

Causes
Microcephaly is a type of cephalic disorder. It has been classified in two types based on the onset:

Congenital
Isolated
Familial (autosomal recessive) microcephaly
Autosomal dominant microcephaly
X-linked microcephaly 
Chromosomal (balanced rearrangements and ring chromosome)
Syndromes
Chromosomal
Poland syndrome
Down syndrome
Edward syndrome
Patau syndrome
Unbalanced rearrangements
Contiguous gene deletion
4p deletion (Wolf–Hirschhorn syndrome)
5p deletion (Cri-du-chat)
7q11.23 deletion (Williams syndrome)
22q11 deletion (DiGeorge syndrome)
Single gene defects
Smith–Lemli–Opitz syndrome
Seckel syndrome
Cornelia de Lange syndrome
Holoprosencephaly
Primary microcephaly 4
Wiedemann-Steiner syndrome
Acquired
Disruptive injuries
Ischemic stroke
Hemorrhagic stroke
Death of a monozygotic twin
Vertically transmitted infections
Congenital cytomegalovirus infection
Toxoplasmosis
Congenital rubella syndrome
Congenital Varicella Syndrome
Zika virus (see Zika fever#Microcephaly) 
Drugs
Fetal hydantoin syndrome
Fetal alcohol syndrome
Other
Radiation exposure to mother
Maternal malnutrition
Maternal phenylketonuria
Poorly controlled gestational diabetes
Hyperthermia
Maternal hypothyroidism
Placental insufficiency
Craniosynostosis

Postnatal onset
Genetic
Inborn errors of metabolism
Congenital disorder of glycosylation
Mitochondrial disorders
Peroxisomal disorder
Glucose transporter defect
Menkes disease
Congenital disorders of amino acid metabolism
Organic acidemia
Syndromes
Contiguous gene deletion
17p13.3 deletion (Miller–Dieker syndrome)
Single gene defects
Rett syndrome (primarily girls)
Nijmegen breakage syndrome
X-linked lissencephaly with abnormal genitalia
Aicardi–Goutières syndrome
Ataxia telangiectasia
Cohen syndrome
Cockayne syndrome
Acquired
Disruptive injuries
Traumatic brain injury
Hypoxic-ischemic encephalopathy
Ischemic stroke
Hemorrhagic stroke
Infections
Congenital HIV encephalopathy
Meningitis
Encephalitis
Toxins
Chronic kidney failure
Deprivation
Hypothyroidism
Anemia
Congenital heart disease
MalnutritionGenetic mutations cause most cases of microcephaly. Relationships have been found between autism, duplications of genes and macrocephaly on one side. On the other side, a relationship has been found between schizophrenia, deletions of genes and microcephaly. Several genes have been designated "MCPH" genes, after microcephalin (MCPH1), based on their role in brain size and primary microcephaly syndromes when mutated. In addition to microcephalin, these include WDR62 (MCPH2), CDK5RAP2 (MCPH3), KNL1 (MCPH4), ASPM (MCPH5), CENPJ (MCPH6), STIL (MCPH7), CEP135 (MCPH8), CEP152 (MCPH9), ZNF335 (MCPH10), PHC1 (MCPH11) and CDK6 (MCPH12). Moreover, an association has been established between common genetic variants within known microcephaly genes (such as MCPH1 and CDK5RAP2) and normal variation in brain structure as measured with magnetic resonance imaging (MRI)‍—‌i.e., primarily brain cortical surface area and total brain volume.The spread of Aedes mosquito-borne Zika virus has been implicated in increasing levels of congenital microcephaly by the International Society for Infectious Diseases and the US Centers for Disease Control and Prevention. Zika can spread from a pregnant woman to her fetus. This can result in other severe brain malformations and birth defects. A study published in The New England Journal of Medicine has documented a case in which they found evidence of the Zika virus in the brain of a fetus that displayed the morphology of microcephaly.

Microlissencephaly
Microlissencephaly is microcephaly combined with lissencephaly (smooth brain surface due to absent sulci and gyri). Most cases of microlissencephaly are described in consanguineous families, suggesting an autosomal recessive inheritance.

Historical causes of microcephaly
After the dropping of atomic bombs "Little Boy" on Hiroshima and "Fat Man" on Nagasaki, several women close to ground zero who had been pregnant at the time gave birth to children with microcephaly. Microcephaly was present in 7 children from a group of 11 pregnant women at 11–17 weeks of gestation who survived the blast at less than 1.2 km (0.75 mi) from ground zero. Due to their proximity to the bomb, the pregnant womens in utero children received a biologically significant radiation dose that was relatively high due to the massive neutron output of the lower explosive-yielding Little Boy. Researchers studied 286 additional children who were in utero during the atomic bombings, and after a year they found these children had a higher incidence of microcephaly and mental retardation.

Other relations
Intracranial volume also affects this pathology, as it is related with the size of the brain.

Pathophysiology
Microcephaly generally is due to the diminished size of the largest part of the human brain, the cerebral cortex, and the condition can arise during embryonic and fetal development due to insufficient neural stem cell proliferation, impaired or premature neurogenesis, the death of neural stem cells or neurons, or a combination of these factors. Research in animal models such as rodents has found many genes that are required for normal brain growth. For example, the Notch pathway genes regulate the balance between stem cell proliferation and neurogenesis in the stem cell layer known as the ventricular zone, and experimental mutations of many genes can cause microcephaly in mice, similar to human microcephaly. Mutations of the abnormal spindle-like microcephaly-associated (ASPM) gene are associated with microcephaly in humans and a knockout model has been developed in ferrets that exhibits severe microcephaly. In addition, viruses such as cytomegalovirus (CMV) or Zika have been shown to infect and kill the primary stem cell of the brain—the radial glial cell, resulting in the loss of future daughter neurons