Microcephaly means "smallheadedness", whereas "Microencephaly" means "small brain". Because the size of the brain is mostly determined by the size of the head, microencephaly is implied when discussing microcephaly. Microcephaly is a neurodevelopmental disorder. It 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. Microcephaly may be congenital or it may develop in the first few years of life. The disorder may stem from a wide variety of conditions that cause abnormal growth of the brain, or from syndromes associated with chromosomal abnormalities. In general, life expectancy for individuals with microcephaly is reduced and the prognosis for normal brain function is poor. The prognosis varies depending on the presence of associated abnormalities.

Figure 1: Zika fever symptoms of babies born with microcephaly.

Affected newborns generally have striking neurological defects and seizures. Severely impaired intellectual development is common, but disturbances in motor functions may not appear until later in life. 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. 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.

In those with severe weakness, prompt treatment with intravenous immunoglobulins or plasmapheresis, together with supportive care, will lead to good recovery in the majority. Some may experience ongoing difficulty with walking, painful symptoms, and some require long-term breathing support. Guillain–Barré syndrome is rare, at one to two cases per 100,000 people every year. The syndrome is named after the French neurologists Georges Guillain and Jean Alexandre Barré, who described it with André Strohl in 1916.

Generally, no specific cure is known for microcephaly. Treatment is symptomatic and supportive. The spread of mosquito-borne Zika virus has been implicated in increasing levels of congenital microcephaly by the CDC.

Figure 2: A baby with microcephaly (left) compared to a baby with a typical head size.