This condition is marked by an abnormally large, heavy, usually malfunctioning brain. The head may be enlarged at birth or become abnormally large in the early years. A rare variation on this condition, unilateral megalencephaly (also called hemimegalencephaly), involves enlargement of half of the brain. This results in a large, often asymmetrical head.

Causes of Megalencephaly

Megalencephaly may be caused by a disturbance in the process that controls the way nerve cells divide to form new cells (called nerve cell proliferation). Males are more likely to be afflicted than females.

Signs and Symptoms of Megalencephaly

Symptoms may include delayed development, convulsions, seizures, and brain cortex/spinal cord dysfunction. Unilateral megalencephaly usually results in mental retardation and severe seizures.

Treatment for Megalencephaly

Treatment is symptomatic.

Prognosis for Infants with Megalencephaly

Prognosis varies according to the severity of associated disorders. Prognosis for those afflicted with unilateral megalencephaly is poor.


In microcephaly, the circumference of the head is markedly smaller than normal for the infant's age and sex. Microcephaly can be congenital or can develop during the first years of life.

Causes of Microcephaly

The condition may be caused by factors related to abnormal brain growth or from chromosomal abnormalities.

Signs and Symptoms of Microcephaly

At birth, the head of a microcephalic infant may be either normal or abnormally small. In many cases the head fails to grow as the face develops at a normal rate, resulting in the development of a small head with a large face, receding forehead, and scalp with loose, wrinkled, skin. The smallness of the skull becomes increasingly evident with age. The child's entire body may be underweight and dwarf-like. Motor functions often are poor, ranging from simple clumsiness to spastic quadriplegia (affecting all four limbs). Speech development often is subnormal. The child may suffer convulsions and be hyperactive or mentally retarded.

Treatment for Microcephaly

Treatment for microcephaly is symptomatic and supportive.

Prognosis for Infants with Microcephaly

Prognosis varies according to the severity of the child's associated abnormalities. Life expectancy usually is short.


In porencephaly a cyst or cavity develops in a cerebral hemisphere. The disorder can occur before or after birth.

Causes of Porencephaly

Porencephaly may have a number of different causes, including absence of brain development and lesions that destroy brain tissue.

Signs and Symptoms of Porencephaly

Symptoms include delayed growth and development, slight or partial paralysis, decreased muscle tone (hypotonia), seizures or spasms, macrocephaly or microcephaly, epilepsy, hydrocephalus, shrinkage or shortening of muscles, and mental retardation. Speech development may range from poor to absent.

Diagnosis of Porencephaly

Porencephalic cysts are sometimes detected in infancy. Severely affected infants usually display symptoms shortly after birth, and diagnosis usually is confirmed before age 1 by transillumination (passing light through) of the skull, CT scan, MRI scan, or ultrasonography.

Treatment for Porencephaly

Treatment may include physical therapy, medication for seizures, and a shunt for hydrocephalus.

Prognosis for Infants with Porencephaly

The location and extent of the brain damage determines the prognosis. Some patients develop normal intelligence and experience only minor neurological problems; others may be severely disabled. Many die before age 10.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Jan 2000

Last Modified: 25 Jan 2016