A small brain, known medically as microcephaly, is a condition where a baby’s head is significantly smaller than expected, as the skull expands to accommodate the developing brain during pregnancy and infancy. Microcephaly can be present at birth or may develop within the first couple of years of life. It is a rare neurological condition, occurring in approximately 1 in every 1,150 births in the United States.
Medical Definition of a Small Brain
Clinicians diagnose microcephaly by measuring a baby’s head circumference and comparing it to standardized growth charts for the same age and sex. A measurement that falls more than two standard deviations below the average, or is smaller than the 3rd percentile, is defined as microcephaly. The condition’s severity can vary, with severe microcephaly defined as a head size more than three standard deviations below the mean.
This condition is classified into two main types based on when it appears. Congenital microcephaly means the baby is born with a small head size. In contrast, postnatal or acquired microcephaly describes a situation where an infant is born with a normal head circumference, but head growth slows or stops after birth.
Factors Contributing to a Small Brain
The origins of microcephaly are diverse and often linked to issues that affect brain development during pregnancy. Genetic abnormalities are a significant factor, with conditions like Down syndrome and Cornelia de Lange syndrome interfering with typical growth processes. In some cases, a smaller head size can be a familial trait, which is typically mild and doesn’t lead to neurological symptoms. The specific cause, however, remains unknown in many cases.
Prenatal infections contracted by the mother can also lead to microcephaly in a developing fetus. Viruses such as Zika, rubella, cytomegalovirus (CMV), and toxoplasmosis are known to disrupt fetal brain development. Exposure to harmful substances during pregnancy is another risk. This includes maternal use of alcohol or certain drugs, as well as contact with toxic chemicals.
Complications during pregnancy or birth can restrict the resources needed for proper brain growth. For instance, severe maternal malnutrition can deprive the fetus of necessary nutrients. A reduction in the oxygen supply to the baby’s brain during development or delivery, a condition known as cerebral anoxia, is another potential cause.
Developmental and Health Implications
The effects of microcephaly on a child’s development exist on a wide spectrum, largely dependent on the severity of the underlying brain abnormality. Some individuals with the condition may experience only mild developmental differences and have typical intelligence. However, those with more severe forms often face a range of health challenges.
Many children with microcephaly experience developmental delays, affecting milestones like speech, sitting, standing, and walking. Intellectual disabilities and learning difficulties are common associated challenges. Problems with movement and balance, sometimes diagnosed as cerebral palsy, can also occur, along with hyperactivity. A distinct, high-pitched cry is sometimes noted in infants with the condition.
Beyond developmental delays, microcephaly can be associated with other neurological and physical problems. Seizures are a frequent complication for these children. Issues with feeding, such as difficulty swallowing (dysphagia), may also be present. Furthermore, hearing loss and vision problems can arise due to abnormal development of the eyes and ears.
Diagnosis and Monitoring
Microcephaly can sometimes be identified before a baby is born. A prenatal ultrasound, particularly one performed late in the second or early in the third trimester, may reveal that the fetal head is not growing at a normal rate.
The most common method for diagnosis occurs after birth during a routine physical examination. A healthcare provider will measure the distance around the baby’s head and plot this head circumference measurement on a growth chart.
Following a diagnosis, further tests are often recommended to understand the underlying structure of the brain. Neuroimaging tests, such as an MRI or CT scan, can provide detailed pictures of the brain and reveal any structural abnormalities. Genetic testing may also be used to identify a specific syndrome or chromosomal cause. Because microcephaly is a lifelong condition without a cure, ongoing monitoring by a team of pediatric specialists is necessary to manage associated health problems and support the child’s development.