MedFriendlyTM TM

MACROCEPHALY

Macrocephaly is a rare condition in which the head is abnormally large. Spaces between the bones of the head that hold the brain, known as fontanels, are very wide in people with macrocephaly. Compare macrocephaly to microcephaly, which is a rare condition in which the head is abnormally small.

HOW LARGE DOES THE HEAD HAVE TO BE TO QUALIFY AS MACROCEPHALY?

Technically, any head in which the skull is able to hold more than 1450 milliliters qualifies as macrocephaly. A milliliter is one thousandth of a liter, meaning that it takes one thousand milliliters to equal one liter. One liter is equal to about one fourth of a gallon. Now that you know how much is in a liter, you can tell that a milliliter is a very small amount of liquid. The normal range for the amount of milliliters that can fit in the skull is 1350 to 1450 milliliters. Heads in this range are known as mesocephalic.

The head of an individual with macrocephaly is two or more standard deviations above the average head size of a normal individual that is the same age, race, and sex. A standard deviation is a measure of the degree in which something varies from the average. Not all people have the exact same head sizes. However, most people's head sizes fall within a normal range. The normal range is typically defined as falling within one standard deviation below the average and one standard deviation above the average. The more standard deviations away from the average that something gets, the more abnormal it is.

WHEN DOES MACROCEPHALY DEVELOP AND WHAT CAUSES IT?

Macrocephaly can be present from birth, but it can also develop during the first few years of life. Thus, it is possible for the head to be normal in size at birth, but to grow at a more rapid rate afterwards, causing it to be too large. Macrocephaly can be due to the following causes:

• HYDROCEPHALUS: A condition in which the head grows too large due to a big increase in cerebrospinal fluid, and is usually accompanied by increased pressure within the bones that surround the brain. Cerebrospinal fluid is a liquid that cushions and protects brain and the spinal cord. People with hydrocephalus have a thin scalp (the skin that covers the skull). As a result, the veins in the scalp are easily seen in people with hydrocephalus. Hydrocephalus can cause the sutures (places where two bones join together) of the skull to separate. The eyes of people with hydrocephalus are usually turned downwards and the whites of the top of the eyes are clearly seen. In addition, the front part of the head appears swollen.

  Hydrocephalus can be caused by infection when the mother is still pregnant, narrowing of a passageway for cerebrospinal fluid, and an Arnold-Chiari malformation. An Arnold-Chiari malformation is an abnormal condition in which parts of the back, bottom area of the brain, push down through the top of the opening that contains the spine.

  HYDRANENCEPHALUS: The combination of hydrocephalus (see above) and the destruction or failed development of some parts of the top area of the brain (known as the cerebrum). This condition is often associated with a large head. When a light is passed through the head of someone with hydranencephalus in a dark room, the head glows like a jack-o'-lantern. This condition can be causes by something that blocks the blood supply to the inside of the uterus. The uterus is a hollow organ in a female's body where the egg is implanted and the baby develops. Hydranencephalus can also be caused by diseases that destroy both sides of the brain, such as toxoplasmosis and cytomegalovirus (CMV). In hydranencephaly, the brain and head become enlarged because the lack of tissue in the brain prevents it from resisting the pressure of the ventricles. Ventricles are openings in the brain that produce the cushiony fluid that protects the brain and spine. As the ventricles get bigger, they push outwards and the brain gets larger, as does the head. MACRENCEPHALY: Excessive brain growth. This can be due to certain diseases that make the brain larger, such as Alexander disease, later phases of Tay-Sachs disease, and spongy degeneration of infancy.

• SUBDURAL HEMATOMA: A collection of blood below the outer covering of the brain. This can cause to head to get bigger because the pressure from the blood pushes the bones of the skull outwards. Subdural hematomas can cause the sutures (places where two bones join together) of the skull to separate. Children with this condition are often irritable, difficult to nourish, and have a lack of energy.

• NEUROFIBROMATOSIS: A condition present at birth in which numerous benign tumors (known as neurofibromas) and light tan spots are on the skin are present on the nerves and skin, and the bones, muscles, and certain organs inside the body develop abnormally. Tumors are tissues that grow more rapidly than normal. Benign tumors are tumors that are not immediately harmful.

• ACHONDROPLASIA: A disorder in which the skull and cartilage (a type of tissue) in the ends of long bones grows abnormally.

• OSTEOGENESIS IMPERFECTA: An inherited disorder in which connective tissue does not develop properly. Hydrocephalus (see above) seems to be responsible for the enlargement of the head in this condition.

• DESTRUCTIVE BRAIN DISEASES: Diseases that destroy parts of the brain, such as Greenfield's disease and Schilder's disease, which both destroy myelin in the brain. Myelin is a fatty nerve covering that sends impulses quickly.

• AGENESIS OF THE CORPUS CALLOSUM: When the corpus callosum (a group of nerve fibers that connect both sides of the brain) is not present. This condition is sometimes inherited.

• DEFECTIVE FORMATION: A defect in formation during while the baby is developing during the first 8 weeks of pregnancy.