ACEPHALY literally means absence of the head. It is a much rarer condition than anencephaly. The acephalic fetus is a parasitic twin attached to an otherwise intact fetus. The acephalic fetus has a body but lacks a head and a heart; the fetus’s neck is attached to the normal twin. The blood circulation of the acephalic fetus is provided by the heart of the twin. The acephalic fetus can not exist independently of the fetus to which it is attached.
EXENCEPHALY is a condition in which the brain is located outside of the skull. This condition is usually found in embryos as an early stage of anencephaly. As an exencephalic pregnancy progresses, the neural tissue gradually degenerates. It is unusual to find an infant carried to term with this condition because the defect is incompatible with survival.
MACROCEPHALY is a condition in which the head circumference is larger than average for the age and gender of the infant or child. It is a descriptive rather than a diagnostic term, and is a characteristic of a variety of disorders. Macrocephaly also may be inherited. Although one form of macrocephaly may be associated with mental retardation, in approximately one-half of cases mental development is normal. Macrocephaly may be caused by an enlarged brain or hydrocephalus. It may be associated with other disorders such as dwarfism, neurofibromatosis, and tuberous sclerosis.
MICRENCEPHALY is a disorder characterized by a small brain and may be caused by a disturbance in the proliferation of nerve cells. Micrencephaly may also be associated with maternal problems such as alcoholism, diabetes, or rubella (German measles). A genetic factor may play a role in causing some cases of micrencephaly. 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.
OCTOCEPHALY is a lethal condition in which the primary feature is agnathia–a developmental anomaly characterized by total or virtual absence of the lower jaw. The condition is considered lethal because of a poorly functioning airway. In octocephaly, agnathia may occur alone or together with holoprosencephaly.