Other Cephalic Disorders
Acephaly is usually associated with the development of a parasitic "twin" attached to an otherwise normal fetus, usually at the neck. The acephalic fetus has no head or heart and cannot survive independently; blood circulation comes from the heart of the otherwise normal "twin."
Exencephaly usually is found in embryos as an early stage of anencephaly. It is characterized by development of the brain outside the skull. In most cases, the brain tissue degenerates as pregnancy progresses. Exencephalic embryos rarely survive to term.
Macrocephaly is characterized by an abnormally large head circumference. It may be inherited and may or may not be associated with other disorders, such as dwarfism, neurofibromatosis, mental retardation, or tuberous sclerosis. Some cases may be caused by an enlarged brain or by hydrocephalus.
Micrencephaly is characterized by an abnormally small brain. It can be caused by a genetic factor but is more often associated with maternal alcoholism, diabetes, and exposure to German measles (rubella). Newborns with micrencephaly usually have profound neurological defects, seizures, and greatly impaired intellectual development. Motor function problems may appear later in life.
Octocephaly is characterized by agnathia, the total or virtual absence of a lower jaw, and a poorly functioning airway. This is a fatal condition that may occur alone or with holoprosencephaly.
Craniostenoses are an extremely rare group of cephalic disorders, characterized by skull deformities caused by premature fusion of various fibrous joints (cranial sutures) connecting the bones of the head.
- Scaphocephaly: fusion of the sagittal suture, the joint connecting the two parietal (top and sides of the head) bones of the skull, causing a long, narrow head. This is the most common of the craniostenoses.
- Plagiocephaly: unilateral fusion (joining of one side) of the fibrous joint between the occipital and parietal bones, causing asymmetrical (one side) flattening of the skull. It is thought to be caused by a brain malformation, an excessively small or tight intrauterine environment, or a spasm or tightening of the neck muscles.
- Brachycephaly: fusion of the joint between the skull's frontal and parietal bones, causing foreshortened front-to-back diameter of the skull.
- Trigonocephaly: fusion of the metopic suture, part of the frontal joint between the two halves of the skull's frontal bone, causing a V-shaped deformity at the front of the skull, a triangular prominence of the forehead, and closely set eyes.
- Oxycephaly: closure of the coronal suture and any other suture or fusion of all cranial sutures. This is the most severe form of craniostenoses.