Acalvaria is a rare malformation consisting of the absence of the calvarial bones, dura mater and associated muscles in the presence of a normal

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

base and normal facial bones. The

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

is usually unaffected. The presumed pathogenesis of acalvaria is the faulty migration of the membranous neurocranium with normal placement of the embryonic ectoderm, resulting in the absence of the calvaria, but with an intact layer of skin over the brain

parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

. In other words, instead of having a skull cap protecting the brain, there is only skin covering it."Acalvaria." Right Diagnosis. Health Grades Inc., n.d. Web. 27 Nov. 2012. The size of the area that is missing the skull cap can vary from case to case. In extreme cases, the entire top part of the cranium that is dome-shaped may be absent."Acalvaria." Orphanet. N.p., n.d. Web. 18 Nov. 2012.

Signs and symptoms

There are four main signs of acalvaria: absence of the flat bones of the cranial vault, absence of the dura mater and muscles associated with it, skull abnormalities, and the absence of a skull cap. This condition can be diagnosed prior to birth using ultrasonography. Physicians often use magnetic resonance imaging to confirm the diagnosis because in utero, acalvaria is sometimes confused with

anencephaly Anencephaly is the absence of a major portion of the brain, skull, and scalp that occurs during embryonic development. It is a cephalic disorder that results from a neural tube defect that occurs when the rostral (head) end of the neural tube ...

encephalocele Encephalocele is a neural tube defect characterized by sac-like protrusions of the brain and the Biological membrane, membranes that cover it through openings in the human skull, skull. These defects are caused by failure of the neural tube to cl ...

. A distinguishable difference is that with anencephaly, the cerebral hemispheres are missing, but with acalvaria, all parts of the cerebrum are usually present and developed, whereas parts of the calvarium are missing.Harris, C. P., Townsend, J. J. and Carey, J. C. (1993), Acalvaria: A unique congenital anomaly. Am. J. Med. Genet., 46: 694–699. doi: 10.1002/ajmg.1320460620

Pathogenesis

Currently there is no identified cause of acalvaria. The primary presumed pathogenesis is problematic migration of the membranous

neurocranium In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the cal ...

with respect to the normal positioning of the immature

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

. When an embryo develops normally, the anterior neural pore closes about the fourth week. After this occurs, mesenchymal tissue migrates under the ectoderm. This ectoderm underlies where the cerebral hemisphere will eventually be. When a fetus has acalvaria, the embryonic ectoderm is in its correct place, but the mesenchymal migration does not occur correctly. Therefore, acalvaria is considered to be a postneurulation defect. Because it is a postneurulation defect, it must develop after embryonic stage 11, between 24 and 26 days after conception.

ACE inhibitor Angiotensin-converting-enzyme inhibitors (ACE inhibitors) are a class of medication used primarily for the treatment of high blood pressure and heart failure. This class of medicine works by causing relaxation of blood vessels as well as a decr ...

s and

angiotensin II receptor antagonists Angiotensin II receptor blockers (ARBs), formally angiotensin II receptor type 1 (AT1) antagonists, also known as angiotensin receptor blockers, angiotensin II receptor antagonists, or AT1 receptor antagonists, are a group of pharmaceuticals tha ...

are reported to cause prenatal hypocalvaria.

Treatment

Because this malformation is rare and there are extremely few individuals living with this condition, treatment is limited. Treatment consists of carefully managing the condition in a controlled manner. Proceeding with a

bone graft Bone grafting is a type of transplantation used to replace missing bone tissue or stimulate the healing of fractures. This surgical procedure is useful for repairing bone fractures that are extremely complex, pose a significant health risk to the ...

when the child reaches school age is also recommended.

Prognosis

Usually babies with this malformation do not survive past birth. However, there have been cases of survival. As of 2004, there were only two reported living cases. Of these two, one was severely cognitively impaired and physically disabled. The status of the other was unreported. If the fetus progresses to full term there is the risk that it will have head trauma from the pressure applied to the head while being delivered. A few other cases of acalvaria have been reported that did not progress to birth. In addition to the lack of the skull cap, there were brain malformations in each case and all the pregnancies were terminated either electively or the fetuses were spontaneously aborted.

Epidemiology

Acalvaria usually occurs in less than 1 of every 100,000 births. By way of epidemiological data, it is thought that females are more prone to have this defect. Currently, acalvaria is not thought to have much of a risk of recurrence.

References

External links

* {{Congenital malformations and deformations of nervous system Congenital disorders of nervous system