Acalvaria
What is Acalvaria?
The human skull consists of a number of bones that have fused together. Among these bones are the frontal bone which makes up the forehead, the temporal bones, and parietal bones which make up the sides and the top of the skull, and the occipital bone which makes up the base of the skull. Acalvaria is a rare malformation in which parts of these bones are absent at birth. The overlying skin may or may not be present. The brain may be unaffected although there have been cases with brain abnormalities. These include the failure of the brain to properly divide into two hemispheres (holoprosencephaly), increased fluid in the brain (hydrocephalus), and the abnormal development of too many small folds in the brain (polymicrogyria). Acalvaria may be associated with other issues such as heart, kidney, or other skeletal problems or an isolated malformation. Diagnosis is often made on prenatal ultrasounds although other imaging techniques may be needed before or after birth. There is no standard treatment as it is a very rare malformation but surgical treatment has been described. Prognosis is poor and affected individuals often pass away shortly after birth, but some have survived into infancy and even childhood.
The human skull consists of a number of bones that have fused together. Among these bones are the frontal bone which makes up the forehead, the temporal bones, and parietal bones which make up the sides and the top of the skull, and the occipital bone which makes up the base of the skull. Acalvaria is a rare malformation in which parts of these bones are absent at birth. The overlying skin may or may not be present. The brain may be unaffected although there have been cases with brain abnormalities. These include the failure of the brain to properly divide into two hemispheres (holoprosencephaly), increased fluid in the brain (hydrocephalus), and the abnormal development of too many small folds in the brain (polymicrogyria). Acalvaria may be associated with other issues such as heart, kidney, or other skeletal problems or an isolated malformation. Diagnosis is often made on prenatal ultrasounds although other imaging techniques may be needed before or after birth. There is no standard treatment as it is a very rare malformation but surgical treatment has been described. Prognosis is poor and affected individuals often pass away shortly after birth, but some have survived into infancy and even childhood.
Less than 1 per 100,000 live births.
Acalvaria is due to a change during the embryological development of the fetus. A specific cause has not been identified for this change, and there are likely a number of different processes that can lead to acalvaria. Among reported cases, there have been cases due to amniotic band syndrome (ABS). The amniotic sac is the fluid-filled sac that surrounds the fetus during pregnancy. ABS occurs when strands of amniotic sac wrap around and strangulate fetal body parts that have developed normally. In severe cases, this can cause the amputation of the strangulated body part due to loss of blood flow. Fingers or toes are most commonly affected but it can involve any body part including the skull in which case it would lead to acalvaria.
Additionally, maternal use of ACE inhibitors, a class of blood-pressure-lowering medications, have also been associated with acalvaria. Of note, folic acid intake has not been shown to prevent acalvaria. This contrasts with a group of disorders known as neural tube defects which are birth defects affecting the brain and/or the spinal cord for which expectant mothers are advised to take folic acid supplements. There have been no cases of recurrence in the reported literature which suggests that this is not an inherited condition. Additionally, no specific genetic abnormality has been identified in association with acalvaria.
Acalvaria has been described as the complete or partial absence of frontal, parietal, or temporal bones. The upper part of the occipital bone may also be involved. The number of bones involved and the extent of involvement is variable among patients. The overlying skin may or may not be affected. There have been cases with completely normal scalp and cases with large scalp defects. The dura mater which is a thin protective layer surrounding the brain is usually present. Presentation of acalvaria is very diverse as it can be an isolated finding with no other anomalies or malformations or it may be present in combination with other features.
Other reported features include undescended testes. Testes initially develop in the abdomen of the fetus and gradually descend to the scrotum, however, due to a variety of reasons, this process may fail to happen normally causing undescended testes. While acalvaria with a normal brain has been described, a spectrum of brain anomalies may accompany acalvaria including increased fluid surrounding the brain, improper formation of the structure connecting the left and right hemispheres, and abnormal brain fold development.
While most affected individuals pass away shortly after birth, there have been a few living individuals reported. Findings in these individuals are variable but include both a small or large head, absent eyes, failure to gain weight appropriately, normal development to severe developmental delay, mildly abnormal facial features, cleft lip and palate, inflammation of the optic nerve (the nerve involved in vision), and neurological findings such as seizures and low muscle tones.
Diagnosis is often made on the first-trimester ultrasound although it may be difficult to differentiate acalvaria from other cranial vault defects such as anencephaly, a condition characterized by the absence of parts of the skull and the brain. In such instances, further imaging can be pursued to better characterize the defect before birth.
In addition to the prenatal tests, there may be an obvious skull and scalp defect with the dura-covered brain exposed. In other cases, there may only be a small or no scalp defect and the diagnosis only becomes apparent on palpation of the head. After birth, a CT scan of the head may be performed to assess the exact extent of the bony defect in the skull. Other tests may be done to determine whether there are any other associated abnormalities.
Prenatal ultrasounds are routinely done during all pregnancies during the first, second and third trimesters. These ultrasounds are able to identify abnormalities of the skull but it may be difficult to differentiate between calvaria and other anomalies affecting the skull and the brain. In such cases, a fetal MRI may be done to obtain high-resolution and detailed images of fetal structures. Of note, some cases have been reported with normal prenatal ultrasound. In such cases, the diagnosis was only suspected and confirmed after birth.
After birth, a CT scan of the head is done to better characterize the bony structure of the skull and determine the exact area of the defect. An MRI of the head might also be required to better assess the brain structure and any possible brain involvement. Additional tests that may be done to rule out other associated anomalies include a skeletal survey which is an X-ray examination of the entire bony structure of the body to determine whether any other skeletal abnormalities are present. An echocardiogram which is an ultrasound of the heart may be done to rule out any heart defects. An ultrasound of the abdomen and pelvis can assess the contents of the abdomen and the kidney. Genetic testing may be suggested but so far, no specific genetic abnormality has been associated with acalvaria.
While in most cases, acalvaria leads to early death, surgery has been attempted as a potential treatment for those who survive after birth. This has only been described a few times with different surgical techniques, requiring varying numbers of surgeries. The surgery can involve using the existing skull to create a scaffolding over the exposed brain and skin grafting to cover the missing scalp. In most other cases, the treatment of choice has been non-surgical with close follow-up as well as treatment of any associated anomaly which can require different specialists.
The prognosis of acalvaria is generally poor and most affected children die shortly after birth. Few surviving cases have been reported. The oldest living individual is eleven years old and has severe developmental delays. There are individuals who were treated surgically and in some cases had significant closure of the defect in the skull after the surgery. Of these cases, a 17-month-old child was reported to have near-average development and a 22-year-old child had developmental delays. The long-term outcome of surgical treatment is not known at this time.
https://www.indianpediatrics.net/june2004/june-618-620.htm
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8941675/
https://pubmed.ncbi.nlm.nih.gov/23560003/
https://thejns.org/pediatrics/view/journals/j-neurosurg-pediatr/14/2/article-p200.xml
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8941675/
https://pubmed.ncbi.nlm.nih.gov/18541961/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3611905/
Last updated 1 Aug 2015, 06:48 PM
1 Aug 2015, 06:48 PM
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