Acalvaria

December 31, 2014

Synonyms

Overview

Acalvaria is a rare malformation consisting of absence of the calvarial bones, dura mater and associated muscles in the presence of a normal skull base and normal facial bones. The skull base and facial features are fully formed and usually appear normal.The central nervous system is usually unaffected. The presumed pathogenesis of acalvaria is faulty migration of the membranous neurocranium with normal placement of the embryonic ectoderm, resulting in absence of the calvaria but an intact layer of skin over the brain parenchyma. In other words, instead of having a skull cap protecting the brain, there is only skin covering it. 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.

The cause of acalvaria is still unknown. Acalvaria can be distinguished from anencephaly, the most common differential diagnosis, by the presence of a layer of skin overlying the brain matter and normal cerebral hemispheres. This malformation is most often lethal at birth due to other associated anomalies or to trauma during delivery, but a few surviving infants have been reported. Prenatal diagnosis via transvaginal ultrasound and/or magnetic resonance imaging is critical for better pregnancy management.

Symptoms

Absence of flat bones of cranial vault
Absence of duramater and associated muscles
Skull abnormalities
Absence of skull cap
Anencephaly
Aplasia/Hypoplasia of the cerebellum
Primary adrenal insufficienc

Causes

Acalvaria is a rare malformation usually regarded as a postneurulation defect. It consists of absense of the calvarial bones, dura mater and associated muscles in the presence of a normal skull base and normal facial bones. The condition is frequenctly confused by prenatal ultrasonography with anencephaly or an encephalocele. Whereas the cerebral hemispheres are absent in anencephaly, the cranial contents in acalvaria are generally complete, though some neuropathological abnormality is often present. The presumed pathogenesis of acalvaria is faulty migration of the membranous neurocranium with normal placement of the embryonic ectoderm, resulting in absence of the calvaria but an intact layer of skin over the brain parenchyma. We describe 2 cases of acalvaria, one misdiagnosed ultrasonographically as an occipital encephalocele prenatally. The brain in one fetus demonstrated semilobar holoprosencephaly and micropolygyria, but in the other, was structurally and histologically normal with the exception of hydrocephalus.

Diagnosis

This condition can be diagnosed prior to birth using ultrasonography. Physicians often use magnetic resonance imaging to confirm the diagnosis because in utero, acalvaria sometimes confused with anencephaly or encephalocele. 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.

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, which did not progress to birth. In addition, to the lack skull cap, there were brain malformations present in each case, and all of the pregnancies were terminated either electively or the fetuses were spontaneously aborted.

Treatment

The initial treatment is conservative, mainly aimed at supportive care and management of any associated anomalies, if present. Infants with acalvaria are managed conservatively in the newborn period because spontaneous bone growth has been seen in some newborns with other skull abnormalities, such as aplasia cutis congenita. Skull reconstruction by bone grafting and cranioplasty at school age has been discussed in the literature.