What is Acalvaria?
Acalvaria is an extremely rare congenital malformation characterized by an absence of flat bones of skull, dura mater, and associated muscles in the presence of normal cranial contents and facial bones.
Although its pathogenesis is unclear it has been explained by the theory of post-neurulation defect.
It has been considered a fatal congenital anomaly and only a handful of cases have been reported in English literature till date.
Thus, we herein present the present case which to the best of our knowledge is third living case of acalvaria.
The pertinent literature is reviewed with the aim to make physicians aware regarding the clinical features of this uncommon fatal clinical anomaly so that undue associated morbidity could be avoided by prompt antenatal diagnosis and subsequent management.
The etiopathogenesis of acalvaria is still unknown. It is considered a heterogenous disorder without any genetic predilection and correlation to folic acid intake.
A review of literature suggests that different investigators have explained the embryological basis of this rare malformation by theory of post-neurulation defect.
Normally, around 4th week of gestation and after closure of anterior neural pore, there is migration of mesenchymal tissue under ectoderm which forms skin and scalp, but the mesenchymal tissue develops into skull bones and associated muscles
Thus, faulty migration of mesenchymal tissue in presence of normal development of ectoderm results in an absence of flat bones of skull and associated muscles.
Although not widely accepted, but still some investigators consider acalvaria to be a part of spectrum of anencephaly, whereas others explain its pathogenesis on the basis of failure of primary closure of neural tube.
The clinical presentation as experienced in present and reported cases is heralded by the presence of soft, lax skin covered skull due to absence of flat bones of cranial vault and associated muscles.
The facial bones and cranial contents are usually normal, although some neuropathological abnormality has also been reported.
Since acalvaria remains a fatal congenital malformation, an insight into its radiological features demands special emphasis because a timely antenatal diagnosis followed by an appropriate intervention could prevent the undue morbidity, mortality, and psychological trauma resulting at term.
Although different craniofacial anomalies including anencephaly, hydrocephalous, osteogenesis imperfecta, hypophosphatasia, etc., remain important radiological differential diagnosis, but still as experienced by different investigators, a vigilant transvaginal ultrasound scan examination around 12 weeks of gestation can accurately diagnose acalvaria in most of the cases.
The presence of normal cerebral hemispheres remains main sonographic feature differentiating acalvaria from anencephaly which remains most common antenatal differential diagnosis.
Among laboratory investigations, alpha-fetoprotein level is usually raised, whereas unconjugated estradiol levels are usually undetectable.
The initial treatment of acalvaria is mainly conservative aimed at supportive care and management of any associated anomalies if present.
Being a rare anomaly with only two reported living cases, the surgical treatment for correcting the skull defect has not been discussed in literature till date. The presence of spontaneous bone growth as observed in few cases of scalp defect, such as cutis aplasia, highlights the importance of initial conservative management and supportive care in patients with acalvaria. They can subsequently undergo skull reconstruction by bone grafting and cranioplasty at school going age.
Since the only living case of this fatal anomaly with long-term follow-up is severely mentally retarded and disabled requiring specialized medical and social care, so we conclude that antenatal diagnosis of this fatal anomaly by a vigilant radiological evaluation is extremely important so that the morbidity, mortality, and the associated psychological trauma resulting at the time of birth and on subsequent management could be prevented.
Antenatal diagnosis
Prenatal diagnosis is made by ultrasonography and usually confirmed by magnetic resonance imaging because the condition may be confused with anencephaly or encephalocele.
Prognosis
The malformation is most often lethal at birth, but a few surviving infants have been reported.
