3 DISCUSSION
Acalvaria is an extremely rare congenital malformation involving the cranial vault bones, the dura matter and associated muscles. The development of the facial and the base bones is normal with a complete cranial content.1-3 However association with other abnormalities is often reported, including hydrocephalus, holoprosencephaly, micropolygyria, encephalocele or amniotic band syndrome resulting premature death, most often prenatally.1-6 The few survival cases deal with a mental retardation but with motor weakness or seizure has not been reported.
This is the second case reported in Africa so far after the one from South Africa who was a 6-month of age girl presenting head softness with no other clinical symptoms but brain CT-scan showed the absence of major skull bones with normal facial and skull base bones. However, no follow-up data beyond the 6 months were given.5
The patient presented focal neurological deficit and seizures but no intellectual disability. Brain CT-scan showed an absence of the upper part of the frontal and the parietal bones and a dilatation of the occipital horns of the lateral ventricles which may be related to this region’s atrophy. Although, the lesions found on the brain CT-scan didn’t correlate with the clinical findings, brain malformations or other brain lesions from perinatal brain related trauma that went unnoticed could explain these symptoms. Antenatal diagnosis is possible from 11 weeks of amenorrhea with obstetrical ultrasound; but an elevated Alpha foetoprotein and non-detected unconjugated estradiol could help with the diagnosis.2,7,8 However, this patient was diagnosed very lately. An unusual longevity that could be due to mild brain abnormalities or yet to identify genetic or environmental factors. This presentation should be differentiated from anencephaly, exencephaly and acrania.
Anencephaly differs from acalvaria as it presents with a total or partial absence of the brain and scalp.8,9 Exencephaly is considered as precursor to anencephaly.8,9 Acrania is often used as the synonym of acalvaria but in this condition, the absence of the skull bones may extend beyond the vault bones and is often accompanied by anencephaly.8,10
The pathophysiology of acalvaria is still unclear but a deficit in the mesenchymal migration is the most accepted hypothesis.1-3 Normally, mesenchymal migration occurs after the neural tube closure around the fourth week of the embryonic development.2,3,11 This lack of migration would explain the absence of bones and muscles while the brain tissue and the skin of ectodermal origin are present.2-3 However, some authors argued that this could be a variant of the anomalies linked to the neural tube closure defect.2 The etiology of acalvaria is not identified but a female predominance is reported in the literature as the case we report here.2 To date, there is no consensus on its treatment which remains largely conservative consisting of parents’ reassurance, monitoring and symptomatic treatment of any associated condition.2,3,5 As in the present study, continued bone development has been observed in other conditions with cranial vault absence such as Aplasia cutis congenita.12 It has therefore been suggested surgical treatment including bone grafting or plastic surgery should be reserved for school age children.5,12 An early diagnosis could help to limit the complications, especially the antenatal ones with a good planning of the delivery.