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.