Figure 3. Evolution of the hypotrophy of the lower lip at 24
months (3A). Smile at 27 months showing no asymmetry (3B).
While much research has been done to understand and explain the
embryological mechanism of upper cleft lip, the aetiology of paramedian
lower cleft lip is still unclear and there is no consensus on its
embryogenesis.
There are several hypotheses. The first is a fusion defect between the
mandibular prominences of the first branchial arches. According to
Warbrick 9, it is due to three depressions in the
mandibular process during the foetal period. He suggested that a
disturbance in the embryological formation of the mandibular process
could lead to a disturbance in one of these depressions, resulting in
paramedian defects.
Secondly, clefts may result from failure of mesodermal migration and
penetrance. In addition, growth centres within the developing mandible
may be required for formation. Partial or complete failure of growth
centre differentiation may contribute to mandibular defects, rather than
a simple failure of mandibular protrusions to fuse at the midline10.
Thirdly, there is a belief that intrauterine trauma may be responsible
for cleft lip and palate 5,11. In 2018, Chauvel-Picard
et al observed a paramedian cleft of the lower lip in a child whose
mother had undergone fetal reduction for a multiple pregnancy. They
believed that the introduction of a transabdominal or transvaginal
needle could cause craniofacial abnormalities. Burton et al studied 394
fetuses and infants whose mothers had undergone chorionic villus
sampling. Thirteen (3.3%) had major congenital anomalies, including
four with missing limbs or parts of limbs and three with cleft lip, with
or without cleft palate 12. In our case, the mother
didn’t experience any trauma or fetal reduction. However, it is
important to note that our patient was sucking his thumb on the first
and second fetal scan. Prolonged intrauterine lip pressure could explain
this anomaly, but it would probably have been seen in many other babies.
Fourthly, two main syndromes could explain this anomaly. Amniotic band
syndrome and Van der Woude syndrome. The amniotic band syndrome is known
to be responsible for many forms of clefting. It wasn’t considered here
because there were no ringstrings and no amputations of digits or limbs.
Van der Woude syndrome involves paramedian pits or clefts. This syndrome
is highly variable and has been reported as lip pits in combination with
a cleft. However, the present case has neither the characteristic lip
pits nor the cleft. It has therefore been excluded.
A fifth hypothesis could be a vascular origin. Ischaemia of the
stapedial arterial system, which arises from the second pharyngeal arch
and is responsible for the vascularisation of the face during
embryogenesis, could eventually lead to facial malformation. In 1973,
Poswillo attempted to demonstrate this vascular theory in an animal
model by injecting triazene into the carotid artery, causing a haematoma
in the distribution area of the stapedial artery and eventually leading
to the formation of macrostomia and hemifacial microsomia. This theory
remained controversial and was criticised in 1995 by Louryan et al.13
Our case has many similarities with cases of paramedian inferior cleft
lip, but the anatomical analysis of the lip deformity led us to believe
that it could not be formally classified as a 28-29 Tessier cleft. Thus,
the mechanism may be completely novel.
Conclusion:
This may be the only report of a congenital partial hypoplasia of the
lower lip that does not quite fit the Tessier classification, as it does
not have the shape of a common cleft. However, the associated anomalies
(mucosal bridle, gingival notch, difficulty in eruption of teeth) can be
seen in usual cleft lips and suggest a longer follow-up of this young
patient to evaluate speech and dentition development. Surgical lip
correction will certainly be performed by minimally invasive lipofilling
with autologous fat graft or hyaluronic acid.