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.