*4.1.1. Arhinia*

**Definition:** Absence of the nose.

**Etiology:** Unknown. It can either be an isolated malformation or be part of a malformation complex, such as holoprosencephaly or mandibulofacial dysostosis (Treacher Collins syndrome) [23].

**Embryology:** Around the 6th week of gestation, the primitive nasal and oral cavities communicate freely using an opening, which will close progressively when the palate starts developing. When the lateral palatine processes fuse with the nasal septum in the middle, the oral and the two nasal cavities are formed and separated; this takes place around the 12th week of gestation. The external nose starts at the lower portion of the frontonasal prominence, merging on both sides with the maxillary processes (**Figure 9**). If the frontonasal prominence does not fully develop, the result is partial or complete nasal aplasia. This anomaly is part of a more complex spectrum of midfacial defects, which, in the holoprosencephalic sequence, are considered to appear from a primitive defect of the prechordal mesenchyma, the tissue responsible for the induction of both facial and cerebral structures [24, 25].

**Prognosis** depends on the associated anomalies; however, isolated arhinia is not life incompatible.
