Embryology: During embryonic development ectodermal thickenings called the nasal placodes begin to form at the end of the fourth week. These deepen due to active growth of the placode epithelium and proliferation of mesenchyme, forming the nasal groove. The edges of the placode form the lateral then medial nasal process in the early fifth week. As the medial and lateral nasal processes grow, the nasal groove deepens and ultimately becomes the nasal sacs by late fifth week. Over the next few weeks, the medial and lateral nasal processes fuse with the maxillary processes to create the nasal tip, columella, and nasal alae, as well as portions of the upper lip and primary palate .
Differential diagnosis: The differential diagnosis for congenital arrhinia is limited. Patients with holoprosencephaly can present without a nose, though these patients have associated brain anomalies. We also considered Oral-Facial-Digital syndrome which can present with anomalies of the oral cavity (Cleft lip and palate, clefts of the jaw and tongue in the area of the lateral incisors and canines, lobulated tongue with hamartomas), face (aplasia of the alar cartilages, hypertelorism, microphthalmia, coloboma, micrognathia), and extremities (syndactyly, clinodactyly, brachydactyly and occasionally postaxial polydactyly). Additionally, there can be defects in the development of the central nervous system (Mental retardation, agenesis of the corpus callosum, seizures), alopecia, polycystic kidney disease and growth retardation . Our case did not have the cleft lip/palate or tongue, significant brain anomalies, and growth retardation that is more commonly seen with Oral-Facial-Digital syndrome. Additionally, the genital findings in our case are less common in Oral-Facial-Digital syndrome.
Clinical consequences: Infants with Bosma Arrhinia Microphthalmia syndrome often have normal intellect and can grow up to study and be employed . However, in the neonatal period, they can suffer from respiratory distress requiring tracheostomy. Some are even provided tube feedings via a gastrostomy in order to lower the risk of aspiration while feeding . During puberty, these patients often require hormone therapy to develop normally and can suffer from reduced bone density, with a propensity for fractures [12, 14].
Etiology: The etiology of Bosma Arrhinia Microphthalmia syndrome is still under investigation. While a genetic cause is thought to play a role, the karyotype in these patients is usually normal with a few exceptions [19, 20]. There have been several familial cases reported including two sets of siblings born with arrhinia and microphthalmia [9, 21], and an aunt and niece presenting with variable manifestations of arrhinia, choanal atresia, microphthalmia, and hypertelorism . Initially, Pax6 gene was investigated as the etiology of BAM syndrome as mice with homozygous mutations of this gene fail to form nasal placodes and have defective ocular and nasal development . However, the Bosma phenotype was not found to correlate with PAX6 mutations in humans [11, 23]. In 2017, Shaw et al developed an international consortium of 40 patients with arrhinia, 84% of which met BAM criteria and found that mutations in SMCHD1 were associated with isolated arrhinia and BAM (15). These findings were further investigated by Gordon et al who found that BAM was associated with missense mutations mapping to the extended ATPase domain of SMCHD1 . This gene has also been associated with facioscapulohumeral muscular dystrophy (FSHD), a myopathy characterized by progressive and often asymmetric weakness and atrophy of the facial and upper extremity muscles . Mul et al investigated whether these different phenotypes were part of a spectrum and found that none of the patients with FSHD2 demonstrated features found in BAM syndrome, concluding that these conditions are likely caused by complex oligogenic or multifactorial mechanisms that only partially overlap at the level of SMCHD1 .
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