Moebius syndrome (MBS) is considered a congenital cranial dysinnervation disorder (CCDD) [
1]. Its clinical features are impaired ocular motility, lagophthalmos, and lack of facial expression; these features are related to congenital, nonprogressive nerve palsy of the 6th and 7th cranial nerves that typically affect newborns bilaterally. MBS is diagnosed according to the "Bethesda Diagnostic Criteria", which have been recently updated to include genetic testing to ascertain a diagnosis [
2‐
5]. The minimum clinical diagnostic criteria for MBS are as follows: “A congenital, uni- or bilateral, nonprogressive facial weakness with limited abduction of the eye(s) and full vertical motility” [
2‐
4]. Patients who do not meet these criteria are labeled “Moebius-like” and are considered affected by a separate congenital disorder. This is of particular importance since the clinical features of MBS overlap with those of many of other CCDDs with well-described genetic bases, such as congenital fibrosis of the extraocular muscles (CFEOM), Duane’s syndrome, and horizontal gaze palsy with progressive scoliosis (HGPPS) [
1]. The differential diagnosis of MBS in the early perinatal period may be complex and should consider different neurological disorders that result in an MBS-like phenotype with myopathic facies, abnormalities of the palate and feeding difficulties. To this end, cerebral MRI is a tool to be considered.
At more than a century after the first description of the disease, the etiology of MBS is still unclear; recent studies have postulated a multifactorial pathogenesis in which fetal toxic exposure acts on a genetic predisposition for vascular terminal instability and focal microcirculatory failure in the lower brainstem [
6‐
9]. However, it is not clear what causes these changes and why they specifically disrupt the development of the 6th and 7th cranial nerve nuclei; even less is known about the causes of the extraophthalmological signs and symptoms associated with MBS (e.g., lingual and palate dysfunction, hypoplasia of the hand, clubfoot, and thoracic abnormalities). The exact incidence and prevalence of MBS are not clear; the syndrome is considered a "rare disease", as it affects a very small number of people. Clinicians and researchers estimate that this condition affects 1 in 50,000 to 1 in 500,000 newborns, but this estimate is based on only their personal experience with MBS patients, with no epidemiological basis [
3,
9‐
11]. In a Dutch series, the estimated prevalence of MBS was 0.002% of births (4 per 189,000 newborns); this evidence was obtained in 1996 without the present diagnostic criteria for MBS [
12]. The Orphanet Report Series, Rare Disease Collection 2019 reports the estimated prevalence/incidence per 100,000 as "unknown", with only 300 cases described in the literature [
13]. Other epidemiological estimates from series reported worldwide are anecdotal, with no statistical basis. It is difficult to plan an epidemiological study of MBS for many reasons: (1) despite the new diagnostic criteria, the disease is often over- and misdiagnosed in newborns; (2) like many other rare diseases, MBS does not have a regional register from which to derive data for epidemiological purposes; (3) there are no referral centers that provide multidisciplinary care with consequent dispersion of cases; (4) few physicians have expertise in MBS, and they may be difficult to reach; and (5) MBS, like other genetic disorders, may carry social stigma, leading affected individuals to self-marginalize. This study reports the epidemiology of MBS in a well-defined population over a very long period; furthermore, we investigated whether there were geographical differences in MBS incidence/prevalence to identify factors that may cause or contribute to its development.