# Vitamin D and Primary Ciliary Dyskinesia: A Topic to Be Further Explored
## 1 Introduction
In approximately half of the patients harboring PCD, there are defects in the nodal cilia, which cause _situs inversus_[8, 11, 12]. PCD had some very well know eponyms and synonyms, including \"Kartagener Syndrome\" (bronchiectasis, chronic sinusitis, and _situs inversus totalis_) and \"Immotile Cilia Syndrome\" (after detection of immotile respiratory cilia and immotile sperm in subjects with Kartagener Syndrome) [13]. The different mutations cause different ciliary phenotypes, which also impact the clinical features of the disease. For example, a mutation that affects some components of the ODAs and IDAs may result in immotile cilia mutation that affects the radial spoke proteins resulting in cilia with circular motions. On the other side, mutations that affect CCDC39 and CCDC40 genes (Coiled-Coil Domain Containing proteins 39 and 40) induce a more severe clinical phenotype. The Coiled-Coil Domain-Containing Protein 114 gene (CCDC114) is associated with the biogenesis of cilia. It is crucial for the attachment of the outer dynein arms (ODAs) to the axoneme of cilia. Recently, mutations in the CCDC114 gene have been identified in a subtype of PCD named ciliary dyskinesia 20 (CILD20) [14, 15, 16, 17, 18]. In 78 patients with PCD, [PERSON] et al. investigated 76 patients using ciliary ultrastructural analysis [19]. The multiple PCD phenotypes included four major categories of structural ciliary defects. They are (1) outer dynein arm (ODA) alone, (2) inner dynein arm (IDA) alone, (3) defects in both outer and inner dynein arms, and (4) central apparatus defects. This last category is typically subdivided into defects of the central pair, radial spokes, or nexin links. They found defects in ODA alone, IDA alone, ODA and IDA combined, and central apparatus in 43%, 29%, 24%, and 4%, respectively [19]. Although heavily debated due to reproducibility issues in a clinical setting, the central doublet orientation deviations of over 20 degrees from each other may indicate the cilia's failure to move in a coordinated wave-like fashion [5, 6, 20, 21, 22, 23]. Although the ultrastructural analysis of cilia is quite well established worldwide, this assay has limited detection sensitivity. Not all PCD mutations cause cilia defects visualized by electron microscopy. The method is subject to artifacts due to numerous technical biases [9, 24]. The functional analysis represents another critical approach toward our current understanding of ciliary motility. Still, this investigation remains technically challenging, and the relationship between ciliary motion and mucus transport has not been well characterized. The lifelong burden of PCD to pediatric patients changes probably slightly when they become adults. The focus moves on bronchiectasis, and the otologic disturbances become quite attenuated. However, the burden can be focused on fertility issues, mainly when the couple is attempting to have a child. Despite the extensive and increasing understanding of the spectrum of cilia structural defects in PCD, only a few reports concentrate on vitamin D's role in PCD. The scope of this narrative review is attempting to fill this gap in the medical literature.