REVIEW Congenital problems of mucociliary clearance: primary ciliary dyskinesia* Summary Mucociliary clearance is a primary defence mechanism of the airway that can be altered in congenital diseases such as primary ciliary dyskinesia and cystic fibrosis, as well as acquired conditions. This article focuses on primary ciliary dyskinesia and the diag- nostic approach to it, which is still evolving. Key words: mucociliary clearance, primary ciliary dyskinesia, cilia, Kartagener’s syndrome 353 Introduction Mucociliary clearance (MCC) is a primary defence mechanism of the airway, whereby secreted mucus and the substances within it are cleared (1) . It can be disturbed by infections, toxins and congenital disorders such as cystic fibrosis and primary ciliary dyskinesia (2) . Afzelius originally described ciliary ultrastructure in 1959 (3) . He was the first to identify an ultrastructural abnormality in four men with ‘living but immotile’ sperm (4) . ‘Immotile cilia syndrome’ was formally described in 1977, in male patients with chronic respiratory symptoms, infertility and absent dynein arms (5) . As it has now been shown that cilia are not necessarily completely immotile, but may have abnormal motility, the name ‘primary ciliary dyskinesia’ (PCD) seems a better alternative (6) . Afzelius also suggested that ciliary movement influences em- bryonic visceral rotation, and that in the absence of functional cilia this rotation will occur by chance (4) . Cilia are now known to be important in many organs during embryogenesis, and congenital ciliopathies may be associated with polycystic renal or hepatic disease, nephronophthisis, hydrocephalus and rare syndromes such as Bardet-Biedl, Alstrom’s and oro-facial-digital syndromes (7) . Whilst motile cilia are a vital part of primary respi- ratory defence systems, it is sensory cilia that appear important in the development of these other organ systems. Mucociliary clearance Respiratory epithelium consists of approximately 80% ciliated cells and 20% goblet cells (8) . Airway surface fluid has two layers, an outer mucus ‘gel’ layer and an inner thin ‘sol’ layer (9) . This classical model has been challenged with an alternative model of a tangled mucin layer extending to the epithelial surface (10) . Mucus traps inhaled particles and pathogens and is then propelled by ciliary movement towards the gastrointestinal tract for elimination, the so-called ‘mucociliary escalator.’ The lower airways and nose can also clear mucus with a cough or a sneeze respectively, but the sinuses depend on mucociliary activity alone. Effective MCC requires the production of appropriate mucus and periciliary fluid as well as coordinated ciliary activity (11) . Normal MCC has been reported as 4.5-7mm/minute in the nose and 10.7 ± 3.5mm/minute in the trachea and large lower airways (12,13) . Joanne Rimmer Department of Otolaryngology, Royal National Throat Nose & Ear Hospital, London, United Kingdom Rhinology 50: 353-359, 2012 DOI:10.4193.Rhino.12.126 *Received for publication: July 18, 2012 Accepted: September 30, 2012