Clinical phenotype and current diagnostic criteria for primary ciliary dyskinesia
Eleonora Dehlink1, Claire Hogg2, Siobhan B. Carr1, and Andrew Bush 1,3
Department of Paediatrics, Royal Brompton Hospital, London, UK
Department of Paediatric Respirology, Royal Brompton Hospital, London, UK
Department of Paediatric Respirology, Imperial School of Medicine at National Heart and Lung Institute, London, UK
Abstract:
Introduction: Primary ciliary dyskinesia (PCD) is a rare, mostly autosomal-recessive disorder of motile cilia, characterized by chronic lung disease, rhinosinusitis, hearing impairment, and subfertility. PCD is still often missed or diagnosed late since symptoms overlap with common respiratory complaints, but should be considered if one or more of the cardinal clues are present.
Areas covered:We provide an overview on clinical presentations of PCD and clues for when to consider PCD, these include unexplained neonatal respiratory distress, persistent rhinitis from the first days of life, situs anomalies, or otorrhoea following tympanostomy tube insertion. Diagnosis is on the basis of clinical suspicion, and an algorithm of nasal nitric oxide, ciliary beatpattern and frequency , transmission electron microscopy , immunofluorescence of ciliary proteins and genetic studies.However, there is no one gold-standard testas yet.We reviewed the current literature based on PubMed and Ovid databases literature search.
Expert commentary:There is a need for increased awareness about PCD beyond specialist respiratory clinicians and a need for standardization of PCD diagnostics internationally. Early diagnosis means that inappropriate treatment based on misdiagnosed conditions can be avoided, and the onset of bronchiectasis may be delayed.
Key words: heterotaxy, rhinorrhea, neonatal respiratory distress, infertility, congenital heart disease, chronic cough
1 Introduction:
In 1976, Bjorn Afzelius described “A human syndrome caused by immotile cilia”, describing six men and a woman with frequent bronchitis and sinusitis, four of them had situs inversus and the men had immotile spermatozoa,[1]. Both sperm tails and respiratory cilia were lacking dynein arms and had impaired motility. Afzelius hypothesized that these symptoms were linked by inborn defects of motile cilia, thus demonstrating the defect underlying the triad of signs and symptoms described by Kartagener almost half a century earlier: the classical triad of sinusitis, bronchiectasis, and situs inversus[2]. The clinical picture of primary ciliary dyskinesia (PCD) has since evolved from his observations. It has since been discovered that a number of other conditions and syndromes are caused by pathology in other types of cilia, referred to as ciliopathies. Here we will focus on the clinical features and diagnostic clues for primary ciliary dyskinesia (PCD), the congenital condition caused by defects of motile cilia, which is almost exclusively inherited as an autosomal-recessive trait. Although an inborn defect, PCD patients may be diagnosed late due to the non-specific symptoms of PCD and the overlap with other, more common respiratory ailments. In contrast to PCD, external factors like infection or tobacco smoke exposureresult in transient impairment of ciliary motility and -ultrastructure [secondary ciliary dyskinesia]. We will elaborate on the current diagnostic criteria of PCD and recent advances in diagnostic techniques. We will also briefly discuss the approach to patients with other ciliopathies where respiratory complications co-exist.
1.1 Ciliary ultrastructure and function
Ciliary biology is complex and a detailed description is beyond the scope of this review; we give a summary here, in order to allow a better understanding of the pathology of PCD and the characteristic findings in ciliary diagnostics.
Cilia are hair like organelles projecting from cells. They are formed by a cytoskeletal scaffold, the axoneme, built of doublets of microtubules, covered by a membrane contiguous with the plasmalemma. The basal body anchors and orientates the cilium in the cell membrane. Microtubules are linked by a number of proteins crucial for ciliary function.
Cilia can be divided into three classes: primary, which are non-motile mechano- and chemosensory organelles widely distributed throughout the body; nodal, which are found in the embryonic node where they determine organ situs; and motile, which propel fluids along epithelial surfaces or single organisms through a liquid medium. This type of cilia is dysfunctional in PCD. Motile cilia extend from the epithelial surface of many mucus membranes numbering hundredsper cell (Figure 1 A and B) and create a unidirectional, coordinated beat. They are made up of nine outer doublets and a central pair of microtubules, often referred to as the classic 9+2 structure (Figure 1 C and D). Each outer microtubule pair is connected to the central pair by radial spokes. Inner (IDA) and outer dynein arms (ODA) protrude from the outer doublets. Dynein composition varies along the axoneme. For example, the ODA heavy chain, DNAH9, localizes only to the distal end of the cilium [3].The Dynein arms contain ATPases which drive motility by sliding microtubule doublets relative to each other, creating a cycle of movement consisting of a forward beating stroke and a whip-lash backward recovery beat. Dynein motor activity, i.e. ciliary movement, is regulated by the Nexin-dynein regulatory complex (DRC), connecting outer microtubule doublets. During the forward beating stroke, the tips of the cilia engage with the respiratory mucus and whip back in the periciliary fluid layer during the recovery phase [4]. Respiratory cilia typically beat at a frequency of 8 to 20 Hz, which is dependent on temperature, and chemical and mechanical stimulation. Lower temperatures will reduce the beat frequency, hence temperature-controlled microscope stage chambersare usually used for high-speed video microscopy analysis [5]. Structural and functional defects of motile ciliawithin the lower respiratory tract, Eustachian tube and nasal sinuses, and the oviductslead to reduced mucociliary clearance or reduced fluid movement (there are motile cilia in the cerebral ependyma, but these do not move mucus). The resultis chronic infections, such as otitis media with effusion associated with conductive hearing loss, sinusitis, and chronic suppurative lung disease characterized by recurrentbacterial infection, and bronchiectasis. The role of motile cilia in the cerebral ventricles is not completely understood. It is presumed that they effect normal flow of cerebro-spinal fluid, hence the association with ventriculomegaly and hydrocephalus. However, most PCD patients do not have hydrocephalus.
Recent research suggests that ciliary beating is not a perpetual, monotonous pattern, but is modulated by external stimuli sensed via specific receptors. Motile cilia on airway epitheliaexpress bitter taste receptors of the T2R family. Activation of these receptors by bitter compounds denatonium, thujone, salicin, quinine, and nicotine, triggers calcium-ion influx and increases ciliary beat frequency by 25% in vitro[6]. In cystic fibrosis (CF), the Pseudomonas aeruginosa-derived quorum sensing molecule N-(3-oxododecanoyl)-l-homoserine lactone has been shown to activate the bitter taste receptor T2R38 [7]. Hence in vivo, increased ciliary beat frequency, upon sensing bacterial products, via these receptors may be the first line of defense by improving clearance of bacteria from the airway.
Sperm flagella have a similar structure to respiratory cilia, therefore ciliary defects often impair sperm motility leading to infertility. However, not all PCD is associated with immotile spermatozoa, suggesting that they are either under different genetic control or there is genetic redundancy [8, 9].
Nodal cilia in the embryonic node lack the central pair and radial spokes, which results in a circular movement in contrast to the uni-directional whip-lash movement of motile cilia. These cilia determine organ laterality during embryonic development. Since they lack the central pair and radial spokes, PCD causing gene mutations affecting these two structures are not associated with laterality defects[10].
Primary cilia generally have nine outer microtubule pairs, no central pair [9+0], and (unlike nodal and motile cilia) are usually immotile[11]. There are some types of primary cilia that have a central pair of microtubules, e.g. the inner ear sensory cilia [12]. Primary cilia have numerous functions as mechano-, chemo-, osmo-, and photo-sensors [13, 14].They are also critically involved in a number of signaling pathways during development, e.g. the sonic hedgehog (SHH)- Wnt-pathways [15], the signaling cascades of platelet-derived growth factor receptor α (PDGFRα), fibroblast growth factor (FGF), or mTOR [16]. Defects in primary cilia can cause a wide range of diseases, termed ciliopathies. Ciliopathies may manifest in defects of neural tube formation, bone structure, sensory organs such as the retina, inner ear, the olfactory system, anddevelopment of polycystic kidneys, liver and pancreas. The underlying defects are often associated with disruption of the complex intraflagellar transport. This mechanism is crucial for the building, maintenance, and signaling function of cilia, since they lack the machinery to manufacture proteins and all proteins must be transported from the cytoplasm to the tip of the cilium (anterograde), and from the tip back to the cytoplasm again (retrograde) [17, 18]: In retinitis pigmentosa, defects in the Retinitis Pigmentosa GTPase Regulator (RPGR), a protein crucially involved in trafficking in rod and cone cilia, lead to death of photoreceptors and consequent blindness. In Bardet-Biedlsyndrome, disruptions of the Bardet-Biedl syndrome protein complex, which shuttles membrane proteins into the primary cilium, gives rise to a variety of organ manifestations (Table 2).
In clinical practice, the dichotomous classification of cilia is often not as clear. In Usher’s syndrome,forexample, although it is classified as a nonmotile ciliopathy, nasal ciliary function is also impaired [19], suggesting single mutations can affect both, motile and primary cilia. Recently, motile cilia dysfunction and low nasal NO have also been described in patients with Sensenbrenner syndrome and Jeune syndrome, two ciliopathies that may have respiratory problems due to restrictive lung diseases caused by thoracic deformities [20]. These patients havecompound heterozygous mutations in WDR35, a gene that had only been known to be required for primary cilia function. However, WDR35 siRNA knockdown in human respiratory epithelial cultures disrupted motile ciliogenesis, indicating that mutations in WDR35 can perturb both motile and primary cilia function.
1.2 Epidemiology
The prevalence of PCD is difficult to determine, with an estimated incidence of 1 in 15,000 to 1 in 30,000 live births according to Orphanet [21]; others suggest between 1:10,000 [22]and 1:40,000 [23]. These figures are likely to underreport the true prevalence, since milder cases and particularly those without situs inversus may not get diagnosed. A survey across European centers showed that reported cases in children between the age of 5 and 14 years were highest in Cyprus, Switzerland and Denmark, with 111, 47, and 46 per million, respectively. Overall, 48 % of these patients had situs inversus[24]. Currently, international databases are being established to gather more information about the prevalence of this condition [25].
2. Clinical phenotypes of PCD
2.1 When to suspect PCD?
PCD can present at any time from antenatal to adult life. Hence all clinicians, but especially obstetricians, paediatricians, ENT specialists, pulmonologists, fertility specialists, cardiologists, neurologists and neurosurgeons should be aware of the diagnostic clues of PCD and refer patients for testing. It is not uncommon that problems outside a particular specialty are missed or recognized late, resulting in missed or late diagnosis of PCD, e.g. that the snuffly child with complex congenital heart disease has underlying PCD may be missed.
Because many clinical symptoms of PCD are rather non-specific and there is significant overlap with common respiratory ailments, PCD is often considered quite late and referral may be delayed significantly. The other important implication of the non-specific presentation of PCD is that clinical skills are increasingly important in determining who to refer for the ever more complex array of diagnostic tests; it is unfeasible for every child with a cough or runny nose to have transmission electron microscopyperformed, for example. A recent study by the North American Genetic Disorders of Mucociliary Clearance Consortium has identified laterality defect, unexplained neonatal respiratory distress, early onset, year round nasal congestion, and early onset, year-round wet cough as clinical features statistically predictive of PCD in children and young people referred due with a high suspicion ofPCD [26]. These and other diagnostic clues for PCD at different ages are summarized in Table 1.
Although there is no study proving that early diagnosis improves outcome, it is likely that diagnosis and treatment before the development of irreversible bronchiectasis would be beneficial; and it could also prevent the deployment of inappropriate treatment, for example inhaled corticosteroids for a mistaken diagnosis of asthma. There is growing evidence that lung function is significantly worse in patients who are diagnosed late or in adulthood [27, 28]. Alanin et al. found that patients who were diagnosed after preschool age (>6 years) were more frequently colonized with Pseudomonas aeruginosa[29]. It should be noted though that the detrimental effect of Pseudomonas aeruginosa on longitudinal lung function in PCD is less well established than in CF[30].
2.2 Antenatal features
As described above, cilia in the embryonic node determine organ situs. Hence mirror image arrangement of organs, heterotaxy, polysplenia (left isomerism) or asplenia (right isomerism) on antenatal ultrasound scanning can be a diagnostic clue to PCD, especially in association with complex congenital heart disease [31]. These babies should be referred for PCD diagnostics after birth. It is important to counsel parents though that not all individuals with mirror image arrangement actually have PCD [32]. In our diagnostic clinic, only 25% of individuals with situs inversus referred for testing actually have PCD, and this is widely reflected in other centers. This is because heterotaxy has a more complex, largely non-monogenic etiology [33, 34]. Mirror image arrangement is found in 40-50% of PCD patients [31, 35], and around 12.1% of PCD patients show heterotaxy of some degree [36]. Ciliary dysfunction can affect the development of the cerebral ventricles, hence babies with cerebral ventriculomegaly should be considered for referral as well, although hydrocephalus due to ciliary impairment, which is well described in animal models [37, 38], is rare in humans [39-42].A positive family history of PCD or other ciliopathies should always prompt a referral for testing of the newborn, even in the absence of antenatally detected situs anomalies. Chorionic villus sampling for genetic testing can be considered if the relative’s genotype is known.
2.3 Neonatal presentation
All babies with situs anomalies, whenever the diagnosis is made, should be investigated for PCD. PCD with heterotaxy is also strongly associated with complex congenital heart disease (CHD). Early assessment of ciliary function in CHD patients with heterotaxy is important for their pre- and postoperative management, as well as early institution of treatment to try to prevent bronchiectasis. CHD patients with heterotaxy have a higher rate of postsurgical respiratory complications, and morbidity and mortality than those with normal situs [43, 44]. This may be because they have defects of motile cilia underlying their cardiovascular diagnosis. However, it must be said that there is no study showing beneficial effects of an early diagnosis of ciliary dysfunction on postsurgical outcomes.
In the absence of situs anomalies, diagnostic clues for PCD can beless obvious. An important indicator of PCD is unexplained neonatal respiratory distress in otherwise healthy term infants; a recent case-control study showed that up to 84% of PCD patients have a history of unexplained oxygen requirement after birth [45]. Children who were diagnosed with PCD later in life had a longer duration of oxygen requirement than controls (mean 15.2 days versus 0.8 days), a higher frequency of lobar collapse (48% versus 0%), and a later onset of respiratory distress (12 hours versus 1 hour). Chronic rhinorrhea from the first day of life is almost a hallmark of PCD, and most parents will remember if a baby had a runny nose from the first day of life. This must be distinguished from the baby who gets a first viral cold and rhinitis in the first few weeks of life.
2.4 Presentation in Childhood
Many PCD patients will present in childhood. A review of medical records in a British paediatric PCD referral center showed that the median age at referral was 4 years [46]. A survey of 223 PCD centers across 26 European countries revealed that the median age at diagnosis in Europe was 5.3 years. The age at diagnosis seems to be largely affected by two factors: whether or not the child presents with obvious signs like situs inversus and where in Europe the child presents. In children with mirror image arrangement the median age at diagnosis was 3.5 years (still a very long delay!), whereas children with situs solitus were diagnosed significantly later, at median 5.8 years of age. With regards to regional differences, the survey showed that children in Western Europe, including Britain, were diagnosed at a younger age (5.0 and 4.8 years) than children in regions with lower health expenditures, e.g. Eastern and Southern Europe (6.8 and 6.5 years) [24].
In the absence of situs anomalies, the presentation of PCD in childhood is often non-specific, and many features of PCD are so commonly seen in preschool children that PCD is often considered late and delayed referral is common. It needs to be emphasized that situs anomalies are not a prerequisite of PCD, and taking a focused history is essential in the diagnostic workup. Clues from the patient’s history are neonatal dyspnea/distress, pneumonia or rhinorrhea starting in the neonatal period (above), recurrent ear infections and middle ear effusions resulting in hearing loss and speech delay.Persistent malodorous otorrhoea following insertion of ventilation tubes with minimal impact on hearing loss [47, 48], a history of atypical asthma or wheeze not responsive to conventional treatment, as well as a family history of PCD or atypical asthma should all warrant further investigation. Agenesis of paranasal sinuses may also be found [49]. As a mostly autosomal recessive trait, PCD is more common in consanguineous families.