Congenital Pulmonary Airway Malformations With a Reconsideration and Current Perspective on the Stocker Classification.

Congenital cystic pulmonary lesions (CCPLs) are represented by the following entities: congenital pulmonary airway malformation (CPAM), formerly congenital cystic adenomatoid malformation, extra- and intralobar sequestration (EIS), congenital lobar emphysema (overexpansion), and bronchogenic cyst. The developmental model of CPAM histogenesis by Stocker proposed perturbations designated as CPAM type 0 to type 4 without known or specific pathogenetic mechanisms along the airway from the bronchus to the alveolus. This review highlights mutational events either at the somatic level in KRAS (CPAM types 1 and possibly 3) or germline variants in congenital acinar dysplasia, formerly CPAM type 0, and pleuropulmonary blastoma (PPB), type I, formerly CPAM type 4. The potential for overt malignant progression exists in the case of PPB type I and CPAM type 1 in some cases to well-differentiated mucinous adenocarcinoma. On the other hand, CPAM type 2 is an acquired lesion resulting from interruption in lung development secondary to bronchial atresia. The latter is also regarded as the etiology of EIS whose pathologic features are similar, if not identical, to CPAM type 2. These observations have provided important insights into the pathogenetic mechanisms in the development of the CPAMs since the Stocker classification.

Congenital lung malformations: Dysregulated lung developmental processes and altered signaling pathways.

Congenital lung malformations comprise a diverse group of anomalies including congenital pulmonary airway malformation (CPAM, previously known as congenital cystic adenomatoid malformation or CCAM), bronchopulmonary sequestration (BPS), congenital lobar emphysema (CLE), bronchogenic cysts, and hybrid lesions. Little is known about the signaling pathways that underlie the pathophysiology of these lesions and the processes that may promote their malignant transformation. In the last decade, the use of transgenic/knockout animal models and the implementation of next generation sequencing on surgical lung specimens have increased our knowledge on the pathophysiology of these lesions. Herein, we provide an overview of normal lung development in humans and rodents, and we discuss the current state of knowledge on the pathophysiology and molecular pathways that are altered in each congenital lung malformation.

SARS-CoV-2 Exposed Mesenchymal Stromal Cell from Congenital Pulmonary Airway Malformations: Transcriptomic Analysis and the Expression of Immunomodulatory Genes.

The inflammatory response plays a central role in the complications of congenital pulmonary airway malformations (CPAM) and severe coronavirus disease 2019 (COVID-19). The aim of this study was to evaluate the transcriptional changes induced by SARS-CoV-2 exposure in pediatric MSCs derived from pediatric lung (MSCs-lung) and CPAM tissues (MSCs-CPAM) in order to elucidate potential pathways involved in SARS-CoV-2 infection in a condition of exacerbated inflammatory response. MSCs-lung and MSCs-CPAM do not express angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TRMPSS2). SARS-CoV-2 appears to be unable to replicate in MSCs-CPAM and MSCs-lung. MSCs-lung and MSCs-CPAM maintained the expression of stemness markers MSCs-lung show an inflammatory response (IL6, IL1B, CXCL8, and CXCL10), and the activation of Notch3 non-canonical pathway; this route appears silent in MSCs-CPAM, and cytokine genes expression is reduced. Decreased value of p21 in MSCs-lung suggested no cell cycle block, and cells did not undergo apoptosis. MSCs-lung appears to increase genes associated with immunomodulatory function but could contribute to inflammation, while MSCs-CPAM keeps stable or reduce the immunomodulatory receptors expression, but they also reduce their cytokines expression. These data indicated that, independently from their perilesional or cystic origin, the MSCs populations already present in a patient affected with CPAM are not permissive for SARS-CoV-2 entry, and they will not spread the disease in case of infection. Moreover, these MSCs will not undergo apoptosis when they come in contact with SARS-CoV-2; on the contrary, they maintain their staminality profile.

Effect of TGF-ß1 on eosinophils to induce cysteinyl leukotriene E4 production in aspirin-exacerbated respiratory disease.

Cysteinyl leukotriene (cysLT) overproduction and eosinophil activation are hallmarks of aspirin-exacerbated respiratory disease (AERD). However, pathogenic mechanisms of AERD remain to be clarified. Here, we aimed to find the significance of transforming growth factor beta 1 (TGF-ß1) in association with cysteinyl leukotriene E4 (LTE4) production, leading to eosinophil degranulation. To evaluate levels of serum TGF-ß1, first cohort enrolled AERD (n = 336), ATA (n = 442) patients and healthy control subjects (HCs, n = 253). In addition, second cohort recruited AERD (n = 34) and ATA (n = 25) patients to investigate a relation between levels of serum TGF-ß1 and urinary LTE4. The function of TGF-ß1 in LTE4 production was further demonstrated by ex vivo (human peripheral eosinophils) or in vivo (BALB/c mice) experiment. As a result, the levels of serum TGF-ß1 were significantly higher in AERD patients than in ATA patients or HCs (P = .001; respectively). Moreover, levels of serum TGF-ß1 and urinary LTE4 had a positive correlation (r = 0.273, P = .037). In the presence of TGF-ß1, leukotriene C4 synthase (LTC4S) expression was enhanced in peripheral eosinophils to produce LTE4, which sequentially induced eosinophil degranulation via the p38 pathway. When mice were treated with TGF-ß1, significantly induced eosinophilia with increased LTE4 production in the lung tissues were noted. These findings suggest that higher levels of TGF-ß1 in AERD patients may contribute to LTE4 production via enhancing LTC4S expression which induces eosinophil degranulation, accelerating airway inflammation.

Pulmonary Manifestations of Genetic Disorders in Children.

Congenital bronchopulmonary malformations are relatively common and arise during various periods of morphogenesis. Although some are isolated or sporadic occurrences, others may result from single gene mutations or cytogenetic imbalances. Single gene mutations have been identified, which are etiologically related to primary pulmonary hypoplasia, lung segmentation defects as well as pulmonary vascular and lymphatic lesions. Functional defects in cystic fibrosis, primary ciliary dyskinesias, alpha-1-antitrypsin deficiency, and surfactant proteins caused by gene mutations may result in progressive pulmonary disease. This article provides an overview of pediatric pulmonary disease from a genetic perspective.

Massive ascites and severe pulmonary hypoplasia in a premature infant with meconium peritonitis and congenital cytomegalovirus infection.

Background: Meconium peritonitis (MP) is an infrequent prenatal complication. Association between MP and pulmonary hypoplasia has never been reported. Case Report: A female infant with antenatally diagnosed MP and ascites was delivered at 36 gestational weeks. She died shortly after birth due to pulmonary insufficiency. Autopsy confirmed the presence of MP and ascites, and additionally revealed intestinal malrotation, volvulus, necrosis and perforation, and pulmonary hypoplasia. Congenital cytomegalovirus (CMV) infection was also noted. This was an unexpected finding as the mother was tested negative for CMV IgM at 26 gestational weeks after sonographic detection of fetal ascites. Conclusions: This is the first reported case of lethal pulmonary hypoplasia in a neonate with MP-associated ascites complicated by congenital CMV infection. This case illustrates that a negative maternal CMV IgM might not be sufficient to rule out congenital CMV, and that a concomitant infectious etiology should always be considered even when a primary cause for fetal ascites (e.g., MP in this case) is identified.

Complete Tracheal Ring Deformity. A Translational Genomics Approach to Pathogenesis.

Rationale: Complete tracheal ring deformity (CTRD) is a rare congenital abnormality of unknown etiology characterized by circumferentially continuous or nearly continuous cartilaginous tracheal rings, variable degrees of tracheal stenosis and/or shortening, and/or pulmonary arterial sling anomaly.Objectives: To test the hypothesis that CTRD is caused by inherited or de novo mutations in genes required for normal tracheal development.Methods: CTRD and normal tracheal tissues were examined microscopically to define the tracheal abnormalities present in CTRD. Whole-exome sequencing was performed in children with CTRD and their biological parents ("trio analysis") to identify gene variants in patients with CTRD. Mutations were confirmed by Sanger sequencing, and their potential impact on structure and/or function of encoded proteins was examined using human gene mutation databases. Relevance was further examined by comparison with the effects of targeted deletion of murine homologs important to tracheal development in mice.Measurements and Main Results: The trachealis muscle was absent in all of five patients with CTRD. Exome analysis identified six de novo, three recessive, and multiple compound-heterozygous or rare hemizygous variants in children with CTRD. De novo variants were identified in SHH (Sonic Hedgehog), and inherited variants were identified in HSPG2 (perlecan), ROR2 (receptor tyrosine kinase-like orphan receptor 2), and WLS (Wntless), genes involved in morphogenetic pathways known to mediate tracheoesophageal development in mice.Conclusions: The results of the present study demonstrate that absence of the trachealis muscle is associated with CTRD. Variants predicted to cause disease were identified in genes encoding Hedgehog and Wnt signaling pathway molecules, which are critical to cartilage formation and normal upper airway development in mice.

The oculoauriculofrontonasal syndrome: Further clinical characterization and additional evidence suggesting a nontraditional mode of inheritance.

The oculoauriculofrontonasal syndrome (OAFNS) is a rare disorder characterized by the association of frontonasal dysplasia (widely spaced eyes, facial cleft, and nose abnormalities) and oculo-auriculo-vertebral spectrum (OAVS)-associated features, such as preauricular ear tags, ear dysplasia, mandibular asymmetry, epibulbar dermoids, eyelid coloboma, and costovertebral anomalies. The etiology is unknown so far. This work aimed to identify molecular bases for the OAFNS. Among a cohort of 130 patients with frontonasal dysplasia, accurate phenotyping identified 18 individuals with OAFNS. We describe their clinical spectrum, including the report of new features (micro/anophtalmia, cataract, thyroid agenesis, polymicrogyria, olfactory bulb hypoplasia, and mandibular cleft), and emphasize the high frequency of nasal polyps in OAFNS (56%). We report the negative results of ALX1, ALX3, and ALX4 genes sequencing and next-generation sequencing strategy performed on blood-derived DNA from respectively, four and four individuals. Exome sequencing was performed in four individuals, genome sequencing in one patient with negative exome sequencing result. Based on the data from this series and the literature, diverse hypotheses can be raised regarding the etiology of OAFNS: mosaic mutation, epigenetic anomaly, oligogenism, or nongenetic cause. In conclusion, this series represents further clinical delineation work of the rare OAFNS, and paves the way toward the identification of the causing mechanism.

Fetal lung transcriptome patterns in an ex vivo compression model of diaphragmatic hernia.

BACKGROUND: The purpose of this study was to employ a novel ex vivo lung model of congenital diaphragmatic hernia (CDH) to determine how a mechanical compression affects early pulmonary development. METHODS: Day-15 whole fetal rat lungs (n = 6-12/group) from nitrofen-exposed and normal (vehicle only) dams were explanted and cultured ex vivo in compression microdevices (0.2 or 0.4 kPa) for 16 h to mimic physiologic compression forces that occur in CDH in vivo. Lungs were evaluated with significance set at P < 0.05. RESULTS: Nitrofen-exposed lungs were hypoplastic and expressed lower levels of surfactant protein C at baseline. Although compression alone did not alter the α-smooth muscle actin (ACTA2) expression in normal lungs, nitrofen-exposed lungs had significantly increased ACTA2 transcripts (0.2 kPa: 2.04 ± 0.15; 0.4 kPa: 2.22 ± 0.11; both P < 0.001). Nitrofen-exposed lungs also showed further reductions in surfactant protein C expression at 0.2 and 0.4 kPa (0.53 ± 0.04, P < 0.01; 0.69 ± 0.23, P < 0.001; respectively). Whereas normal lungs exposed to 0.2 and 0.4 kPa showed significant increases in periostin (POSTN), a mechanical stress-response molecule (1.79 ± 0.10 and 2.12 ± 0.39, respectively; both P < 0.001), nitrofen-exposed lungs had a significant decrease in POSTN expression (0.4 kPa: 0.67 ± 0.15, P < 0.001), which was confirmed by immunohistochemistry. CONCLUSIONS: Collectively, these pilot data in a model of CDH lung hypoplasia suggest a primary aberration in response to mechanical stress within the nitrofen lung, characterized by an upregulation of ACTA2 and a downregulation in SPFTC and POSTN. This ex vivo compression system may serve as a novel research platform to better understand the mechanobiology and complex regulation of matricellular dynamics during CDH fetal lung development.

Novel Molecular and Phenotypic Insights into Congenital Lung Malformations.

RATIONALE: Disruption of normal pulmonary development is a leading cause of morbidity and mortality in infants. Congenital lung malformations are a unique model to study the molecular pathogenesis of isolated structural birth defects, as they are often surgically resected. OBJECTIVES: To provide insight into the molecular pathogenesis of congenital lung malformations through analysis of cell-type and gene expression changes in these lesions. METHODS: Clinical data, and lung tissue for DNA, RNA, and histology, were obtained from 58 infants undergoing surgical resection of a congenital lung lesion. Transcriptome-wide gene expression analysis was performed on paired affected and unaffected samples from a subset of infants (n = 14). A three-dimensional organoid culture model was used to assess isolated congenital lung malformation epithelium (n = 3). MEASUREMENTS AND MAIN RESULTS: Congenital lung lesions express higher levels of airway epithelial related genes, and dysregulated expression of genes related to the Ras and PI3K-AKT-mTOR (phosphatidylinositol 3-kinase-AKT-mammalian target of rapamycin) signaling pathways. Immunofluorescence confirmed differentiated airway epithelial cell types throughout all major subtypes of congenital lung lesions, and three-dimensional cell culture demonstrated a cell-autonomous defect in the epithelium of these lesions. CONCLUSIONS: This study provides the first comprehensive analysis of the congenital lung malformation transcriptome and suggests that disruptions in Ras or PI3K-AKT-mTOR signaling may contribute to the pathology through an epithelial cell-autonomous defect.

Association of Single Nucleotide Polymorphisms of PADI4 and HLA-DRB1 Alleles with Susceptibility to Rheumatoid Arthritis-Related Lung Diseases.

OBJECTIVE: Lung diseases (LD) are common extra-articular manifestations in rheumatoid arthritis (RA). However, little is known about factors associated with susceptibility to rheumatoid arthritis-related lung diseases (RA-LD). The aim of the present study was to investigate whether the single nucleotide polymorphisms (SNPs) of PADI4 and HLA-DRB1 alleles were associated with RA-LD. METHODS: Blood samples and clinical data were collected from 116 consecutive RA patients who satisfied the 1987 American College of Rheumatology classification criteria. RA-LD was diagnosed using high-resolution computed tomography of the chest. All patients were genotyped for SNPs of PADI4 and HLA-DRB1 alleles and analyzed for full amino acid sequence of the HLA protein corresponding to a 4-digit HLA typing. Data were analyzed by independent t test (or Mann-Whitney test) for continuous variables, Chi-square test (or Fisher's exact test) and trend test for categorical variables, and logistic regression analysis. RESULTS: Ninety-four (81.0 %) RA patients had LD, of which eight (6.9 %) had interstitial lung disease (ILD) and 92 (79.3 %) had airway abnormalities in which 64 (55.2 %) showed bronchiectasis and 47 (40.5 %) revealed bronchial wall thickening. The recessive genotype of padi4_92 was susceptible to airway abnormalities (OR = 2.22, 95 % CI = 1.05-4.49, p = 0.034). Tryptophan at position 9 of HLA-DRB1 sequence was associated with the susceptibility to RA-ILD (OR = 22.89, 95 % CI = 1.20-432.56, p = 0.037). CONCLUSION: PADI4 polymorphisms and HLA-DRB1 alleles could attribute differently to the development of airway abnormalities and ILD, respectively, in RA.

Respiratory system involvement in Costello syndrome.

Costello syndrome (CS) is a multisystem disorder caused by heterozygous germline mutations in the HRAS proto-oncogene. Respiratory system complications have been reported in individuals with CS, but a comprehensive description of the full spectrum and incidence of respiratory symptoms in these patients is not available. Here, we report the clinical course of four CS patients with respiratory complications as a major cause of morbidity. Review of the literature identified 56 CS patients with descriptions of their neonatal course and 17 patients in childhood/adulthood. We found that in the neonatal period, respiratory complications are seen in approximately 78% of patients with transient respiratory distress reported in 45% of neonates. Other more specific respiratory diagnoses were reported in 62% of patients, the majority of which comprised disorders of the upper and lower respiratory tract. Symptoms of upper airway obstruction were reported in CS neonates but were more commonly diagnosed in childhood/adulthood (71%). Analysis of HRAS mutations and their respiratory phenotype revealed that the common p.Gly12Ser mutation is more often associated with transient respiratory distress and other respiratory diagnoses. Respiratory failure and dependence on mechanical ventilation occurs almost exclusively with rare mutations. In cases of prenatally diagnosed CS, the high incidence of respiratory complications in the neonatal period should prompt anticipatory guidance and development of a postnatal management plan. This may be important in cases involving rarer mutations. Furthermore, the high frequency of airway obstruction in CS patients suggests that otorhinolaryngological evaluation and sleep studies should be considered. © 2016 Wiley Periodicals, Inc.

A complete corticotropin releasing factor system localized in human fetal lung.

OBJECTIVE: The Corticotropin Releasing Factor (CRF) system (neuropeptides CRF, Ucn I, II, III and binding sites CRFR1, CRFR2, CRF-BP) is responsible for stress regulation and the homeostasis of an organism. Herein we study the CRF system in human normal and pathological fetal lungs. DESIGN: Lung tissues from 46 archival human fetuses were divided into Group A (normal), Group B (chromosomal abnormalities) and Group C (congenital disorders). Presence of elements of the CRF system was evaluated using immunohistochemistry and was correlated to pathology, lung developmental stage and clinicopathological characteristics. RESULTS: Immunoreactivity for all antigens was found in both epithelial and mesenchymal lung cells of the bronchi and alveoli. Ucn I and CRFR1 were more frequently present in Group A. Ucns were more frequently localized at the pseudoglandular stage. There was a positive correlation between the presence of the CRF neuropeptides and between CRFR1 and CRF. Two fetuses with lung malformations showed low or no detectable presence of the CRF system. CONCLUSIONS: We report the presence of a complete CRF system in human fetal lungs correlating its developmental stage and several pathologies. Our results are in agreement with findings in experimental animal models, implicating the CRF system in fetal lung development, its action being more significant in the early stages.

Neurodegeneration in drop-dead mutant drosophila melanogaster is associated with the respiratory system but not with Hypoxia.

Mutations in the gene drop-dead (drd) cause diverse phenotypes in adult Drosophila melanogaster including early lethality, neurodegeneration, tracheal defects, gut dysfunction, reduced body mass, and female sterility. Despite the identification of the drd gene itself, the causes of early lethality and neurodegeneration in the mutant flies remain unknown. To determine the pattern of drd expression associated with the neurodegenerative phenotype, knockdown of drd with various Gal4 drivers was performed. Early adult lethality and neurodegeneration were observed upon knockdown of drd in the tracheal system with two independent insertions of the breathless-Gal4 driver and upon knockdown in the tracheal system and elsewhere with the DJ717-Gal4 driver. Surprisingly, rescue of drd expression exclusively in the tracheae in otherwise mutant flies rescued the neurodegenerative phenotype but not adult lethality. Gut dysfunction, as measured by defecation rate, was not rescued in these flies, and gut function appeared normal upon tracheal-specific knockdown of drd. Finally, the hypothesis that tracheal dysfunction in drd mutants results in hypoxia was tested. Hypoxia-sensitive reporter transgenes (LDH-Gal4 and LDH-LacZ) were placed on a drd mutant background, but enhanced expression of these reporters was not observed. In addition, manipulation of drd expression in the tracheae did not affect expression of the hypoxia-induced genes LDH, tango, and similar. Overall, these results indicate that there are at least two causes of adult lethality in drd mutants, that gut dysfunction and neurodegeneration are independent phenotypes, and that neurodegeneration is associated with tracheal expression of drd but not with hypoxia.

Oculoauriculofrontonasal syndrome: case series revealing new bony nasal anomalies in an old syndrome.

Frontonasal Dysplasia (FND) and Oculo-auriculo-vertebral spectrum (OAVS) are two well-recognized clinical entities. With features of both FND and OAVS, the term oculoauriculofrontonasal syndrome (OAFNS) was coined in 1981. The OAFNS phenotype combines elements of abnormal morphogenesis of the frontonasal and maxillary process (derived from forebrain neural crest) with abnormal development of the first and second branchial arches (derived from hindbrain neural crest). We present a case series of 33 children with OAFNS ascertained from a comprehensive review of the literature and report an additional retrospective series of eight patients displaying features consistent with OAFNS. Notably, in a subset of our cases, we have observed abnormalities in nasal ossification and bony structures of the maxilla that have not previously described in OAFNS and are not seen in either FND or OAVS. We present the phenotype and novel naso-maxillary findings and explore potential etiologic and developmental pathways for OAFNS. We highlight the differences in phenotypic characteristics of OAFNS compared to OAVS and FND. These observations support the classification of OAFNS as a discrete syndrome. Further phenotypic refinements of OAFNS are needed to understand pathogenesis of this syndrome and the newly described nasal malformation may help identify the etiology.

The spectrum of pulmonary malformation in trisomy-21 patient. A review with emphasis on the molecular-genetic basis.

Gene overexpression has been identified as a primary determining factor for the distinct Down syndrome (DS) phenotypes. Previous genetic research has identified a spectrum of gene expressions responsible for many of the observed traits in DS patients including cardiovascular, brain, and GI anomalies. However, the molecular/genetic basis underlying pulmonary anomalies are yet to be identified, even though respiratory complications represent the leading cause of morbidity and mortality in DS patients. In this article, we will discuss the Etiopathogenesis and spectrum of pulmonary anomalies in DS patients.

[Congenital lung malformations: natural history and pathophysiological mechanisms]. / Malformations pulmonaires congénitales: histoire naturelle et hypothèses pathogéniques.

INTRODUCTION: Congenital lung lesions comprise a broad spectrum of various malformations including congenital cystic adenomatoid malformation (CCAM), bronchopulmonary sequestration (BPS), congenital lobar emphysema, bronchial atresia and bronchogenic cyst. This review aims at the description of their natural history, and of the underlying pathophysiological mechanisms. STATE OF THE ART: Congenital lung lesions are frequently diagnosed antenatally and many remain asymptomatic after birth. In the absence of antenatal identification, they are usually revealed by the occurrence of infection. In some cases, spontaneous resolution of the malformation can occur. Different pathogenic hypotheses are discussed for the origin of these abnormalities, and common processes appear likely to all of these malformations. Factors involved in the process of branching seem to play a particularly important role. PERSPECTIVES: Prospective follow-up of operated and unoperated children would complete our knowledge about the natural history of these lesions. The contribution of experimental models has led to advances in the understanding of pathogenic mechanisms. Further studies are needed to identify the factors initiating the malformative process.

High prevalence of respiratory ciliary dysfunction in congenital heart disease patients with heterotaxy.

BACKGROUND: Patients with congenital heart disease (CHD) and heterotaxy show high postsurgical morbidity/mortality, with some developing respiratory complications. Although this finding is often attributed to the CHD, airway clearance and left-right patterning both require motile cilia function. Thus, airway ciliary dysfunction (CD) similar to that of primary ciliary dyskinesia (PCD) may contribute to increased respiratory complications in heterotaxy patients. METHODS AND RESULTS: We assessed 43 CHD patients with heterotaxy for airway CD. Videomicrocopy was used to examine ciliary motion in nasal tissue, and nasal nitric oxide (nNO) was measured; nNO level is typically low with PCD. Eighteen patients exhibited CD characterized by abnormal ciliary motion and nNO levels below or near the PCD cutoff values. Patients with CD aged >6 years show increased respiratory symptoms similar to those seen in PCD. Sequencing of all 14 known PCD genes in 13 heterotaxy patients with CD, 12 without CD, 10 PCD disease controls, and 13 healthy controls yielded 0.769, 0.417, 1.0, and 0.077 novel variants per patient, respectively. One heterotaxy patient with CD had the PCD causing DNAI1 founder mutation. Another with hyperkinetic ciliary beat had 2 mutations in DNAH11, the only PCD gene known to cause hyperkinetic beat. Among PCD patients, 2 had known PCD causing CCDC39 and CCDC40 mutations. CONCLUSIONS: Our studies show that CHD patients with heterotaxy have substantial risk for CD and increased respiratory disease. Heterotaxy patients with CD were enriched for mutations in PCD genes. Future studies are needed to assess the potential benefit of prescreening and prophylactically treating heterotaxy patients for CD.

Complex dynamic upper airway collapse: associations between abnormalities in 99 harness racehorses with one or more dynamic disorders.

REASONS FOR PERFORMING STUDY: Many horses demonstrate dynamic collapse of more than one upper respiratory tract (URT) structure during high-speed treadmill videoendoscopy (HSTV). OBJECTIVES: To report the frequency of complex dynamic URT collapse in harness racehorses and determine if an association exists between occurrence of certain disorders. METHODS: Retrospective study of 99 Standardbreds and Coldblooded trotters that had one or more dynamic URT disorders identified during HSTV between 1998 and 2006. The horses underwent HSTV using a protocol that included periods of free head carriage and poll flexion until fully fatigued. Dynamic abnormalities were classified as: dynamic laryngeal collapse (DLC) associated with poll flexion; axial deviation of the aryepiglottic folds (ADAF); dynamically flaccid epiglottis (FE); caudal palatal instability (PI); intermittent dorsal displacement of the soft palate (iDDSP); pharyngeal collapse (PC); alar fold collapse (AFC) and nasal flutter (NF). Cluster analysis and Fisher's exact test was performed between groups. Significance was set at P<0.05. RESULTS: The overall incidence of complex dynamic URT collapse was 69.7%. Axial deviation of the aryepiglottic folds was significantly associated with PI (P<0.0001) and FE (P<0.0001); iDDSP was significantly associated with PI (P = 0.004) only. Dynamic laryngeal collapse associated with poll flexion was significantly associated with FE (P = 0.002). Cluster analysis confirmed strong associations between the above diagnosis groups and weaker associations between others. CONCLUSION: Caudal PI seems to result in 2 diagnosis groupings: one that leads to ADAF and the other to iDDSP. Dynamic collapse of the margins of the epiglottis was a common finding in this study strongly associated with DLC, PI and ADAF. POTENTIAL CLINICAL RELEVANCE: The associations between certain dynamic diagnoses identified in this study seems best explained as due to local anatomic and functional weaknesses that influence other structures through the Bernouilli principle, rather than being due to a generalised neurological disorder.