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Compendium July 2012 (Vol 34, No 7)

Brachycephalic Airway Syndrome: Pathophysiology and Diagnosis

by Dena L. Lodato, DVM, Cheryl S. Hedlund, DVM, MS, DACVS

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    Abstract

    Brachycephalic airway syndrome (BAS) is a group of abnormalities that result in upper airway obstruction. Primary malformations include stenotic nares, elongated soft palate, and hypoplastic trachea, which cause an increase in negative pressure within the upper airways that can eventually lead to secondary abnormalities such as everted laryngeal saccules, everted tonsils, and laryngeal and tracheal collapse. Abnormal nasopharyngeal turbinates are also encountered, but have not been classified as primary or secondary. BAS is readily diagnosed, and quality of life is improved with appropriate medical and/or surgical management. This is the first of two articles on BAS; the second article will be published in the August 2012 issue.

    In brachycephalic breeds, the lower jaw is of normal length, but the upper jaw is much shorter in proportion to body size than normal.1–3 This is a result of early ankylosis of the basicranial epiphyseal cartilage, leading to a shortened longitudinal skull axis.1–3 All of the associated oral tissues are still present in the shortened maxilla; however, they are compressed into a smaller area, resulting in a narrowed upper respiratory tract lumen and increased airway resistance.4 Brachycephalic airway syndrome (BAS) refers to a group of upper respiratory tract abnormalities that can include stenotic nares, an elongated soft palate, and tracheal hypoplasia.1,2,5–18 Abnormal nasopharyngeal turbinates, or nasal turbinates that extend caudally into the nasopharynx from the choanae, have also been observed in brachycephalic dogs and cats with upper airway disease.10  These abnormalities lead to secondary changes, including mucosal edema, everted laryngeal saccules, laryngeal collapse, and tonsil eversion.2,5,7–9,12–17,19 In affected dogs, airway resistance and inspiratory effort increase, leading to increased negative pressure and air turbulence, which cause mucosal inflammation and edema.2,5,7–9,12–17,19 Excessive negative pressure also everts laryngeal saccules and weakens laryngeal cartilages, further obstructing the laryngeal lumen.1–3,7,9,11,12,15,16,19–23 Other abnormalities that may be associated with BAS include a hyperplastic tongue and gastrointestinal (GI) abnormalities.1,2,4–6,8,12,24

    Anatomy

    The nares comprise the alae, or wings, as well as the dorsal lateral nasal cartilages, the ventral lateral nasal cartilages, and the accessory cartilages4 (FIGURE 1). The wings of the nares contain maxillary levator labii and nasolabial levator muscle fibers, allowing them to be maneuvered and widened.1 The nasal cavities house the dorsal, ventral, and ethmoid conchae, which form the common nasal, dorsal, middle, and ventral meatuses that house the nasal turbinates.1,10 These meatuses form the nasopharyngeal ducts and terminate as caudal choanae1 (FIGURE 2).

    Illustration

    Figure 1. Anatomy of the canine nares. Reprinted with permission from Fossum TW, Hedlund CS, Hulse DA, et al. Surgery of the upper airway system. Small Animal Surgery. 2nd ed. St. Louis, MO: Mosby; 2002:716-759.

    In nonbrachycephalic breeds, the soft palate normally extends just past the tip of the epiglottis,1,13 whereas in dogs with BAS, the elongated soft palate extends caudally past the tip of the epiglottis.1 The larynx is composed of muscle and cartilage and controls airflow into the trachea.1 The narrowest area in the upper airway is the rima glottis, which is created dorsally by paired arytenoid cartilages and ventrally by the vocal folds.1,4 The arytenoids comprise two corniculate processes dorsally and two cuneiform processes ventrally, two vocal processes at the ventral aspect where the vocal folds attach, and two muscular processes dorsolaterally at the caudal aspect of these cartilages.4 The laryngeal saccules are located between the vocal and vestibular folds.1,4 The vestibular folds then attach to the cuneiform process1,4 (FIGURE 2).

    Physiology and Pathophysiology

    The size and shape of the choanae determine the path of airflow through the nasal cavities.1 Air travels through the ventral and middle meatuses toward the nasopharynx during inspiration.1 A strong inspiratory effort allows the dorsal meatus to become involved and the olfactory epithelium to be stimulated.1 On expiration, air flows past the caudal choanae, through all the meatuses, and out of the respiratory tract.1 In nonbrachycephalic animals, airflow through the nasal cavities is responsible for 76.5% of total airflow resistance; the larynx contributes 4.5%, and the bronchi and bronchioli contribute 19%.1,15 These values do not differ significantly between inspiration and expiration.1 Nasal cavity resistance increases up to 80% when the airflow is increased.1 In brachycephalic breeds, resistance to airflow is affected by the narrowed airway, which causes an increase in the intraluminal pressure gradient during inspiration.1 When this pressure is excessively greater than the atmospheric pressure, the tissues become inflamed, tonsils and laryngeal saccules evert, and the cartilaginous larynx and trachea become compromised and can collapse, decreasing the tracheal luminal size and further increasing resistance to airflow.1,13 This increased resistance causes turbulent airflow, edema, and, thus, inspiratory noise.1,25

    Panting also increases airflow turbulence within the narrowed airway lumen, leading to inflammation and subsequent swelling of the airways.18 This, in turn, results in obstruction, distress, and potential overheating.18 For this reason, avoidance of heat, humidity, and excessive activity or excitement is important for brachycephalic breeds.18

    Key Facts

    • BAS is a group of primary malformations and secondary abnormalities that can result in upper airway obstruction.
    • This condition is easy to diagnose in brachycephalic breeds with the aid of physical examination and thoracic radiography findings, as well as upper airway endoscopy where available.

    GI lesions have been recognized in some dogs with BAS. At this time, the relationship between these lesions and the respiratory signs of BAS is unknown.24,26,27 It has been hypothesized that (1) an abnormal inspiratory effort results in low negative intrathoracic pressure, which could be sufficient to worsen or even cause a hiatal hernia and gastroesophageal reflux; (2) gastroesophageal disorders, including ptyalism, regurgitation, vomiting, and gastroesophageal reflux, exacerbate the respiratory signs through further impedance of the pharyngeal region, resulting in persistent inflammation; or (3) chronic respiratory depression leads to gastroesophageal reflux.24,26 One study26 evaluated 30 brachycephalic dogs with BAS and digestive disorders and found that 25 dogs (83%) had concurrent esophagitis, most commonly located in the distal third of the thoracic esophagus; some dogs, however, had esophagitis throughout the entire thoracic esophagus. Another study24 evaluated 73 brachycephalic dogs that presented with upper respiratory signs and found that 71 dogs (97.3%) had esophageal, gastric, or duodenal abnormalities. A third study27 found that 98% of examined brachycephalic dogs with BAS had histologic evidence of chronic gastritis. It has been concluded that there is a correlation between the severity of GI signs and respiratory signs among dogs with BAS.24

    History and Clinical Signs

    Typical clinical signs of BAS include stertor, stridor, inspiratory dyspnea, increased respiratory effort, exercise intolerance, vomiting and/or regurgitation, salivation, and, in severe cases, syncope.1,2,4–6,8,12,24 Another common clinical sign is hyperthermia due to the patient’s inability to cool itself efficiently, making it prone to heat stress.4,16,18 BAS is usually diagnosed in dogs aged 2 or 3 years; however, puppies younger than 6 months have been diagnosed with BAS and severe secondary laryngeal changes, demonstrating the need to evaluate immature brachycephalic dogs.11 Additionally, males are typically represented more than females.4,5,11

    Signs of respiratory distress may be mild, moderate, or severe. Animals that present in mild respiratory distress have pink mucous membranes, a normal attitude or restlessness when disturbed, and normal posture.16 Animals that present in moderate respiratory distress have pink mucous membranes, are anxious and restless when undisturbed, and occasionally demonstrate abducted elbows.16 Animals that present in severe respiratory distress may have pale, dusky, or cyanotic mucous membranes; can be disoriented, obtunded, or semicomatose; and have abducted elbows with a generalized use of accessory abdominal musculature.16

    Diagnosis

    BAS abnormalities are often tentatively diagnosed when the dog is first heard and examined. Definitive diagnosis is based on the history, physical examination, diagnostic imaging, blood gas analysis, and upper airway and GI endoscopy findings.4,5,16,18,22 Plethysmography and tidal flow volume loops have been used in research settings to further evaluate BAS and how obstruction affects respiration.3,16,17,21 Early diagnosis of BAS is beneficial because surgical intervention to improve airflow can minimize the progression of secondary changes.4,5,16,18,22

    Physical Examination Findings

    Stenotic nares are easily diagnosed by visualizing the malformed dorsolateral cartilages.6,7,16,21 Thoracic auscultation is impaired by referred upper respiratory noise, which obscures lower airway sounds and limits evaluation of the lungs, but mild to moderate tachycardia and tachypnea have been reported.10,13 Mucous membrane color can be assessed, along with general posture, respiratory pattern, and employment of accessory muscles.16

    Radiographic Findings

    Thoracic radiography is recommended to evaluate for lower airway disease, including tracheal hypoplasia, cardiac, and pulmonary abnormalities (i.e., aspiration pneumonia and noncardiogenic pulmonary edema).5,7,18,21 Noncardiogenic pulmonary edema and aspiration pneumonia should be treated preoperatively to reduce the anesthetic risk and postoperatively to aid in recovery.5,7,18,21 On a properly positioned lateral neck and thoracic radiograph, the distance from the thoracic inlet (TI) at the ventral aspect of the vertebral column at the midpoint of the first rib to the inner surface of the manubrium at its narrowest point is the TI measurement4,18,21 (FIGURE 3). The diameter of the tracheal lumen perpendicular to the long axis of the trachea where the TI line intersects the midpoint of the tracheal lumen provides the tracheal diameter (TD) measurement.4,21 When the TD:TI ratio on a lateral thoracic radiograph is <0.16, the trachea is considered to be hypoplastic.1,6,16,21 This measurement is not influenced by phase of respiration.21 The TD can also be assessed by measuring the diameter of the thoracic trachea at the third rib and the diameter of the third rib at this site.21 If the TD is three times the width of the third rib, it is considered normal.21

    Blood Gas Analysis

    Arterial blood gas analysis is important when evaluating BAS patients because it provides information that will direct treatment (i.e., oxygen supplementation).13,18 If an arterial blood gas sample cannot be obtained, a venous sample can be used to provide the pH and bicarbonate levels and partial pressure of carbon dioxide, or pulse oximetry can be employed as an indirect method of gaining information on oxygen saturation to determine the need for supplemental oxygen.13,18 Blood gas levels may be normal in stable patients.18 Animals may suffer from syncope and collapse when the oxygen saturation falls acutely below 80%; these patients benefit from immediate intervention with oxygen support.18

    Upper Airway Endoscopy

    Endoscopic visualization is the best means of identifying abnormalities associated with BAS. A complete and thorough examination of the upper airway is performed under anesthesia before surgery.7,13,18,21,22 An elongated soft palate, everted laryngeal saccules, laryngeal collapse, everted tonsils, and nasopharyngeal turbinates can all be diagnosed endoscopically.1,4,6,7,8,11,17,20,21 In one study,17 partial collapse of the left mainstem bronchus was a finding in 70% of dogs with BAS; this finding has not been previously described as a component of BAS.

    Gastrointestinal Endoscopy

    Performing esophagoscopy and gastroduodenoscopy may reveal concurrent abnormalities in the GI tract, even in dogs not exhibiting clinical signs of digestive disorders.24 Lesions and physiologic abnormalities have been found in the esophagus, stomach, and duodenum, including esophagitis, esophageal deviation, cardial atony, gastroesophageal reflux, axial hiatal hernia, gastric stasis, mucosal hyperplasia of the pylorus, pyloric stenosis, pyloric atony, inflammation of the corpus or antrum, and diffuse inflammation of the duodenum.24–26

    Conclusion

    BAS can be diagnosed and successfully managed with knowledge of the abnormalities and treatment options. Animals that have BAS can lead a more normal life with medical and surgical management.

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    References

    1. Koch DA, Arnold S, Hubler M, Montavon PM. Brachycephalic syndrome in dogs. Compend Contin Educ Pract Vet 2003;25(1):48-55.

    2. Reicks TW, Birchard SJ, Stephens JA. Surgical correction of brachycephalic syndrome in dogs: 62 cases (1991-2004). J Am Vet Med Assoc 2007;230(9):1324-1328.

    3. Robinson NE. Airway physiology. Vet Clin North Am Small Anim Pract 1992;22(5):1043-1064.

    4. Fossum TW, Hedlund CS, Hulse DA, et al. Surgery of the upper airway system. Small Animal Surgery. 2nded. St. Louis, MO: Mosby; 2002:716-759.

    5. Seim HB. Brachycephalic syndrome. Proc Atl Coast Vet Conf 2001.

    6. Dupre G. Brachycephalic syndrome: new knowledge, new treatments. Proc 33rd World Small Anim Vet Assoc 14th FECAVA 2008.

    7. Ellison GW. Alapexy: an alternative technique for repair of stenotic nares in dogs. J Am Anim Hosp Assoc 2004;40:484-489.

    8. Davidson EB, Davis MS, Campbell GA, et al. Evaluation of carbon dioxide laser and conventional incisional techniques for resection of soft palates in brachycephalic dogs. J Am Anim Hosp Assoc 2001;219(6):776-781.

    9. Huck JL, Stanley BJ, Hauptman JG. Technique and outcome of nares amputation (Trader’s technique) in immature shih tzus. J Am Anim Hosp Assoc 2008;44(2):82-85.

    10. Ginn JA, Kumar MS, McKiernan BC, Powers BE. Nasopharyngeal turbinates in brachycephalic dogs and cats. J Am Anim Hosp Assoc 2008;44(5):243-249.

    11. Pink JJ, Doyle RS, Hughes JM, et al. Laryngeal collapse in seven brachycephalic puppies. J Small Anim Pract 2006;47(3):131-135.

    12. Torrez CV, Hunt GB. Results of surgical correction of abnormalities associated with brachycephalic airway obstruction syndrome in dogs in Australia. J Small Anim Pract 2006;47(3):150-154.

    13. Hobson HP. Brachycephalic syndrome. Semin Vet Med Surg (Small Anim) 1995;10(2):109-114.

    14. Harvey CE. Soft palate resection in brachycephalic dogs. II. J Am Anim Hosp Assoc 1982;18:538-544.

    15. Hoffman AM. Airway physiology and clinical function testing. Vet Clin North Am Small Anim Pract 2007;37:829-843.

    16. Aron DN, Crowe DT. Upper airway obstruction: general principles and selected conditions in the dog and cat. Vet Clin North Am Small Anim Pract 1985;15(5):891-916.

    17. Bernaerts F, Talavera J, Leemans J, et al. Description of original endoscopic findings and respiratory functional assessment using barometric whole-body plethysmography in dogs suffering from brachycephalic airway obstruction syndrome. Vet J 2010;183(1):95-102.

    18. Hendricks JC. Brachycephalic airway syndrome. Vet Clin North Am Small Anim Pract 1992;22(5):1145-1153.

    19. Harvey CE. Everted laryngeal saccule surgery in brachycephalic dogs. III. J Am Anim Hosp Assoc 1982;18:545-547.

    20. Brdecka D, Rawlings C, Howerth E, et al. A histopathological comparison of two techniques for soft palate resection in normal dogs. J Am Anim Hosp Assoc 2007;43:39-44.

    21. Wykes PM. Brachycephalic airway obstructive syndrome. Probl Vet Med 1991;3(2):188-197.

    22. Harvey CE. Overview of results. IIIV. J Am Anim Hosp Assoc 1982;18:567-569.

    23. Harvey CE. Tracheal diameter: analysis of radiographic measurements in brachycephalic and nonbrachycephalic dogs. J Am Anim Hosp Assoc 1982;18:570-576.

    24. Poncet CM, Dupre GP, Freiche VG, et al. Prevalence of gastrointestinal tract lesions in 73 brachycephalic dogs with upper respiratory syndrome. J Small Anim Pract 2005;46(6):273-279.

    25. Rozanski E, Chan DL. Approach to the patient with respiratory distress. Vet Clin North Am Small Anim Pract 2005;35:307-317.

    26. Lecoindre P, Richard S. Digestive disorders associated with the chronic obstructive respiratory syndrome of brachycephalic dogs: 30 cases (1999-2001). Revue Méd Vét 2004;155(3):141-146.

    27. Poncet CM, Dupre GP, Freiche VG, Bouvy BM. Long-term results of upper respiratory syndrome surgery and gastrointestinal tract medical treatment in 51 brachycephalic dogs. J Small Anim Pract 2006;47(3):127-142.

    References »

    NEXT: Clinical Snapshot: Unusual Skull Mass in a Mixed-Breed Dog

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    didyouknow

    Did you know... Radiography is the initial diagnostic modality of choice for tracheal obstruction, and a lateral view is generally adequate if the patient cannot tolerate dorsal or ventral recumbencyRead More

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