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Wheezing is described as a constant noise lasting more than 0.25 s that is caused by the oscillation of the opposing air way walls. It may occur during inspiration or expiration and may be caused by narrowing of the airway. Stridor is a high-frequency sound caused by air turbulence inside the upper airway. Though wheezing and stridor are clinically different, they are often mistaken.
One of the causes of stridor is laryngotracheal stenosis resulting from an abnormal narrowing along the glottis and the carina. Before 1960 the condition was a congenital eti ology, but with the wide use of orotracheal intubation, its incidence rose from 0.9%1 to 24.5%.2 The most frequent benign cause is tracheal intubation and the most frequent malignant cause is squamous cell carcinoma. The most widely accepted stenosis-associated factor following endotracheal intubation is the duration of the intubation. Not with standing the exist ing evidence about its relevance, the occurrence of cases after short endotracheal intubations suggests that laryngeal lesions are multifactorial, involving patient as well as intubation-associated factors.3
Furthermore, wheezing secondary to exercise-induced bronchoconstriction is the primary indicator of asthma exa cerbations and the last to be resolved.4 In case of an asthmatic patient, a differential diagnosis shall be done between inspiratory symptoms during exercise, typically occurring at the end of the exercise session, and exercise-induced asthma, with onset of symptoms between 3 and 15 min after exercise. Finally, the possibility of exercise-induced laryngeal obstruc tion, associated to adduction of the vocal folds as a secondary phenomenon shall also be kept in mind.5
When studying asthma, the flow-volume curve evidences highly suggestive changes, although it is a rare cause of dysp nea and wheezing during exertion. However, without a clinical suspicion, the symptoms may be confused for asthma. Delays in making the diagnosis may result in increased morbidity.6
Lung function tests are useful for diagnosing unexplained respiratory symptoms and for monitoring patients with known respiratory disease. Generally speaking, patients over 5 years of age are considered able to cooperate with the proper performance of the procedure. A FVC ratio ‹70% in adults and ‹80% in children suggests obstructive pathology and the bronchodilator response must be evaluated. If the FVC is more than 12% between 5 and 18 years of age, or more than 12% and >200 ml in adults, a reversible obstruction suggestive of asthma may be considered. However, if these premises are nor met, an irreversible obstruction should be suspected and differential diagnoses must be evaluated. When the flow -volume curve is flat, the most frequent cause is submaximal effort, but then a central or upper airway obstruction must be considered8,9
The Empey10 index has been classically used to assess airway obstruction using spirometry. It measures the ratio between VEF1 (ml)and PEFR(l/min); if the ratio is over10, this is considered evidence of laryngotracheal stenosis.
Properly diagnosing upper or lower respiratory disease is vitally important in the pre-surgical setting, to prevent unex pected difficult airway cases and getting the surgical team ready to optimize patient outcomes.
Clinical case
This case is a 13-year old female patient with a history of patent ductus arteriosus surgically corrected 7 days after birth. The patient did not require orotracheal intubation following the procedure. Additionally, the patient has a his tory of asthma and is managed with inhaled steroids, with persistent symptoms when she exercises and follow-up by pulmonology. She came to the ENT department of the Fun dación Hospital de la Misericordia because of hearing loss and repeated episodes of otitis media during her childhood. A sub-total right tympanic perforation was identified, and the patient was scheduled for type 1 tympanoplasty as an ambulatory procedure. During the pre-anesthesia evaluation, the patient was classified as ASA II because of her history of asthma and continued management was recommended. Following the anesthetic induction, difficulty for intubation with a #6.0 cuffed endotracheal tube was reported, with evi dence of subglottic concentric stenosis during laryngoscopy; the anesthetist then switched to a #4.0 cuffed tube. Since the patient was admitted to a high-complexity hospital, she simultaneously underwent laryngeal micro endoscopy and bronchoscopy, evidencing a Cotton-Myer III11 concentric sub-glottic stenosis affecting 70% of the subglottic lumen (Fig. 1), that required laryngeal dilatation and mitomycin and triamcinolone infiltration as per the institutional protocol (Fig. 2).The pre-surgical spirometry was studied, evidencing a curve with discrete flattening of the expiratory peak, reporting all param eters within the normal limits, except for a 60% PEF, with poor response to beta2-agonists (Fig. 3).
Discussion
The British Thoracic Society12 recommends that unless an alternate diagnosis is more likely, patients with symptoms suggestive of asthma shall be initiated on medical manage ment and further testing is advised if the response is poor.
When clinically evaluating the differential diagnoses, the upper airway obstruction usually involves stridor, contrary to asthma that presents with wheezing. However, there may be some cases that are impossible to differentiate and in the pres ence of dyspnea and exertion symptoms, maybe mistaken for asthma. The upper airway obstruction may be due to altered mobility of the vocal folds (paralysis, paradoxical movement), congenital abnormalities such as laryngeal membranes or vascular compressions, subglottic stenosis for multiple rea sons, masses or pressure at the subglottic or tracheal level. Keep in mind that gastroesophageal reflux is one of the con ditions most frequently associated with a poor diagnosis of asthma.
Whilst the differential diagnosis may be relatively easy using the flow-volume curve, the Empey11 index and the expi ratory disproportion index13 (FEV1[L]/PEFR[L-s] x100) are also helpful. The latter provides a high specificity and sensitivity of >50 for diagnosing laryngotracheal stenosis.
This is a case of asthma refractory to management, with absence of the typical loop curve in spirometry suggestive of subglottic stenosis. The spirometry values reported as normal are probably due to the enormous muscle effort the patient was able to produce as a result of her body size, reaching close to normal levels, affecting only the PEF and hence hindering an accurate diagnosis. The estimated expiratory disproportioned index was 50.4 in this case, indicating a high suspicion of laryngotracheal stenosis. Despite the difficulty in interpre tation, the spirometry findings could have been suggestive of the diagnosis, leading to additional airway testing and hence preventing the potential complications of a difficult airway and the emergent surgical management. We believe it is crit ical that the surgical team as a whole makes an accurate interpretation of pulmonary functional tests during the pre-anesthesia and pre-surgical evaluation, in all patients with a diagnosis of asthma.
Conclusions
A delayed diagnosis of laryngotracheal stenosis may affect up to 10% of patients, increasing the respiratory morbidity and the risk of respiratory failure.14 Notwithstanding the fact that this is a low frequency disease among the general popula tion, this case reminds us of the need to be suspicious in case of a diagnosis of asthma refractory to management and it is the duty of the surgical team as a whole, to be aware of the interpretation of the spirometry for early identification of any laryngotracheal stenosis cases and to prevent any anesthe sia or surgery-associated complications. In accordance with the recommendations of the British Thoracic Society,12 the authors believe that any patient with a questionable diag nosis of asthma, particularly in the case of exercise-induced asthma, and with a history of previous intubations, requires an evaluation of vocal folds' movement and subglottic patency. A judicious preoperative evaluation must comprise this aspect. In this particular case, both pathologies were simultaneously treated, since the patient was admitted to a high complexity pediatric hospital; however, in a different setting, probably the outcome would have been less favorable.
Ethical disclosures
Protection of human and animal subjects. The authors declare that no experiments were performed on humans or animals for this study.
Confidentiality of data. The authors declare that no patient data appear in this article.
Right to privacy and informed consent. The authors declare that no patient data appear in this article.
