An increasingly breathless patient.

Sir,

We read the recent article by Karkhanis and colleagues entitled “Flow volume loops as a diagnostic marker” with great interest.[1] The authors describe the case of a young girl who developed exertional dyspnea after a period of mechanical ventilation. Flow-volume loops demonstrated a “box-pattern” suggesting fixed upper airways obstruction. Subsequently, the diagnosis of post-intubation tracheal stenosis was made. We would like to present another case of fixed upper airway obstruction, of a different etiology, reinforcing the importance of the use of simple flow-volume loops.

A 56-year-old male attended a respiratory clinic appointment for follow-up of a suspected lower respiratory tract infection (LRTI) 6 weeks previously for which he had been admitted to our Medical Assessment Unit. The patient had initially been admitted with complaints of breathlessness and dry cough for the preceding 2 weeks associated with a degree of chest tightness. D-dimer assay performed on his first admission was negative and his chest X-ray [Figure 1] was unremarkable. The presumptive diagnosis of LRTI was made in view of a raised C-reactive protein of 41 mg/L and the patient discharged on antibiotics. Since that time, the patient's symptoms had not improved, and had in fact gradually worsened when reviewed in the respiratory clinic 6 weeks later. The patient described marked breathlessness that had been present for 2 or 3 months only; he had had no symptoms before this time. He did not report any expectoration, hemoptysis, orthopnea, ankle edema or any systemic symptoms. A small goitre was apparent, which the patient reported having noticed for the previous few weeks, but no stridor was audible at this time. Otherwise, the general physical and systemic examination was unremarkable. A simple spirometry performed in the clinic demonstrated an FEV1/FVC ratio of 64% and an FEV1 that was 73% predicted. The peak expiratory flow rate was 290 L/min (56% predicted). The disproportionate decrease in peak expiratory flow as compared with the FEV1 in conjunction with the clinical history raised a suspicion of upper airway obstruction. Flow-volume loops and a computed tomography (CT) scan were organized. The flow-volume loop demonstrated flattened inspiratory and expiratory curves consistent with fixed extrathoracic airway obstruction. Empey's index and midvital capacity ratios were also in keeping [Figure 2 and Table 1]. The CT scan demonstrated marked compression of the trachea in the lower neck due to an enlarged thyroid. At its narrowest point, the trachea measured just 0.6 × 1.2 cm in diameter [Figure 3]. A left thyroid lobectomy was subsequently performed, which resulted in prompt amelioration of his breathlessness. Histology subsequently was consistent with a diagnosis of follicular carcinoma of the thyroid. His case was then referred for discussion at the multi-disciplinary team meeting for further management.

Figure 1

The patient's normal chest X-ray

Figure 2

The patient's flow-volume loop. Solid line - patient's results. Dashed line - reference. Initially normal expiration becomes lung volume independent until the terminal portion and inspiration is obstructed in a similar way as expiration, indicative of a fixed extrathoracic obstruction

Table 1

Spirometry results suggestive of fixed extrathoracic airflow obstruction

Figure 3

(a) Sagittal and (b) axial computed tomography images showing the enlarged thyroid compressing the trachea (white arrows)

Extrathoracic airway obstruction is an uncommon but not a rare cause of breathlessness. Some of the more prevalent causes of upper airway obstruction that present with breathlessness include post-traumatic strictures, bilateral vocal cord paralysis, chronic inflammatory foci and goitres.[23] Upper airways obstruction is a well-recognized complication of thyroid disease.[2] In this context, evidence of airway obstruction on flow-volume loops may well be an incidental finding with no clinical implications. Twenty-five percent of patients with large goitres report the symptom of breathlessness.[4] Case series have reported that up to one-third of patients with goitres have upper airway obstruction and that most improve post-thyroidectomy.[2]

The flow-volume loop is a simple non-invasive test and is the best method of evaluating upper airways obstruction and distinguishing it from obstructive pulmonary disease.[4] As discussed by Karkhanis et al., the Empey's index and ratio of maximal expiratory flow at 50% of vital capacity and maximal inspiratory flow at 50% of the vital capacity (MEF50/MIF50) are also useful indices to help make this diagnosis. A fixed upper airway obstruction will limit maximal flow to the same level regardless of lung volumes.[5] The flow will become lung volume independent apart from the initial period of inspiration and end of expiration, where lower lung volumes exert less traction on the small airways allowing the smaller airways to narrow and become the rate-limiting factor in determining flow.[5] This gives a clipped or “square box” appearance of the flow-volume loop with flow limited equally in both inspiration and expiration. As discussed by Karkhanis et al., variations in this clipping can give a clue as to the cause of upper airway obstruction.[1] In addition, flow-volume loops can provide an estimate of the size of the patent airway by changing the diameter of the mouth piece used until it becomes the rate-limiting component.

However, it should be noted that clinical history predicts obstruction poorly, as exemplified by this case. In one study of patients with large goiters, 29% of patients with goitre symptoms were found to have no airway obstruction on flow-volume loops; conversely, obstruction was demonstrated in 21% of patients with no symptoms.[6] Furthermore, clinical assessment of thyroid size demonstrates no relation to likelihood of airway obstruction.[4] The retrosternal extent of a goitre is not amenable to clinical examination. Imaging techniques such as a CT scan will both confirm and quantify the presence and extent of airway compromise. If the clinical suspicion is not high, appropriate investigations may not be requested and the diagnosis may remain elusive. Upper airway obstruction presenting as breathlessness is uncommon but should be borne in mind by physicians as the diagnosis can be effectively made with the use of flow-volume loops and imaging and symptoms easily ameliorated with correct treatment.