What is the most likely diagnosis for a patient with normal Forced Expiratory Volume in 1 second (FEV1) and Forced Vital Capacity (FVC) but a flattened inspiratory loop?

Variable Extrathoracic Upper Airway Obstruction

The most likely diagnosis is variable extrathoracic upper airway obstruction, with vocal cord dysfunction being the most common etiology in this clinical scenario. 1

Key Diagnostic Features

The combination of normal FEV1 and FVC with a flattened inspiratory loop is pathognomonic for variable extrathoracic upper airway obstruction. 1 This pattern occurs because:

• The obstruction worsens during inspiration due to negative intraluminal pressure collapsing the extrathoracic airway 1
• Expiratory parameters remain normal because positive intraluminal pressure during expiration keeps the airway open 1
• The European Respiratory Society confirms that a repeatable plateau of forced inspiratory flow with preserved expiratory flow indicates variable extrathoracic central or upper airway obstruction 1

Confirmatory Diagnostic Approach

Direct laryngoscopy is the definitive diagnostic test for evaluating this condition and should be performed to identify the specific etiology. 1 This allows direct visualization of:

• Vocal cord dysfunction (most common cause) 2
• Paradoxical vocal cord motion during inspiration 1, 3
• Laryngeal pathology 1
• Dynamic airway collapse 1
• Structural abnormalities 1

Supporting Spirometric Ratios

Additional spirometric indices that support this diagnosis include:

• FEV1/PEF ratio >8-10 ml/L/min suggests central or upper airway obstruction 1, 3, 4
• MEF50/MIF50 ratio >4 is highly suggestive of variable extrathoracic obstruction, particularly from vocal cord pathology 3, 4
• In bilateral vocal cord paralysis specifically, an FEV1/PEF ratio >10 ml/L/min combined with MEF50/MIF50 >4 is characteristic 3

Common Etiologies to Consider

In the appropriate clinical context, consider:

• Vocal cord dysfunction - the most frequently identified diagnosis when abnormal inspiratory curves are properly evaluated 2
• Bilateral vocal cord paralysis - characterized by paradoxical vocal cord behavior during forced respiration 3
• Functional laryngeal obstruction 1
• Subglottic stenosis 1
• Laryngeal pathology 1

Critical Pitfalls to Avoid

• Do not diagnose asthma based on dyspnea alone, as the normal expiratory findings and FEV1/FVC ratio effectively rule out lower airway obstruction 1
• Ensure maximal patient effort during spirometry, as submaximal inspiratory effort can create a pseudo-plateau that mimics true obstruction 1
• Review multiple flow-volume loops - if one inspiratory curve shows abnormality, examine all curves from that testing session, as consistently abnormal curves warrant both anatomical and functional evaluation 2
• Bronchodilator testing is not indicated in this scenario, as the problem is anatomic or functional upper airway obstruction, not bronchospasm 1

Clinical Context

Studies show that abnormal inspiratory curves occur in approximately 4.6% of pulmonary function tests, yet evaluation of the underlying etiology is undertaken in only 17% of all patients and 30% of those with consistently abnormal curves. 2 This represents a significant missed diagnostic opportunity, as a specific etiology can be identified in 52% of properly evaluated patients. 2