How does vocal dysfunction affect Pulmonary Function Tests (PFTs)?

Impact of Vocal Dysfunction on Pulmonary Function Tests (PFTs)

Vocal cord dysfunction (VCD) can significantly affect pulmonary function test results by causing abnormal flow-volume loop patterns, particularly in the inspiratory phase, which may lead to misdiagnosis if not properly identified and evaluated. 1

Characteristic PFT Findings in Vocal Cord Dysfunction

Flow-Volume Loop Abnormalities

• The most common spirometric finding in VCD is a plateau or flattening in the inspiratory limb of the flow-volume loop 1, 2
• Importantly, up to 81% of patients with confirmed VCD may also show a plateau in the expiratory curve 2
• These abnormalities are most likely to be observed during symptomatic periods or after provocation tests

Specific Spirometric Parameters

• The ratio of forced inspiratory flow at 25% to 75% of inspired volume (FIF25%/75%) may be predictive of VCD 3
• Traditional diagnostic criterion of MEF50%/MIF50% ratio >2.2 (mid-expiratory to mid-inspiratory flow) may not be reliable, as it was present in only 36% of confirmed VCD cases in one study 2

Diagnostic Challenges

Limited Predictive Value

• Despite characteristic patterns, flow-volume loop abnormalities alone have poor diagnostic performance for VCD 3
• Normal flow-volume loops do not rule out VCD and should not prevent further evaluation when clinically suspected 3
• Agreement among pulmonologists about which flow-volume loops suggest VCD may be moderate to good (kappa 0.55-0.76), but these assessments often don't correlate with actual laryngoscopic findings 3

Recommendations for Testing

When VCD is suspected:

1. Perform complete spirometry with full FVC maneuvers rather than abbreviated expiratory maneuvers 1
2. Include forced inspiratory vital capacity (FIVC) maneuvers in the assessment 1
3. Evaluate for spontaneous or provocation-induced inspiratory flow limitation that appears as a plateau on the inspiratory curve 1
4. Consider exercise challenge testing to provoke symptoms, as VCD is frequently exercise-induced 4
5. Methacholine challenge testing may reveal abnormal flow-volume loops in VCD patients (60% developed abnormal patterns after methacholine in one study) 4

Clinical Context

• VCD is often misdiagnosed as asthma due to similar presenting symptoms 5, 4
• Female sex and obesity (BMI >30 kg/m²) are independent predictors of VCD 3
• 15% of patients with exertional dyspnea may have VCD 4
• VCD can coexist with asthma, complicating diagnosis and management 2

Definitive Diagnosis

• Direct laryngoscopy during symptoms remains the gold standard for VCD diagnosis 3, 5
• Newer techniques such as Multidimensional Voice Program (MDVP) analysis may help identify VCD by detecting abnormal voice patterns, particularly in the soft phonation index (SPI) and variation in fundamental frequency (vFo) 5

Practical Implications

• PFT technicians should be trained to recognize potential VCD patterns
• When VCD is suspected, document any inspiratory stridor during testing
• Ensure quality control of spirometric maneuvers, as poor technique can mimic VCD patterns
• Consider VCD in patients with treatment-resistant "asthma" or disproportionate dyspnea

Remember that while PFTs can provide valuable clues to the presence of VCD, they should be interpreted in the context of the complete clinical picture, and normal PFT results should not preclude further evaluation when VCD is clinically suspected.