What can cause complex restriction on a Pulmonary Function Test (PFT)?

Causes of Complex Restriction on Pulmonary Function Tests

Complex restriction on a PFT is characterized by disproportionate reduction in FVC compared to TLC, where the TLC-FVC percent predicted difference exceeds 10%. 1 This pattern differs from simple restriction where TLC, FVC, and FEV1 are reduced to a similar degree.

Primary Causes of Complex Restriction

Neuromuscular Disorders

• Respiratory muscle weakness 1
• Diaphragmatic dysfunction 1
• Myopathies and neurodegenerative diseases

Obesity and Body Habitus

• BMI > 40 kg/m² or < 18.5 kg/m² 1
• Chest wall restriction

Airway Disorders with Air Trapping

• Bronchiectasis 1
• Occult airflow obstruction 1
• Cystic fibrosis with airway involvement 2
• Small airway disease

Vascular and Circulatory Issues

• Pulmonary hypertension 1
• CT mosaic attenuation pattern 1

Combined Disorders

• Combined pulmonary fibrosis and emphysema (CPFE)
  • Upper lobe emphysema with lower lobe fibrosis 3, 4
  • Typically shows preserved lung volumes despite severe pathology 4
  • Severely reduced DLCO despite near-normal spirometry 5

Specific Disease Entities

• Acid sphingomyelinase deficiency (Niemann-Pick disease) 6
• Osteogenesis imperfecta with thoracic cage restriction 6
• Sarcoidosis with both restrictive and obstructive components 7
• Hypersensitivity pneumonitis 7
• Lymphangioleiomyomatosis 7
• Pulmonary Langerhans cell histiocytosis 7

Diagnostic Considerations

PFT Pattern Recognition

• TLC below lower limit of normal (confirming restriction) 6
• FVC percent predicted disproportionately reduced compared to TLC percent predicted 1
• TLC-FVC difference > 10% defines complex restriction 1
• Classes of severity based on TLC-FVC discrepancy:
  • Class 1: 10-15% difference
  • Class 2: 15-20% difference
  • Class 3: 20-25% difference
  • Class 4: >25% difference 1

Additional Testing

• Body plethysmography to accurately measure TLC and RV/TLC ratio 7
• DLCO measurement (often severely reduced in CPFE) 4
• Maximum inspiratory and expiratory pressures to evaluate respiratory muscle strength 6
• Peak cough flow to assess cough effectiveness 6
• Chest imaging (HRCT) to evaluate for:
  • Parenchymal lung disease
  • Bronchiectasis
  • Emphysema
  • Interstitial abnormalities 7

Clinical Implications

Complex restriction is associated with:

• Impaired lung emptying 1
• Air trapping (elevated RV/TLC ratio) 3
• Exercise limitation and desaturation 3
• Potentially worse prognosis in certain conditions (e.g., CPFE with elevated pulmonary pressures) 3

Common Pitfalls in Diagnosis

• Misinterpreting complex restriction as simple restriction or mixed defect
• Failing to recognize CPFE due to preserved spirometry 5
• Relying solely on spirometry without lung volumes or DLCO measurement
• Not considering neuromuscular weakness in patients with normal chest imaging
• Overlooking the possibility of occult obstruction contributing to restriction

Complex restriction requires comprehensive evaluation beyond basic spirometry to identify the underlying cause and guide appropriate management.