Diaphragmatic Paralysis — Restrictive Spirometry & Sniff Test MCQ — NEET PG Practice Question | NEETPGAI
Diaphragmatic Paralysis — Restrictive Spirometry & Sniff Test
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stethoscope Medicine
A 45-year-old man with a history of cardiac surgery 6 months ago presents with progressive dyspnea and orthopnea. Spirometry shows reduced FVC (55% predicted) and reduced TLC, with a normal FEV1/FVC ratio of 0.82. When supine FVC is measured, it drops by 42% compared to upright FVC. The structure marked **A** in the diagram represents this restrictive ventilatory pattern. Which of the following clinical findings would MOST strongly support bilateral diaphragmatic paralysis as the underlying cause?
A. Reduced DLCO out of proportion to lung volumes, with normal diaphragmatic movement on sniff test and elevated maximal expiratory pressure
B. Paradoxical cephalad movement of both hemidiaphragms on sniff test fluoroscopy, with markedly reduced maximal inspiratory pressure and normal DLCO
Normal maximal inspiratory pressure with preserved diaphragmatic excursion bilaterally, and elevated DLCO corrected for alveolar volume
C.
D. Increased residual volume with reduced FEV1/FVC ratio, and unilateral paradoxical diaphragmatic movement on sniff test
Explanation
Why "Paradoxical cephalad movement of both hemidiaphragms on sniff test fluoroscopy, with markedly reduced maximal inspiratory pressure and normal DLCO" is right
Bilateral diaphragmatic paralysis produces the restrictive pattern marked A — reduced FVC and TLC with normal/elevated FEV1/FVC ratio and a pathognomonic ≥30% supine FVC drop. The sniff test is the gold-standard bedside confirmatory test: in bilateral paralysis, BOTH hemidiaphragms move paradoxically CEPHALAD (sucked upward by negative intrathoracic pressure generated by accessory muscles alone), whereas normal diaphragms move briskly caudad. Maximal inspiratory pressure (MIP) is markedly reduced (<30 cm H₂O) because the diaphragm normally generates 60–75% of resting tidal volume. DLCO remains NORMAL (corrected for alveolar volume) because the lung parenchyma is intact — this neuromuscular restriction differs fundamentally from parenchymal interstitial lung disease. This constellation is pathognomonic for bilateral diaphragm paralysis (McCool FD, Tzelepis GE, NEJM 2012).
Why each distractor is wrong
Reduced DLCO out of proportion to lung volumes, with normal diaphragmatic movement on sniff test and elevated maximal expiratory pressure: DLCO reduction out of proportion to volumes indicates parenchymal interstitial lung disease (e.g., IPF, sarcoidosis), not neuromuscular restriction. Normal diaphragmatic movement excludes paralysis. This pattern does not match the anchor.
Increased residual volume with reduced FEV1/FVC ratio, and unilateral paradoxical diaphragmatic movement on sniff test: Increased RV and reduced FEV1/FVC ratio indicate obstructive or mixed obstructive-restrictive disease (patterns B or C), not the pure restrictive pattern A. Unilateral paradoxical movement indicates unilateral paralysis, not bilateral.
Normal maximal inspiratory pressure with preserved diaphragmatic excursion bilaterally, and elevated DLCO corrected for alveolar volume: Normal MIP and preserved bilateral diaphragmatic movement exclude diaphragmatic paralysis entirely. This would be seen in healthy individuals or in parenchymal restriction where the diaphragm functions normally.
High-YieldNEET PG
Bilateral diaphragm paralysis = restrictive pattern + ≥30% supine FVC drop + bilateral paradoxical sniff test + reduced MIP + normal DLCO. The supine drop is pathognomonic because abdominal contents push the paralyzed diaphragm cephalad, and accessory muscles work less effectively supine.
