A 56-year-old man with motor neuron disease (ALS) presents with progressive orthopnea and inability to sleep flat. Sitting spirometry shows FVC 65%. When spirometry is repeated in the supine position, the FVC drops to 38%. The finding marked **B** in the diagram—a supine loop with reduced VC >25%—is characteristic of which underlying pathophysiology?

Explanation

## Why option 1 is correct The finding marked **B**—a supine loop with reduced VC >25%—is the diagnostic hallmark of bilateral diaphragmatic paralysis. The diaphragm normally provides 60–70% of tidal volume at rest. When paralyzed bilaterally, the patient loses this primary inspiratory muscle and must rely entirely on accessory muscles (intercostals, scalenes). In supine position, abdominal contents push the flaccid diaphragm cephalad, worsening ventilation and causing a dramatic fall in FVC (>25% positional drop is diagnostic). This positional worsening with orthopnea is pathognomonic for diaphragmatic weakness. The patient's ALS is a neuromuscular cause of phrenic nerve dysfunction, leading to this characteristic spirometric pattern (ATS/ERS Statement on Respiratory Muscle Testing 2002; Murray & Nadel 7th ed). ## Why each distractor is wrong - **Option 2 (Pulmonary fibrosis)**: Restrictive lung disease causes uniform reduction in all lung volumes in both sitting and supine positions. There is NO positional variation (>25% drop) because the lungs themselves are stiff, not because of diaphragmatic weakness. Fibrosis does not explain orthopnea or paradoxical abdominal motion. - **Option 3 (Upper airway obstruction)**: Upper airway obstruction preferentially affects expiratory flow rates and is worse in supine position due to gravity-dependent airway collapse, but it does NOT cause a selective reduction in vital capacity or the characteristic orthopnea-with-paradoxical-breathing pattern of diaphragmatic paralysis. The flow-volume loop pattern differs fundamentally. - **Option 4 (Cardiac dysfunction/pulmonary edema)**: While cardiac pulmonary edema can cause orthopnea, it produces global reduction in all spirometric parameters in both positions and is accompanied by other cardiac signs (elevated JVP, S3 gallop, peripheral edema). It does NOT produce the specific >25% positional FVC drop or paradoxical abdominal motion diagnostic of diaphragmatic paralysis. **High-Yield:** Orthopnea + supine FVC drop >25% = diaphragmatic weakness until proven otherwise; check for paradoxical abdominal motion (Hoover sign) and measure maximal inspiratory pressure (MIP <60 cm H₂O abnormal). [cite: ATS/ERS Statement on Respiratory Muscle Testing 2002; Murray & Nadel 7th ed]