Restrictive Spirometry in Neuromuscular Weakness (ALS)

A 62-year-old man with a 14-month history of progressive limb weakness and dysarthria is diagnosed with ALS. He presents with exertional dyspnea, early-morning headaches, and orthopnea. Spirometry is performed in both sitting and supine positions. Seated FVC is 2.20 L (58% predicted) with FEV1/FVC ratio 0.84 (normal). Supine FVC drops to 1.55 L. The finding marked **B** in the diagram—a fall in FVC >20 percent from sitting to supine—is observed in this patient (30% drop). Which of the following best explains the clinical significance of this finding in ALS?