Limitations of sniff nasal pressure in patients with severe neuromuscular weakness

Background: Inspiratory muscle strength in patients with neuromuscular disorders can be assessed using sniff inspiratory nasal pressure (Pn(sn)) and maximum inspiratory mouth pressure (PImax). However, the relative merits of Pn(sn) against PImax are not known in patients with severe neuromuscular disease. Objective: To investigate whether severity of disease modifies the relation between Pn(sn) and PImax. Methods: Vital capacity (VC), Pn(sn), and PImax were measured in 258 patients with neuromuscular disorders. Results: Data were analysed from 241 patients, 17 being unable to perform PImax or Pn(sn) manoeuvres. The correlation between Pn(sn) and PImax was +0.94 (p<0.0001), with a mean (SD) difference between Pn(sn) and PImax of 24.8 (21.2) cm H2O ( the limits of agreement were 37.6 and -47.2 cm H2O). VC (% predicted) was positively correlated with Pn(sn)/PImax (r=+0.86; p<0.0001), with a lower Pn(sn)/PImax value in patients with a VC<40% of predicted than in those with a VC>40% (0.80 (0.35) v 1.04 (0.41); p<0.0001). Conclusions: PImax is greater than Pn(sn) in patients with a severe restrictive ventilatory defect caused by neuromuscular disease. Pn(sn) may not accurately reflect inspiratory muscle strength in such patients and it is thus advisable to use both tests.