Optimal method for assessment of respiratory muscle strength in neuromuscular disorders using sniff nasal inspiratory pressure (SNIP)

BACKGROUND: The ability to accurately determine respiratory muscle strength is vitally important in patients with neuromuscular disorders (NMD). Sniff nasal inspiratory pressure (SNIP), a test of inspiratory muscle strength, is easier to perform for many NMD patients than the more commonly used determination of maximum inspiratory pressure measured at the mouth (MIP). However, due to an inconsistent approach in the literature, the optimal technique to perform the SNIP maneuver is unclear. Therefore, we systematically evaluated the impact of performing the maneuver with nostril contralateral to the pressure-sensing probe open (SNIP(OP)) versus closed (SNIP(CL)), on determination of inspiratory muscle strength in NMD patients as well as control subjects with normal respiratory muscle function. METHODS: NMD patients (n = 52) and control subjects without respiratory dysfunction (n = 52) were studied. SNIP(OP,) SNIP(CL), and MIP were measured during the same session and compared using ANOVA. Agreement and bias were assessed with intraclass correlation coefficients (ICC) and Bland-Altman plots. RESULTS: Mean MIP values were 58.2 and 94.0 cmH2O in NMD and control subjects, respectively (p<0.001). SNIP(CL) was greater than SNIP(OP) in NMD (51.9 ±31.0 vs. 36.9 ±25.4 cmH(2)O; p<0.001) as well as in controls (89.2 ±28.1 vs. 69.2 ±29.2 cmH(2)O; p<0.001). In both populations, the ICC between MIP and SNIP(CL) (NMD = 0.78, controls = 0.35) was higher than for MIP and SNIP(OP) (NMD = 0.53, controls = 0.06). In addition, SNIP(CL) was more often able to exclude inspiratory muscle weakness than SNIP(OP). CONCLUSIONS: SNIP(CL) values are systematically higher than SNIP(OP) in both normal subjects and NMD patients. Therefore, SNIP(CL) is a useful complementary test for ruling out inspiratory muscle weakness in individuals with low MIP values.