Effect of nostril occlusion and mouth sealing in the measurement of sniff nasal inspiratory pressure

Sniff nasal inspiratory pressure (SNIP) is used to assess respiratory muscle strength in neuromuscular diseases like amyotrophic lateral sclerosis (ALS). The effect of contralateral nostril occlusion and mouth sealing on SNIP measurement are unclear. 81 participants were included (16 healthy, 39 patients with limb-onset ALS and 26 patients with bulbar-onset ALS). SNIP was obtained with combinations of mouth open/sealed and contralateral nostril open/occluded. Occluding the contralateral nostril (with mouth closed) increased SNIP by 12 cmH₂O (95% CI 4, 20; p=0.003) in the healthy participants, by 9 cmH₂O (95% CI 5, 12; p<0.001) in the limb-onset cohort and by 10 cmH₂O (95% CI 5, 14; p<0.001) in the bulbar-onset cohort. Opening the mouth decreased SNIP by 19 cmH₂O (95% CI 5, 34; p<0.009) in healthy participants, by 8 cmH₂O (95% CI 4, 13; p<0.001) in the limb-onset cohort and by 13 cmH₂O (95% CI 7, 19; p<0.001) in the bulbar-onset cohort. With contralateral nostril occlusion, 11% fewer individuals would have qualified for non-invasive ventilation. In conclusion, contralateral nostril occlusion increased SNIP compared with standard technique, likely reflecting true strength. Opening the mouth reduced SNIP, emphasising the need for good mouth sealing. Documenting SNIP technique is important for longitudinal assessments and clinical decision-making.