Longitudinal analysis of sniff nasal inspiratory pressure assessed using occluded and un-occluded measurement techniques in amyotrophic lateral sclerosis and primary lateral sclerosis.

Objective: Sniff nasal inspiratory pressure (SNIP) is a commonly used clinical measure of respiratory impairment in amyotrophic lateral sclerosis (ALS), which is used to guide the initiation of noninvasive ventilation (NIV). SNIP can be completed with either an occluded or an un-occluded contralateral nostril. The aim of this study was to compare occluded and un-occluded SNIP measurements and to examine the decline in occluded SNIP over time compared to the ALSFRS-R respiratory subscore. Methods: This was a prospective longitudinal study examining occluded and un-occluded SNIP scores in ALS and PLS patients recorded between 2001 and 2018. Bland and Altman graphs were plotted for occluded vs. un-occluded SNIP measurements taking account of the repeated measures nature of the data. Longitudinal models were constructed as linear mixed effects multi-level models with follow-up in ALS limited to 6 years. Results: SNIP measured with an occluded contralateral nostril was systematically higher than with an un-occluded nostril. SNIP measured using both methods declined non-linearly, particularly after 2-3 years. The best fit model for decline in occluded SNIP included a main effect and interaction between site of onset and time, with age and diagnostic delay as independent variables. This showed a linear decline in spinal onset with a floor effect in bulbar-onset ALS. Conclusion: SNIP measured with an occluded and un-occluded contralateral nostril is not interchangeable, which is relevant in interpreting thresholds for initiation of NIV. SNIP declines non-linearly, which is explained in spinal onset ALS by age and diagnostic delay, but an apparent floor effect remains in bulbar onset.