Heterogeneity of predictors of nocturnal hypoventilation in amyotrophic lateral sclerosis

Objective: In amyotrophic lateral sclerosis (ALS), early recognition of nocturnal hypoventilation (NH) is essential to start noninvasive ventilation (NIV), but nocturnal transcutaneous PCO (PtcCO) is difficult to monitor. Usefulness of respiratory and muscular function test in the prediction of NH has been explored without distinguishing among ALS phenotypes. We evaluated cross-sectional relationships between functional tests and nocturnal PCO, and the best predictors of NH, separately in patients with spinal and bulbar onset of ALS. Methods: ALS patients candidate to NIV were recruited. Diurnal respiratory and muscular function tests and nocturnal polysomnography with PtcCO monitoring were performed. NH was defined as peak PtcCO >49 mm Hg. Results: Thirty-six patients with spinal and 11 with bulbar onset ALS were included. Nocturnal oxygen saturation and PtcCO, and proportion of subjects with NH were similar in each group (spinal: 50%; bulbar: 45.5%). Significant differences between groups were found in forced vital capacity (p = 0.03), maximal inspiratory pressure (p = 0.01) and sniff nasal inspiratory pressure (SNIP) (p = 0.007), but not in diurnal arterial blood gases. In the spinal group, SNIP and Base Excess (BE) independently predicted nocturnal PtcCO (R 0.59, p < 0.0001). In the bulbar group only SNIP was correlated to PtcCO, but it varied little in relationship to PtcCO changes. Conclusions: Respiratory and muscle function parameters are differently related to NH in ALS patients with spinal and bulbar presentation. SNIP and BE may be helpful to reveal NH in spinal patients, while in bulbar patients no respiratory or muscle function tests may reliably predict NH.