Transcutaneous Carbon Dioxide Monitoring in Amyotrophic Lateral Sclerosis: A Scoping Review.

BACKGROUND

Respiratory failure is the leading cause of death in amyotrophic lateral sclerosis (ALS). Conventional pulmonary function tests are effort-dependent and may fail to detect early nocturnal hypoventilation. Transcutaneous carbon dioxide (PtcCO2) measurement is a non-invasive method for assessing ventilation, but its role in ALS remains unclear. We performed a scoping review to map the current evidence.

METHODS

PubMed, Web of Science, Scopus, and Google Scholar were searched from inception to April 2026. Original studies evaluating PtcCO2 monitoring in ALS were included. Due to heterogeneity in study designs, populations, and outcomes, a narrative synthesis was performed, categorized by study characteristics, measurement protocols, and clinical endpoints.

RESULTS

Twenty-one studies were included. Most were observational and conducted in tertiary centres. Twelve studies evaluated PtcCO2 as a surrogate marker for nocturnal hypoventilation, mainly to guide initiation or titration of non-invasive ventilation. Definitions of hypoventilation and reported PtcCO2 metrics varied widely. Only three studies evaluated PtcCO2 in relation to respiratory impairment or hypercapnia, but none reported formal measures of diagnostic accuracy. Evidence regarding the prognostic value was limited and inconsistent. Three studies showed that nocturnal hypercapnia preceded abnormalities in spirometry or daytime hypercapnia.

CONCLUSIONS

PtcCO2 monitoring shows promise as a non-invasive method for assessing nocturnal ventilatory abnormalities in ALS, including nocturnal hypoventilation. However, due to methodological heterogeneity, its diagnostic and prognostic roles remain uncertain. Current evidence is insufficient to support PtcCO2 as a standalone criterion for NIV initiation. Large-scale prospective studies with standardized protocols are essential to delineate its clinical utility.