Uncoupling of apnea-hypopnea index and oxygen desaturation in als: characterization and diagnostic implications of an "AHI-SpO₂ dissociation" phenotype.

backgroundIn Amyotrophic Lateral Sclerosis (ALS), sleep-disordered breathing (SDB) is primarily driven by neuromuscular hypoventilation. Traditional metrics like the Apnea-Hypopnea Index (AHI) may therefore lack sensitivity. This study aimed to characterize an “AHI-SpO₂ Dissociation” phenotype—low event frequency despite significant desaturation—and explore its associations.

methodsWe retrospectively analyzed polysomnography and pulmonary function data from 43 ALS patients. The “AHI-SpO₂ Dissociation” phenotype was defined as AHI < 5 events/h with Minimum SpO₂ <90%. Spearman correlations assessed relationships between sleep parameters and Vital Capacity (VC) grade.

resultsThe AHI-SpO₂ Dissociation phenotype was identified in 32.6% (n = 14) of patients. Despite a “normal” AHI (2.8 vs. 2.1 events/h, p > 0.05 compared to controls), these patients exhibited nocturnal hypoxia comparable to the Obstructive Sleep Apnea group (Min SpO₂ 84.5% vs. 82.1%). Minimum SpO₂ showed a strong positive correlation with VC impairment grade (ρ = 0.68, p < 0.001), while AHI did not (ρ=-0.15, p = 0.322). Relying solely on AHI ≥ 5 would have failed to classify 32.6% of the cohort (all AHI-SpO₂ Dissociation patients) as having SDB.

conclusionA dissociation between AHI and nocturnal oxygenation is prevalent in ALS. Minimum SpO₂ correlates strongly with restrictive ventilatory deficit, suggesting it may complement AHI in identifying patients with significant respiratory compromise who might otherwise be missed. These findings highlight the need to integrate oximetry metrics into the diagnostic assessment of SDB in ALS.