Ultrasound assessment of diaphragmatic function in patients with amyotrophic lateral sclerosis.

Background: Evaluation of diaphragm function in Amyotrophic Lateral Sclerosis (ALS) is critical in determining when to commence non-invasive mechanical ventilation (NIV). Currently, forced vital capacity (FVC) and sniff nasal inspiratory pressure (SNIP) are volitional measures for this evaluation, but require collaboration and are poorly specific. The primary aim of this study was to assess whether diaphragmatic thickness measured by ultrasound (US) correlates with lung function impairment in ALS patients. The secondary aim was then to compare US diaphragm thickness index (Tdi) with a new parameter (Tmax index). Methods: 41 patients with ALS and 30 healthy subjects were enrolled in the study. All subjects underwent spirometry, SNIP and diaphragm US evaluation, while arterial blood gases were measured in patients only. US assessed diaphragm thickness (Tdi) at tidal volume (Vt) or TLC, and their ratio (Tmax) were recorded. Changes (Δ) in Tdi indices during tidal volume (TdiVt) and maximal inspiration (TdiTLC) were also assessed. Results: TdiTLC (p <0.001) and Tmax (p= 0.007), but not TdiVt, differed between patients and controls. Significant correlation (p<0.05) was found between TdiTLC, Tmax and FVC. The ROC curve analysis for comparison of individual testing showed better accuracy with Δtmax than with ΔtdiTLC for FVC (AUC 0.76 and 0.27) and SNIP (AUC 0.71 and 0.25). Conclusions: Diaphragm thickness assessed by ultrasound significantly correlates with global respiratory alterations in patients with ALS. Tmax represents a new US index of early diaphragmatic dysfunction, better related with the routinely performed lung function tests.