LAUSR

To Editor in Chief, Diaphragm muscle function is often impaired in ALS, leading to a restrictive respiratory pattern and respiratory failure. Actually, PFTs are the gold standard to evaluate the development of respiratory muscle weakness but there is normally a significant variation due to lack of cooperation or volition where there is comorbid depression or cognitive impairment. The diaphragm can be clearly identified using ultrasound with minimal interobserver variability in its assessment. Diaphragmatic thickening is assessed by the thickening fraction (TF), defined as “thickness at end inspiration – thickness at end expiration/thickness at end expiration.” In literature, the relationship between TF and lung volume has been reported normally to be linear [1]. We have read with interest Sartucci et al.’s [2] work about the correlation in ultrasound measurement thickness and the variation during inspiration and expiration, and the measurement was correlated to respiratory function and disease severity also in severe bulbar disease. First, in literature, several studies have shown a relationship between dynamic diaphragm thickness and respiratory function with positive correlation with impaired vital capacity and inverse correlation with pCO2, indicating atrophy and impaired diaphragmatic contractility in those patients with hypoventilation. Dynamic diaphragm thickness is also correlated to functional assessments in ALS using the ALSFRS-R. This is different by Pinto et al. [3] where there was a difference in the bulbar-onset subgroup in diaphragmatic ultrasound measurements that not correlate with PFTs. In the bulbaronset patient, probably due to the PFTs’ unreliability, diaphragm ultrasound may represent a superior marker of respiratory dysfunction. Secondly, patients with severe bulbar weakness were excluded from the majority of studies. There is some evidence for abnormalities of central respiratory drive in bulbar-onset ALS which would not be reflected in direct muscle-based measurement and may be partial drivers of hypoventilation. In Sartucci’s study, BMI is not evaluated due to the complexity of visualizing the diaphragm even in obese patients where the muscle is located relatively superficially. Second, in this study, the correlation between thickening and cough efficacy is not evaluated. The function of inspiratory muscles is crucial for effective cough as well as expiratory muscles in patients with ALS. Cough is a defensive mechanism, and the early observation of initial failure is very important. The weakened respiratory muscles can neither fully expand the lungs up to the maximal capacity nor compress them to the point of the smallest residual volume, leading to the reduction of chest wall compliance through shortening and stiffening of the unstretched tissue and fibrosis of dystrophic muscles. The compliance of the lungs is also reduced by the spreading of micro atelectasis in the lungs. This respiratory disability decreases lung capacity and causes impairment of the ability to cough. If we can observe diaphragm thickness, we can do an optimal cough assist program and perform an optimal use of positive or negative pressure [4, 5]. Third, it is a noninvasive bedside test that is easily accessible, also at home, and can be readily repeated to allow serial monitoring of respiratory muscle weakness and therefore potentially has a role in assessing the progression of the disease. It also allows dynamic assessment of diaphragmatic motion * Giuseppe Fiorentino [email protected]