Introduction

Evaluation and management of respiratory dysfunction are critical components of Amyotrophic Lateral Sclerosis (ALS) care. Death is often caused by progressive respiratory failure, and dyspnea is a distressing symptom. Noninvasive ventilation (NIV) improves quality of life through amelioration of dyspnea and improvement of sleep quality, with studies demonstrating a survival benefit. ^{Reference Ackrivo, Hsu, Hansen-Flaschen, Elman and Kawut1,Reference Bourke, Tomlinson, Williams, Bullock, Shaw and Gibson2} Evidence suggests that early initiation of NIV confers an improved survival advantage. ^{Reference Carratù, Spicuzza and Cassano3}

Li et al. ^{Reference Li, Ding, Lu, Sun and Xu4} utilized polysomnography (PSG) in people with normal daytime function and sitting forced vital capacity (FVC) to determine whether abnormal apnea hypopnea index (AHI) and reduced oxygen saturation (SpO₂) could be markers of early respiratory dysfunction. This longitudinal study found that abnormalities of AHI and SpO₂ predicted risk of dyspnea within 12 months, and low overnight SpO₂ on PSG correlated with low orthopnea sub-scores on the ALSFRS-R rating scale.

We commend the authors on their efforts to identify early markers of respiratory insufficiency. We agree that it is crucial to ensure the opportunity for early use of NIV by identifying the first signs of respiratory compromise. PSG can be a useful tool to do so. One important limitation of this study design was the lack of supine FVC, mean inspiratory pressure (MIP) and sniff nasal pressure (SNIP) measurements to determine diaphragmatic function and inspiratory muscle strength. A significant drop in supine FVC compared to sitting can demonstrate early diaphragmatic dysfunction.

Supine FVC, MIP or SNIP are routinely obtained at bedside in ALS clinics or pulmonary function labs. Judicious use of supine FVC, MIP and SNIP, particularly for those who endorse orthopnea, could obviate the need for PSG to identify first signs of respiratory compromise. Mobility and transfers can be challenging, and sleep labs may not be well equipped to manage the complex needs of people with ALS. Some patients with mobility concerns choose not to attend additional appointments and prefer tests done in the clinic.

The Canadian ALS guidelines recommend measuring either supine FVC, MIP or SNIP in addition to sitting FVC for routine evaluation. ^{Reference Shoesmith, Abrahao and Benstead5} Interested readers can refer to the guideline for comprehensive information on respiratory testing and management. PSG or nocturnal oximetry (which is less logistically challenging) is recommended when symptomatic sleep disordered breathing is suspected and other daytime indications for NIV initiation are lacking. Li et al. provide evidence that PSG can be an early detector of respiratory compromise in ALS; however, it may not be needed if other tests are utilized as recommended. We thank the authors for drawing attention to the importance of respiratory management in ALS.