The role of radiologic assessment in evaluating and monitoring respiratory function in amyotrophic lateral sclerosis (ALS) patients: a narrative review

Background and Objective: Respiratory failure is the primary cause of mortality in amyotrophic lateral sclerosis (ALS), usually caused by progressive neuromuscular respiratory weakness. Standard pulmonary function tests (PFTs) such as maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), and both supine and upright forced vital capacity (FVC) are crucial for objective measurements of diaphragmatic weakness but have limitations, including dependence on the patient's performance and the inability to detect early, subclinical diaphragmatic impairment or be used effectively in patients with bulbar symptoms. Radiological assessments, particularly dynamic imaging, have emerged as potential objective tools for evaluating respiratory function. This review comprehensively summarizes findings on the use of diaphragmatic ultrasound (DUS), dynamic chest magnetic resonance imaging (MRI) and deep learning (DL)-based chest computed tomography (CT) for assessing lung function in ALS patients. Methods: Key radiological metrics include diaphragm thickness (DT), thickening fraction during inspiration, real-time diaphragmatic excursion, lung diameter changes and changes in pulmonary length and area. These measures have been compared with conventional PFTs in various studies to validate their use for diagnostic accuracy, particularly in early stages of disease. Key Content and Findings: DUS is a non-invasive, widely available tool that strongly correlates with PFT measurements, especially FVC, MIP, and sniff nasal inspiratory pressure (SNIP). Dynamic measures, such as excursion and velocity, appear more sensitive to early dysfunction than thickness alone. Chest dynamic MRI has also shown significant correlations with spirometric parameters. Small cohort studies indicate that dynamic chest MRI is a superior, sensitive tool for detecting early respiratory impairment in asymptomatic patients with normal spirometry. Conclusions: Radiological assessments, primarily DUS, DL-based chest CT and dynamic MRI, offer valuable, objective, and non-invasive methods for monitoring respiratory muscle strength in ALS. These techniques serve as complementary tools to traditional PFTs, particularly in selected clinical scenarios such as ALS patients with early disease, bulbar involvement and unable to perform PFTs. Further longitudinal research with larger cohorts is needed to standardize protocols and validate their role as early parameters to guide the timely initiation of supportive interventions like non-invasive ventilation (NIV).