Multimodal strategies for diagnosis, stratification, and therapeutic monitoring in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder of motor neurons (MN) that is currently diagnosed through a prolonged process of exclusion, often delaying intervention. This review provides an overview of fluid, imaging, electrophysiological, and genetic biomarkers, explicitly linking each modality to early detection, patient stratification, disease monitoring, therapeutic development, and clinical trial design. Fluid biomarkers (i.e., neurofilament light chain, phosphorylated neurofilament heavy chain, inflammatory cytokines, microRNAs, and proteins in blood or cerebrospinal fluid) reflect neuronal injury and/or disease activity, enabling early identification of pres-ymptomatic individuals and longitudinal tracking of neurodegeneration. Imaging biomarkers, such as structural and diffusion MRI of the motor cortex, corticospinal tracts, and spinal cord, as well as PET imaging neuroinflammation or metabolism, provide objective measures of MN degeneration and extra-motor involvement. Electrophysiological biomarkers, including high-density electromyography, motor unit number, transcranial magnetic stimulation, and electrical impedance myography, quantitatively assess upper and lower MN loss and functional reserve. Genetic biomarkers, encompassing variants in genes such as C9orf72, SOD1, FUS, and TARDBP, enable presymptomatic screening and molecular stratification. In this context, transposable elements have emerged as an additional layer linking genomic variation and RNA dysregulation. We highlight the importance of multimodal and stage-specific biomarker integration to improve diagnostic accuracy and illuminate distinct disease phases. This approach supports stratification by progression rate or molecular subtype, enrichment of clinical trial cohorts, and the development of surrogate endpoints. We conclude by discussing current challenges, including disease heterogeneity and assay standardization, and outline future directions toward biomarker-driven precision medicine in ALS.