Pathophysiology of ALS
ALS is a complex neurodegenerative disorder characterized by progressive degeneration of both upper and lower motor neurons in the brain, brainstem, and spinal cord, driven by multiple interconnected pathological mechanisms including oxidative stress, glutamate excitotoxicity, mitochondrial dysfunction, protein misfolding and aggregation, neuroinflammation, and apoptosis 1.
Core Pathological Mechanisms
Motor Neuron Degeneration
The disease fundamentally involves degeneration of upper motor neurons (in the motor cortex and corticospinal tracts) and lower motor neurons (in the brainstem and spinal cord anterior horns), representing approximately 85% of all motor neuron diseases 2. This dual motor neuron involvement produces the characteristic clinical picture of combined upper motor neuron signs (hypertonicity, hyperreflexia, spasticity) and lower motor neuron signs (muscle fasciculations, weakness, atrophy, flaccidity) 2.
Molecular and Cellular Mechanisms
Glutamate Excitotoxicity: Dysregulation of glutamate homeostasis leads to excessive calcium-mediated neurotoxicity. Motor neurons exhibit diminished capacity to regulate calcium influx, and astrocytic dysfunction reduces glutamate clearance, creating a vicious cycle of neuronal damage 3, 4.
Oxidative Stress and Mitochondrial Dysfunction: Motor neurons show impaired mitochondrial function with excessive reactive oxygen species (ROS) production. This oxidative stress exacerbates glutamate imbalance and establishes a detrimental cycle of neuronal damage and glial dysfunction 1, 3.
Protein Misfolding and Aggregation: The disease involves accumulation of ubiquitinated proteins in affected motor neurons. Key proteins include TDP-43 (found in most sporadic ALS cases), SOD1, FUS, and others. These misfolded proteins disrupt cellular function and contribute to neuronal death 5, 3, 6.
Neuroinflammation: Astrocytic stress responses and heightened microglia reactivity drive motor neuron susceptibility. Dysregulated inflammatory pathways, including NF-kB and other pro-inflammatory transcription factors, maintain chronic neuroinflammation 3, 7.
Axonal Transport Abnormalities: Disruption of axonal transport mechanisms compromises the delivery of essential proteins and organelles to distal neuronal compartments 3, 6.
Genetic Factors
Approximately 85-90% of cases are sporadic without known genetic mutations, while 10-15% are familial 2, 8. Over 60 genes have been associated with ALS, most following autosomal dominant inheritance patterns 8:
- C9orf72: Accounts for 40% of familial ALS cases
- SOD1: Found in 20% of familial ALS cases
- TARDBP (encoding TDP-43): Common in sporadic cases
- FUS: Associated with both ALS and frontotemporal dementia
- CHCHD10: Affects mitochondrial function
These genetic variants encode proteins that regulate mitochondrial homeostasis and function, directly linking genetic factors to the core pathophysiological mechanisms 5.
Clinical Manifestations and Disease Progression
The pathophysiology manifests in two primary forms 1:
Bulbar onset (25-35% of patients): Degeneration of bulbar neurons causes dysarthria, dysphagia, difficulty chewing, and prolonged meal times. Almost 80% develop severe swallowing problems.
Spinal/limb onset (65%): Spinal motor neuron injury presents as focal limb weakness (hand weakness or foot drop) that progressively spreads.
The relentless neurodegeneration leads to progressive skeletal muscle atrophy, including respiratory muscles. Mean survival is 3-5 years, with only 5-10% living longer than 10 years 1, 8. Patients with bulbar onset and older age have the shortest life expectancy. Death typically results from respiratory failure, with malnutrition and dehydration as contributing factors 1.
Systemic Complications
Metabolic Alterations: Despite reduced lean body mass, patients often have increased energy requirements due to increased work of breathing, lung infections, and other poorly understood factors 1.
Cognitive Dysfunction: 20-50% of cases develop cognitive impairment, mainly frontotemporal dementia, reflecting the overlap between ALS and frontotemporal lobar degeneration pathology 1, 9.
Imaging Correlates
MRI findings reflect the underlying pathophysiology 2:
- Abnormal T2/FLAIR signal in corticospinal tracts from subcortical white matter to pons
- "Snake eyes" appearance in spinal cord anterior horns (T2/STIR signal abnormality in lateral corticospinal tracts)
- These findings correspond to motor neuron degeneration but may only appear later in disease course
The multifactorial etiology and heterogeneous presentation of ALS reflect the complex interplay between genetic susceptibility, environmental factors, and the various molecular mechanisms driving progressive motor neuron death 3, 6, 10.