Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease of the motor system in which upper and lower motor neurons are lost, causing relentless weakness, muscle wasting, and spasticity that typically progresses to respiratory failure. It is the most common form of adult motor-neuron disease and overlaps clinically and pathologically with frontotemporal dementia.

Definition

Amyotrophic lateral sclerosis is a progressive neurodegenerative disorder characterised by the degeneration of upper motor neurons in the motor cortex and lower motor neurons in the brainstem and spinal cord, producing progressive weakness, muscle atrophy, and spasticity, and pathologically associated in most cases with aggregated TDP-43.

Scope

This topic covers amyotrophic lateral sclerosis as a clinical and biological entity: the combined upper and lower motor-neuron involvement that defines it, its TDP-43 pathology and genetic causes, its diagnostic framing, its overlap with frontotemporal dementia, and its epidemiology. It is a reference overview and does not provide diagnostic protocols or treatment guidance.

Core questions

• How do combined upper and lower motor-neuron signs define the disease?
• What molecular and genetic mechanisms drive motor-neuron loss?
• How does ALS overlap with frontotemporal dementia?
• Why is the clinical course so variable between patients?

Key concepts

• Upper and lower motor-neuron degeneration
• TDP-43 pathology
• C9orf72, SOD1, and other genetic causes
• Bulbar versus limb onset
• ALS-FTD overlap
• Progressive respiratory involvement
• El Escorial diagnostic criteria

Key theories

TDP-43 proteinopathy

Most cases of amyotrophic lateral sclerosis are characterised by cytoplasmic aggregates of TDP-43 in motor neurons, the same protein implicated in much of frontotemporal lobar degeneration, providing a molecular link between the two disorders.

ALS-FTD genetic and clinical spectrum

Shared genetic causes, most notably C9orf72 repeat expansions, and shared TDP-43 pathology support the view that amyotrophic lateral sclerosis and frontotemporal dementia lie on a continuum rather than being entirely separate diseases.

Mechanisms

Amyotrophic lateral sclerosis involves the progressive loss of both upper motor neurons in the motor cortex and corticospinal tracts and lower motor neurons in the brainstem and spinal cord, producing a combination of spasticity and brisk reflexes with weakness, wasting, and fasciculations. In most cases motor neurons contain cytoplasmic aggregates of TDP-43, linking the disease molecularly to frontotemporal lobar degeneration; proposed contributors to neuronal death include disturbed RNA processing, protein aggregation, excitotoxicity, oxidative and mitochondrial stress, and impaired axonal transport. A minority of cases are familial, with C9orf72 repeat expansions and SOD1 mutations among the recognised causes (Neumann et al., 2006; Feldman et al., 2022; Dugger & Dickson, 2017).

Clinical relevance

Amyotrophic lateral sclerosis is the principal adult motor-neuron disease, and understanding its combined upper and lower motor-neuron pathology and its TDP-43 and genetic basis informs how it is recognised and distinguished from mimics. This entry describes how the disease is defined and studied; it is not a basis for individual diagnosis or treatment decisions.

Epidemiology

Amyotrophic lateral sclerosis is uncommon, with incidence on the order of a few cases per 100,000 person-years, typically presenting in late-middle to older age and slightly more often in men. Most cases are sporadic, with roughly a tenth familial; survival is usually limited by progressive respiratory muscle involvement, though the rate of progression varies considerably (Feldman et al., 2022).

History

Jean-Martin Charcot characterised amyotrophic lateral sclerosis in the 1870s, linking the clinical picture of combined upper and lower motor-neuron signs to degeneration of the corticospinal tracts and anterior horn cells. The later identification of SOD1 mutations and then of TDP-43 as the major aggregating protein, along with the discovery of C9orf72 expansions, reframed the disease molecularly and connected it to frontotemporal dementia, while diagnostic criteria such as the El Escorial framework standardised its classification (Feldman et al., 2022; Neumann et al., 2006; Brooks et al., 2000).

Debates

Are ALS and frontotemporal dementia one disease or two?

Overlapping TDP-43 pathology and shared genetic causes such as C9orf72 expansions support a continuum view, while distinct predominant clinical presentations sustain the practice of classifying them separately.

Key figures

• Jean-Martin Charcot
• Eva Feldman
• Benjamin Brooks
• Manuela Neumann

Seminal works

• feldman-2022
• neumann-2006
• brooks-2000

Frequently asked questions

What distinguishes ALS from other motor-neuron diseases?

Amyotrophic lateral sclerosis is defined by degeneration of both upper and lower motor neurons, producing a combination of spasticity and weakness with wasting; this combined involvement separates it from forms of motor-neuron disease that affect only one of these populations.

Is ALS inherited?

Most cases are sporadic, but about one in ten is familial, and several genes, including C9orf72 and SOD1, are recognised causes; the C9orf72 expansion also links ALS to frontotemporal dementia.