Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disease that destroys motor neurons in the brain and spinal cord, leading to the loss of voluntary muscle control. This page covers the definition and classification of ALS, the cellular and physiological mechanisms driving motor neuron death, the clinical scenarios in which the disease presents, and the diagnostic and prognostic boundaries that distinguish ALS from related conditions. Understanding ALS is clinically and regulatorily significant: the U.S. Food and Drug Administration (FDA) has approved a limited roster of disease-modifying drugs for ALS, and the disease is recognized by the Social Security Administration (SSA) as a Compassionate Allowance condition — meaning disability benefits are fast-tracked at diagnosis.

Definition and scope

ALS is classified by the World Health Organization (WHO) International Classification of Diseases (ICD-11) under code 8B60.0 as a motor neuron disease (MND) affecting both upper motor neurons (UMNs) originating in the motor cortex and lower motor neurons (LMNs) originating in the brainstem and anterior horn of the spinal cord. The simultaneous involvement of both neuron populations is the defining feature that separates ALS from other MNDs such as primary lateral sclerosis (PLS), which affects only UMNs, and spinal muscular atrophy (SMA), which affects only LMNs.

Epidemiologically, ALS affects approximately 5 out of every 100,000 people in the United States, according to the Centers for Disease Control and Prevention (CDC) National ALS Registry. The CDC Registry, established under the ALS Registry Act (Public Law 110-373), has identified approximately 16,000–18,000 Americans living with ALS at any given time. Mean age of onset is between 55 and 65 years, and median survival after diagnosis is 2 to 5 years, though approximately 10 percent of patients survive beyond 10 years (CDC National ALS Registry).

ALS is further subclassified by genetic origin:

  1. Sporadic ALS (SALS) — Accounts for roughly 90 to 95 percent of all cases; no clear hereditary pattern.
  2. Familial ALS (FALS) — Accounts for roughly 5 to 10 percent of cases; associated with inherited mutations including those in SOD1, C9orf72, TARDBP, and FUS genes.
  3. ALS-FTD — A recognized overlap syndrome in which ALS co-occurs with frontotemporal dementia (FTD), reflecting shared TDP-43 proteinopathy across both conditions.

The regulatory context for neurological conditions — including FDA drug approval pathways, orphan drug designation, and SSA disability determinations — is directly relevant to ALS management and patient access to care.

How it works

ALS involves the selective degeneration of both corticospinal tract (upper motor) neurons and anterior horn (lower motor) neurons. The precise mechanism of neuronal death is multifactorial and incompletely understood, but research supported by the National Institute of Neurological Disorders and Stroke (NINDS) has identified the following converging pathways:

Upper motor neuron death produces spasticity, hyperreflexia, and the Babinski sign. Lower motor neuron death produces flaccid weakness, fasciculations, and atrophy. The co-presence of both UMN and LMN signs in the same patient at the same anatomical level is the clinical signature that satisfies El Escorial diagnostic criteria, the internationally recognized standard developed by the World Federation of Neurology (WFN).

Diagnostic confirmation relies on EMG and nerve conduction studies, which detect active denervation and reinnervation patterns characteristic of LMN degeneration. MRI of the brain and spine is used not to confirm ALS directly, but to rule out structural mimics such as cervical myelopathy or brainstem lesions.

Common scenarios

ALS presents differently depending on the anatomical site of disease onset:

Limb-onset ALS (spinal onset) — The most common presentation, accounting for approximately 70 percent of cases (NINDS). Patients typically report unilateral hand weakness, foot drop, or difficulty with fine motor tasks such as buttoning clothing. Asymmetric onset is characteristic.

Bulbar-onset ALS — Accounts for roughly 25 to 30 percent of cases. Initial symptoms involve speech (dysarthria) and swallowing (dysphagia), reflecting degeneration of brainstem motor nuclei. Bulbar-onset disease carries a shorter median survival than limb-onset disease, in part due to earlier respiratory compromise.

Respiratory-onset ALS — A rare presentation in which diaphragmatic weakness precedes limb or bulbar involvement. Patients may present to pulmonology before receiving a neurological diagnosis.

Conditions that neurologists must actively exclude before confirming an ALS diagnosis include multifocal motor neuropathy (MMN), Kennedy disease (spinal and bulbar muscular atrophy, SBMA), cervical spondylotic myelopathy, myasthenia gravis, and inclusion body myositis. For context on the broader category of myasthenia gravis and neuromuscular disorders, distinct diagnostic boundaries separate these conditions from ALS despite overlapping weakness presentations.

Decision boundaries

The El Escorial criteria, revised as the Awaji-Shima criteria in 2008 by the WFN, establish four levels of diagnostic certainty:

  1. Clinically Definite ALS: UMN and LMN signs in 3 or more body regions (bulbar, cervical, thoracic, lumbosacral).
  2. Clinically Probable ALS: UMN and LMN signs in 2 or more regions, with UMN signs rostral to LMN signs.
  3. Clinically Probable — Laboratory-Supported: UMN signs in 1 region; LMN signs defined by EMG in at least 2 regions.
  4. Clinically Possible ALS: UMN and LMN signs in 1 region only, or UMN signs in 2+ regions without meeting higher criteria.

Genetic testing through certified laboratory panels (such as those meeting CLIA standards under 42 CFR Part 493) is indicated when familial history is present or when onset age is below 45 years, given the higher probability of identifiable SOD1, C9orf72, or FUS mutations in younger patients.

FDA-approved pharmacological treatments as of 2023 include riluzole (1995), edaravone (Radicava, 2017), and the sodium phenylbutyrate–taurursodiol combination (Relyvrio, 2022; subsequently withdrawn by the manufacturer in 2024 following a confirmatory trial failure). The FDA's accelerated approval pathway, applied to edaravone and Relyvrio, is outlined under 21 CFR Part 314. Respiratory management, including non-invasive ventilation (NIV), is guided by the American Academy of Neurology (AAN) practice parameters, which recommend initiating NIV when forced vital capacity (FVC) falls below 50 percent of predicted.

The broader landscape of neurological disease management — including the full scope of conditions evaluated and treated by neurologists — is indexed at the neurologicalauthority.com site index.

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)