Amyotrophic Lateral Sclerosis (ALS)

1. Overview

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease or motor neuron disease (MND), is a progressive neurodegenerative disorder that selectively attacks upper motor neurons (UMNs) in the motor cortex and lower motor neurons (LMNs) in the brainstem and spinal cord. The relentless degeneration of these neurons leads to progressive muscle weakness, atrophy, fasciculations, and spasticity, ultimately resulting in paralysis and death, most commonly from respiratory failure. ALS is the most common form of motor neuron disease in adults.

The disease was first described by French neurologist Jean-Martin Charcot in 1869, but it gained widespread public awareness in the United States after it struck the legendary baseball player Lou Gehrig in 1939. The name "amyotrophic" refers to muscle atrophy and weakness ("a-" without, "myo-" muscle, "trophic" nourishment), while "lateral sclerosis" describes the hardening of the lateral columns of the spinal cord where the corticospinal tracts degenerate.

ALS is classified into several subtypes based on clinical and genetic features:

Sporadic ALS (sALS) — accounts for approximately 90-95% of cases with no clear family history
Familial ALS (fALS) — accounts for 5-10% of cases with an identifiable genetic mutation inherited in an autosomal dominant pattern
Limb-onset ALS — begins with weakness in the arms or legs (approximately 65% of cases)
Bulbar-onset ALS — begins with speech or swallowing difficulties (approximately 25% of cases)
Respiratory-onset ALS — rare presentation beginning with respiratory insufficiency (approximately 3-5% of cases)
ALS-FTD — ALS co-occurring with frontotemporal dementia, seen in approximately 15% of patients

2. Epidemiology

ALS affects approximately 2 per 100,000 people per year worldwide, with a prevalence of 5-7 per 100,000. In the United States, an estimated 30,000 individuals are living with ALS at any given time, with roughly 5,000 new cases diagnosed annually. The disease is approximately 1.3-1.5 times more common in men than in women, although this sex difference diminishes with advancing age.

The peak age of onset for sporadic ALS is between 55 and 75 years, with a mean age of onset of approximately 63 years. Familial ALS tends to present earlier, with a mean onset around 46 years. ALS is rare before age 25, though juvenile-onset cases do occur. The incidence appears relatively uniform across European and North American populations but is notably higher in certain geographic clusters, including the Kii Peninsula of Japan, western New Guinea, and Guam, where it is associated with the ALS-parkinsonism-dementia complex (ALS-PDC).

The lifetime risk of developing ALS is estimated at approximately 1 in 300 to 1 in 400. The incidence in Europe and North America has been relatively stable over the past several decades, though some studies suggest a modest increase, likely attributable to improved diagnostic ascertainment and aging populations. Military veterans, particularly those who served in the Gulf War, have been reported to have a nearly two-fold increased risk of developing ALS.

3. Pathophysiology

The pathophysiology of ALS is multifactorial and involves a complex interplay of molecular and cellular mechanisms that converge to produce motor neuron degeneration. The disease selectively targets motor neurons while relatively sparing sensory neurons, autonomic neurons, and oculomotor neurons.

Protein Misfolding and Aggregation

A hallmark of ALS pathology is the accumulation of misfolded protein aggregates within motor neurons. The most common inclusion is TDP-43 (TAR DNA-binding protein 43), found in approximately 97% of all ALS cases. TDP-43 is normally a nuclear RNA-binding protein, but in ALS it becomes hyperphosphorylated, ubiquitinated, and mislocalized to the cytoplasm, where it forms insoluble aggregates. In cases caused by SOD1 mutations, misfolded SOD1 protein aggregates are the primary inclusion. FUS (fused in sarcoma) protein aggregates are found in a smaller subset of cases.

Glutamate Excitotoxicity

Motor neurons in ALS are subjected to excessive glutamate signaling, leading to sustained calcium influx through AMPA and NMDA receptors. This excitotoxicity is partly due to reduced expression and function of the excitatory amino acid transporter 2 (EAAT2/GLT-1) on astrocytes, which normally clears synaptic glutamate. The resulting intracellular calcium overload activates destructive enzymatic cascades including calpains, endonucleases, and phospholipases. This mechanism forms the basis for the therapeutic action of riluzole.

Oxidative Stress

Motor neurons exhibit heightened vulnerability to oxidative damage due to their high metabolic demands, large cell body size, and long axonal projections. Mutations in SOD1 (superoxide dismutase 1) lead to a toxic gain-of-function that generates reactive oxygen species (ROS), causing lipid peroxidation, protein oxidation, and DNA damage. Mitochondrial dysfunction further amplifies oxidative stress through impaired electron transport chain activity and increased superoxide production.

Neuroinflammation

Activated microglia and reactive astrocytes play a dual role in ALS pathogenesis. Initially, microglia may be neuroprotective (M2 phenotype), but as the disease progresses, they shift to a neurotoxic (M1 phenotype) state, releasing pro-inflammatory cytokines including TNF-alpha, IL-1beta, and IL-6. Reactive astrocytes lose their supportive functions and acquire toxic properties, actively contributing to motor neuron death through mechanisms involving NF-kB signaling.

Impaired Axonal Transport and Cytoskeletal Dysfunction

Motor neurons depend on efficient axonal transport to shuttle organelles, proteins, and trophic factors along axons that can extend over one meter in length. ALS disrupts both anterograde and retrograde axonal transport through damage to neurofilaments, dynein-dynactin complexes, and kinesin motors. Accumulation of neurofilaments in proximal axons is an early pathological finding and contributes to axonal dysfunction and eventual degeneration.

C9orf72 Repeat Expansion Pathology

The GGGGCC hexanucleotide repeat expansion in C9orf72 is the most common genetic cause of both familial and sporadic ALS. This expansion leads to disease through three non-mutually exclusive mechanisms: loss of C9orf72 protein function (haploinsufficiency), RNA toxicity from sense and antisense RNA foci that sequester RNA-binding proteins, and dipeptide repeat protein (DPR) toxicity from unconventional repeat-associated non-ATG (RAN) translation producing five different DPR species (poly-GA, poly-GP, poly-GR, poly-PA, poly-PR).

4. Etiology and Risk Factors

Genetic Factors

More than 40 genes have been associated with ALS. The most significant include:

C9orf72 — hexanucleotide repeat expansion; accounts for 40% of familial and 7% of sporadic ALS cases; also associated with frontotemporal dementia
SOD1 (superoxide dismutase 1) — the first ALS gene identified (1993); over 180 mutations known; accounts for approximately 12-20% of familial ALS
TARDBP (TDP-43) — mutations account for approximately 4% of familial ALS
FUS (fused in sarcoma) — mutations account for approximately 4% of familial ALS; often associated with younger onset
UBQLN2 (ubiquilin 2) — X-linked dominant inheritance
VCP (valosin-containing protein) — also causes inclusion body myopathy and Paget disease
OPTN (optineurin) — associated with both ALS and glaucoma
TBK1 (TANK-binding kinase 1) — involved in autophagy and neuroinflammation pathways

Environmental Risk Factors

Cigarette smoking — the most consistently identified environmental risk factor; approximately 1.4-fold increased risk
Military service — particularly Gulf War veterans; ALS is recognized as a service-connected disease by the U.S. Department of Veterans Affairs
Heavy metal exposure — lead, mercury, and manganese have been implicated in some epidemiological studies
Pesticide and herbicide exposure — organophosphates and other agricultural chemicals
Head trauma — repeated traumatic brain injury, particularly in contact sports athletes
Intense physical activity — professional athletes, particularly soccer players, show elevated risk in some studies, though the mechanism is debated
Cyanobacterial toxins (BMAA) — beta-methylamino-L-alanine, proposed as the environmental factor in Western Pacific ALS clusters

Demographic Risk Factors

Age — peak incidence between 55-75 years
Sex — males at slightly higher risk (ratio approximately 1.3-1.5:1)
Ethnicity — higher incidence in Caucasian populations compared to African, Asian, and Hispanic populations
Family history — 5-10% of cases are familial with autosomal dominant inheritance being most common

5. Clinical Presentation

Upper Motor Neuron Signs

Spasticity — velocity-dependent increase in muscle tone
Hyperreflexia — exaggerated deep tendon reflexes
Pathological reflexes — Babinski sign (extensor plantar response), Hoffman sign
Pseudobulbar affect — involuntary, uncontrollable episodes of laughing or crying disproportionate to the emotional stimulus
Clonus — rhythmic, involuntary muscular contractions
Jaw jerk reflex — exaggerated in bulbar UMN involvement

Lower Motor Neuron Signs

Muscle weakness — progressive, asymmetric, beginning focally and spreading contiguously
Muscle atrophy — visible wasting, particularly in the intrinsic hand muscles (thenar and hypothenar eminences), forearms, and tongue
Fasciculations — spontaneous, involuntary twitching of muscle fibers visible under the skin
Hyporeflexia — diminished reflexes in severely atrophied muscles
Muscle cramps — painful involuntary contractions, often an early symptom
Flaccid weakness — decreased muscle tone in affected segments

Bulbar Symptoms

Dysarthria — slurred, nasal, or strained speech; eventual anarthria
Dysphagia — difficulty swallowing, choking on liquids, and progressive inability to manage oral secretions
Tongue atrophy and fasciculations — a characteristic finding on examination
Sialorrhea — drooling due to impaired swallowing of saliva rather than overproduction
Laryngospasm — sudden vocal cord spasm causing stridor

Respiratory Symptoms

Dyspnea on exertion — progressing to dyspnea at rest
Orthopnea — difficulty breathing when lying flat due to diaphragmatic weakness
Sleep-disordered breathing — nocturnal hypoventilation causing morning headaches and daytime somnolence
Weak cough — impaired secretion clearance increasing pneumonia risk

Cognitive and Behavioral Changes

Up to 50% of ALS patients exhibit some degree of cognitive or behavioral impairment. Approximately 13-15% meet criteria for comorbid frontotemporal dementia (FTD), characterized by executive dysfunction, behavioral disinhibition, apathy, and language deficits. This ALS-FTD overlap is particularly common in patients carrying the C9orf72 repeat expansion.

6. Diagnosis

ALS remains a clinical diagnosis, as there is no single definitive diagnostic test. The average time from symptom onset to diagnosis is approximately 10-16 months, and patients often see multiple physicians before receiving the correct diagnosis.

Revised El Escorial Criteria (World Federation of Neurology)

The diagnosis requires the presence of:

Evidence of lower motor neuron degeneration by clinical, electrophysiological, or neuropathological examination
Evidence of upper motor neuron degeneration by clinical examination
Progressive spread of symptoms within a region or to other regions
Absence of electrophysiological or pathological evidence of other disease processes

Diagnostic certainty categories:

Clinically definite ALS — UMN and LMN signs in three or more regions
Clinically probable ALS — UMN and LMN signs in two regions with UMN signs rostral to LMN signs
Clinically probable ALS, laboratory-supported — UMN and LMN signs in one region with EMG evidence of LMN involvement in two or more limbs
Clinically possible ALS — UMN and LMN signs in one region, or UMN signs in two or more regions

Electrophysiological Studies

Electromyography (EMG) — shows active and chronic denervation (fibrillation potentials, positive sharp waves, fasciculation potentials, large motor unit potentials with reduced recruitment) in clinically affected and unaffected muscles
Nerve conduction studies (NCS) — motor and sensory nerve conduction velocities are typically normal or near-normal, helping to exclude peripheral neuropathies and multifocal motor neuropathy

Laboratory Studies

Serum creatine kinase (CK) — mildly to moderately elevated in most patients
Neurofilament light chain (NfL) — elevated in serum and CSF; increasingly used as a diagnostic and prognostic biomarker
Genetic testing — recommended for patients with family history; screening panels available for C9orf72, SOD1, TARDBP, FUS, and other genes
Exclusionary labs — thyroid function, vitamin B12, copper, parathyroid hormone, serum protein electrophoresis, anti-GM1 antibodies

Neuroimaging

MRI of the brain and spinal cord — primarily used to exclude structural lesions, myelopathy, and other mimics; may show T2 hyperintensity along the corticospinal tracts
PET and functional MRI — research tools showing motor cortex hypometabolism and altered connectivity

Pulmonary Function Testing

Forced vital capacity (FVC) — baseline measurement and serial monitoring; FVC below 50% predicted indicates significant respiratory compromise
Maximal inspiratory pressure (MIP) — more sensitive for early diaphragmatic weakness
Sniff nasal inspiratory pressure (SNIP) — useful in patients with facial weakness who cannot form an adequate seal for standard spirometry

7. Treatment

ALS treatment is multidisciplinary and focuses on slowing disease progression, managing symptoms, maintaining function, and optimizing quality of life. Care is best delivered through specialized multidisciplinary ALS clinics, which have been shown to improve survival and quality of life.

Disease-Modifying Therapies

Riluzole (Rilutek) — the first FDA-approved drug for ALS (1995); a glutamate antagonist that modestly extends survival by approximately 2-3 months; dose is 50 mg twice daily; available in oral tablet and liquid formulation (Tiglutik)
Edaravone (Radicava) — FDA-approved in 2017; a free radical scavenger that slows functional decline by approximately 33% as measured by the ALS Functional Rating Scale-Revised (ALSFRS-R) in a select patient population; administered intravenously or orally
Sodium phenylbutyrate-taurursodiol (Relyvrio/AMX0035) — FDA-approved in 2022; targets endoplasmic reticulum stress and mitochondrial dysfunction; shown to slow functional decline in clinical trials
Tofersen (Qalsody) — FDA-approved in 2023 for SOD1-ALS; an antisense oligonucleotide (ASO) that reduces SOD1 protein production; administered intrathecally

Symptom Management

Spasticity — baclofen, tizanidine, benzodiazepines, dantrolene
Sialorrhea — glycopyrrolate, atropine sublingual drops, botulinum toxin injections to salivary glands, suction devices
Pseudobulbar affect — dextromethorphan-quinidine (Nuedexta), the only FDA-approved treatment for this symptom
Pain — NSAIDs, gabapentin, pregabalin, opioids for refractory cases
Depression and anxiety — SSRIs, SNRIs, mirtazapine
Muscle cramps — mexiletine, quinine (limited evidence), magnesium
Insomnia — trazodone, zolpidem, melatonin
Constipation — stool softeners, stimulant laxatives, adequate hydration

Respiratory Support

Non-invasive ventilation (NIV/BiPAP) — recommended when FVC falls below 50% or symptoms of nocturnal hypoventilation develop; extends survival by a median of 7-13 months and significantly improves quality of life
Mechanical insufflation-exsufflation (cough assist) — helps clear secretions and reduces pneumonia risk
Invasive mechanical ventilation (tracheostomy) — considered when NIV is no longer sufficient; can sustain life for years but requires 24-hour care; chosen by approximately 5-10% of ALS patients in the United States
Diaphragm pacing — no longer recommended based on negative clinical trial results

Nutritional Support

Dietary modifications — high-calorie, high-protein diet; modified food textures for dysphagia
Percutaneous endoscopic gastrostomy (PEG) — recommended when swallowing becomes unsafe or weight loss exceeds 10% of baseline; stabilizes weight and simplifies medication administration
Radiologically inserted gastrostomy (RIG) — alternative for patients with low FVC who may not tolerate sedation for PEG placement

Rehabilitation and Assistive Devices

Physical therapy — gentle range-of-motion exercises, stretching, and adaptive exercise programs
Occupational therapy — adaptive equipment, home modifications, energy conservation strategies
Speech therapy — communication strategies, voice banking, augmentative and alternative communication (AAC) devices, eye-tracking technology
Assistive technology — ankle-foot orthoses (AFOs), wheelchairs, hospital beds, environmental control units

8. Complications

Respiratory failure — the leading cause of death in ALS; progressive diaphragmatic and intercostal muscle weakness
Aspiration pneumonia — resulting from dysphagia and impaired cough reflex
Malnutrition and cachexia — due to dysphagia, increased metabolic demands, and hypermetabolism
Venous thromboembolism — due to immobility; deep vein thrombosis and pulmonary embolism
Pressure ulcers — from immobility and malnutrition
Falls and fractures — resulting from progressive weakness and impaired balance
Depression and anxiety — affecting up to 44% of patients
Social isolation — due to communication difficulties and mobility limitations
Caregiver burden — significant psychological, physical, and financial strain on family caregivers
Frontotemporal dementia — cognitive and behavioral decline complicating care decisions and advance planning
Laryngospasm — sudden airway obstruction causing significant patient distress
Pain syndromes — musculoskeletal pain, joint contractures, spasticity-related pain

9. Prognosis

ALS is invariably fatal, with a median survival of 2-5 years from symptom onset. Approximately 50% of patients die within 30 months of symptom onset, and approximately 80-90% die within 5 years. However, about 5-10% of patients survive beyond 10 years, and rare individuals survive 20 years or more, as exemplified by physicist Stephen Hawking, who lived 55 years after diagnosis.

Factors associated with poorer prognosis include:

Bulbar-onset disease — median survival approximately 2-2.5 years
Older age at onset — particularly onset after age 75
Rapid progression — significant decline in ALSFRS-R score in the first 3 months
Low FVC at diagnosis — particularly below 70% predicted
Comorbid frontotemporal dementia — reduces median survival by approximately 6-12 months
Low BMI and weight loss — malnutrition is an independent predictor of shorter survival
Diagnostic delay — longer delay paradoxically correlates with slower progression (selection bias)

Factors associated with better prognosis include:

Younger age at onset — particularly onset before age 40
Limb-onset disease — median survival approximately 3-5 years
Predominantly LMN phenotype (progressive muscular atrophy)
Flail arm or flail leg variants — slower progression phenotypes
Multidisciplinary clinic care — associated with 7-10 months longer survival
Early use of NIV — extends survival by a median of 7-13 months

10. Prevention

There are no proven strategies to prevent ALS, as the etiology in most cases remains unknown. However, modifiable risk factors that may reduce risk include:

Smoking cessation — smoking is the most consistently identified modifiable risk factor
Minimizing toxic exposures — reducing occupational exposure to heavy metals, pesticides, and industrial chemicals
Head injury prevention — use of helmets and safety equipment in sports and occupational settings
Genetic counseling — for family members of individuals with familial ALS; presymptomatic genetic testing is available for known ALS genes

For individuals carrying known ALS gene mutations, presymptomatic monitoring with serial neurological examinations, neurofilament light chain levels, and electrophysiological studies may allow earlier detection and future access to gene-targeted therapies such as tofersen for SOD1 carriers.

Overview
Epidemiology
Pathophysiology
Etiology and Risk Factors
Clinical Presentation
Diagnosis
Treatment
Complications
Prognosis
Prevention
Research Papers
Connections
Featured Videos

Back to Table of Contents

Research Papers

Curated PubMed topic searches on ALS (Amyotrophic Lateral Sclerosis). Each link opens a live PubMed query so the result set stays current as new studies are indexed.

Back to Table of Contents

Connections

Back to Table of Contents

Featured Videos

Why is it so hard to cure ALS? - Fernando G. Vieira

Signs and Symptoms of ALS

What I Would Do If I Had ALS (Amyotrophic Lateral Sclerosis): Lou Gehrig’s Disease – Dr. Berg