Amyotrophic Lateral Sclerosis (ALS)

01Pathophysiology

ALS is a relentlessly progressive neurodegenerative disease that selectively destroys both upper motor neurons in the motor cortex and lower motor neurons in the brainstem nuclei and spinal cord anterior horn cells. This dual involvement is the defining feature of the disease and the single most important concept tested on exams. No other common condition produces combined UMN and LMN findings in the absence of sensory loss.

Glutamate excitotoxicity is the most widely accepted pathogenic mechanism. Motor neurons are particularly vulnerable to excessive glutamate signaling at NMDA and AMPA receptors. Sustained glutamate stimulation causes intracellular calcium overload, mitochondrial dysfunction, generation of reactive oxygen species, and ultimately neuronal death. This is the rationale for using riluzole, which inhibits presynaptic glutamate release and blocks sodium channels to reduce excitotoxic injury.

In familial ALS (5 to 10% of cases), the two most commonly tested mutations are SOD1 (superoxide dismutase 1), which leads to toxic gain-of-function protein aggregation and oxidative stress, and C9orf72, a hexanucleotide repeat expansion that is the single most common genetic cause of both familial ALS and frontotemporal dementia (FTD). This genetic overlap explains why roughly 15% of ALS patients develop concomitant FTD, presenting with behavioral changes and executive dysfunction alongside motor neuron degeneration.

The reason the patient develops fasciculations, atrophy, and flaccid weakness is straightforward: lower motor neurons die, and denervated muscle fibers begin firing spontaneously (fasciculations) and then waste away (atrophy). The reason the same patient also develops hyperreflexia, spasticity, and a positive Babinski sign is that upper motor neurons, which normally modulate and inhibit spinal reflex arcs, are simultaneously degenerating. The loss of descending inhibitory input "releases" spinal reflexes, producing brisk tendon jerks and extensor plantar responses. This paradox of atrophy (LMN) coexisting with hyperreflexia (UMN) in the same limb is the hallmark finding that clinches the diagnosis.

Sensory neurons, autonomic neurons, and the neurons controlling extraocular muscles and sphincters are characteristically spared. That is why the vignette patient never has numbness, tingling, bladder dysfunction, or eye movement abnormalities. If any of these are present, you must consider an alternative diagnosis.

02Classification and Clinical Manifestation

Onset Subtypes

Revised El Escorial Diagnostic Categories

The Awaji-Shima criteria modified these categories by treating fasciculation potentials on EMG as equivalent to fibrillation potentials and positive sharp waves for evidence of active denervation. This increased diagnostic sensitivity, particularly in bulbar-onset ALS, without sacrificing accuracy.

Clinical Signs by Body Region

A notable exam-relevant sign is the split-hand sign: preferential wasting of the thenar eminence and first dorsal interosseous muscle with relative preservation of the hypothenar muscles. This dissociated pattern of hand wasting is relatively unique to ALS and is frequently described in vignettes.

03Diagnostic Workup

There is no single confirmatory "gold standard" test for ALS. The diagnosis is clinical, supported by electrodiagnostic studies and the exclusion of mimics. This is itself a testable concept: if a question asks for the "most accurate test," the answer is the combination of clinical criteria (El Escorial or Awaji) supported by EMG findings, not a single laboratory value.

The first step after clinical suspicion is EMG with nerve conduction studies (NCS). The EMG will demonstrate evidence of active denervation (fibrillation potentials and positive sharp waves) and chronic denervation with reinnervation (large-amplitude, long-duration motor unit potentials and reduced recruitment) across multiple myotomes. Fasciculation potentials are often prominent. The critical finding is that sensory nerve conduction must be normal. If sensory responses are abnormal, the diagnosis is not ALS, and you must consider a polyneuropathy or other condition.

MRI of the brain and the entire spinal cord is ordered not to confirm ALS but to exclude structural lesions that can mimic it. Cervical spondylotic myelopathy is the most common structural mimic: it can produce both UMN signs (from cord compression) and LMN signs (from root compression) in the upper limbs. A clean MRI in a patient with widespread UMN and LMN signs and no sensory deficits strongly supports the diagnosis of ALS.

Bloodwork serves a similar exclusionary purpose. A mildly elevated creatine kinase (typically 1.5 to 3 times the upper limit of normal, rarely exceeding 10 times) is common in ALS due to ongoing muscle fiber breakdown but is non-diagnostic. Thyroid function, B12, and serum protein electrophoresis help rule out metabolic and paraproteinemic neuropathies. Anti-GM1 ganglioside antibodies should be checked because multifocal motor neuropathy with conduction block (MMN) can closely mimic ALS but is treatable with intravenous immunoglobulin.

Pulmonary function testing, particularly the upright and supine FVC, establishes a respiratory baseline. A disproportionate drop in supine FVC suggests diaphragmatic weakness and is an early indicator of respiratory compromise, even when the upright FVC is still relatively preserved.

04Management and Treatment

Riluzole is the cornerstone tested answer for disease-modifying therapy in ALS. It works by reducing glutamate-mediated excitotoxicity. Its survival benefit is modest (approximately 2 to 3 months of extended median survival and a higher probability of being alive at 12 months), but it remains the most commonly examined pharmacologic intervention. The most important side effect to know for exams is hepatotoxicity: liver function tests must be monitored, and the drug is contraindicated in patients with baseline transaminases above five times the upper limit of normal.

Edaravone is a free radical scavenger that reduces oxidative stress. It was initially approved based on data showing a slower rate of functional decline in a subset of patients with early ALS and preserved respiratory function ($FVC\ge 80\mathrm{\%}FVC\; \backslash geq\; 80\backslash \%$ predicted). It is administered intravenously in treatment cycles. An oral formulation became available to reduce the burden of infusion visits. On exams, edaravone is the second disease-modifying agent to know after riluzole.

Tofersen is an antisense oligonucleotide that targets SOD1 mRNA and is approved only for patients with confirmed SOD1 mutations. It is delivered via intrathecal injection. While it reduces cerebrospinal fluid neurofilament light chain (a biomarker of neuronal injury), its clinical benefit on functional outcomes is still being studied. Know that it exists as a genotype-directed therapy, which makes it a potential exam topic when the vignette provides a family history and mentions genetic testing results.

Non-invasive ventilation (NIV) with BiPAP is the single most effective intervention for prolonging survival in ALS, adding 7 to 12 months or more in compliant patients. It is initiated based on symptoms of respiratory insufficiency (orthopnea, morning headaches from CO2 retention, daytime somnolence) or when objective testing shows predicted, , or nocturnal oximetry demonstrating desaturation. If a vignette describes a patient with known ALS presenting with morning headaches and daytime fatigue, the next best step is order pulmonary function tests and/or initiate BiPAP, not increase riluzole or add another drug.

PEG tube placement should be performed proactively, ideally while the FVC is still above 50%, because the procedural risk increases with worsening respiratory function. Weight loss exceeding 10% of baseline or recurrent aspiration episodes are strong indications.

End-of-life care is integral. ALS is uniformly fatal, with most patients dying of respiratory failure. Palliative care discussions, advance directive planning, and hospice referral should be addressed early. This is a tested concept: recognizing when the appropriate next step is a goals-of-care discussion rather than another intervention.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students get ALS questions wrong is by failing to recognize that UMN and LMN signs must coexist in the same patient without sensory involvement. When a vignette describes fasciculations, atrophy, and weakness (LMN features) alongside hyperreflexia, spasticity, and a Babinski sign (UMN features), and explicitly states that sensation is intact, the diagnosis is ALS. Students who focus only on the LMN findings may select a peripheral neuropathy or motor neuron disease variant like spinal muscular atrophy. Students who focus only on the UMN findings may select multiple sclerosis or a myelopathy. The tested competency is the ability to synthesize both sets of findings and recognize that their simultaneous presence, in the absence of sensory loss, narrows the diagnosis to ALS.

A second common trap involves the bulbar-onset presentation. A vignette may describe an older adult with progressive slurred speech, difficulty swallowing, and tongue wasting. Students may gravitate toward myasthenia gravis. The discriminator is that myasthenia gravis produces fatigable weakness (worse at end of day, better after rest) and does not cause fasciculations or muscle atrophy early in the course. ALS bulbar symptoms are progressive and non-fatigable, and tongue fasciculations are a strong clue.

Third, students are often tested on the next best step when respiratory symptoms appear. A patient with known ALS presenting with orthopnea and morning headaches is describing early respiratory failure due to diaphragmatic weakness. The answer is not intubation (which raises end-of-life considerations) and not a new medication. The answer is pulmonary function testing (FVC) and initiation of non-invasive ventilation (BiPAP).

Finally, know that riluzole is the tested pharmacologic answer for disease-modifying therapy. It does not reverse symptoms or dramatically extend life, but it is the first-line agent that exam-writers expect you to select. Do not confuse it with rivastigmine (used for Alzheimer disease) or rizatriptan (used for migraine). The mechanism (glutamate inhibition) and the required monitoring (liver function tests) are both testable.

The core competency being assessed across ALS questions is pattern recognition: identifying the combination of upper and lower motor neuron degeneration, confirming the absence of sensory and autonomic involvement, selecting the correct electrodiagnostic study (EMG/NCS), and knowing the stepwise management from riluzole initiation through respiratory support with BiPAP.