Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disorder that targets motor neurons in the brain and spinal cord, leading to muscle weakness, atrophy, and eventual paralysis. Because ALS symptoms can overlap with other conditions, Electromyography (EMG) provides objective evidence of motor neuron damage to support the diagnosis. EMG findings, combined with a clinical examination, are used to meet standardized diagnostic benchmarks, such as the revised El Escorial or Awaji criteria.
The Role of Electromyography in Diagnosis
The primary purpose of the EMG is to electrically visualize the consequences of lower motor neuron death defining ALS. It measures the electrical activity of muscles, both at rest and during contraction, providing a functional assessment of the motor unit—the nerve cell and the muscle fibers it controls. This helps neurologists determine if muscle weakness stems from a problem with the nerve, the muscle, or the connection between them.
ALS affects motor neurons throughout the body, requiring the EMG to demonstrate widespread involvement. The test often detects abnormalities in muscles that still appear normal on a physical exam. This evidence of damage must be distributed across multiple distinct body regions, such as the cervical (neck/arms), lumbosacral (lower back/legs), thoracic (trunk), and bulbar (face/throat) areas, to support the diagnosis.
How the EMG and Nerve Conduction Study Are Perform
An EMG is a combination of two tests: the Nerve Conduction Study (NCS) and the needle electromyography. The NCS is performed first, using surface electrodes placed on the skin over a nerve to deliver a brief, mild electrical shock. This test measures how fast and how strongly the nerve carries the electrical signal, assessing the health of the nerve fibers.
The needle EMG is a separate part of the exam where a needle electrode is inserted directly into various muscles. The needle records the muscle’s electrical activity while it is relaxed and again when the patient gently contracts it. The specialist tests muscles across different body segments, including those not yet showing symptoms, to check for subclinical evidence of lower motor neuron involvement.
A key finding from the NCS in ALS is that the sensory nerves are typically spared. Motor nerve conduction studies may show a reduction in the strength of the muscle response, known as the compound muscle action potential (CMAP), reflecting the loss of motor axons. However, the sensory nerve action potentials (SNAPs) remain normal, which helps distinguish ALS from many types of peripheral neuropathy where sensory nerves are also affected.
Specific Electrical Signatures of ALS
The needle EMG reveals a complex pattern of electrical activity that reflects both the ongoing destruction of motor neurons and the body’s attempt to repair the damage. This combination of “active” and “chronic” denervation is the hallmark electrophysiological signature of ALS. Active denervation is detected by the spontaneous electrical firing of muscle fibers that have lost their nerve connection, indicating that motor neurons are currently dying.
Two specific signs of active denervation are fibrillation potentials and positive sharp waves. Fibrillation potentials are tiny, rapid electrical spikes that occur when a single muscle fiber fires spontaneously. Positive sharp waves are signals that appear sharp and positive on the EMG screen. The presence of these signals in multiple regions is strong evidence of widespread, ongoing lower motor neuron loss.
Another finding important in diagnosis is the presence of fasciculation potentials. Fasciculations represent the spontaneous firing of an entire motor unit and are often seen clinically as visible muscle twitches. While fasciculations can occur in healthy people, their widespread presence on EMG, combined with other signs of denervation, supports an ALS diagnosis.
The EMG also captures evidence of chronic denervation and reinnervation. When a motor neuron dies, the axons of neighboring, surviving motor neurons sprout new connections to re-supply the orphaned muscle fibers. This process results in Motor Unit Potentials (MUPs) that are abnormally large in amplitude and long in duration, often called “giant MUPs.” These large MUPs indicate that a single, surviving motor neuron is now responsible for controlling a much larger number of muscle fibers than normal.
Differentiating ALS from Other Neuromuscular Conditions
The primary function of the EMG in ALS is to confirm motor neuron loss and rule out other conditions that can mimic its symptoms. The ALS pattern is characterized by widespread active and chronic denervation in the muscles, combined with normal or near-normal sensory nerve function.
For example, the pattern found in peripheral neuropathy is distinctly different. The NCS usually shows either slowing of nerve conduction or a reduction in the sensory nerve response, indicating sensory nerve damage. Since ALS selectively affects the motor neurons while sparing the sensory pathways, the preservation of sensory nerve function on the NCS provides a clear distinction.
In conditions that affect the muscle itself, such as myopathy, the EMG typically shows small, low-amplitude MUPs, which is the opposite of the large, “giant MUPs” seen in ALS. Furthermore, a condition like Multifocal Motor Neuropathy (MMN), which can resemble ALS, often presents with conduction block on the NCS, where the electrical signal fails to pass through a segment of the nerve. The absence of significant conduction block in ALS is a defining electrophysiological feature that helps exclude MMN.