Electromyography (EMG) is a diagnostic procedure used to assess the health and function of muscles and the nerve cells that control them, known as motor neurons. This test measures the electrical activity produced by skeletal muscles, helping physicians determine if muscle weakness, numbness, or pain is caused by a problem in the muscle itself or in the nerve supply leading to it. Understanding the meaning of these metrics is the first step toward interpreting the results.
Understanding the Baseline: Normal EMG Findings
The interpretation of any EMG study relies on comparing the patient’s results against established normal reference values. These baseline values are standardized based on factors like age, limb temperature, and the specific muscle or nerve being tested.
A healthy nerve conduction study (NCS) shows results within a specific normal range for both the speed and the strength of the electrical signal. For example, the conduction velocity in major nerves is expected to be above a certain threshold, and the amplitude, which reflects the signal strength, must also exceed a minimum threshold.
When a needle electrode is inserted into a healthy muscle at rest, the recording should be electrically silent, meaning no spontaneous electrical activity is detected. A brief burst of activity is expected only upon the initial movement of the needle, which quickly subsides. During a minimal voluntary contraction, the muscle produces stable, appropriately shaped electrical signals known as Motor Unit Action Potentials (MUAPs).
Interpreting Nerve Conduction Study Results
The Nerve Conduction Study (NCS) evaluates the function of the peripheral nerves by applying a small electrical stimulus and measuring the nerve’s response. Three primary metrics are analyzed to determine the type and location of any nerve damage.
Amplitude
The amplitude, or height, of the electrical signal measures how many nerve fibers successfully transmit the signal. A low amplitude in the Compound Muscle Action Potential (CMAP) or Sensory Nerve Action Potential (SNAP) suggests Axonal Loss, which is damage to the core nerve fiber itself. This results in a weaker overall signal because fewer nerve fibers are available to conduct the electrical impulse. Severe axonal loss indicates damage to the nerve structure and may correlate with slower recovery.
Conduction Velocity
Conduction velocity measures the speed at which the electrical impulse travels along the nerve segment. A significantly slow velocity suggests Demyelination, which is damage to the myelin sheath, the fatty insulation surrounding the nerve fiber. Damage to the myelin causes the signal to slow down considerably. While demyelination is the primary cause of severe slowing, mild reductions in velocity can also occur in severe axonal loss due to the selective loss of the fastest conducting fibers.
Latency
Latency is the time it takes for the electrical signal to travel from the point of stimulation to the point of recording. This measurement is relevant for diagnosing focal nerve compressions, such as carpal tunnel syndrome. Prolonged latency suggests the signal is taking too long to arrive, which can be due to demyelination or nerve compression at a specific site. Comparing the latency of the affected nerve to an unaffected nerve over the same distance is often more informative than relying on an absolute value.
Interpreting Needle Electromyography Results
The needle EMG component involves inserting a fine needle electrode directly into the muscle to record its electrical activity at rest and during contraction. This technique provides detailed information about the muscle fibers and the motor units that control them.
Resting Activity
A healthy muscle is electrically silent at rest, but abnormal spontaneous activity can be detected in damaged tissue. The presence of Fibrillation Potentials (Fibs) and Positive Sharp Waves (PSWs) indicates denervation, meaning the muscle has lost its nerve supply. These potentials represent individual muscle fibers firing spontaneously because they are no longer under the control of the nerve. They signify the muscle’s response to an acute or chronic loss of innervation.
Motor Unit Action Potentials (MUAPs)
MUAPs are the electrical signals generated when a motor unit is activated during voluntary muscle contraction; their shape and size are key to interpretation. Abnormalities are classified into neuropathic and myopathic changes. Neuropathic changes, related to nerve damage, typically result in MUAPs that are large in amplitude and long in duration. This occurs because surviving nerve axons sprout and reinnervate orphaned muscle fibers, causing a single motor unit to control a much larger territory than normal.
Conversely, Myopathic changes, related to primary muscle disease, are characterized by MUAPs that are small in amplitude and short in duration. This morphology arises because the muscle fibers are damaged, and fewer are available to contribute to the overall motor unit signal. In myopathic conditions, the MUAPs may also appear polyphasic, meaning they have an increased number of phases, due to the asynchronous firing of the remaining muscle fibers.
Recruitment Pattern
Recruitment refers to the number of motor units activated and their firing rate as muscle force increases. During a maximal voluntary contraction, the electrical activity should be dense, forming a “full” interference pattern. In cases of nerve damage, or decreased recruitment, fewer functional motor units are available, resulting in a sparse interference pattern.
In myopathic conditions, the recruitment pattern is often described as early or rapid. Since each motor unit is weakened by muscle fiber damage, the central nervous system must activate an excessive number of motor units to generate even a small amount of force. Consequently, a full interference pattern is achieved much earlier and at lower levels of effort than in a healthy muscle.
Connecting Findings to Diagnostic Categories
The most meaningful aspect of the EMG is the synthesis of NCS and needle EMG findings to establish an overall pattern of disease. This combined analysis helps classify the disorder into a general category of damage.
Pattern 1: Primary Neuropathy (Nerve Disease)
A primary neuropathy is characterized by clear abnormalities in both parts of the test. The NCS will show either significant slowing (demyelination) or low amplitude (axonal loss). The needle EMG reinforces this finding by revealing abnormal spontaneous activity, such as fibrillations, and displaying the characteristic large, long-duration MUAPs indicative of nerve reinnervation. This pattern helps localize the problem to the peripheral nerve or the motor neuron.
Pattern 2: Primary Myopathy (Muscle Disease)
A myopathic pattern points to a problem originating in the muscle tissue itself. The NCS results are typically normal because the nerve controlling the muscle is undamaged. However, the needle EMG is abnormal, showing small, short-duration, and often polyphasic MUAPs. This is accompanied by the early or rapid recruitment of these small units during minimal effort.
Pattern 3: Neuromuscular Junction Disorders
Disorders affecting the neuromuscular junction (NMJ), where the nerve meets the muscle, require specialized testing because the nerve and muscle may appear structurally normal. These conditions, including Myasthenia Gravis, often present with normal standard NCS results. Diagnosis relies on Repetitive Nerve Stimulation (RNS), where the nerve is stimulated multiple times in quick succession. In some post-synaptic disorders, RNS reveals a greater than 10% decrease in the CMAP amplitude between the first and fourth electrical responses.