What Does a Conduction Block Mean on an EMG for MMN?

When you read the report from your EMG (electromyography) appointment, you may see the term conduction block. Simply put, a conduction block means that an electrical signal traveling down your nerve is stopped or significantly weakened before it can reach your muscle ^[1].

While people often call the entire appointment an “EMG,” a conduction block is specifically found during the Nerve Conduction Study (NCS) portion of the test, rather than the needle EMG portion.

The Kinked Hose Analogy

To understand what a conduction block is, imagine your nerve is like a garden hose, and the electrical signal is the water flowing through it. A conduction block is like a kink in the hose. The hose itself is physically intact, but the water (the electrical signal) is temporarily blocked from reaching the nozzle (the muscle). Because the signal doesn’t reach the muscle, the muscle cannot contract properly, causing physical weakness.

Another way to think about it involves an electrical wire. Your nerves are wrapped in a protective coating called myelin, similar to wire insulation. In many nerve diseases, the immune system simply strips this myelin away. However, Multifocal Motor Neuropathy (MMN) is more complex. The immune system attacks the nodes of Ranvier—specialized, uninsulated gaps between the myelin layers—and the membrane of the nerve fiber itself ^[2][3]. This targeted immune attack disrupts the microscopic ion channels needed to pass the electrical signal forward, causing the current to fail and stopping the signal ^[4].

How the Test Detects a Block

During a nerve conduction study, a specialist stimulates your nerve at two different points: one closer to your muscle (distal) and one further up your arm or leg (proximal). They then measure the electrical response of the muscle, which is recorded as a waveform called a CMAP (Compound Muscle Action Potential).

If the nerve is healthy, the muscle’s response will be roughly the same regardless of where the nerve was stimulated. However, if there is a conduction block, the signal from the stimulation point further up the limb will be significantly smaller when it reaches the muscle. Depending on the specific criteria your laboratory uses, the signal’s amplitude or area might drop by 30% or more compared to the closer stimulation point ^[5][6].

It is important to know that a drop in the CMAP does not automatically “prove” you have a conduction block or MMN. The neurologist must carefully interpret the waveforms to rule out technical factors or temporal dispersion—a phenomenon where the electrical signal simply spreads out and slows down rather than being truly blocked ^[6][7].

Key Features of a Conduction Block in MMN

• It typically affects only motor nerves: In MMN, conduction blocks typically stop the signals that tell your muscles to move. The sensory nerves (which feel pain, temperature, and touch) usually remain healthy and unaffected ^[8]. If you have significant sensory nerve blocks, your doctor may consider other diagnoses.
• It occurs at unusual spots: Common nerve injuries, like Carpal Tunnel Syndrome, happen because a nerve is physically pinched at a tight, “compressible” spot. In contrast, MMN frequently causes blocks at “non-compressible” sites along the nerve. Finding blocks in these unusual locations strongly supports an MMN diagnosis ^[9].
• It can sometimes be reversed: In the early stages of MMN, the inner nerve fiber (the axon) beneath the immune attack is still intact. This means the block is a temporary failure that can sometimes be reversed or improved with treatments like IVIg ^[10].
• The risk of permanent damage: If the immune attack continues untreated, the nerve fibers themselves can eventually suffer permanent damage, known as secondary axonal loss ^[10][11]. Once an axon is lost, that specific nerve pathway cannot easily recover, which is why early diagnosis and treatment are critical.

A Finding, Not a Diagnosis

A conduction block is an electrodiagnostic finding, not a final diagnosis on its own. Your neurologist will combine the results of your nerve conduction study with the pattern of your weakness, your sensory exams, and the exclusion of other diseases to determine if MMN is the true cause of your symptoms.