Diagnosing myasthenia gravis (MG) can be complicated. For many years, doctors used the Tensilon test to help confirm a diagnosis of myasthenia gravis. However, in 2018, the U.S. Food and Drug Administration (FDA) discontinued the test due to its limitations and the availability of safer, more accurate diagnostic methods.

So, what was the Tensilon test, and why isn’t it used anymore? Here’s what you need to know.

What Was the Tensilon Test?

Introduced in 1952, the Tensilon test was designed to help diagnose myasthenia gravis by assessing how muscles responded to a drug called edrophonium (sold under the brand name Tensilon). If a person’s muscle weakness temporarily improved after receiving Tensilon, doctors considered this a sign of MG. While the test provided a quick diagnosis, it was not always reliable.

Understanding What Causes MG

A neurotransmitter is a chemical messenger that helps nerve cells communicate with muscles to make movement possible. Acetylcholine (ACh) is the neurotransmitter responsible for triggering muscle contractions — a process needed for movements like standing, lifting an arm, chewing, or opening the eyes. For acetylcholine to work, it must bind to special receptors on muscle cells.

In most people with MG, the immune system mistakenly produces autoantibodies — proteins that attack the body’s own cells instead of harmful invaders. These autoantibodies block or destroy acetylcholine receptors (AChRs), making it harder for nerves to send signals to muscles. Over time, this leads to progressive muscle weakness (weakness that worsens with activity).

How Did the Tensilon Test Work?

Edrophonium, the active ingredient in Tensilon, temporarily boosted nerve-to-muscle communication by blocking an enzyme that breaks down acetylcholine. This allowed more acetylcholine to reach its receptors, improving muscle strength — at least for a short time. If a person with MG had anti-AChR antibodies, the test would provide temporary symptom relief, helping doctors confirm the diagnosis.

How Was the Tensilon Test Performed?

The Tensilon test was a simple procedure, but it required careful medical supervision due to potential side effects. Over time, doctors adjusted the testing method to improve safety.

Initial Assessment

Before the test, a doctor would evaluate the person’s muscle strength and symptoms. This included checking for common signs and symptoms of MG, such as ptosis (drooping eyelids) or dysphagia (difficulty swallowing) and dysarthria (trouble speaking).

Administration of Edrophonium

To perform the test, a doctor would slowly inject edrophonium into a vein (intravenously, or through an IV), starting with a low dose and gradually increasing it. This step-by-step approach allowed the doctor to monitor for any unexpected reactions.

Monitoring the Response

After receiving edrophonium, the person was observed for any immediate changes in muscle strength. Usually within about a minute, those with MG often experienced temporary improvement — for example, they might be able to lift an arm or hold up their head more easily. A positive response helped confirm an MG diagnosis.

If the person experienced serious side effects — such as trouble breathing, irregular heartbeat, or fainting — the doctor would quickly give another medication, atropine, to counteract the effects.

Positive and Negative Results

If a person’s muscle strength temporarily improved after receiving edrophonium, the test was considered positive, suggesting myasthenia gravis. This improvement typically lasted only a few minutes and indicated that muscle weakness was due to a lack of acetylcholine at the neuromuscular junction — the site where nerves and muscles communicate. Since disrupted signaling at this junction is the main cause of MG symptoms, a positive test supported an MG diagnosis.

If there was no change in muscle strength, the test was considered negative. In these cases, doctors suspected that the muscle weakness was due to another condition rather than MG.

Problems With the Tensilon Test

Although the Tensilon test was widely used for many years, significant drawbacks limited its reliability as a diagnostic tool.

False Positives

One reason the Tensilon test fell out of use is that it wasn’t perfect — it sometimes produced false positives and false negatives.

The test’s specificity — or its ability to correctly rule out people who didn’t have MG — was 97 percent. This meant that 3 percent of people without MG could receive a false-positive result, meaning the test incorrectly suggested they had MG.

False positives occurred when people without MG showed temporary muscle improvement after receiving edrophonium. This could happen due to the placebo effect or observer bias. As a result, some people were misdiagnosed with MG when they actually had another condition.

False Negatives

Sensitivity refers to a test’s ability to correctly identify all people who actually have a disease. The Tensilon test’s sensitivity was 88 percent, meaning that 12 percent of people with MG received false-negative results, incorrectly suggesting they didn’t have the condition.

False negatives occurred when people with MG didn’t show any muscle improvement after receiving edrophonium. This happened because not everyone with MG has autoantibodies to AChRs. Some people have a different type of autoantibody, called muscle-specific receptor kinase (MuSK) antibodies, instead of AChR antibodies. Additionally, about 10 percent of people with MG have no detectable autoantibodies at all, a form of the disease known as seronegative MG.

Because of these inconsistent results, doctors found it difficult to rely solely on the Tensilon test for an accurate MG diagnosis, leading to its eventual discontinuation.

Side Effects and Risks

The Tensilon test also posed potential risks to the person being tested. Edrophonium, the drug used in the test, could cause a range of side effects, including:

• Bradycardia (slow heart rate)
• Bronchospasm (closing of the small airways in the lung)
• Seizures
• Trouble speaking or swallowing
• Cough and wheezing
• Shortness of breath
• Nausea, vomiting, and diarrhea
• Sweating
• Frequent urination
• Muscle twitching
• Loss of bowel control

Due to these risks, the test required close monitoring and wasn’t safe for everyone. For example, people with urinary or digestive tract blockages were not eligible for the test. Additionally, testing individuals with heart or lung conditions required extreme caution.

Ultimately, because of its limited accuracy and potential risks, the FDA discontinued the Tensilon test in 2018 as a diagnostic tool for MG.

Safer and More Accurate Tests To Diagnose MG

In the United States, the edrophonium test has been replaced by modern diagnostic techniques, including advanced blood tests and nerve function studies.

Advances in Blood Tests

Recent scientific advances have led to simple blood tests that detect autoantibodies linked to MG, including AChR and MuSK antibodies. These tests provide a safe, accurate way to diagnose MG without the risks associated with the Tensilon test.

Electromyography and Repetitive Nerve Stimulation

Other tests, like electromyography (EMG) and nerve conduction studies, allow doctors to assess how well nerves and muscles function. EMG measures the electrical activity in muscles to see if they are responding properly to nerve signals. Nerve conduction studies check how fast and how well electrical signals travel along nerves to detect any nerve damage or dysfunction.

These tests provide detailed data on both the presence and severity of neuromuscular dysfunction, helping to confirm an MG diagnosis.

How Is Edrophonium Used Today?

Although the Tensilon test is no longer used to diagnose MG in the U.S., it may still be performed in other countries where newer diagnostic methods are less accessible.

In the U.S., edrophonium is now used for other medical purposes. For example, doctors may use it to reverse the effects of certain medications or to keep muscles from contracting (tightening) during surgery.

Talk to Your Healthcare Provider

If you’re experiencing symptoms of MG, it’s important to consult with a healthcare professional, such as a neurologist (specialist in conditions of brain and nervous system). They can guide you through the most current and reliable testing options available. Early diagnosis means earlier and more effective treatment. While the Tensilon test once played an important role in diagnosing MG, there are safer and more accurate diagnostic tools available today.

Read more about how MG is diagnosed.

Find Others Who Understand

On MGteam, the social network for people living with myasthenia gravis and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with myasthenia gravis.

How was your myasthenia gravis diagnosed? Share your experience in the comments below, or start a conversation by posting on your Activities page.