Single-Fiber EMG

Single-fiber electromyography (SFEMG) is the most sensitive diagnostic test for myasthenia gravis. It can detect abnormal neuromuscular transmission even when antibody tests and standard EMG are normal.

How It Works

Normal Neuromuscular Transmission

When a nerve signals a muscle to contract:

1. Nerve releases acetylcholine
2. Acetylcholine binds to muscle receptors
3. Muscle fires with consistent timing

The time between nerve signal and muscle response should be very consistent.

Abnormal Transmission in Myasthenia

In myasthenia gravis:

• Antibodies damage acetylcholine receptors
• Transmission becomes unreliable
• Timing varies (increased "jitter")
• Sometimes transmission fails completely ("blocking")

What SFEMG Measures

Jitter: Variation in timing between nerve signal and muscle response

• Normal: Very consistent timing
• Abnormal: Variable timing (increased jitter)

Blocking: Complete failure of transmission

• Indicates more severe involvement

When SFEMG Is Ordered

• Suspected myasthenia gravis with negative antibodies
• Ocular myasthenia (often seronegative)
• Normal repetitive nerve stimulation but clinical suspicion high
• Confirming diagnosis when other tests inconclusive

The Procedure

Preparation

• May need to stop pyridostigmine temporarily (ask your doctor)
• No special preparation otherwise
• Takes 30-60 minutes

During the Test

• Small needle electrode inserted into muscle
• Usually tested in forehead (frontalis) or forearm (extensor digitorum)
• Multiple muscle fiber pairs examined
• You'll be asked to gently contract the muscle
• May feel mild discomfort

After the Test

• Minimal discomfort
• Can resume normal activities
• No special care needed

Interpreting Results

Abnormal SFEMG

• Increased jitter values
• May have blocking
• Supports diagnosis of myasthenia gravis
• But not specific-other conditions can cause abnormal jitter

Normal SFEMG

• Makes myasthenia gravis less likely
• But doesn't completely rule it out
• May be normal in very mild or localized disease

Sensitivity and Specificity

For ocular myasthenia:

• SFEMG sensitivity ~85-90%
• Much higher than antibody tests (~50%)

Limitations

Not Specific for Myasthenia

Other conditions can cause increased jitter:

• Other neuromuscular diseases
• Motor neuron disease
• Some neuropathies
• Recent botulinum toxin injection

Operator Dependent

• Requires specialized training
• Results depend on examiner skill
• Not available at all centers

Muscle Selection

• Results may vary by muscle tested
• Clinically affected muscles may be harder to test
• Frontalis often used for suspected ocular MG

Comparison with Other Tests

vs. Repetitive Nerve Stimulation (RNS)

vs. Antibody Testing

• SFEMG more sensitive than AChR antibodies
• Antibodies more specific when positive
• Tests are complementary

What to Expect from Results

If Abnormal

• Supports myasthenia diagnosis
• Further workup may include CT chest (thymoma)
• Treatment discussion

If Normal

• Makes myasthenia less likely
• Consider other diagnoses
• May repeat if clinical suspicion remains

Frequently Asked Questions

Does a positive single-fiber EMG prove myasthenia gravis?

It strongly supports a neuromuscular junction problem, but it is not perfectly specific. The result has to match the symptoms, exam, antibody testing, and medication history.

Why might my antibody tests be negative but SFEMG abnormal?

Ocular myasthenia can be seronegative, meaning standard blood tests do not detect an antibody even when the disease is present. SFEMG can pick up the transmission problem directly.

Should I stop pyridostigmine before the test?

Only if the testing neurologist tells you to. Some centers ask patients to hold pyridostigmine before testing; others adjust interpretation based on medication timing.