SFEMG is a specialized diagnostic procedure that investigates the connection between a nerve and a muscle, known as the neuromuscular junction. It precisely measures how reliably nerve impulses are transmitted to individual muscle fibers. SFEMG is valuable when doctors suspect a disorder causing muscle weakness or fatigue, such as Myasthenia Gravis, especially when other common tests have returned inconclusive results.
Preparing for the Single-Fiber EMG
Preparing for an SFEMG involves practical steps to ensure accurate results. Wear loose-fitting clothing, especially if the muscles in your arms or legs are being examined, for easy access. Avoid applying lotions, oils, or creams to your skin on the day of the test, as they can interfere with electrical recordings.
Managing certain medications is a significant consideration, particularly acetylcholinesterase inhibitors like Pyridostigmine (Mestinon). These drugs temporarily improve muscle function and can lead to a falsely normal test result if taken too close to the procedure. You will likely be instructed by your referring physician to stop taking these specific medications for at least 8 to 12 hours before your appointment. Discuss all medications, including blood thinners, with the clinic in advance, as adjustments may be necessary to proceed safely.
What Happens During the Procedure
The SFEMG procedure is performed while you are comfortably lying down on an examination table, and the entire process typically takes between 30 and 90 minutes. The technologist will first clean the skin over the muscle being studied to prepare the area for the electrode insertion. Muscles in the face, such as those above the eyebrow, are frequently tested due to their small, easily accessible fibers.
The examination involves the use of a specialized, very fine needle electrode, which is much thinner than typical needles and contains a small, 25-micrometer recording surface near its tip. This electrode is carefully inserted a short distance into the target muscle, which you will feel as a brief, sharp sensation similar to a scratch. The purpose of this unique electrode is to isolate and record the electrical activity from just one or two muscle fibers belonging to the same motor unit.
Once the electrode is in position, the test proceeds in one of two ways. In the “volitional” method, you will be asked to maintain a very slight, steady contraction of the muscle, such as looking upward or holding a gentle grip. Alternatively, in the “stimulated” method, a separate electrode delivers small electrical pulses to the nerve, causing the muscle to twitch slightly. In both methods, the goal is to record the paired electrical signals from two adjacent muscle fibers repeatedly.
The recording equipment measures the minute variation in the time interval between the firing of these two muscle fibers, a phenomenon known as “jitter.” You may experience a feeling of mild discomfort or a slight twitching sensation during the recording process, but the test is generally well-tolerated. The needle may be repositioned several times to sample activity from approximately 20 different pairs of muscle fibers to gather sufficient data for a thorough analysis.
Interpreting the Test Findings
The data collected during the SFEMG provides the doctor with an objective measure of neuromuscular transmission stability. The test specifically focuses on the jitter, which represents the temporal variability in the firing of the two muscle fibers belonging to the same motor unit. This variability reflects the slight fluctuations in the time it takes for the chemical signal at the neuromuscular junction to trigger an electrical response.
A small degree of jitter is considered normal, as the transmission process is not perfectly instantaneous. However, an increase in the mean jitter value beyond the established normal limits for that muscle indicates that the signal transmission is unstable or delayed. In more pronounced cases of dysfunction, the electrical impulse may fail to elicit a response from the second muscle fiber entirely, which is referred to as “blocking.”
Both increased jitter and blocking are definitive indicators of disturbed neuromuscular transmission. The neurologist analyzes the results, often calculating the mean consecutive difference (MCD) of the jitter values. If the jitter is abnormal in more than 10% of the sampled fiber pairs, the test is considered positive for a neuromuscular junction disorder. The complete report is then sent to your referring physician, typically within a few days to two weeks, who will discuss the findings and next steps with you.