Single-Fiber Electromyography (SFEMG) is a specialized diagnostic procedure used to investigate the function of the neuromuscular junction, the point where a nerve communicates with a muscle fiber. This technique is more sensitive than a standard EMG because it isolates and measures the electrical activity of individual muscle fibers. The primary purpose of SFEMG is to detect subtle abnormalities in how nerve signals are transmitted to muscle cells, diagnosing disorders of neuromuscular transmission. SFEMG is frequently used when conditions like Myasthenia Gravis are suspected, especially when other tests have produced inconclusive results.
Preparing for the Test
Preparation for the Single-Fiber EMG test involves discussing current medications, especially acetylcholinesterase inhibitors like Pyridostigmine (Mestinon), with your physician in advance. These medications enhance nerve-to-muscle signaling and can mask abnormalities, potentially leading to a false normal result. You may be instructed to skip your morning dose or stop the medication for up to 24 hours before the test, but only under specific medical guidance.
Avoid applying lotions, oils, or creams to your skin on the day of the procedure, as these interfere with the electrical contact of the electrodes. Wear loose-fitting clothing that allows easy access to the muscles being examined, which often include those in the arms, legs, or face. Keeping your hands and feet warm, such as by wearing socks or gloves, can also improve the quality of the recordings.
What Happens During the Test
The SFEMG procedure is performed by a neurologist or neurophysiology specialist in a clinical setting. You will be asked to lie down comfortably while the physician cleans the skin over the muscle to be tested. The test duration depends on the number of muscles examined.
The procedure uses a specialized single-fiber needle electrode, which is much finer than a standard EMG needle. This electrode isolates the electrical signals from one or two muscle fibers at a time. The insertion of this needle may cause brief, mild discomfort or a feeling of pressure, but nothing is injected into the muscle.
The recording process uses two methods: voluntary activation or nerve stimulation.
Voluntary Activation
In voluntary SFEMG, you perform a slight, steady muscle contraction, such as gently holding your finger or maintaining a light frown. This requires precise, sustained cooperation to keep the targeted muscle fibers firing at a slow, constant rate. The physician will guide you continuously through these actions.
Nerve Stimulation
In some cases, particularly when testing facial muscles, stimulated SFEMG is used. This involves applying a small electrical pulse to the nerve supplying the muscle. This pulse triggers the muscle fibers to fire, causing a brief, uncomfortable tingling or tapping sensation on the skin.
The purpose of both techniques is to capture the electrical relationship between muscle fibers belonging to the same motor unit. The needle is repositioned several times within the muscle to gather data from approximately 20 different muscle fiber pairs.
Interpreting the Findings
The results are not immediately available after the procedure, as they require specialized analysis by the neurophysiologist. The central focus of the SFEMG analysis is a measurement called “Jitter,” which quantifies the variability in the timing of muscle fiber firing. Jitter measures the tiny fluctuations in the time interval between the electrical signals of two muscle fibers belonging to the same motor unit.
When a nerve signal reaches the neuromuscular junction, it should consistently trigger muscle fibers. An increased jitter value indicates instability at the neuromuscular junction, meaning the timing of the signal transmission is unreliable. This finding reflects a compromised ability of the junction to successfully transmit the signal.
The study is considered abnormal if the mean jitter value exceeds the established normal limit for that muscle, or if more than 10% of the recorded muscle fiber pairs show abnormally high jitter. This points directly to a defect in neuromuscular transmission.
While increased jitter is a highly sensitive indicator of conditions like Myasthenia Gravis, it is not specific to a single disease and can also be seen in other neuromuscular disorders, such as Lambert-Eaton Myasthenic Syndrome. The final report, including the Jitter analysis, is sent to your referring physician, who correlates the results with your clinical symptoms and other test findings to establish a diagnosis.