The Repetitive Nerve Stimulation (RNS) test is a diagnostic procedure in neurology. It assesses the function of nerves and the muscles they control. This test helps neurologists understand how electrical signals are transmitted between nerves and muscles, which can indicate underlying neurological conditions.
Understanding the RNS Test
The RNS test works by delivering a series of small electrical impulses to a motor nerve. Electrodes placed on the skin over a corresponding muscle record the muscle’s electrical response, known as a compound muscle action potential (CMAP). The test measures how the strength of the muscle’s electrical response changes over a sequence of these repeated stimulations, observing how the nerve’s ability to transmit signals to the muscle changes with repeated activation. When a nerve is stimulated repeatedly, the amount of neurotransmitter, acetylcholine, released at the neuromuscular junction can temporarily decrease. In healthy individuals, the muscle’s response remains stable despite this depletion.
Conditions Diagnosed by RNS
The RNS test is commonly used to diagnose disorders affecting the neuromuscular junction, the specialized site where nerve cells communicate with muscle fibers. Myasthenia Gravis (MG) is a common autoimmune condition where the immune system attacks acetylcholine receptors on muscle fibers, leading to muscle weakness and fatigue. The RNS test helps reveal a characteristic “decremental” response in MG, meaning the muscle’s electrical response gets progressively smaller with repeated stimulation. Lambert-Eaton Myasthenic Syndrome (LEMS) is another neuromuscular junction disorder that RNS can help identify; unlike MG, LEMS shows an “incremental” response, where the muscle’s electrical activity increases after a brief period of intense muscle contraction or high-frequency stimulation. The RNS test can also help diagnose botulism, a condition caused by toxins that impair nerve-muscle communication.
What to Expect During an RNS Test
Preparation for an RNS test is minimal. Patients should avoid applying lotions, creams, or oils to their skin on the day of the test to ensure proper electrode contact; fasting is not required, and wearing loose-fitting clothing allows easy access to the limbs being tested. During the test, a neurophysiology technologist or a neurologist places small metal disc electrodes on the skin over the nerve and the muscle, and a brief series of electrical shocks are delivered to the nerve. Patients may experience a sensation described as a quick zap, tingling, or mild discomfort during the electrical impulses; the test is well-tolerated, and the sensation lasts only a few milliseconds. The procedure takes about 30 to 60 minutes, depending on the number of nerves and muscles being assessed.
Interpreting RNS Test Findings
A neurologist carefully analyzes the RNS test results. In a healthy individual, the muscle’s electrical activity (CMAP amplitude) remains consistent with each successive electrical stimulation, indicating normal signal transmission across the neuromuscular junction. An abnormal RNS test result shows a significant change in the CMAP amplitude over successive stimulations. For Myasthenia Gravis, a “decremental” response is characteristic, where the amplitude of the muscle’s electrical response decreases by more than 10% between the first and fourth or fifth stimuli during low-frequency stimulation (e.g., 2-5 Hz); conversely, in Lambert-Eaton Myasthenic Syndrome, an “incremental” response is observed, meaning the CMAP amplitude significantly increases by more than 100%, after high-frequency stimulation or a brief period of voluntary muscle contraction. These specific patterns, combined with clinical symptoms and other diagnostic information, help the neurologist reach an accurate diagnosis.