This is Dr. Goyal, and today we will be discussing single fiber EMG with a concentric needle and reviewing a waveform of increased jitter with blocking. Today we have a 65-year-old female who presents to us for the evaluation of right ptosis, double vision, and generalized fatigue. Her examination shows that she has right ptosis with suggestion of fatigue on prolonged upgaze, subjective double vision without any obvious extraocular movement abnormalities, and normal power testing. She has had blood work for acetylcholine receptor, MUSK, and LRP4 antibodies, which has returned negative. The repetitive nerve stimulation of the facial nerve is normal. There is no obvious clinical response to a periodostigmine treatment. She then goes on to have a single fiber EMG. Here we can see one abnormal pair with increased jitter and blocking in her single fiber EMG. So to review, a motor nerve fires and sends a signal distally and excites all the muscle fibers within its motor unit at more or less the same time. The variation in the time between the firing of two single muscle fibers within the same motor unit is termed jitter and reflects variation in neuromuscular junction transmission time. If there is abnormal neuromuscular junction transmission, then it may take longer for the signal to lead to muscle fiber depolarization, and thus there will be greater variability between the firing of the fibers, which is called increased jitter. If the prolongation is so severe that the muscle fiber does not depolarize, this will result in blocking of the muscle fiber. As you observe this slide, you can see that these pairs demonstrate increase in jitter. Normal jitter for a single pair in the frontalis is less than 37.8 microseconds, and this fiber demonstrates jitter of 190 microseconds. You will also note that there is blocking, and at times, as demonstrated by the red line, the second potential is no longer present. This is blocking. These findings support abnormal neuromuscular junction transmission and can be seen in diseases such as Myasthenia gravis.

Dr. Goyal

65-year-old female

ptosis

double vision

fatigue