The difference in these recording techniques is dependent largely upon the recording surface area of each needle. Single fibre EMG needles are characterized by a very small side-port that is 25 μm [1] in size and serves as the recording surface. This gives rise to a very small recording radius enabling potentials to be recorded from only 1–2 muscle fibers at a time. Conventional EMG utilizes needles with a larger recording surface area that may be either concentric or monopolar. Concentric needles are characterized by their beveled tips that contain a side port with a recording area of 150 × 580 μm [1]. Monopolar needles are characterized by their pointed tip with a recording surface area of 640 μm [1]. Of note, when concentric needles are used, a single ground surface electrode is required, while the use of a monopolar needle requires both ground and reference surface electrodes. MacroEMG needles are unique since they possess a recording surface area that is many times larger than these other needles. Although the needle contains a side-port similar to single fiber EMG, the distal 15 mm of the macroEMG needle is un-insulated, which gives rise to the larger recording surface area allowing it to perform analysis of all muscle fibers belonging to an individual motor unit (Table 13.1 [1, 2]).

Some of the traditional motor unit action potential (MUAP) measurement and analysis techniques described by Buchtal in the 1950s [3, 4] are time-consuming and pose significant challenges even in cooperative adult patients. Pediatric neurophysiologists are frequently called upon to differentiate normal versus abnormal neuromuscular pathology without the luxury of time and patient cooperation. In infants and younger children, the term “burst” EMG analysis is sometimes used to describe the sampling that occurs with a patient who is not able to follow commands consistently, in contrast to the traditional motor unit analysis technique used in older children, adolescents, and adults. Though experienced pediatric neurophysiologists are usually able to obtain accurate and valuable data from these smaller samplings, there are circumstances in which the amount of data collected is not optimal.

Compounding this difficult undertaking is that concentric needle electrodes record from about 10–20 muscle fibers of a motor unit at a time. While in adults one can overcome this limitation by moving the needle electrode inside the corridor of a motor unit’s territory to explore other areas, in children, especially infants and toddlers, this luxury of needle motor unit territory exploration is not always consistently attainable. Figure 13.2 shows a schematic representation of a concentric needle electrode inside the muscle, and the contribution of the nearby muscle fibers belonging to the same motor unit to the MUAP.

Macro EMG as described by Stålberg [5] is a well-accepted technique for estimating the electrical size of a motor unit. It makes use of a large recording surface that makes it possible to sample the majority of muscle fibers of a motor unit, in contrast to the 10–20 muscle fibers sampled by the tip of a concentric needle electrode. As useful as the Stålberg technique is, a practical limitation is that it uses a single fiber needle electrode to record the cannula potential and does not afford a concomitant view of the concentric MUAP from which the Macro potential is derived. Another practical limitation is the significant cost associated with the purchase of macroEMG needles. To overcome these limitations, a technique known as concentric macro EMG or ConMac was developed by Jabre [6] that uses the cannula of a standard concentric needle electrode to record the concentric electrode’s cannula potential, referenced to a nearby surface electrode, allowing for a simultaneous view of the concentric MUAP and its corresponding macro potential [7]. To clearly distinguish between macro potentials derived from these two electrode types, we refer to the single fiber electrode cannula derived potential as the Macro potential, and to the concentric electrode cannula derived potential as the ConMac potential.

In this chapter, we will describe the ConMac EMG technique and its applicability to quickly and easily distinguish between normal, neurogenic, and myopathic motor unit changes.

In our experience, ConMac EMG is a fairly simple technique to learn and can be performed in under 5 minutes with a conventional concentric needle electrode using an EMG machine that allows for two channel recording.