Needle EMG guidance is useful

Botulinum toxin (BTX) is used increasingly in neurologic practice. Needle electromyographic (EMG) analysis of muscles in disorders treated with BTX is often useful in diagnosis, assessing treatment response, and searching for denervation when neutralizing antibodies are suspected. These are important uses of needle EMG. Although needle EMG guidance for injection of BTX is an accepted technique, some argue over whether this technique increases treatment efficacy. Needle EMG guidance is more important in some diseases than others. For example, it is hard to argue in support of needle EMG–guided BTX injection for the treatment of cosmetic wrinkles (although a recent review in the dermatologic literature claimed benefits for its use). By contrast, it would be difficult to treat laryngeal dysphonia without it. The essential difference between these examples is the location and accessibility of the target muscles, being easily accessible in patients with wrinkles, but difficult in the setting of spasmodic dysphonia. The main advantage of needle EMG guidance is precision of toxin placement. Does precision improve treatment efficacy? There are theoretical advantages of needle EMG– guided delivery of BTX. First, clinical examination alone is often incapable of detecting dystonic muscles. Examination has a sensitivity of only 0.35 and specificity of 0.74 — a finding that holds even for superficial muscles such as the trapezius and scalenus posterior. The sensitivity of clinical examination would be lower for deeper muscles, where needle EMG guidance would likely improve injection accuracy. The importance of targeting the motor endplate has been emphasized in animal models. Using rat anterior tibialis muscle, Shaari and Sanders demonstrated that toxin injection into the motor endplate region produces the greatest paralysis. Injections only 0.5 cm away from the region resulted in a 50% decrease in paralysis. In a canine model, placement of BTX at the motor endplate potentiated the toxic effect and decreased muscle force generation. These results support the premise that the closer the toxin is placed to the motor endplate, the greater the effect and the lower the toxin requirement. It is known that BTX diffuses outside of target muscles even when these muscles are localized by electrical stimulation. This spread is estimated to be 2.5–4.5 cm, with the size of the diffusion field being proportional to the amount of toxin delivered, and can occur across fascial planes. It makes sense that the toxin should be delivered to the target muscle; the further off-target the injection, the more that uninvolved, nondystonic muscles will be weakened. In one series of patients with upper limb dystonia, weakness of uninjected muscles adjacent to those injected was found in 63% of patients. Spread to and weakness of the uninjected muscles was the primary cause of a suboptimal response in 15% of these patients. Is exact localization of muscles advantageous? The answer probably depends on the disease. In blepharospasm and hemifacial spasm (HFS), the involved muscles are superficial and needle EMG may not be necessary. The small lower facial muscles in HFS may be hard to locate exactly, and any misinjection may lead to more extensive diffusion of toxin along fascial planes and increase unwanted cosmetic side effects. Limb dystonia, such as occupational cramp, often involves small extrinsic and intrinsic hand muscles that lie in close proximity to one another. Inappropriate toxin placement can worsen the functional outcome by weakening nondystonic adjacent muscles. Furthermore, it is occasionally important to weaken particular fascicles of individual Abbreviations: BTX, botulinum toxin; EMG, electromyography; HFS, hemifacial spasm