US-guided BoNT injections require a US instrument or machine and one or more transducers (Figure 7.7A-C). A linear transducer with a lower range of frequencies of 4 to 5 mHz (for deep muscles in larger patients) and a higher frequency range of 12 to 17 mHz will allow visualization of most muscles in pediatric and adolescent patients. A lower frequency range curvilinear transducer may be needed for deeply seated muscles in larger teenagers or adult patients. A hockey stick transducer is useful when imaging small patients, irregular surfaces, and for salivary gland injections. Other US supplies include US gel and transducer cleansers. Sterile transducer covers and gel packets are available, if desired. Other procedural supplies include alcohol or another skin cleanser,
hypodermic needles of various lengths, gloves, skin cleansers, gauze, and band aids/plasters. If the physician is also using EMG or E-Stim then monopolar insulated injection electrodes/surface electrodes, a cable to connect the reference electrode to the stimulator, and a stimulator or EMG machine are also required.
CLEANING AND MAINTENANCE
US machines and transducers should be cleaned as per the manufacturer's recommendations. Alcohol-based cleaners should not be used to clean transducer heads as this will damage the membrane (14).
ADVANTAGES AND LIMITATIONS OF US TO GUIDE BoNT INJECTIONS
As with the other localization techniques used to guide BoNT injections, there are advantages and disadvantages to using US guidance (2,4,45,48).
• US provides a detailed view of the muscle location, depth, and structures in the region of interest.
• The needle can be tracked continuously during the procedure to a position within the target muscle.
• The volume of injectate can be visualized during the procedure preventing overdistention or injection at one site.
• US decreases the risk of inadvertent needle penetration and/or injection of untargeted muscles, vessels, nerves, or other structures.
FIGURE 7.7 (A) Phillips IU 22 ultrasound; (B)Terason 3200 uSmart ultrasound; (C) ultrasound transducers.
• US localization/visualization of muscles can be performed quickly and is painless.
• Standard hypodermic needles, which may be used with US-guided procedures, may be less painful to insert than insulated monopolar needles.
• Viewing the US image may provide distraction to the patient during the procedure.
• Many procedures can be done with the patient unsedated.
Disadvantages of US Guidance for BoNT Injections
• Cost of the equipment is high.
• Physicians unfamiliar with US have a steep learning curve.
• Access to hands-on training, particularly training specific for BoNT injections may be limited.
• Until the physician is familiar with and facile with this technique, using US for BoNT injections may increase the time required for the procedure.
• US may not provide information on the activity of a muscle, for example, whether it is contributing to the patient's problem. However, US can be combined with EMG to obtain this information, if desired.
EVIDENCE SUPPORTING OR REFUTING THE USE OF US TO GUIDE BoNT INJECTIONS
There is increasing evidence that US is more accurate at localizing targets for BoNT as well as reducing adverse events and outcomes with BoNT injections. See the section "Clinical Studies; Comparison of Anatomic Localization With Other Localization Techniques for BoNT in
TABLE 7.1 ADVANTAGES AND DISADVANTAGES OF VARIOUS GUIDANCE TECHNIQUES FOR CHEMODENERVATION PROCEDURES
Limb Muscles," for evidence reviews for manual, EMG, and E-Stim guidance.
US guidance provides detailed information on the location of the target structure, the position of the needle, and location of the injectate during interventional procedures including BoNT injections. The evidence to support the superiority of US over other guidance techniques is increasing. While there is a steep learning curve when learning to use this technology, it is a technique physicians should consider learning and adding to their tool boxes.
When selecting a guidance or localization technique for BoNT procedures, physicians should be aware of the advantages and disadvantages of each of the available techniques (Table 7.1). This knowledge allows the physician to select the most appropriate guidance method for a given patient and procedure. Optimally physicians should be trained in and have access to all of these techniques. Additional head-to-head comparative studies are needed to further define and determine which of the discussed techniques is superior, and if so, for which muscles and/or procedures (59-61).