ELECTRICAL STIMULATION GUIDANCE FOR BoNT INJECTIONS

E-Stim is only useful for BoNT injections in muscles and is not required when targeting superficial muscles of the face. When using E-Stim for chemodenervation procedures, a nerve trunk can be stimulated, which will activate the entire muscle or group of muscles innervated by the stimulated nerve. Alternately, E-Stim can be used to stimulate distal motor nerve branches to activate muscle fascicles, for example, motor point stimulation or for MoEP targeting (1,40). Nerve trunk stimulation is typically used for diagnostic nerve blocks using local anesthetics or for phenol chemoneurolysis procedures. When using E-Stim to guide BoNT injections, physicians typically use motor point or endplate targeting techniques. These techniques are also used for phenol chemoneurolysis.

TECHNIQUE

When using E-Stim to guide BoNT injections, selection of the most appropriate site for needle insertion begins with inspection of the limb or body part, PROM, and AROM. To determine this site, many physicians also use published information on the location of endplates within a muscle, if this information is available (see the previous section on endplate location/targeting).

Once the location of the needle insertion is identified, the skin is cleaned or disinfected using the physician or institution's standard protocol. The physician then inserts the needle electrode through the skin advancing the needle toward the muscle target. When the physician estimates that the needle is near or within the muscle target, the stimulator is turned on and the intensity of stimulation is adjusted up to produce a muscle twitch, typically 1 to 3 mA. The needle is then advanced, redirected, or repositioned so that successive reductions in stimulation intensity continue to produce a maximum twitch in the desired muscle. When performing motor point blocks, the reported goal for stimulation intensity is 0.025 to 0.5 mA (1). If upon stimulation, contraction occurs in several muscles or a muscle other than the target, the clinician should reposition the needle to isolate the target muscle. Care must be taken to avoid overstimulation, which may lead to volume conduction.

EQUIPMENT FOR E-STIM GUIDANCE

A nerve stimulator (either a portable handheld unit or the stimulator from an electrodiagnostic machine) is used (Fig 7.1 A, C, D). The stimulator unit must have a port to which the cable of the insulated injecting electrode (needle) is connected. Other supplies include monopolar insulated injection electrodes/needles of various lengths, surface electrodes, a cable to connect the reference electrode to the stimulator, gloves, skin cleansers, gauze, and band aids/plasters.

ADVANTAGES AND LIMITATIONS OF E-STIM FOR BoNT INJECTIONS

Advantages

• The primary advantage of E-Stim when compared to using anatomic or EMG guidance is that E-Stim produces a direct visual feedback confirming that the needle is in the target muscle (i.e., muscle twitch/contraction and/or joint movement).

• E-Stim can be used in a patient for whom mass synergy or cocontraction limits the usefulness of EMG.

• E-Stim can be used when patients are sedated, where EMG is often not helpful.

• E-Stim may help isolate muscles that are difficult to isolate with EMG (deep or overlapping muscles, thin muscles such as the rectal sphincter).

• Studies have shown that E-Stim is more accurate than manual needle placement (24,44).

Disadvantages

There are a number of disadvantages to E-Stim not the least of which is that electrical stimulation increases the pain of the procedure (3,25,44,45).

• The stimulation and repeated insertions and adjustments of the needle that are necessary with E-Stim often cause pain and may be difficult for patients to tolerate, even cooperative adults.

• Most if not all pediatric patients will require general sedation and analgesia.

- Sedation may increase the risk of BoNT procedures, particularly in compromised patients.

- Sedation increases the time and cost of the BoNT procedure.

- Sedation increases the time away from work or school for the patient and/or parent.

• When the needle is outside of the target muscle, overstimulation (excessive current) can lead to volume conduction and therefore contraction in the target muscle. The resulting muscle twitch may lead the physician to falsely conclude that the needle is in the target muscle when in fact it is located elsewhere.

• When stimulation occurs for a motor nerve branch prior to its insertion in the targeted muscle, stimulation will lead to a muscle twitch but the needle tip is outside of the muscle. The physician may falsely conclude that the needle is in the target muscle and toxin may be injected at the wrong site.

EVIDENCE

Although E-Stim is commonly used to guide chemo-denervation procedures, there is limited evidence from controlled trials to support or refute the superiority of E-Stim over other guidance techniques for BoNT procedures.

• See the previous section, "Clinical Studies; Comparison of Anatomic Localization With Other Localization Techniques for BoNT in Limb Muscles," for studies comparing E-Stim to manual, EMG, and US guidance techniques.

• Surface E-Stim, following BoNT injections in calf muscle, has been shown to potentiate or increase the effects of the toxin in the stimulated muscle (46,47).

 
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