ELECTROMYOGRAPHY GUIDANCE FOR BoNT INJECTIONS

EMG is the most commonly reported or recommended localization technique used for BoNT injections in muscle targets, other than anatomic guidance (1,22, Botox PI, Dysport PI, Xeomin PI, Myobloc PI). Obviously, EMG is not helpful for localizing or injecting nonmuscle targets and is not necessary for most muscles of the face.

When using EMG to guide BoNT injections, available equipment options include a standard electrodiagnostic machine or one of the inexpensive commercially available small portable EMG audio or audiovisual amplifier units. EMG-guided injections require the use of sterile, single-use, insulated injecting needle electrodes. These needles are available in a range of lengths (25-75 mm). The choice of needle length/size is determined by the depth of the target muscle(s).

TECHNIQUE FOR EMG-GUIDED BoNT PROCEDURES

When EMG is used to guide BoNT injections, identifying the optimal site for needle insertion begins with inspection of the limb followed by palpation and ROM. PROM or AROM while palpating the limb or body part may facilitate muscle localization. Once the site for needle insertion has been selected, the skin is then disinfected or cleansed and the needle electrode is inserted through the skin, advancing the needle to the target while listening for audible EMG activity. To facilitate muscle localization, the patient should be instructed to either relax or contract the target muscle (if he or she has adequate selective motor control or can cooperate with this request). If the patient has impaired selective motor control, asking him or her to contract (or relax) a given muscle may result in mass synergy or cocontraction in multiple muscles, which may limit the accuracy of EMG guidance. In patients with impaired motor control, another option is for the examiner to perform PROM while recording EMG. The movement of the muscle may trigger activation of motor units or insertional activity as the needle moves within the muscle during passive motion (2).

When using EMG to guide BoNT injections, the clinician listens as the recording needle electrode is advanced into the target muscle. As the needle nears an actively contracting muscle fiber, the tone associated with EMG activity will change from a dull or low-pitch to the crisp high-pitched sound, which is characteristic when recording nearby motor unit action potentials (MUAPs). This tonal change from dull to crisp indicates that the needle electrode is near the firing muscle fiber. If the tone remains dull, then the needle position should be adjusted until a crisp tone is heard.

Localizing a target muscle using EMG requires simultaneous contraction in the target and relaxation in antagonist muscles and/or adjacent muscles, which may have a similar action. While this process of contraction and relaxation may be facile for some patients, it is often difficult for many patients, particularly those with UMNS, impaired motor control, or cognitive impairments. Patients with UMNS and spasticity may have impaired reciprocal inhibition. This may lead to firing in muscles at rest or when the muscle is moved in the direction of the antagonist and firing in multiple muscles. Cocontraction, mass synergy, and the loss of reciprocal inhibition often limit the usefulness of EMG to isolate a specific muscle for injection. For example, when a needle electrode is inserted into the forearm to target the FDS, there may be cocontraction or firing in the other flexor digitorum profundus (FDP) or wrist flexors. Recording EMG activity in this circumstance only indicates the needle is in an active muscle but it may not be possible to determine which muscle is firing. PROM of the various muscles may be useful in this circumstance as the needle electrode may reveal increased firing in the stretched muscle. E-Stim may also be more useful than EMG for muscle localization when cocontraction is present. Careful inspection of the patient's abnormal posture may help determine which muscle is contributing to the pattern (1,3).

EQUIPMENT FOR EMG GUIDANCE

Electrodiagnostic machines, EMG audio amplifiers, and EMG audio/liquid-crystal display (LCD) units (Figures 7.1A-C) are used as equipment. Other supplies include monopolar insulated injection electrodes/needles of various lengths, surface electrodes, gloves, skin cleansers, gauze, and band aids/plasters.

(A) EMG machine; (B) EMG audio amplifier; (C) EMG E-Stim combined; (D) E-Stim unit.

FIGURE 7.1 (A) EMG machine; (B) EMG audio amplifier; (C) EMG E-Stim combined; (D) E-Stim unit.

ADVANTAGES AND LIMITATIONS OF EMG FOR BoNT PROCEDURES

Advantages

• One of the main advantages of EMG is that it provides auditory feedback indicating the level of activity or overactivity in a muscle.

• In adult patients with cervical dystonia, EMG is often useful to determine whether a muscle is active and therefore contributing to the patient's abnormal head posture (21,25).

• In adult patients with focal limb dystonia, EMG is often useful to isolate individual muscle fascicles.

Limitations

• While EMG may be useful to isolate a muscle or muscle fascicle in patients with focal dystonia, it may be less useful in patients with UMNS-related muscle overactivity (spasticity, dystonia) where;

- The presence of cocontraction, mass synergy, and loss of reciprocal inhibition may lead to diffuse activation of multiple muscles.

- EMG activity in muscles surrounding the target may make it impossible to correctly isolate a muscle. For example, when a needle electrode is inserted into the forearm of a patient with flexion synergy, it may only be possible to state that the needle is in an active muscle, but it may not be possible to determine which muscle.

• The usefulness of EMG may also be limited when patients are sedated for BoNT injections, particularly when general anesthesia is used.

• Patients often report more pain with the insertion of Teflon-coated needle electrodes than with standard hypodermic needles.

• Insulated needles are more costly than hypodermic needles.

EVIDENCE

There is limited evidence, particularly in pediatric patients, to support or refute whether EMG improves the efficacy or safety of BoNT injection or of its superiority over other techniques.

• See the previous section, "Clinical Studies; Comparison of Anatomic Localization With Other Localization Techniques for BoNT in Limb Muscles," for details from several studies, which showed superiority of BoNT injections guided by EMG compared to anatomic guidance in adult patients with spasticity and focal hand dystonia (21,22).

• In a 2012 study, Hong et al. compared the incidence of adverse events (dysphagia) when BoNT injections were guided by EMG versus US. The incidence of dysphagia was 34.7% in patients where the procedure was guided by EMG and 0% in the same patients when the procedure was guided by US (41).

• In a 1996 methodological study comparing EMG to E-Stim for adults with focal hand dystonia, there was no significant difference in outcome measures between the two techniques (42).

• In a study of adults with cervical dystonia, the authors reported that EMG guidance may reduce the number and severity of side effects following BoNT injections (43). Additional studies comparing the EMG and other guidance are reviewed in the sections following.

SUMMARY

EMG may provide useful information as to whether a muscle is active and/or contributing to the clinical problem. It may be challenging or difficult to determine if the EMG needle is actually in the target muscle. The combination of US and EMG may be useful as US can direct the needle to a target muscle and EMG will determine if the muscle is active.

 
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