MOTOR ENDPLATE TARGETING OR LOCALIZATION TECHNIQUE
It is well established that to exert its action BoNT avidly binds to and is internalized at cholinergic nerve terminals including at the neuromuscular junction. Knowing this, clinicians and researchers have suggested (or investigated) that targeting the endplate zones or motor points may enhance the uptake of the toxin (29-32). The technique of motor point or endplate targeting requires knowledge of the location and distribution of motor end-plates (MoEPs) in human skeletal muscle.
The location of motor points or endplate zones in mammalian muscles has been studied in animal models and in humans using histochemical staining and electrophysiologic methods (32-35).
STUDIES ON THE LOCATION OF MOTOR ENDPLATE LOCATION AND DISTRIBUTION
In the 1950s, two researchers published data on the location and distribution of MoEP in human skeletal muscles. Coers described three types or arrangements of MoEP in human muscle: (a) muscles having a single innervation band, (b) muscles with multiple innervation bands, and (c) muscles where the innervation bands were scattered throughout (35). Christensson published data on the distribution and pattern of MoEP in stillborn infants. She reported that MoEPs were distributed in a single transverse band at the midpoint of unipennate muscles and in a concave band in bipennate muscles (gastrocnemius) (34).
In the last decade, additional anatomical studies detailing the location of MoEP have been published (29,36,37). Kim et al. reported that the MoEPs in gastrocnemius and soleus muscles were distributed along the length of the muscle. They reported the location of the most proximal of MoEPs in the medial gastrocnemius, lateral gastrocnemius, and soleus at 9.6% (+/- 3.5%), 12.0% (+/- 3.4%), and 20.5% (+/- 3.9%, of the lower leg length. The most distal MoEPs were reportedly located at 37.5% (+/- 5.5%), 37.9% (+/-2.3%), and 46.7% (+/- 3.6%) of lower limb length, respectively (36).
In the biceps brachia muscle, an inverted V arrangement of MoEP has been reported (37). The authors reported the MoEP zone to be 1 cm in width, laterally located 7 cm superior to the olecranon, in the midline located 11 cm superior to the olecranon, and medially 8 cm proximal to the olecranon. The authors also reported the ratio of MoEP location to total olecranon-acromion length: 0.25 at the lateral edge, 0.39 at midline, and 0.28 medially.
The location and distribution of MoEP in the psoas muscle of adult cadavers were reported in a 2010 study. The authors reported an average of 3.7 (range 2-7) nerve branches from the lumbar plexus innervating the long pennate psoas muscle, which was made up of converging fibers of variable length. The area of the MoEP zone was reported to correlate with a zone between 30.83% and 70.25% of the distance from T12 to the inguinal ligament. Therefore the majority of the MoEPs were proximal to the sacral promontory (29).
MoEP TARGETING FOR BoNT INJECTIONS
Clinicians have suggested or recommended targeting MoEP for BoNT injections, for decades (30,31,38). There is limited data from trials comparing MoEP targeting to other techniques.
In a 2014 double-blind randomized controlled trial (DB-RCT), Im et al. compared the efficacy of a fixed dose of BoNT with injections placed within the MoEP zone suggested by anatomical studies (2/10 and 3/10 of calf length) to injections below the mid-belly of the muscle. Both groups improved and there were no statistical differences between the two groups in either clinical or electrophysiologic measures (39). In a 2011 review, Van Campenhout et al. published the location of MoEPs in lower limb muscles and anatomical guidelines for the use of MoEP targeting
for BoNT injections. Based on the location of MoEP, the authors recommended an optimal injection zone for the gastrocnemius, soleus, tibialis posterior, semitendinosus, semimembranosus, biceps femoris, gracilis, rectus femoris, adductors longus, brevis, magus, and psoas muscles.
In a 2009 DB-RCT, Grades et al. used published information on the location of the MoEP in the biceps brachii to compare the effectiveness of a fixed dose of onabotulinumtoxinA using MoEP targeting, standard dilution (100 units in 1 ml), and high volume dilution (100 units in 5 ml). The authors reported a greater benefit with injections using MoEP targeting and those with high volume dilution of onabotulinumtoxinA (40).
MoEP TARGETING TECHNIQUES
The use of MoEP targeting can be incorporated into conventional targeting techniques used to guide BoNT injections. When using manual or blind needle guidance for BoNT injections, physicians can make use of published motor point maps or the information (where available) as to the location of MoEPs or MoEP zones (29,36,37). When using EMG, physicians can target MoEP by listening for endplate noise and injecting toxin in this zone/ location (30). If using E-Stim, MoEPs are targeted by repositioning the needle within the muscle and maintaining a maximal or visible twitch while reducing the stimulation intensity (40). While MoEP cannot be visualized with US or other imaging-based guidance techniques, the published information on the location of MoEPs in muscles can be used when using US to guide BoNT injections.
ADVANTAGES AND LIMITATIONS OF MoEP TARGETING FOR BoNT THERAPY
Advantages of MoEP Targeting
Targeting of MoEP when performing BoNT chemodenervation procedures has at least a theoretical advantage over injections placed in areas away from the MoEP. The data on the superiority of MoEP targeting is limited and additional controlled trials are needed to determine whether all BoNT injections should be performed using this technique.
Disadvantages of MoEP Targeting
A potential disadvantage of MoEP targeting is that it may increase the time required to perform a BoNT procedure.
Using this technique requires measuring the limb and marking MoEP zones. However, this information is memorized or can be transcribed to a template or form, which can be used in clinical practice to speed up this process.
If MoEP targeting is not superior, then the extra time and effort required for this method is unnecessary.
MoEP targeting, as currently described, is not useful for nonmuscle targets. Studies of the location of other cholinergic nerve terminals in organs targeted for BoNT injections may become available in the future.
While there is limited data on the superiority of MoEP targeting over injection at other sites in skeletal muscles, this practice requires minimal time or effort. Therefore, clinicians should consider incorporating this technique into whatever guidance technique or techniques that they currently use to guide BoNT injections into muscles.