It is also clear that low frequency T.E.N.S. has a high degree of specificity when utilized for craniofacial pain (Andersson, 1979; Eriksson et al., 1984; Chapman et al., 1979; Andersson et al., 1977; Andersson and Holmgren, 1975; Sjolund et al., 1982; Phero, 1987; Lasagna et al., 1986; Thomas, 1986; Pantaleo et al., 1983; Wessberg and Dinham, 1977; Konchak et al., 1988). (Over 44 studies internationally).
Evoked response while using wire EMG electrodes
Choi and Mitani at Osaka Dental University in 1973 applied the Myomonitor to 15 subjects and monitored the evoked response using wire EMG electrodes. The study concluded “The evoked EMG was recorded from the anterior portion of the temporal, the masseter, the anterior ventral of the digastric, and obicularis oris and the buccinator muscles…The Myo-monitor pulse stimulates the nerve trunks of the fifth and seventh cranial nerves at the superior mandibular notch percutaneously and it appeared to have afferent and efferent effects.”
Myo-monitor Stimulus is Transmitted Neurally
Using accepted intensity-duration methodology Jankelson, et al., 1975 demonstrated that the chronaxy values for Myo-monitor generated curves were well below those for direct muscle stimulation. Further verification of neural mediation resulted from the study of Williamson and Marchall, 1986 using succinylcholine. The study concluded “Succinuylcholine acts by competing with acetylcholine at the myoneural end plate and, therefore, no neurally stimulated muscle contraction under such conditions is by direct depolarization of the muscle itself. With the Myo-monitor evoking electrical impluses, there was no muscle contraction in either instance. This information would support the conclusion that they Myo-monitor stimulus is transmitted neurally.”
Multiple Site Monitoring
Fujii 1977 at the University of Osaka used multiple site monitoring to distinguish M wave and H wave response. Using multiple anatomically separate recording sites the study concluded “Two kinds of response were obtained with latencies of about 2.0 msec. and about 6.0 msec. respectively. The former was assumed to be a direct potential (M wave) and the latter a monosynaptic reflex potential (H wave).” The use of recording sites anatomically distant from the input stimuli is essential for valid conclusions using this methodology. In a 1988 study of Myo-monitor stimulation, Dao, Feine and Lund for unexplained reasons placed the recording needle proximate to the electrode stimuli site.
Stimulation is Neurally Mediated
McMillan et al., 1987 at the University of Hong Kong concluded that “Contraction of muscles of the upper and lower eyelids, the lateral aspect of the nose and the upper lip indicates stimulation of the facial nerve, in particular its zygomatic and buccal branches. The results of our anatomic investigation indicate that this effect is produced by the stimulation of the branches of the upper division of the facial nerve as they pass in a more or less direct anterior course over the preauricular region. These branches will then be directly beneath a surface electrode placed according to the standard protocol. Propagation of the Myo-monitor stimulus along branches from the buccal anastomotic loops of the nerve would ensure contraction of muscles of the upper lip and angles of the mouth…This observation supports electromyographic evidence and results of intensity duration tests that indicate muscle contraction resulting from Myo-monitor stimulation is neurally mediated.”
Latency and Conduction Velocity of Peripheral Motor Nerves
Goodgold and Eberstein examined eight individual investigative studies and found that normal distal latency and conduction velocity of peripheral motor nerves ranged from 2.1 to 5.6 msec. and 44.8 to 67.9 msec., respectively. They concluded that the latency to the obicularis oris which is innervated by the facial nerve in response to stimulation at the angle of the jaw, averages 2.5 to 3.0 msec. Basmajian summarized the results of six studies conducted by separate authors on peripheral nerve conduction velocity and found a range of conduction velocity between 37 and 73 meters/sec. Assuming the distance between the stimulation electrode and the wire recording electrode was approximately 2 cm, it should have taken .27 to .54 msec. for the pulse to travel this distance if the muscles were stimulated directly. This time interval is much less than the 1.85 to 4.4 msec. measured in the Dao study. This suggests the pulse must have traveled a much longer distance. A neurally mediated pulse would have: 1) .5 msec. charching the dermal capacitance, 2) neural conduction time of .7 msec. assuming a neural conduction pathway of 4 cm and conduction velocity of 55 meters/sec. which is the average of Basmajian’s review, 3) residual latency (delay at the myoneural junction) of .6 msec., 4) intermuscular delay of approximately .4 msec. depending upon electrode placement. Adding the sum of these phenomena we find the latency of 1.8 to 4.04 msec. as measured by Dao, et al. is well within the rage of neurally mediated response, despite their electrode placement.