There are a variety of approaches that have been investigated which employ afferent (i.e., sensory) stimulation for the reduction of spasticity in people with SCI. As noted previously, electrical stimulation, in particular TENS, is one of the preferred methods for providing afferent stimulation. This technique differs from the approaches noted in the previous sections that involve stimulation to the motor system, whether via muscles or motor nerves. TENS may involve the stimulation of large, low threshold afferent nerves (Goulet et al. 1996) or dermatomal stimulation which is directed towards cutaneous skin receptors supplying the skin in the dermatome of the muscle of interest (Bajd et al. 1985; van der Salm et al. 2006). These methods are aimed at altering motor-neuron excitability through sensory reflex arcs, thereby reducing spasticity. An alternate approach employing electrical stimulation involves rectal probe stimulation, developed originally to overcome an ejaculation in males and heretofore a technique employed only within fertility clinics (Halstead & Seager 1991).
In addition, a variety of methods of mechanical or thermal stimuli to various afferent systems have also been studied. These include therapeutic massage over the spastic muscle (Goldberg et al. 1994), penile vibration (Laessoe et al. 2004; Alaca et al. 2005), the application of cold (i.e., cryotherapy) to reduce local muscle spasticity (Price et al. 1993) and also irradiation of the skin overlying sensory nerves with a helium-neon laser purported to induce photochemical reactions which may trigger neural activity (Walker, 1985). It should be noted that the article examining cryotherapy (i.e., Price et al. 1993) did not meet the review criteria of having 50% of subjects with SCI. The article was included in the review as individual results were presented for all subjects with SCI (N=7), enabling independent discernment of the effects on SCI (thereby meeting review criteria for studies having SCI N³3).
Transcutaneous Electrical Nerve Stimulation (TENS)
Aydin et al. (2005) employed a RCT design to compare oral Baclofen (N=10) and TENS (N=11) to the bilateral tibial nerves (i.e., innervating gastrocnemius muscle) in reducing lower limb spasticity. Most important, this is the only trial examining multiple applications of TENS. Fifteen minute sessions of TENS were applied to the tibial nerve over 15 days demonstrating significantly reduced spasticity as indicated by reductions in the following measures assessed immediately after the last treatment session: Ashworth Scale, Spasm Frequency Scale, deep tendon reflex score, FIM and Functional Disability Scores and H-reflex amplitude. In addition, there were also some lasting effects over the next 24 hours as repeat testing the next day indicated continued spasticity reductions although to a lesser degree. It should also be noted that significant reductions were obtained on some measures (but not all) following a single session. However, the long-term effects were more profound than those obtained following a single session.
Possover et al. (2010) examined the effect of continuous TENS to the pudenal nerves and sacral nerve roots S3 and S4. They reported that a frequency of 20 Hz was sufficient to completely control spasticity of the lower extremities and reflex incontinuence. In this study, it appears that continuous TENS was effective in reducing spinal spasticity.
Other researchers have examined the effects of TENS following a single session (Bajd et al. 1985; Goulet et al. 1996; Chung et al. 2010; Possover et al. 2010; van der Salm et al. 2006). Goulet et al. (1996) employed a single 30 minute bout of TENS over the common peroneal nerve in an attempt to reduce plantar flexor spasticity in 14 individuals with SCI. This study showed significant decreases in scores for the modified Ashworth scale and the Achilles tendon reflex but no significant changes were seen for H-reflex amplitude. A trend for decreased clonus scores was observed but this was not statistically significant. Chung et al. (2010) also examined the effect of 60 min TENS sessions over the common peroneal nerve. The authors reported a significant reduction on the Composite Spasticity score (p=0.017), a reduction in full range passive ankle dorsiflexion (p=0.024), and ankle clonus (p=0.023). In these two studies, TENS appears to be effective in reducing spinal spasticity following a single session.
In contrast, van der Salm et al. (2006) and Bajd et al. (1985) each examined dermatomal TENS as opposed to direct nerve stimulation with conflicting results. Bajd et al. (1985), in a small pre-post trial (n=6) reported reduced spasticity in 3 subjects as indicated by increased relaxation indexes associated with the pendulum test although provided no mean data or group statistical analysis. Van der Salm et al. (2006) conducted a more thorough analysis of the effect of a single 45 minute session of TENS to the L1 dermatome in 10 individuals with longstanding SCI (mostly AIS A) and obtained no short-term effects, although as noted in the previous section obtained benefits with motor stimulation. A critical element within these investigations of single-session effects is the precise time of assessment, relative to treatment, a detail not always precisely reported in the various studies, although it is clear that van der Salm et al. (2006) assessed individuals as close as possible to treatment end.
Rectal Electrical Stimulation
Halstead et al. (1993) have evaluated another form of electrical stimulation, rectal electrostimulation, when they observed patients undergoing this procedure, for the purpose of sperm retrieval, reporting improved spasticity. These investigators conducted a prospective pre-post trial examining the effects of a minimum of 6 sessions of rectal probe electrostimulation on various clinical measures of spasticity including the Ashworth scale, Penn Spasm Frequency Scale, deep tendon reflexes and ankle clonus. Although they achieved good to excellent effects in more than half of the patients examined including significant reductions in the Ashworth scale (p<0.01) and with the effects outlasting the intervention by a mean of 8.2 hours according to patient self-report further therapeutic development of this approach has not continued.
Afferent stimulation may also be produced via mechanical means. Goldberg et al. (1994)have employed therapeutic massage over the triceps surae muscle and assessed H-reflex amplitude to demonstrate that µ-motor neuron excitability is reduced significantly during a short 3 minute period of massage and somewhat reduced 3 minutes after but not 6 minutes after. Reductions in µ-motor neuron excitability are indicative of decreased spasticity.
Penile vibration has also been investigated as a method of providing sensory stimulation to reduce spasticity (Laessoe et al. 2004; Alaca et al. 2005). In particular, Læssoe et al. (2004) employed an unblinded, crossover RCT design (N=9) in which male participants either received penile vibration or not followed by the opposite condition. The modified Ashworth scale and Penn Spasm Frequency Scale were conducted in addition to an EMG assessment in which ongoing muscle activity was recorded over a 24 hour period. Penile vibration was shown to be effective in reducing spasticity as indicated by reductions in modified Ashworth scale scores (p<0.01) and a slight trend for reduced Penn Spasm Frequency Scale Scores (p=0.26). These were not maintained over 24 hours. The EMG analysis showed that reduced muscle activity was most apparent in the first hour post-stimulation, and had returned to baseline by 3 hours suggesting the effect lasted no more than 3 hours. The authors attempted to include female subjects involving clitoral vibratory stimulation but were only able to recruit 2 subjects willing to submit to the procedure. Both women reported similar reductions in spasticity although evaluation of the effectiveness of the stimulation was more difficult (i.e., no confirmatory ejaculation). The results of Alaca et al. (2005) confirmed the overall study findings as penile vibratory stimulation in 10 males resulted in significant reductions in Ashworth Scale scores as assessed 3 hours after stimulation (p=0.001) and maintained at 6 hours (p=0.03) with a trend for reduced values still apparent at 24 hours (p=0.08). The longer carry over effect in this study may have been due to a prolonged stimulus period as Alaca et al. (2005) employed 6, 3 minute periods of stimulation (separated by 1 minute) whereas Læssoe et al. (2004) used a single 5 minute period.
The short-term effect of cryotherapy was investigated by Price et al. (1993) who used a biomechanical approach similar to that described earlier (i.e., Seib et al.1994) to monitor ankle viscoelastic stiffness through measurements of resistance torque to repetitive sinusoidal ankle movements. Although the majority of subjects were individuals with stroke or head injury, 5 of 7 people with SCI showed a significant reduction in spasticity both immediately following cryotherapy and also at 1 hour after the cold stimulus was removed.
Helium-neon Laser Stimulation
Walker (1985) employed a helium-neon laser to irradiate the skin overlying sensory nerves and demonstrated a similar beneficial effect of suppressing clonus as seen with electrical stimulation of sensory nerves. This investigator employed a RCT design with a variety of small group control conditions (N=5 to 7), but failed to report several important experimental details (i.e., method of concealment, method of analysis and statistical comparisons). This approach has not been investigated since this brief 1985 report.
It should be noted that several of the modalities noted in this section have not been employed in regular clinical practice and may be deemed as more investigational in nature. For example, helium-neon laser irradiation has only been employed in one investigation and has not been considered as a viable therapeutic approach. Similarly, penile and rectal stimulation, first noted as delivering potential benefits within fertility clinic investigations, may not be acceptable forms of therapy to individuals from either a safety or a psychological perspective. Other therapies might simply be impractical to implement. For example, hippotherapy requires access to a suitable equine facility with appropriately trained individuals.
Ness et al. (2009) examined the effect of whole-body vibration at 50 Hz on individuals with SCI for four weeks. Sessions occurred 3 times per week with 4 bouts of vibration occurring each day for 45 second each. A significant reduction in quadriceps spasticity was reported, as determined by first swing excursion, and persisted for at least 8 days.
There is level 1b evidence from a single RCT and level 4 evidence from a case series that an ongoing program of TENS acts to reduce spasticity as demonstrated by clinical and electrophysiological measures.
There is level 1b evidence from a single RCT that reductions in spasticity with ongoing programs of TENS may persist for up to 24 hours.
There is level 1a evidence from two RCTs that a single treatment of TENS acts to reduce spasticity but to a lesser degree than that seen with ongoing programs of TENS. This evidence is muted somewhat by conflicting results with a null result (level 2) compared with 2 positive results (level 4).
There is level 4 evidence from a single pre-post study that several sessions of rectal probe stimulation reduces lower limb muscle spasticity for up to 8 hours.
There is level 4 evidence from a single pre-post study that short periods of massage (e.g., 3 minutes) of the triceps surae results in reduced H-reflexes with the effect lasting no longer than a few minutes.
There is level 1b evidence from a single RCT supported by a single pre-post study that a single bout of penile vibration acts to reduce spasticity lasting for at least 3 hours and possibly up to 6 hours.
There is level 4 evidence from a single pre-post study that cryotherapy may reduce muscle spasticity for up to 1 hour after removal of the cold stimulus.
There is level 2 evidence from a single low quality RCT that helium-neon irradiation of sensory nerves may suppress ankle clonus for up to 60 minutes following 40 seconds of stimulation.
Ongoing (TENS) transcutaneous electrical nerve stimulation programs result in short-term reductions in spasticity which may last for up to 24 hours.
Penile vibration and rectal probe stimulation may be effective at reducing lower limb muscle spasticity for several hours.
Other forms of afferent stimulation including massage, cryotherapy, helium-neon irradiation, and whole-body vibration may result in immediate spasticity reduction but require more research to examine long-term effects.