TENS and Blood Flow

The effects of TENS on blood flow have been investigated in a number of studies. The possible mechanisms that have been suggested for an increase in blood flow following TENS treatment, include: segmental inhibition of sympathetic vasoconstriction, release of vasodilator peptides from sensory neurons, and the muscle pump action of contracting muscles.

Wikstrom et al ⁽³⁾, in their study, concluded that skin blood flow in participants who were healthy was increased with TENS of low frequency (2 Hz) but not with TENS of high frequency (100 Hz). In another study, in patients with chronic leg ulcers, low-frequency TENS (2 Hz) of the highest tolerable intensity, but not by low-intensity stimulation, increased microcirculation.⁽⁵⁾ However, in the tissue surrounding the ulcers, the blood flow increase was substantially less.

In a preclinical study on rats by Vance et al (Jan, 2007), it was shown that TENS was ineffective in reducing primary hyperalgesia in the early, acute phase of inflammation, 4 hours after induction.However, of note, secondary hyperalgesia was reduced in this early, acute phase of inflammation. Thus, clinically, TENS could be effective in reducing radiating pain and secondary hyperalgesia but likely not primary hyperalgesia shortly after injury. However, 24 hours later, when inflammation was still acute, and 2 weeks later, when inflammation was chronic, TENS reduced both primary hyperalgesia and secondary hyperalgesia. Thus, clinically, after the early, acute phase of inflammation, TENS may be more effective in reducing pain and hyperalgesia.

TENS following a Peripheral Nerve Injury

Complex regional pain syndrome type II (CPSII) is defined by the International Association for the Study of Pain as a chronic condition that can develop following a peripheral nerve injury.⁽⁴⁾ Most frequently, the injury involves the median, ulnar, sciatic, or tibial nerve, and the condition is characterized by spontaneous pain in the limb of the damaged nerve. The pain is described as constant and burning and is accompanied by a lowering of the pain threshold for mechanical and thermal stimulation (allodynia). In addition, painful mechanical and thermal stimulation are perceived as inordinately painful (hyperalgesia). These symptoms continue after the initial injury has healed and may become severe, spreading beyond the site of initial injury and ultimately reducing the normal use of the affected extremity.

The pain of CPSII is managed by pharmacological or nonpharmacological interventions. Pharmacotherapy includes the use of antidepressants, antiepileptics, and opioids. Although these treatments are somewhat effective, they are associated with side effects which can include dizziness, sedation, gastrointestinal dysfunction, dry mouth, ataxia, constipation and physical dependence.^(5-10)

Transcutaneous electrical nerve stimulation (TENS) is a nonpharmacological intervention that is used to reduce the pain of CPSII. The modality is delivered to peripheral sensory nerves through surface electrodes and is believed to produce analgesia by both peripheral and central nervous system mechanisms^(11-13). Like pharmacological interventions, TENS is somewhat effective at reducing the pain of CPSII. Application of TENS to humans with neuropathic pain substantially reduced the pain in 53% to 81% of those treated^(14-16).

Although TENS and pharmacological intervention are both variably effective when used to manage the pain of CPSII, TENS produces none of the side effects associated with drug therapy. There is a risk of skin irritation or an allergic reaction from application of electrodes to the skin, but these problems are relatively rare and are easily managed by shifting the electrode position⁽¹⁴⁾. Therefore, TENS represents a viable, nonpharmacological intervention for the management of CPSII.

There is no consensus on how TENS should be applied to best relieve neuropathic pain. Transcutaneous electrical nerve stimulation may be applied at high frequency (80-110 Hz) or low frequency
(2-10 Hz),^(17,18) but it is unknown which of these frequencies will best prevent or alleviate the pain of CPSII. Electrodes to deliver TENS may be positioned on
skin located ipsilateral or contralateral to a nerve injury,⁽¹⁹⁾ but it also is unknown which of these locations would best prevent or reduce neuropathic pain. Although the effects of frequency and electrode positioning on CPSII-like pain has not been examined, there are reasons to suspect that these parameters may influence treatment effectiveness.

High-frequency (80-110 Hz) and low-frequency (2-10 Hz) TENS^(17,18) differ in their ability to relieve pain and in the central nervous system alterations they produce. Thirty minutes of high-frequency TENS applied to the upper extremity in humans who were healthy produced an increase in the mechanical pain threshold when assessed in the ipsilateral hand⁽²⁰⁾.

Because high- and low-frequency TENS differ in their mechanism of action and in their ability to relieve pain, it is conceivable that perhaps one frequency of TENS will relieve the painful symptoms of CPSII better than the other frequency of stimulation. It is also conceivable, that high- and low-frequency TENS may differentially alter individual painful symptoms associated with the syndrome, but more research is needed in this area.

Somers et al directly compared the ability of high- and low-frequency TENS to prevent the development of the painful symptoms of CPSII. They concluded that early intervention with high- and low-frequency TENS can reduce the development of mechanical and thermal allodynia, respectively.

There is good reason to suspect that what was observed in the rats used in their study also would be true of humans treated with daily TENS following a nerve injury. The differential effect of high- and low-frequency TENS on mechanical and thermal allodynia suggests that both treatment strategies may be necessary in order to comprehensively reduce allodynia in humans developing CPSII. Moreover, their data suggest that treatment contralateral to the nerve injury, rather than ipsilateral to the nerve injury, may be the best strategy for beginning treatment with daily TENS. Finally, it is important to note that based on their data, their recommendations for managing the pain of CPSII are intended only as starting points for the use of the modality. To reiterate, Somers et al believe that their data presented suggest that both high- and low-frequency stimulation delivered through electrodes positioned contralateral to a nerve injury may be the best starting point for TENS treatment of humans developing CPSII.

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References

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   Surg Hand Surg. 1999;33:195-201.
2. Cosmo P, Svensson H, Bornmyr S, Wikstrom SO. Effects of transcutaneous nerve stimulation on the microcirculation in chronic leg ulcers. Scand J Plast Reconstr Surg Hand Surg. 2000;34:61-64.
3. Vance, Carol G.T.; Radhakrishnan, Rajan; Skyba, David A.; Sluka, Kathleen A. Transcutaneous electrical nerve stimulation at both high and low frequencies reduces primary hyperalgesia in rats with joint inflammation in a time-dependent manner. Physical Therapy, January, 2007.
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21. Somers, David L.; Clemente, F. Richard. Transcutaneous electrical nerve stimulation for the management of neuropathic pain: the effects of frequency and electrode position on prevention of allodynia in a rat model of complex regional pain syndrome type II. Physical Therapy, May, 2006.