CONTACT: JENNIFER BROWN
Iowa City IA 52242
(319) 335-9917; fax(319) 384-4638
Release: Oct. 2, 2001
UI studies advance understanding of common pain treatment
IOWA CITY, Iowa -- University of Iowa researchers are gaining a better understanding
of how a common pain-relief therapy, used clinically for decades to treat
chronic pain such as arthritis, low back pain, ankle injuries and muscle pain,
works. Transcutaneous electrical nerve stimulation (TENS), which involves
electrical stimulation of nerves through the skin, is non-invasive, inexpensive
and safe. However, TENS is not universally accepted as an effective pain treatment.
"There is a lot of controversy about whether TENS works," said
Kathleen Sluka, Ph.D., UI associate professor of physical therapy and rehabilitation
sciences. "And, if it does work, it is not clear what type of pain it
treats best or how long-lasting the effects are."
Sluka believes that much of the controversy stems from a lack of understanding
of the basic science behind this technique and from the complicated nature
of measuring pain and assessing pain relief. Sluka and her team decided to
investigate in detail the effect of TENS on pain signaling pathways in animal
"By understanding the mechanisms of TENS we can utilize combinations
of the stimulation with pharmaceutical agents to enhance pain relief and make
this treatment more effective," Sluka said.
TENS equipment consists of electrodes, which attach to the skin, and a small
battery-powered unit which controls the frequency, intensity and duration
of the electrical stimulation. Patients can use this portable device to help
them control pain and allow them to lead normal lives.
TENS stimulation can be applied at varying frequencies. The UI team studied
the effects of low-frequency and high-frequency TENS on pain relief in animal
The team found that both low- and high-frequency stimulation inhibit pain
sensitivity away from the site of an injury. However, only high frequency
TENS has any effect on pain sensitivity at the site of injury and it is only
a small effect. This finding suggests that how effective TENS is as a pain
relieving therapy depends on the type of pain being targeted.
Proteins called opiate receptors control biochemical pathways that inhibit
pain. These proteins interact with chemicals called opiates, and one consequence
of this interaction is pain relief. It had been suggested that TENS might
interact with opiate pathways.
Through their studies, Sluka and her colleagues discovered that both low-frequency
and high-frequency TENS acts on this biochemical pathway.
Opiate drugs such as morphine are commonly used to treat pain. Sluka and
her team wanted to know how TENS and opiates work together given that the
evidence indicates that these two treatments work on the same pain-signaling
The UI researchers found that both low- and high-frequency TENS in combination
with morphine produced greater pain relief than either TENS or the drug alone.
This means that by adding TENS to a treatment regime, one can lower the
dose of opiates required to achieve adequate pain control, Sluka said. This
is an advantage because these drugs have some undesirable side effects.
However, there are several different types of opiate receptor and the UI
team also discovered that different frequencies of TENS selectively target
different opiate receptors to produce pain relief.
Investigating the effect of TENS on animals that had developed a tolerance
to morphine, the UI researchers found that low-frequency TENS no longer relieved
the animals' sensitivity to pain, but high-frequency TENS still worked. This
finding makes sense given the previous studies, which had shown that low-frequency
TENS works on the same opiate receptor as morphine, whereas high-frequency
TENS activates different opiate receptors.
Our research shows the importance of understanding which opiate receptors
are activated by various treatments so that you can rationally combine treatments
to optimize pain relief, Sluka said.
An understanding of how TENS affected the opiate pathway led to another
discovery, which also could have implications for pain treatment.
Clonidine is a drug used to treat pain. It is not an opiate and produces
pain relief by acting on a different kind of receptor. Previous studies had
shown that if clonidine's receptor and opiate receptors are activated at the
same time, then the pain-relieving effect is synergistic. In other words the
combined effect is greater than the sum of the individual effects.
Sluka and her team found that low-frequency TENS and clonidine produced
the greatest pain relief. This result was exciting, Sluka noted, because the
experiments measured decreased pain sensitivity at the site of injury where
TENS is usually not effective.
"Low-frequency TENS usually has no effect on pain sensitivity at the
site of injury," Sluka said. "However, in combination with a really
low dose of this drug, low-frequency TENS eliminated the increased sensitivity
to pain at the injury site."
Sluka noted that future studies would be needed to determine whether her
team's results from animal studies are mirrored by success in humans.
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