Electrical Stimulation Therapy
continued from page-1
.
Interference Current
Interference current is based on the summation of two
alternating-current signals of slightly different
frequency. This results in current having a recurring
modulation of amplitude, based on the difference in
frequency between the two signals.
TYPES OF ELECTRICAL STIMULATION
Iontophoresis
Iontophoresis, the process of
increasing the penetration of drugs into the skin by
application of an electric current, is commonly used by
physical therapists for the purpose of delivering
anti-inflammatory medications such as corticosteroids.
The groundwork for iontophoresis dates back to the early
1900s, with initial scientific experiments performed by
a researcher named LeDuc.
The majority of units consist of a compact
phoresor that operates with a 9-volt battery
and two wire leads, each connected to an electrode. One
electrode is the drug-delivery electrode intended for
the anti-inflammatory, and the other is used as a
dispersive electrode charged opposite to the
anti-inflammatory ion. When the electrodes contain
solutions of ions, negatively charged anions are
repelled from the cathode into the body and positively
charged cations are repelled into the targeted body area
from the anode.
This effect is specific for ions of the same polarity
as the electrode and, conversely, ions of the opposite
polarity are not transferred into the body. Physical
therapists use iontophoresis based on this penetration
and distribution of ions primarily for controlling and
reducing inflammation. This is applied while minimizing
the systemic concentration caused by circulatory removal
of the desired medication from the targeted area.
Two typical prerequisites for treatment with
iontophoresis are that the medication must be charged
(or modified to carry a charge) and that the
inflammatory process be near the body surface (i.e. a
superficial muscle or tendon rather than a deeper muscle
tendon bursa) (Costello, 1995).
The effectiveness of the ion transport system remains
controversial. For example, some researchers have
proposed that all the material delivered through the
skin with iontophoresis is removed by the subcutaneous
circulation and circulated around the body, providing
little if any local concentration to the intended
region. Conversely, other researchers have shown with
animal studies that ions and other substances do
penetrate and do provide local concentration.
In the physical therapy setting, constant direct
current has been commonly used in iontophoresis
applications. However because of concern over pH
changes, some researchers contend that a method of
producing a more "consistent" constant current should be
used to provide current while the skin resistance is
changing. Because of potential skin charge accumulation
and skin irritation due to pH changes, modulated
currents have been used with success on laboratory
animals. Pulsed currents have proved to be as effective
or more effective in the delivery of small ions. Such
studies indicate the need for physical therapists to
consider and investigate the use of currents other than
the traditional continuous monophasic current for
iontophoresis.
Corticosteroids are the principal drugs used with
iontophoresis in physical therapy because they have an
anti-inflammatory effect and are relatively inexpensive.
Dexamethasone is available in a somewhat more stable
dissolved form and is therefore often used with
iontophoresis. Some clinicians recommend treatments
using a current of 4 mA for 10 minutes. This current is
thought necessary to penetrate into the deeper tissues;
however, treatment times greater than 10 minutes are
less likely to achieve any greater tissue concentration
due to circulatory removal of the medication.
Still other clinicians propose a current of 2.0 mA
for 20 minutes for more superficial areas with a chronic
inflammatory condition. More recent advances in this
technology have introduced a disposable single-use
iontophoresis system with an internal battery and
current limiting circuitry. This method provides a
constant drug delivery for an 80 mA-minute treatment and
can deliver both negatively and positively charged drug
ions. It operates at a low current and is worn for 24
hours to deliver the desired dose. The unit is designed
to begin a treatment as soon as it is hydrated and
applied to the skin, and stop the treatment at
approximately 80 mA-minutes (Morris, 2003; Reena Rai,
2005).
Transcutaneous Electrical Nerve Stimulation (TENS)
Transcutaneous electrical nerve stimulation
(TENS) is one of the most commonly used forms
of electrostimulation for pain relief. Numerous clinical
reports exist regarding the use of TENS for conditions
such as low back pain, myofascial and arthritic pain,
neurogenic pain, and postsurgical pain. The method of
pain reduction produced by TENS is explained by the gate
control theory proposed by Melzack and Wall in 1965. The
"gate" between the level of the spinal cord and the pain
centers of the brain usually is closed, thereby
inhibiting constant nociceptive transmission by way of C
fibers from the periphery to the T cell.
When painful peripheral stimulation does occur, the
information carried by C fibers reaches the T cells and
opens the gate, allowing pain transmission centrally to
the thalamus and cortex, where it is interpreted as
pain. Recall that the gate control theory
postulated a mechanism by which the gate is closed
again, preventing further central transmission of the
nociceptive information to the cortex. The proposed
mechanism for closing the gate is inhibition of the
C-fiber nociception by impulses in activated myelinated
fibers (eMedicine Clinical Knowledge Base, 1996; Gersh,
1992; Noback,1991).
A TENS unit consists of one or more electric signal
generators, a battery, and a set of electrodes. The
units are small and programmable, and the generators can
deliver uninterrupted forms of stimuli with variable
current strengths, pulse rates, and pulse widths. The
preferred waveform is biphasic, which helps avoid the
electrolytic and iontophoretic effects of a
unidirectional current. A variety of newer
transcutaneous or percutaneous electrical stimulation
modalities are emerging as technology advances (Jarzem,
2005).
Interferential Current Therapy (IFC)
Interference current, utilized as
interferential current therapy (IFC), is based
on the summation of two alternating-current signals of
slightly different frequency. This results in current
having a recurring modulation of amplitude, based on the
difference in frequency between the two signals.
When these signals are in phase, they sum to an
amplitude sufficient to stimulate; when the signals are
out of phase, no stimulation occurs. To determine the
stimulation rate of an IFC unit, you must understand
that the
beat frequency of IFC is equal to the
difference in the frequencies of the two signals. For
example, the beat frequency and, hence, the stimulation
rate of a dual-channel IFC unit with signals set at 3400
and 3300 Hz is the difference of 100 Hz.
Interferential current therapy can deliver high
currents compared to other stimulators, and can use 2,
4, or 6 applicators, arranged in either the same plane
for use on regions such as the back or in different
planes in complex regions such as the shoulder
(eMedicine Clinical Knowledge Base, 1996; Gersh,1992).
Neuromuscular Electrical Stimulation (NEMS)
Neuromuscular electrical stimulation (NMES)
is the application of current to elicit a muscle
contraction. The use of NMES in orthopedic and
neuromuscular rehabilitation has grown significantly in
recent years. A nerve action potential may be elicited
either by a command originating in the motor cortex of
the brain or by an electrically induced stimulus at the
periphery. NEMS is addressed below under Stimulating
Muscle Contraction.
Functional Electrical Stimulation (FES)
Functional electrical stimulation (FES)
is another form of electrotherapy that utilizes
electrical currents to activate the nerves that serve
extremities affected by paralysis. This paralysis can be
the result of a spinal cord injury, head injury, stroke
or other neurologic disorders. The goal of FES is to
help restore function in people with disabilities. The
many possibilities for patient management using FES are
beyond the scope of this course. However, a description
of the use of FES for normalizing gait pattern in
included under Stimulating Muscle Contraction, below.
continue to page -3 of
electrical stimulation
Jeffrey Larson, PT, ATC
