Iontophoresis in Pain Management
There are several common methods that clinicians use to administer medication. One method which has been gaining in popularity is the delivery of medication transdermally. Inherent advantages to this method includes the ability to use a wide array of compounds—having either analgesic or anti-inflammatory actions—applied directly into the target area. This allows for a smaller dose of medication to be used than would be required with other routes of administration. This smaller amount of medication translates into a decreased chance of adverse drug side effects— always a desirable outcome.
This article will focus on iontophoresis, a form of transdermal drug delivery (TDD) that utilizes electrical current to drive or push ionized drugs through the skin’s outermost layer (stratum corneum) which is typically the main barrier to drug transport. Using the rule of ‘likes repel,’ the practitioner chooses medications compatible with the active pole being used so that a positively charged drug is loaded into the positive side of the delivery pad or electrode. Since like charges repel, the positively charged molecules are driven through the skin by the positive current, while negatively charged medications are driven in under the negative pole. Once the drug passes through the skin barrier, natural diffusion and local circulation (perfusion) take over and ultimately determine how the drug is distributed.
Common Uses for Iontophoresis
In rehabilitation medicine, including physical therapy, iontophoresis is used to reduce inflammation that might be seen in musculoskeletal conditions such as lateral epicondylitis, medial epicondylitis, plantar fasciitis, tendo-nitis/bursitis, rheumatoid arthritis, and enthesopathic conditions of various origins. The most popular and well researched iontophoresis application is the use of dexamethasone, a corticosteroid in a sodium phosphate solution. In this form, the drug is composed of negatively charged ions of dexamethasone phosphate and, when loaded into a negatively charged reservoir or electrode pad, the electrical force of the like charges pushes the medication molecules into the desired area. There are two commonly used ways of applying iontophoresis; the first is the traditional method of using a current generator with lead wires connected to an active pad and a dispersive pad (see Figure 1). The medication is loaded in the active pad usually by syringe. The dispersive pad is the polar opposite pad and located away from the treatment site. The treatment is administered clinically for 15-20 minutes per session. In a physical therapy setting, iontophoresis is typically one of several treatment interventions applied, so the addition of another 15-20 minutes to a pre-existing 60 minute program is a time burden for some patients. One ionto-phoresis manufacturer has responded to this situation by introducing a more mobile solution to iontophoresis treatment—sold under the brand name of IontoPatch®—that may be worn by the patient for a 24-hour period. In this way, the clinical time component of this treatment is eliminated altogether and the patient gets a more sustained 24 hour effect. These mobile IontoPatches contain a small flat battery that provides a tiny electrical “push” of the target medication (e.g., dexamethasone; see Figure 2).
The dose period recommended for iontophoresis is calculated based on the amount of current that is flowing, together with the elapsed time of treatment. The dose is expressed as milliampere minutes (mA-min) with the recommended milliampere-minute dose depending on the electrode used. For most electrodes being used today, a typical dosage is 40 mA-min and which can be delivered in a number of ways as long as the product of the two terms equals 40 (e.g., 4 mA x 10 minutes or 2 mA x 20 minutes, etc). The current setting is chosen based on patient comfort and, once the amount of current is determined, the current generator selects the appropriate amount of time required to achieve the pre-set mA-min dose. When using mobile patches, a microampere current is used for much longer periods of time.
Electrode patches have a unique composition that allows ionic movement into human skin tissue. One patch, for example, is made up of embedded zinc and silver chloride electrodes and is activated when the treatment solution bathing one electrode comes into contact with the saline solution bathing the opposite electrode. This creates the necessary charge to drive the molecules into the target tissues. The patches are made up of a semipermeable adhesive membrane that must be in close contact with the skin for optimal penetration.
Iontophoresis for Inflammation
There are a number of drugs that can be driven into the subcutaneous tissues including steroids, NSAIDS, local anesthetics, salicylates, and individual substances such as zinc oxide, iodine, acetic acid, and calcium chloride, to name a few. For a listing of the most commonly used drugs and their applications please refer to Table 1. It is thought that dexamethasone (DEX) can be driven up to 20mm into the tissues with tissue concentration decreasing exponentially with tissue depth. Other factors have been recently identified as being determinants of drug delivery including ratio of skin pore size to drug molecule size, polarity, and the importance of the electric field strength.1 Research indicates that increasing the current density under the membrane can only contribute so much to the total absorption of a specific drug molecule, with passive solvent flow being responsible for a significant portion of the total absorbed amount.2 The more superficial a target structure is, the more drug will be made available for interaction. Conversely the deeper a structure is, the less likely there will be a therapeutic interaction since the drug becomes more diluted as the distance between the electrode and target tissue increases. This makes iontophoresis preferable for treating superficial soft tissue structures.