Although iontophoresis has virtually outdated this modality from a clinical perspective, it is desirable to be knowledgeable regarding its principles. Since iontophoresis is a recognized clinical modality which has been employed as a vital agent in the field of physical medicine, its principles will be reviewed in this writing.
Following the electrochemical phenomenon that unlike charges attract and like charges repel one another, when applied to clinical situations, ions bearing a positive (+) charge are driven, by the electromotive force, into the integument at the anode (+), while those with negative (-) charges are driven in by the cathode (-). Organic compounds do penetrate the systemic circulation rapidly, but attention to their pH is essential. Chantraine et al. found that transfer of water soluble steroids through the skin is not improved by prolonging the DC exposure time. They proposed that the benefit obtained from steroid iontophoresis is a product of exposure to the direct current, not the steroid. For the most part, substances such as crystals, with an electrostatic bond, are more commonly affected by ion transfer. The work of Griffin et al. has suggested that salts are more likely to transfer iontophoretically than covalently bonded molecules, such as enzymes and large molecular structures such as lidocaine. Simply stated, the smaller the molecular weight of a substance (ion), the more rapidly the rate of transfer (flow) into the tissues in ion transfer.
Three basic effects of iontophoresis:
- Physico-chemical effects: Any salt, or medicinal compound, capable of electrolytic dissociation is capable of being transferred ionically through the tissues under the influence of an EMF. This transfer is more efficient in the case of compounds of small molecular weight which should be at the correct pH for optimum transfer.
- Cataphoresis: This principle could be of value in the clinical management of interstitial remnants from trauma with resultant hemorrhage, or in managing hematomatous masses contained by interstitial tissues.
- Electro-osmosis: The shifting of the intra- and extracellular water content through the membrane barriers under the influence of an electromotive force. This principle could be of value in managing the traumatic edema following soft tissue injury.
Burning of the exposed tissue is a major hazard with iontophoresis and is due to the direct current, not the medication. Neither the physician nor the patient may be aware of the presence of the burn due to the numbing effect of the direct current, resulting from flooding of the cutaneous receptors, until the electrodes are removed.
For obvious reasons, it is clinically prudent to administer complex organic molecules, such as steroids, by phonphoresis and, if iontophoresis is performed, to administer small molecular weight inorganic compounds using the appropriate polarity.