Clarifying the Effects of Topical Analgesics

Helping client’s manage pain is an important role for soft-tissue therapists. Consequently, many look to adjunctive treatments or products for assistance in this pursuit. Some of the most common adjunctive treatments used by massage practitioners and recommended to clients are topical analgesics.

Analgesics are substances or agents that relieve pain without causing a loss of consciousness. Most of us are familiar with oral analgesics such as aspirin or ibuprofen. Topical analgesics (TAs) are pain-relieving substances that are placed or rubbed directly on the skin in order to relieve pain.

Topical analgesics are used with great frequency in this country. While most of them do appear to have beneficial pain-relieving effects, they don’t always live up to other marketing claims. Some suggest that topical analgesics owe much of their benefit to the process of rubbing the ointment or cream on during application. If the analgesic benefits derived primarily from applying the substance and not from the compound itself, it would certainly speak well for massage in the treatment of many soft-tissue disorders. However, there is disagreement about the accuracy of that claim. In one study on a particular group of TAs called salicylates, Mason and colleagues found that placebo treatments rubbed on the skin were not as effective as the TAs in producing pain relief.¹

TAs are commonly used as an adjunct treatment along with other procedures such as oral analgesics, physical therapies, support bandages, rest, ice, massage, or compression. They are also particularly helpful in certain pain conditions like musculoskeletal disorders for individuals that aren’t able to tolerate oral analgesics. Among the most common active ingredients in TAs are menthol, methylsalicylate, camphor, and capsaicin. Some of these ingredients will be explored in greater detail below as the different categories of TAs are described.

Categories of Topical Analgesics
There are many different substances that might be used in a topical analgesic preparation.  In a comprehensive review of topical analgesics Sawynok described categories of TAs that include: NSAIDS (non-steroidal anti-inflammatory drugs), opiods, capsaicin, local anesthetics, antidepressants, glutamate receptor antagonists, adrenoreceptor agonists, and a number of other categories based on pharmacological properties.² Most of these categories include substances that contain medications that are not legally dispensed in over-the-counter (OTC) preparations, and subsequently they are not used by massage practitioners either. The more common TAs that massage practitioners use and are familiar with fall into a category that is sometimes called rubefacients.

A rubefacient is an agent that reddens the skin by producing active or passive hyperemia. The majority of rubefacient TAs fall into one of three categories: counterirritants, salicylates, and capsaicin. The physiological properties of each category are described below. Several brand names are mentioned so it is clear which category various products fall into. (Note: all brand names listed are considered trademarked names).

Counterirritants
The majority of TAs used by massage practitioners are counterirritants.  A counterirritant is an agent which produces a superficial irritation in one part of the body that is intended to relieve irritation in another part. The irritation produced by these substances is most commonly a stimulation of the thermal receptors in the skin. Therefore a sensation of either heat or cold is produced. Popular brand names for various counterirritants include: ArthriCare, Eucalyptamint, Atomic Balm, Icy Hot, Prossage Heat, Tiger Balm, Nature’s Chemist, Biofreeze, and Therapeutic Mineral Ice.

The brand names for these various products reflect the emphasis on the sensation of heat or cold experienced from their use. For example, you would probably expect to have a sensation of heat from Prossage Heat or Atomic Balm. Likewise you would expect to have a sensation of cold from Therapeutic Mineral Ice. But what about Icy Hot? Each person’s physiology may respond a little differently to the formula of the analgesic. Therefore some people may have a sensation of heat from Tiger Balm while others have a sensation of cold.

Salicylates
The salicylates are a group of related compounds derived from salicylic acid. They contain similar pharmacological agents as aspirin. Salicylates have several beneficial physiological effects. They are known to inhibit prostaglandin synthesis, reduce inflammatory activity, as well as aiding in fever reduction and pain management. A systematic review of studies related to salicylates indicated they may be beneficial in acute pain and moderately helpful to poor in chronic arthritic and rheumatic pain.¹ Popular brand names for salicylates include: Aspercreme, Ben Gay, Flexall, Mobisyl, and Sportscreme.

Capsaicin
Many people are familiar with a sensation of heat that can be felt on the skin when it is exposed to cayenne pepper. In cold weather climates or sports activities like skiing, participants often put cayenne pepper in their socks to keep their toes warm. The active ingredient in cayenne pepper that produces this warming sensation is capsaicin. It is an alkaloid irritation to the skin and mucous membranes that produces pain-relieving sensations.

Capsaicin binds to the nociceptors of the skin where it will produce an initial excitation of neurons (often felt as burning, itching, or prickling). After the initial sensation there is a degree of desensitization that leads to the analgesic effects. There are adverse effects with some forms of topical capsaicin because it can produce a burning sensation at the site of application. Therefore it is not as popular as many of the other TAs like the counterirritants. Brand names for analgesics containing capsaicin include: Zostrix, Zoxtrix HP, and Capzasin-P.

Therapeutic Effects
The method by which each of these TAs produce pain relief is a complicated physiological process. However, it is important to understand what is actually occurring with these substances to determine when they should be appropriately used. Because the primary TAs used by massage practitioners are counterirritants our focus on the discussion of therapeutic effects will be on this group. For purposes of our analysis the effects of the counterirritants can be grouped into three primary categories: neurological, circulatory, and thermal.

Neurological
The physiological processes by which counterirritants work are not well understood. Interestingly, the neurological responses are the most significant benefit from these substances, yet the least emphasized in most marketing materials. As mentioned before the counterirritants work by chemically stimulating (irritating) sensory receptors in the skin. The irritation of receptors in the skin may also inhibit pain signals that are reported from the nociceptors (pain receptors), thereby blocking pain sensations that are reported to the brain.

Another theory of the effectiveness of counterirritants focuses on neurological phenomena such as the Gate Theory of Pain described by Melzack and Wall.³ Their theory suggests that if the central nervous system is processing a high volume of sensory signals from one location it will essentially ‘close the gate’ on other signals. Although this theory has been challenged and modified since it was originally proposed, many of the concepts still seem to have physiological validity. With this gate being closed by the counterirritant, it is more difficult for the pain signals to arrive at the brain. Keep in mind the ‘gate’ is never fully closed, but it will restrict the traffic of sensory signals that are reported to the central nervous system.

While the receptors that are activated by the TA are located in the skin, the neurological effects are not limited to the skin. For example, there may be beneficial reduction in muscle tone in response to the sensory signals being processed in the skin. In one study an analgesic balm applied to the skin surface decreased the degree of static muscular contraction.⁴ (Ichiyama, Ragan, Bell, & Iwamoto, 2002). The authors concluded that application of TAs to the skin have analgesic effects on signals from receptors located in the muscles. Their findings show evidence for an effective use of TAs by soft-tissue therapists. Reduction of muscular pain and hypertonicity is a goal in most treatments and these substances appear to be helpful in achieving that end.

Circulatory
The chemical stimulation of receptors in the skin produces a superficial vasodilation. The cutaneous vasodilation produces local hyperemia and accounts for the redness that may result from certain analgesics. The increase in circulation is local and only in the superficial tissues, so TAs shouldn’t be considered a means of increasing circulation to damaged tissues at deeper levels.

The increase in circulation may also produce a slight temperature increase due to the increased circulation in the skin. However, this temperature increase is relatively small and it is questionable whether or not the small increase can produce any therapeutic benefits from the heat increase alone. The thermal results of TAs are discussed in greater detail below.

Thermal
Due to the names of various TAs, especially the counterirritants that are used by the general public and massage practitioners so often, one would conclude that the primary effects of these substances are thermal. Advertising and marketing claims often state their effectiveness in heating or cooling damaged tissues. However, there is a difference between a sensation of heat or cold and an actual temperature change within the tissues.

As stated earlier the primary effects of most counterirritants result from chemically stimulating the thermal receptors in the skin. The stimulation of these receptors causes the brain to perceive sensations of heat or cold as a result of applying the analgesic. Yet there is no ingredient in TAs capable of producing a significant temperature change in the tissues.

In the section above on circulation it was mentioned that local circulation increases from TAs may create a small temperature increase in the skin. One study that evaluated Eucalyptamint found the local circulation increase caused a skin temperature rise of approximately 1.5 degrees Fahrenheit.⁵ This slight temperature increase was limited to the skin and did not penetrate to deeper tissues. The therapeutic benefits of heat for soft tissues, especially the deeper tissues don’t occur until temperatures between 104 to 113 degrees Fahrenheit are reached.⁶

An analgesic being applied is most likely at room temperature (about 70 degrees Fahrenheit). Of the five different methods of heat transfer (conduction, convection, conversion, radiation, and evaporation) the only one that is feasible for a topically applied cream to heat tissues is conduction. Therefore, to reach the temperature level considered necessary for therapeutic change in the deep tissues, the TA would have to be at least 30 to 40 degrees warmer than room temperature. Without being at this temperature there may be a sensation of heat but there will not be a significant thermal change in the tissues. The same is true for a “cold” analgesic. It will not actually cause thermal change in the tissues unless the analgesic itself is significantly colder than body temperature.

Does it matter that these substances may produce sensations of heat or cold but not actual temperature changes? When considering therapeutic applications it does. For example, if a practitioner uses a heat sensation TA prior to soft-tissue manipulation techniques with the idea that deeper connective tissues will be more pliable, that practitioner may be overzealous in the application of those techniques and cause additional problems. Likewise, a person who applies a cold sensation TA to an acute injury with the idea of reducing inflammation may, in fact, use it inappropriately by increasing superficial local circulation to the injury site in the acute phase.

Conclusion
It seems that there is a problem of alignment between the physiological effects of TAs and the marketing materials used to promote them. Does that mean they shouldn’t be used? It would be an error to throw the baby out with the bath water in this way. There are clearly benefits in pain reduction that result from the proper use of TAs. They can be a valuable adjunct to many soft-tissue therapies. The neurological effects in particular are valuable for managing numerous musculoskeletal pain conditions. However, the practitioner is advised to accurately examine the true physiological effects of these substances before using them in a therapeutic environment or making recommendations about their use to clients in home care.

Notes

1. Mason L, Moore RA, Edwards JE, McQuay HJ, Derry S, Wiffen PJ. Systematic review of efficacy of topical rubefacients containing salicylates for the treatment of acute and chronic pain. Bmj. 2004;328(7446):995.
2. Sawynok J. Topical and peripherally acting analgesics. Pharmacol Rev. 2003;55(1):1-20.
3. Mendell LM. Constructing and deconstructing the gate theory of pain. Pain. 2014;155(2):210-216. doi:10.1016/j.pain.2013.12.010.
4. Ichiyama RM, Ragan BG, Bell GW, Iwamoto GA. Effects of topical analgesics on the pressor response evoked by muscle afferents. Med Sci Sport Exerc. 2002;34(9):1440-1445.
5. Hong CZ, Shellock FG. Effects of a topically applied counterirritant (Eucalyptamint) on cutaneous blood flow and on skin and muscle temperatures. A placebo-controlled study. Am J Phys Med Rehabil. 1991;70(1):29-33.
6. Myrer JW, Measom GJ, Fellingham GW. Intramuscular Temperature Rises With Topical Analgesics Used as Coupling Agents During Therapeutic Ultrasound. J Athl Train. 2001;36(1):20-25.