Topical Devices and Products for Pain

W. Steven Pray, Ph.D., R.Ph., Joshua J. Pray, Pharm.D. Candidate

US Pharmacist. 2003;28(1) 

Introduction

Back problems affect millions each day, as do muscular strain and overuse syndromes. Dysmenorrheal cramping affects millions of women every month. For each of these problems, there are several different choices for pain control. The pharmacist's challenge is to explain the relevant efficacies and safety questions.

While oral analgesics are safe and effective for many types of minor pain, there are several problems associated with their use in certain situations. For instance, all oral analgesics carry a warning against their use if the patient drinks three or more alcoholic beverages daily.[1]

Salicylates (e.g., aspirin, magnesium salicylate) cause problems with clotting and damage the gastric mucosa.[2]

Acetaminophen is a slightly better alternative for pain than salicylates, because of a reduced risk of interactions and adverse events. However, the FDA has not yet fully approved self-care dosing for those under the age of 2 years. Further, recent data point to acetaminophen as the culprit in acute liver failure when given in accidental overdoses. Acetaminophen is the single most common reason that callers seek the advice of poison control centers.

The only other OTC options for oral analgesic therapy are ibuprofen, sodium naproxen and ketoprofen.[3] Patients who have had ulcers or who have experienced a severe allergy to aspirin or any other analgesics should not take any of these, however. They all carry the same strong warnings against pregnancy as aspirin.

For many patients oral analgesics are not a viable option for pain control. The diverse group of chemicals known as external analgesics is often seen as a valid option. The group includes local anesthetics (e.g., benzocaine, benzyl alcohol) and hydro- cortisone, which are of potential use in minor dermatological conditions such as insect stings, but are not useful in muscle aches, back aches, and dysmenorrhea. Instead, patients often choose counter-irritant products. They affect the epidermis only superficially, creating a warmth, cooling, or irritation that counters or masks the pain originating from deeper tissues. Their use is fraught with problems. Some patients react with rashes and blistering, even with correct use. Any attempt to use them at the same time as a heating pad, hot water bottle, or other thermotherapy device can produce burns. Some (e.g., camphor, methyl salicylate) are toxic if ingested.[4,5,6] Most (e.g., methyl salicylate, menthol, camphor) have pungent odors that are unacceptable to patients; others (e.g., turpentine oil) cause local sensitization.

If the patient experiences an acute injury, immediate application of cryotherapy is required as a component of PRICE (protection, rest, ice, compression, elevation).[3,7] Cryotherapy should be continued for 48 hours. Used in this manner, it helps circumscribe the extent of injury by limiting post-injury inflammation. Cryotherapy products are available as two general nonprescription devices. One is typified by the 3M Nexcare Instant Cold Pack, activated as the patient squeezes the pack firmly until an audible "pop" is heard. This signals rupture of an inner membrane, which allows two chemicals to contact each other, with a resultant cryogenic reaction. As a one-time use product, it may be useful to carry when hiking. The second type of cryogenic device is a reusable gel bag, such as the 3M Cold Comfort Reusable Cold Pack. This type of device cannot generate its own cold, and must be pre-cooled in the lower section of the refrigerator. When an injury occurs, it is removed and immediately applied to the injured area.

Thermotherapy. At one time, thermotherapy was seen mainly as a component of the post-cryotherapy rehabilitative process.[8,9] How-ever, in recent months, new information has emerged demonstrating that thermotherapy allows the patient to attain pain relief through the well-known gate-control theory of noxious signal inhibition, a concept now known as "thermal analgesia."[10] When muscles and tissues are tight, circulation to the area is restricted, resulting in progressive ischemia and increasing pain. Properly applied heat allows muscular tissue to relax, facilitating increased circulation, and relieving pain by allowing metabolic toxins to be removed from the area and increasing tissue oxygenation.

Hot Water Bottles. Hot water bottles are rubber bags with stoppers that are applied to the painful area. While they conform well to various body surfaces, they cannot be easily secured to the body, and their flexibility allows them to slip and slide from the painful area. Further, the water in them cools quickly, requiring the patient to continually refill them.

Heating Pads. The ubiquitous heating pad is usually a safe source of heat. However, the FDA and Consumer Product Safety Commission have logged many cases of injury and death with its use, estimating a total of 1,600 new burns and eight fatalities each year.[10] Many are directly due to the fact that the heating pad is composed of live electrical wires covered with insulation. Others are due to the fact that the heating pad continues to produce heat in whatever level is set by the patient, until it is unplugged. Also, the patient who wishes to work or carry out sports activities while receiving heat cannot use the heating pad, since it is dependent on an electrical source.

One company (Prism Technologies) manufactures a small plastic bag containing a solution of sodium acetate and a small metal disc. The Heat Solution (also known as Zap Pac) is activated by grasping the metal disc inside the bag and clicking it once. This action forms a nucleate crystal, which initiates a cascade of exothermic crystalline precipitation. The heat produced may last as long as one hour. The bag can be indefinitely recharged by placing it in boiling water or a microwave oven. Further, it cools itself, so it is not as prone to burns and injuries as the heating pad. However, it cannot be secured to the body.

Another method to produce heat is to place iron, charcoal, table salt and water in small discs enclosed in a heat wrap that is placed directly over the affected body part. The concept of iron oxidizing to produce therapeutic levels of heat has been used for small hand warmers, but those products did not yield a consistent, safe, or effective level of heat.[10] The problem has been solved with the recent introduction of an innovative ThermaCare heat wrap, which only begins to heat after the package is opened, reaching its target temperature of 104°F in about 30 minutes. It provides a consistent heat over an 8-hour period. Each heat wrap incorporates several discs to deliver heat to the covered area. Three versions are available: a back wrap; neck-to-arm wraps which treat the neck, shoulders, arms, and wrists; menstrual wraps which can be secured to the inner lining of undergarments. The heat wraps conform to body contours, and are fully portable, allowing the patient to carry out normal activities while wearing them.[10]

Comparative studies of oral and topical analgesics are rare. In one, however, investigators compared 12 hours of wearing a heated abdominal patch to 400 mg of oral ibuprofen TID for menstrual pain.[11] Continual low-level topical heat was as effective as ibuprofen for dysmenorrhea. A recent multi-site, randomized, active-controlled, investigator-blinded study compared the heatwrap to oral ibuprofen (400 mg TID) and acetamino-phen (1,000 mg QID) for lower back pain.[12] Topical heat wrap therapy was superior to acetaminophen and/ or ibuprofen in pain relief, lateral trunk flexibility, decreased muscle stiffness, and disability reduction. Further, investigators discovered that the improvements persisted for over 48 hours after removal of the heat wrap, far longer than the duration experienced with nonprescription oral analgesics.

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