[ Editorials | Letters | Selected Articles | Signature Series | Patient Handouts | Index ]

2002 Selected Articles

Heat Therapy: The Next Hot Topic

Roger P. Smith, MD

The use of heat to relieve pain probably dates back to the first availability of warmth and shelter. The history of its medical use (as in fomentation) is less clear, but almost certainly as old. For example, Chapter 7 of the Hippocratic treatise Regimen in Acute Diseases¹ recommends the application of a metal or clay container filled with hot water for pain in the ribs, with something soft placed between the container and skin to prevent burns. It also discusses "dry fomentations" consisting of woolen envelopes filled with heated millet. Indeed, even today patients can buy cloth bags filled with wheat or rice designed to be placed in a microwave oven for 2 to 3 minutes and used like a heating pad or hot water bottle.

In the nineteenth century, when many complaints were attributed to inflammation of the uterus and surrounding tissues ("metritis"), heat and bed rest were routinely prescribed: "Warm poultices of flaxseed or cornmeal should be laid over the hypogastrium, or, instead of these, towels wrung out of hot water and covered by oil-silk may be used."² Heat therapy was employed in many ways for female complaints in the 1800s. As one textbook explains, "Long-continued irrigation of the vagina and cervix with water as hot as could be borne produced a very sedative effect–and the effect was very decided… This syringing or irrigation should be continued at least fifteen or twenty minutes each time, for, if hot water is used but a few moments, the first stimulating action of heat is produced, to be succeeded by congestion; but, when continued, it relieves the parts, and sedation follows. This effect of the hot vaginal irrigation has been well advocated and attested by Dr. Emmett, of New York City, in an article published some time since."³

Therapeutic use of induced heat (eg, diathermy) also has a considerable history. Curtis’ 1931 Text-Book of Gynecology states, "Since 1922 I have employed surgical diathermy as an adjunct to radium in treatment of cervical carcinoma. This form of heat destruction possesses all of the advantages of the actual cautery and is free from many of its disadvantages."⁴ This application for heat in cancer therapy has persisted in one form or another into contemporary use.^5,6

PHYSIOLOGY

Therapeutic heat has physiologic effects that are mediated via neurologic, vascular, and biophysical mechanisms. The neurologic mechanisms of pain relief are thought to occur through the "gate control" theory of pain inhibition.⁷ This theory is based in part on the inhibition ("gating") of noxious signals in the substantia gelatinosa of the dorsal horn of the spinal cord. Topical heat increases the temperature of the skin and underlying tissues, leading to stimulation of thermoreceptors. The resultant increase in small nonmyelinated c-fiber activity in first-order afferent neurons inhibits the concurrent nociceptive signals in other first-order neurons. This interaction is thought to occur at the synapse between first- and second-order neurons, and is termed presynaptic inhibition. Functional brain imaging has shown that gentle warming of the skin activates the thalamus and posterior insula. Touch stimulation of the skin activates the thalamus and S2 region of the cerebral cortex as well. These data suggest that some of the potential benefits provided by topical heat may be mediated by higher centers in the brain in addition to the effects shown in the substantia gelatinosa. Feelings of comfort and relaxation associated with topical heat therapy also mitigate the integration and coherence of the pain experience in the brain.⁸ Locally, vascular mechanisms contribute by increasing blood flow to the tissues, resulting in dilution of prostaglandins, bradykinins, and histamine, which are modulators or instigators of nociception. This local increase in blood flow brings improved oxygenation and nutrients that enhance tissue repair.

Biophysical mechanisms come into play when heat is used to treat musculoskeletal pain. Topical heat increases connective tissue extensibility, supporting a biophysical mechanism of thermotherapy.⁹ This phenomenon is called plastic deformation or elongation. Skeletal muscle spasm is relieved through a reflex action via thermoreceptors and a decline in muscle spindle g fiber activity, decreasing the muscle spindle’s sensitivity to stretching. Heat may also relieve muscle spasm by activating descending pain-inhibitory systems via an unknown mechanism.¹⁰ Decreased a motor neuron activity from the dorsal horn of the spinal cord, resulting in a lowering of muscle tone, has also been postulated.¹¹

DYSMENORRHEA

The application of heat to treat dysmenorrhea has a venerated history of success but low levels of use. Nonetheless, women know that heat is effective, and physicians acknowledge its utility. For centuries, Chinese physicians have treated dysmenorrhea with moxibustion, which involves placement of burning moxa or mugwort (Artemesia vulgaris)æa species of chrysanthemum used as incense)æon acupuncture points on the lower abdomen. Three-thousand-year-old hieroglyphs of acupuncture and moxibustion have been found on bones and tortoise shells from China’s Shang dynasty, indicating that the practice predates that time. Soranus of Ephesus in the second century of the modern era advised local application of heat using an animal bladder filled with hot oil and held over the lower abdomen.¹²

In the late nineteenth century, textbooks encouraged the use of heat for menstrual discomfort: "As soon as menstruation begins, or some hours before if its approach can be recognized, the patient should go to bed and apply warmth by bottle of warm water, warm bricks wrapped in dry flannel, or, as is better, by bags of India rubber filled with warm water to the feet, abdomen, and sacrum alternately."¹³ And as noted in medical books from the early twentieth century, "A word about popular remedies: The hot-water bottle is helpful. Hot sitz baths may be used, but must be employed judiciously."¹⁴

However, heat therapy has not been evaluated rigorously or systematically in the scientific literature,¹⁵ and there are only a few case reports.¹⁶ Even when heat therapy has been endorsed, the practicalities of rapidly cooling hot-water bottles and the inconvenience of electrical cords for heating pads have limited the utility of this option. The recent development of small wearable devices capable of supplying a low level of topical heat at a constant temperature over a prolonged period now make this modality portable enough to be a viable treatment option. A recently published study by Akin and associates¹⁷ showed that continuous, low-level, topical heat was similar or superior to oral ibuprofen therapy for menstrual pain.

This research consisted of a randomized, double-dummy, parallel study over 2 consecutive days using an abdominal patch (self-heating or unheated) for approximately 12 consecutive hours per day and oral medication (placebo or ibuprofen, 400 mg three times a day). Pain relief and pain intensity were recorded at 17 time points. Eighty-four patients were enrolled and 81 completed the study protocol. All patients experienced moderate or greater pain at the start of treatment by protocol, and baseline pain assessments were not significantly different in any of the four study groups. Over the 2 days of treatment, significantly more pain relief was reported by three groupsæthe heat/placebo group (mean pain relief score = 3.27, P < 0.001), the unheated/ibuprofen group (mean = 3.07, P = 0.001), and the combination heat/ibuprofen group (mean = 3.55, P < 0.001)æcompared with the unheated/placebo group (mean = 1.95). While the authors could not document greater pain relief for the combination heat/ibuprofen group compared with the unheated/ibuprofen group (P = 0.096), the time to noticeable pain relief was significantly decreased for the heat/ibuprofen group (median = 1.5 hours) compared with the unheated/ibuprofen group (median = 2.79 hours, P = 0.01). That is, pain relief occurred more rapidly when heat was applied concomitantly with ibuprofen than when ibuprofen was used alone. The rate of complete relief was significantly higher for the heat/ibuprofen group (68% incidence, odds ratio = 4.0, P = 0.02) and the heat/placebo group (70% incidence, odds ratio = 4.3, P = 0.015) than in the unheated/ibuprofen group (55% incidence, odds ratio = 2.3, P = 0.103) and the unheated/placebo group (35% incidence). These results indicated that the odds of receiving complete relief while on heat treatment were four times greater than for unheated/placebo treatment.¹⁷

The topical heat treatment used in this study appeared to have an additive effect with the ibuprofen, as the time to onset of pain relief was significantly shorter with combination heat/ibuprofen therapy compared with ibuprofen treatment alone.¹⁷ In addition, the pain relief in the heat/ibuprofen group on the first day of treatment was directionally higher than that in the unheated/ibuprofen group. Thus, there is finally solid evidence that low-level topical heat therapy can provide relief for dysmenorrhea that is comparable or superior to the therapy with nonsteroidal anti-inflammatory drugs (NSAIDs) that has become the de facto standard.

MECHANISM OF ACTION

The exact mechanism through which heat relieves menstrual pain is unknown. Heat may act as an analgesic through some form of gate control (similar to transcutaneous electrical nerve stimulation or to the effects of heat on musculoskeletal pain), by central alteration of pain thresholds, or through an altered sense of well-being.¹⁸ This may mimic the changes seen during placebo therapy.¹⁹ Warming the skin of the lower thorax increases intestinal blood flow in rats, while heat applied to the anterior abdominal wall has been shown to decrease gastrointestinal (GI) activity, suggesting the possibility of a feedback mechanism that could have a direct relaxing effect on the uterus.¹⁸ The relative contribution from these potential actions of topical heat on the uterus awaits further clarification.

The Akin study¹⁷ raises some interesting questions about the mechanism by which this relief is attained. Their data suggest that, while there is an overall additive effect when heat and ibuprofen are used together, the incremental effect was less than that achieved by either therapy separately. Thus, both modalities may be acting through similar mechanismsæa central analgesic effect, a direct effect on uterine activity, or both. It has been established that NSAIDs act as central analgesics in addition to their direct effect on uterine contractions, although the precise mechanism is largely unknown. Similarly, the ability of NSAIDs to alter uterine activity in primary dysmenorrhea has been well documented.^20-22 The possibility of a feedback mechanism that could have a direct relaxing effect on the uterus is also consistent with these data, and cannot be dismissed.

The Akin study¹⁷ revealed evidence of a "carry-over" effect from heat that was not present when ibuprofen was used either alone or in combination with heat. Patients who received heat alone showed continuing pain relief at time 0 on day 2 of therapy, and this pain relief was statistically different from the results in the unheated/placebo group (P = 0.002). This extended pain relief on the day after heat treatment supports the presence of some form of centrally altered pain threshold, although this remains conjectural. An alteration in the biochemistry of uterine prostaglandin production, action, or degradation is another possibility.

DISADVANTAGES

Traditional heat therapies with heating pads and hot-water bottles are associated with both inconvenience and risk. In the past, burns have been the most significant risk associated with topical heat. The US Food and Drug Administration (FDA) and the Consumer Product Safety Commission (CPSC) have received many reports of injury and death from burns, electric shocks, and fires associated with the use of electric heating pads. These incidents occur in nursing homes, hospitals, and at home. Those at particular risk are infants and persons who may be unable to feel pain because of advanced age, diabetes, spinal cord injury, or medication. In most cases, careful inspection and proper use of the heating pad can prevent such adverse events. Prolonged use of a heating pad on one area of the body can cause a severe burn, even when the pad is set at a low temperature.²³

The CPSC estimates that more than 1,600 heating pad burns are treated annually in hospital emergency departments. Most injuries are direct thermal burns not caused by fire. About 45% of those injured are over the age of 65. Every year, the CPSC receives an average of eight fatality reports associated with the use of heating pads, most due to heating pad fires involving persons over the age of 65.²⁴ Patients should be given general guidelines about using this type of therapeutic heat at home. These include restricting application time to 20 to 30 minutes for devices that heat to an unknown temperature or that heat to more than 40°C. Temperatures above 40°C may result in burns to the skin depending on the length of exposure.

Heat therapy that results in elevated local or core body temperatures may exacerbate systemic conditions such as multiple sclerosis, peripheral vascular disease, spinal cord injury, and rheumatic diseases. Guidelines suggest that local heat therapy above 84°F should be avoided in individuals with multiple sclerosis, as it may induce increased weakness. Temperatures above 92°F in individuals with peripheral vascular disease may exceed tissue oxygen demands, resulting in tissue damage.²⁵

Newly introduced self-heating pads (eg, Procter & Gamble’s ThermaCare^® Air-Activated HeatWraps) avoid the problems associated with electric heating pads by providing a fixed low level of heat over a prolonged period. These devices use a controlled exothermic reaction to produce therapeutic heat. The pads have cells that contain activated carbon and iron powder along with water, vermiculite, and sodium chloride. As long as the pads are enclosed in their wrappers, they are inactive. However, when the patient removes the pad from the wrapper, the iron powder in the cells undergoes controlled oxidation. This "rusting" creates a low level of heat that lasts for more than 8 hours. The technology used to make the pads and cells allows for control of the maximum working temperature, further enhancing both efficacy and safety. The 8-hour duration provides the sustained use missing from other shorter-term thermotherapy options such as hot-water bottles. By using a controlled exothermic reaction, these pads also free the patient from the need for an external source of power.

These products are soft, flexible, and specifically shaped to fit any body contour. This ensures intimate contact between the pad and the skin, maximizing the analgesic effect. The light weight of the pad, coupled with its portability, allows the patient to wear it beneath her clothing while working or engaging in recreation. She only needs to open the package and remove the wrapper, applying it as directed. For menstrual pain, the pad is placed over the lower abdomen and secured by the patient’s underwear. The heat cells warm to their therapeutic temperature range within approximately 30 minutes.

GYNECOLOGIC APPLICATIONS

It is estimated that up to 75% of young women with dysmenorrhea do not seek formal medical care. The availability of inexpensive, safe, effective nonprescription therapy that can be used without consulting a physician means that dysmenorrhea should no longer be the source of disability it once was.

The portability, low cost, and proven efficacy of continuous low-level topical heat therapy make this an attractive option for treating women with menstrual and lower abdominal pain. This modality offers a high degree of efficacy without the systemic and other problems that may be associated with pharmacotherapy. For example, many students are prohibited from bringing or taking medications while at school without a school nurse or other official to dispense them. Topical heat therapy eliminates this problem while providing effective relief and allowing normal physical and social activities for the entire school day.

Unlike NSAIDs, which are more effective for primary versus secondary dysmenorrhea and which may cause GI upset, heat therapy should be equally effective in cases of secondary dysmenorrhea without any side effects. Topical heat therapy can be used safely in those patients with known or suspected GI disorders, which is not the case with NSAIDs. The heat may help with some GI symptoms as well, although trials of heat therapy in patients with GI diseases such as irritable bowel syndrome are lacking.

OTHER APPLICATIONS

Heat therapy in other settings has become well recognized. Recent reports have documented the utility of topical heat in treating everything from arthritis²⁶ to warts.²⁷ One of the most successful familiar uses of heat is for the treatment of musculoskeletal symptoms.

It is estimated that the annual incidence of low back pain is 5%, with a lifetime prevalence of 60% to 90%.²⁸ Roughly half of the patients seen in primary care settings have used self-treatment for an episode of low back pain prior to presenting for care.²⁹ Systematic reviews have shown that acetaminophen and ibuprofen are effective against low back pain, but despite the high prevalence and the long history of both prescription and self-medication, these drugs have not been compared with heat until recently for this disorder.³⁰

Topical heat therapy has demonstrated superior pain relief, improved muscle flexibility, and decreased disability when compared with both acetaminophen and ibuprofen in the treatment of acute muscular low back and neck pain.³¹ Recent studies of low back pain comparing continuous low-level topical heat with oral ibuprofen obtained pain relief curves that are remarkably similar to those in the Akin study.^17,32 Indeed, statistically significant improvements in lateral trunk flexibility and lower side effect rates indicate superiority over the traditional first-line pharmacotherapies.

CONCLUSION

Heat therapy is both one of the oldest and the newest of therapeutic modalities in medicine. The availability of inexpensive, portable, self-heating devices that have clinically proven efficacy make this therapeutic option the new hot topic.

Roger P. Smith, MD, is professor, vice chair, and program director in the Department of Obstetrics and Gynecology at the University of Missouri-Kansas City and Truman Medical Center in Kansas City, Mo.

REFERENCES

1. Littré E, ed. The Hippocratic Corpus, vol. 2. Amsterdam: Kakkert, 1962;268-270.
2. Thomas TG. A Practical Treatise on the Disease of Women. Philadelphia: Henry C. Lea; 1872;233.
3. Thompson JH. Report of Columbia Hospital for Women and Lying-in Asylum, Washington, DC. Washington, DC: Government Printing Office; 1873;280-281.
4. Curtis AH. A Text-Book of Gynecology. Philadelphia: WB Saunders; 1931;105.
5. Hornback NB, Shupe RE, Shidnia H, et al. Advanced stage IIIB cancer of the cervix treatment by hyperthermia and radiation. Gynecol Oncol. 1986;23(2):160-167.
6. Babbs CF, DeWitt DP. Physical principles of local heat therapy for cancer. Med Instrum. 1981;15(6):367-373.
7. Melzack R, Wall PD. Pain mechanisms: a new theory. Science. 1965;150:971-979.
8. Cameron MH. Physical Agents in RehabilitationæFrom Research to Practice. Philadelphia: WB Saunders; 1999; 149-173.
9. Rennie GA, Michlovits SL. Biophysical principles of heating and superficial heating agents. In: Michlovitz, SL, ed. Thermal Agents in Rehabilitation, ed 3. Philadelphia: FA Davis; 1996; 107—135.
10. Cox JS, Andrish JT, Indelicato PA, et al. Heat modalities. In: Drez D, ed. Therapeutic Modalities for Sports Injuries. St. Louis: Mosby-Year Book; 1989;1-23.
11. DePace DM, Newton R. Anatomic and functional aspects of painæevaluation and management with thermal agents. In: Michlovitz SL, ed. Thermal Agents in Rehabilitation, ed. 3. Philadelphia: FA Davis; 1996;30-57.
12. O’Dowd MJ, Philipp EE. The History of Obstetrics and Gynaecology. New York, NY: The Parthenon Publishing Group; 1994:346.
13. Thomas TG. A Practical Treatise on the Disease of Women. Philadelphia: Henry C. Lea; 1872;592.
14. Curtis AH. A Text-Book of Gynecology. Philadelphia: WB Saunders; 1931;219.
15. Lehman JF, de Lateur BJ. Ultrasound, shortwave, microwave, laser superficial heat, and cold in the treatment of pain. In: Wall PD, Melzack R, eds. Textbook of Pain, ed 3. Edinburgh: Churchill-Livingstone; 1994;1239.
16. Vance AR, Hayes SH, Spielholz NI. Microwave diathermy treatment for primary dysmenorrhea. Phys Ther. 1996; 76:1003-1008.
17. Akin MD, Weingand KW, Hengehold DA, et al. Use of continuous low-level topical heat in the treatment of dysmenorrhea. Obstet Gynecol. 2001;97:343-349.
18. On AY, Colakoglu Z, Hepguler S, Aksit R. Local heat effect on sympathetic skin responses after pain of electrical stimulus. Arch Phys Med Rehabil. 1997;78:1196-1199.
19. Smith RP. Objective changes in intrauterine pressure during placebo therapy of primary dysmenorrhea. Pain. 1987; 29:59-66.
20. Smith RP, Powell JR. Intrauterine pressure changes during mefenamic acid treatment of primary spasmodic dysmenorrhea. Am J Obstet Gynecol. 1982;143:286-292.
21. Smith RP, Powell JR. Simultaneous objective and subjective evaluation of meclofenamate sodium in the treatment of primary dysmenorrhea. Am J Obstet Gynecol. 1987;157: 611-616.
22. Smith RP. The dynamics of nonsteroidal anti-inflammatory therapy for primary dysmenorrhea. Obstet Gynecol. 1987; 70:785-788.
23. Bill TJ, Edlich RF, Himel NH. Electric heating pad burns. J Emergency Med. 1994;12:819-824.
24. Hazards associated with the use of electric heating pads. Recent safety information on FDA-regulated products. FDA Medical Bulletin. 1996;26(2).
25. Berger JR, Sheremata NA. Persistent neurological deficit precipitated by hot bath test in MS. JAMA. 1983;249 (13):1751-1753.
26. Welch V, Brosseau L, Shea B, et al. Thermotherapy for treating rheumatoid arthritis (Cochrane Review). Cochrane Database Syst Rev. 2001;2:CD002826.
27. Dvoretzky I. Hyperthermia therapy for warts utilizing a self-administered exothermic patch. Review of two cases. Dermatol Surg. 1996;22(12):1035-1038.
28. Biering-Sorenson F. Physical measurements as risk indicators for low back pain trouble over a one year period. Spine. 1984; 9:106-119.
29. Cherkin DC, Wheeler KJ, Barlow W, et al. Medication use for low back pain in primary care. Spine. 1998;23:6-7,14.
30. Van Tulder MW, Scholten RJPM, Koes BW, et al. Nonsteroidal anti-inflammatory drugs for low back pain. A systematic review with the framework of the Cochrane Collaboration Back Review Group. Spine. 2000;25:2501-2513.
31. Steiner D, Erasala G, Hengehold D, et al. Continuous low level topical heat therapy for trapezius myalgia. Proc Am Pain Society, 19^th Annual Meeting. 2000;171.
32. Nadler SF, Steiner DJ, Erasals GN, et al. Continuous low-level heatwrap therapy provides more efficacy than ibuprofen and acetaminophen for acute low back pain. Spine. (in press).

back to top