The role of heat in (some) painful conditions

Heat a form of thermotherapy has been utilised for hundreds of years as an aid to help reduce pain and improve function. In certain cultures, thermotherapy is even thought to help prevent and cure diseases by reducing invading organisms (1). Patients are often confused about whether they should heat the injured and painful area or not. The purpose of this post is to give the reader an overview of heating and if and when it may be applied.

Under normal circumstances skin temperature is 34°C, core body temperature is 37°C, and the majority of other structures and tissues reside somewhere in-between (2). Rises in tissue temperature induce physiological changes including increased blood flow, cell metabolism, neurological function and tissue extensibility (3). Typical therapeutic heating aims to increase tissue temperatures up to 40°C (4). Heating tissues beyond 40-45°C is avoided in the context of musculoskeletal injury as it can lead to irreversible damage to cells (5).

What is known and what is not

The magnitude of the physiological response is temperature specific, i.e. hotter temperatures result in greater blood flows and cell metabolism. The ideal tissue temperature to promote optimal function, tissue repair and reduce pain is unknown. Furthermore, is unlikely to be the same for specific injuries, body locations and will vary based on individual heat tolerance. This makes it challenging for healthcare professionals to give specific heating recommendations.

Despite orthodox wisdom of exercise increasing tissue temperature, it is unclear how different exercise types and durations increase tissue temperature and subsequently the magnitude of the physiological response. What we do know is that certain muscles are more vulnerable to tearing at lower temperatures (<32°C) (6) and prolonged low-intensity exercise can raise local muscle temperature by 2°C (35°C to 37°C) (7). This would imply, at least muscles mechanical properties are heat sensitive and exercise can increase tissue temperature.

Although there is much to learn about thermal changes in response to exercise, it is widely proven exercise has a plethora of benefits including improving strength, mobility and even reduce the risk of future injury (8, 9,10). Therefore, before reaching for the hot water bottle or sitting in a warm bath, guided exercise should be the foundation of any recovery programme. That been said, there are times when pain or physical capacity constraints stop us from exercising and in these instances, there may be a role for the application of external heat sources.

Heating modality is important

Heating can be categorised into either superficial or deep. Superficial heating is heat applied directly to the skin, which can be in dry or moist forms, Fig1. These applications are unlikely to increase tissue temperatures beyond a 2-3cm depth with deeper tissue heating more slowly and to a lesser extent, due to the insulating effect of fatty tissue (11).

Fig1.

Deep heating occurs through a form of energy conversion, requiring the use of specialist machines such as ultrasound or pulsed short wave diathermy, Fig 2. These modalities can raise tissue temperatures >3cm in depth (12). Clearly, the choice of heating modality will influence the physiological response at varying tissue depths and explains why certain deep tissue complaints may not respond to superficial heating.

Fig2

What does therapeutic heating do?

Therapeutic heat signals are transmitted and processed through the activation of sensory nerve endings and receptors found in the skin, deeper tissues, spinal cord and brain. Once heat signals reach the brain it may trigger a pain-reducing mechanism by which the brain sends messages back down the spinal cord to reduce other painful signals been processed (13) and indirectly promote a sense of relaxation (14). Unfortunately, this mechanism may be less effective in people with damage directly to the nervous system (15), which may explain why therapeutic heat in certain circumstances is not effective and potentially provokes more pain.

It would seem logical that warmer tissues are more compliant and thus the host should be able to move in greater ranges of motion. This theory originates from animal studies but has not been proven in human trials. A complimentary and more likely mechanism to explain increases in the range of motion following heat application is a reduction of pain sensitivity, thus stretch tolerance is increased (16). However, any increases in flexibility rarely last longer than 30 minutes after heat application is removed.

Finally, increases in blood flow result in greater levels of oxygen and nutrients at the injury site, which in combination with higher cell metabolism rates may promote improved tissue repair. This may be desirable in sub-acute (>3 weeks) and chronic (>12 weeks) injuries whereby a stimulus to healing may be deemed beneficial. However, as a rule of thumb, the reader should avoid heat when there is already an increased physiological response such as when the region is hot and swollen or following an acute injury, like a bone fracture and muscle or ligament tear. In these instances, there may be a role for icing, more information on that here.

Who is likely to benefit?

I think most readers will agree minor aches and pains such as tight shoulders and sore feet at the end of a long day feel better after heating. The purpose of this section, however, is to highlight moderate to good quality evidence to justify the use of superficial heating in more problematic conditions. I chose to focus on superficial heating, as it is widely available and can form part of a cost-effective self-care package for patients in pain.

Superficial dry heating reduces pain, improves flexibility and even promotes relaxation and better sleep in people with a mix of acute and chronic low back pain conditions (17 18). Take-home messages from these studies were that a wearable 40°C heat source (actual tissue temperatures unknown) was applied for 4-8 hours daily over 3-5 consecutive days. This is far in excess of the 10-15 minutes commonly prescribed to patients.

Moist heat has been shown to reduce pain, improve flexibility and in some cases strength in patients with osteoarthritis (19 20). Typical heating regimes in these studies involved 10-12, 20-minute sessions over 2 weeks with source temperatures ranging between 38-42°C. Similarly, increases in the immediate range of motion have been reported in patients with post-fracture and traumatic joint stiffness after dry and moist heating (21, 22). Interestingly, moist heat groups had a slightly improved range compared to dry heat groups. This may be explained as the moist heat could exercise during heating in a hydro pool.

Final thoughts

Given there is an acceptable physiological rationale and evidence that superficial heating is an effective tool in the management of low back pain, osteoarthritis and joint stiffness, one can at least try this as part of an overall self-care package for these conditions.

It is important the reader does not think of heating as a cure-all panacea for these problems. It certainly can help improve quality of life but these effects are not long-lasting unless combined with appropriate behaviours and adequate exercise prescription, which I repeat is the foundation of recovering from injury.

I will close this piece by giving some superficial heating guidelines and welcome any questions from the interested reader:

• The heating source should be between 40-42°C

• Apply heat directly to skin over and around the desired area for a minimum of 20 minutes

• Trial for between 3-14 consecutive days before stopping

• Combine with a guided exercise programme

• Exercise during heating if increasing joint range of motion is the priority (hydro pools or wearable heat pads)

• Avoid heating if the painful area is hot and swollen.

• If your pain feels worse during or after heating stop and speak to a healthcare professional

• If you have poor circulation, multiple sclerosis, rheumatoid arthritis, spinal cord injury, altered limb sensation, diabetes, or a skin condition you should discuss heat therapy with a healthcare professional before trialling.

Thanks for reading and see you next time.