Pain, Itch, and Inflammation; Why You Want to Scratch that Healing Scar

Skin is meant to be touched

It's natural for us to touch, hug and stroke those we love. These caring actions don't just maintain our social networks, they are rooted in biological processes which ensure the skin remains healthy, and that immune reactions favour would healing and tissue repair.

Recent research has illuminated the role of light touch mechanoreceptors in the skin, particularly in the context of immune function, wound healing, and connective tissue homeostasis.

The skin is also the major sensory organ of the nervous system, detecting and transmitting information about changes in temperature, chemical exposure, inflammation, or pH levels(1).

Neural connections

Sensory neurons in the skin include; raw dendrites which become nociceptor pathways during injury and inflammation; mechanoreceptors such as Merkel cells and Meissner's corpuscles which allow us to feel vibration and distinguish between light and deep pressure; and thermo-receptors for detecting heat and cold.

These sensory afferents travel via ascending spinal pathways to a variety of brain regions, allowing us to feel sensations like pain, burning, or itching, and alerting the brain to what's happening on the body’s surface. But they don't just provide tactile information, they can also initiate reflex skeletal muscular actions, such as when we instinctively rub over an acutely painful injury, or the impulse to scratch an itch. Stimulation of mechanoreceptors can result in local reflexes too, such as when a light touch makes your hair stand on end.

Sensory Neurons, Pain and Itching

Research shows that itch involves a complex two-way communication system between the skin and the brain. Nerve fibres that react to substances like histamine, connect to particular areas in the spinal cord and send signals to the thalamus, a key relay station in the brain. From there, the itch signals go to the insular cortex, which also handles sensations like internal temperature, hunger, and thirst. Thus the skin “talks” to the brain via these sensory afferents to discriminate information about pain, itch and local inflammation.

Brain imaging studies show that when itch is triggered such as by a histamine injection, multiple brain regions become active. This includes areas involved in arm movement which explains why itch almost always leads to the urge to scratch.

Itch and pain activate many of the same areas in the brain, but they are not identical. Pain activates some areas that itch does not, and itch seems to be more dominant in the left side of the brain.

Both pain and itch are relayed deep within the brain to the periaqueductal gray matter which plays a critical role in motivation and behavioural responses to threats. This is the primary control center for modulating pain via descending pathways, and when pain and itch are triggered together, the sensation of itch is suppressed.

Although itch and pain are different entities, they are closely linked. Both can be soothed by rubbing, which activates mechanoreceptor mediated interruption of ascending pain signals in the spinal cord. Whereas scratching incurs mild pain that can also temporarily block the itch by spinal chord antagonism between pain and itch-processing neurons (1).

High levels of histamines present in acutely inflamed tissue around a healing wound help to explain the desire to scratch the healthy tissue around a healing scab. The rhythmic activity applies the necessary movement into the underlying tissue to stimulate functional connective tissue rehabilitation. However, If we scratch too hard it becomes painful and we stop, as the pain message overrides the itch response.

In chronic conditions, the brain can become overly sensitive to itch in much the same way that it can in chronic pain syndromes. This can be seen in situations such as withdrawal from long term use of topical steroid creams.

Light Touch and Immune Interactions

Studies suggest that light touch may activate specific pathways that enhance local immune activity during wound healing. Mechanoreceptors can stimulate the release of anti-inflammatory cytokines to aid in tissue repair and healing, and this interaction between sensory nerves, manual movement of the healing tissue, and immune function, underscores the physiological basis for the reduction of inflammation during MLD.

Prolonged inflammation and certain substances can over-activate the nociceptors and increase the sensitivity of these pathways, recruiting nerve endings that are usually inactive to become active, or forming inappropriate synapses within the spinal cord.

This can heighten the body’s sensitivity, making even light touch sometimes feel irritating or painful. However caring touch and gentle sensory input can also influence various cellular and molecular immune responses, reduce the local tissue inflammatory load, and modulate the perception of pain.

Harnessing pain modulating processes

The intricate sensory network that informs the brain about the status of the skin can also influence individual responses during treatment. For instance, clients experiencing chronic pain or inflammation may have heightened sensitivity, which could make them either more responsive - or more reactive - to touch and pressure.

Differences between modalities in the specific stretch and shear forces transferred by the therapist's hand to the the skin of the recipient, may explain why MLD can be used in some conditions such as CRPS and allodynia, while lymphatic effleurage may increase sensitivity.

The localised rhythmic action of MLD on the light touch mechanoreceptors may induce the desired anti-inflammatory effect, while even a light stroking moves will be less repetitive and only briefly stimulate each mechanoreceptor, perhaps reinforcing the abnormally sensitive threat/pain pathways.

Understanding the physiological basis of MLD allows the skilled therapist to utilise specific manipulations of the skin and underlying tissue to support the processes conducive to any stage of healing.

Physiological Effects of MLD on Pain, Itch and Wound Healing

The role of proper lymphatic function to maintain the ideal environment in the connective tissue during healing and scar formation cannot be underestimated, and explains the remarkable results that we have observed when MLD is applied correctly. The bi-directional stretch and shear forces created with every MLD movement have been shown to increase the rate and force of stretch-receptor-mediated lymph-motoricity, but it's not just about making the vessels pump harder or faster.

These small, specific movements also; open the gaps in the initial lymph vessels; stimulate the light touch mechanoreceptors; induce cellular changes in lymphatic endothelial cells, fibroblasts, and immune cells; change the thixotropic state of the loose connective tissue to favour tissue drainage; and support functional rehabilitation of the connective tissue fibres during wound healing and scar formation. These factors work synergistically with the neuro-immuno-lymphatic system interactions, which are currently of great interest among researchers.

In the context of wound healing, rhythmic and repetitive sensory input from mechanoreceptors during MLD may help to modulate fibroblast activity, promoting functional organisation during tissue remodeling and preventing conditions like fibrosis.

This is particularly important in the treatment of chronic wounds, where enhanced sensory input may facilitate better healing outcomes.

Bridging Knowledge and Practice

Restoring connective tissue homeostasis during wound healing is a dynamic interaction involving neural, cellular and inflammatory elements which are responsive to gentle stretch and shear forces in the healing tissue. However, the role of tactile stimulation in therapeutic practices is often underestimated and underutilised. In an era where holistic care is paramount, understanding the intricate interplay between sensory input and health will be crucial for recognising the value of therapeutic touch in all health settings.

Modalities such as MLD which apply repetitive pain free manipulations of the skin and underlying tissue can modulate immune response, improve tissue drainage, and promote wound healing.

MLD Therapists have techniques to harness this dynamic interaction and deliver treatment strategies that promote healing, and improve aesthetic and functional outcomes.

Want to Know More?

Join our August #TalkingLymphLive Interactive Webinar where we will review the stages of wound healing to understand the role of mechanical forces in tissue rehabilitation, and discuss traditional and novel scar management modalities and devices.

References

1. Neuronal Control of Skin Function: The Skin as a Neuroimmunoendocrine Organ. Roosterman et al. https://doi.org/10.1152/physrev.00026.2005

2. FIG. 2. Mediators and sensitization pattern of neurons involved in itch and pain processing. During inflammation, mechanoinsensitive “sleeping” nociceptors and itch histamine-sensitive mechanoinsensitive puriceptors and probably mechanosensitive puriceptors transmit the response to the spinal cord. In the spinal cord noxious input can induce central sensitization for pain, and puriceptive input can provoke central sensitization for itch.