Triple response of Lewis

Triple Response of Lewis is a physiological phenomenon observed when the skin is subjected to a firm stroke, typically using a blunt object. This response was first described by Sir Thomas Lewis in 1927 and is used to demonstrate the complex interactions between the nervous system and vascular system in the skin. The Triple Response of Lewis consists of three distinct components: red line, flare, and wheal.

Red Line

The first component of the Triple Response of Lewis is the red line that appears immediately after the skin is stroked. This red line is due to the direct mechanical trauma causing capillary damage, leading to localized redness (erythema). The red line corresponds to the path of the stroke and is attributed to the release of histamine from the damaged cells, which causes vasodilation of the capillaries.

Flare

Following the red line, a flare develops around the stroked area. This flare is more diffuse and extends beyond the immediate area of the stroke. The flare is not due to direct mechanical damage but is a neurogenic response. It is believed to be caused by the axon reflex, where sensory nerves are stimulated, leading to the release of neuropeptides such as substance P and calcitonin gene-related peptide (CGRP). These substances further promote vasodilation and increase the permeability of the blood vessels, leading to the spread of the redness.

Wheal

The third component, the wheal, appears as a localized area of swelling at the site of the stroke. The wheal is due to the accumulation of fluid (edema) in the tissues, which results from the increased vascular permeability induced by the substances released during the flare phase. The wheal typically develops a few minutes after the stroke and may persist for hours.

Clinical Significance

The Triple Response of Lewis is more than a physiological curiosity; it has clinical significance in diagnosing and understanding various dermatological and neurological conditions. For instance, an exaggerated triple response may be observed in individuals with hypersensitivity or allergic conditions, such as urticaria. Conversely, an absent or diminished response can indicate neurological damage or dysfunction, particularly in the sensory nerves.

Mechanism

The underlying mechanism of the Triple Response of Lewis involves a complex interplay between the nervous and vascular systems. The initial red line is a direct vascular response to mechanical trauma, while the flare and wheal involve neurogenic inflammation. This neurogenic inflammation is a result of the activation of sensory nerves, leading to the release of various mediators that affect blood vessels and surrounding tissues.

Conclusion

The Triple Response of Lewis provides valuable insights into the body's response to injury and the intricate relationship between the nervous and vascular systems. Understanding this response is crucial for clinicians and researchers in diagnosing and investigating various skin and neurological conditions.

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