Applications

Wheal and Flare Response

The wheal and flare response (WFR) is the common reference to the triple response described by Lewis (Lewis, 1926): erythema, at site of injury, a spreading flare in the surrounding skin followed by pallor around the centre, as oedema develops, due to leaking capillaries.

Wheal and flare can be induced by local introduction of histamine into the skin by lancet, intra-dermal injection or by micro-dialysis. Laser Doppler imaging (LDI) aids the investigation of mechanisms of WFR and treatments that inhibit it (Denham, 2004).

Laser Doppler and Speckle Contrast Imaging

The microvascular blood flow response is most graphically visualised by laser Doppler Imaging (Clough and Church, 2002) or Speckle imaging (Clough et al, 2006). The dynamic evolution of the response is best observed with the moorLDLS2 or the moorFLPI imagers.

With laser Doppler monitoring the observed responses will be site dependent; a central point will indicate a flare of reduced duration (due to wheal formation) than more distal positions.

Methods

The forearm is a common test site. This region is traversed by underlying veins; the blood flow in these can distort laser Doppler assessments of microvascular blood flow so it is important to select regions of interest that avoid veins. Speckle imaging with the moorFLPI is not affected by underlying veins (except from very thin skin of the very young or elderly) so it will be the method of choice for WFR assessments.

To enable later, accurate assessment of flare area, skin should be marked with black dots separated by 2cm, e.g. using a surgical marker pen). The histamine should be introduced near the centre position between the marker dots. An area of at-least 5cm x 5cm (2” x 2”) should be scanned so that the whole of the flare area can be visualised (depending on dose – take care not to use excess dose, the response ‘seen’ by moorFLPI is much larger than can be seen by eye!).

Analysis

On image review, the flare area can be calculated from the number of image pixels that are significantly greater than the baseline skin blood flow. Baseline skin blood flow is assessed by defining a region of interest (ROI); the software is then used to calculate the average flux, the standard deviation (SD) and other basic statistics. This is used to calculate a level to define flare; e.g. 2 x baseline or baseline + 2 x SD. The area is then derived using either the Histogram function or the Cut-below and statistics functions

The speed of flare development can be assessed by using the software to graph the average flux within up to 16 user-defined regions. Data can be imported to spreadsheet software for further analysis.

References

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