Laser Doppler imaging: a developing technique for application in the rheumatic diseases.

Abstract

Microcirculatory abnormalities contribute to the pathogenesis and pathophysiology of many of the rheumatic diseases. This is best recognized in systemic sclerosis (SSc), in which structural microvessel disease can be well demonstrated using the technique of capillary microscopy [1], and more recently by video and digital capillaroscopy [2, 3]. However, in many other conditions the microvasculature is more subtly involved. By the ‘microvasculature’ we mean the arterioles, the capillaries and the venules. Any inflammatory state is associated with profound microvascular perturbation. For example, in rheumatoid arthritis the synovial microvasculature undergoes major change with formation of new blood vessels (angiogenesis) in the hypertrophied synovium and with lymphocyte trafficking through high endothelial venules. These high endothelial venules are lined by specialized endothelial cells whose formation has been induced during the inflammatory process. [4]. In the study of disease, we must be concerned not only with understanding basic pathophysiology but also with the measurement of disease progression. If we cannot measure the disease, then we cannot assess its progression or responsiveness to treatment. In addition, the ability to measure disease processes can give us indirect insights into pathophysiology by allowing us to assess response to therapeutic interventions which are known to have specific mechanisms of action. Can we measure microvascular disease/involvement by disease, and apply this to the study of rheumatological disorders? As already mentioned, we can examine nailfold capillary structure in certain connective tissue diseases, such as SSc and dermatomyositis, using nailfold microscopy and video capillaroscopy, and one aspect of capillary function (permeability) can be examined by fluoroscopy [5, 6], which is, however, invasive in that it requires an intravenous dye injection. In this review we shall discuss the relatively new technique of laser Doppler imaging (otherwise termed ‘scanning laser Doppler’), which gives a direct measure of microcirculatory flow. We believe laser Doppler imaging affords significant potential in the study of microcirculatory involvement of the rheumatic diseases and it is non-invasive. Background to laser Doppler blood flow monitoring The observed wavelength of electromagnetic radiation is affected by relative motion between the source and observer. This phenomenon (also applicable to sound waves, as in the technique of Doppler ultrasound) is known as the Doppler effect. When low-level laser light, of a few milliwatts, is directed onto the skin’s surface a fraction of the light penetrates the skin and interacts with both static tissue and moving cells (primarily red blood cells). The penetration depth of light is dependent upon the tissue morphology, absorption and the wavelength used [7, 8]. The light that is reflected or randomly scattered from the static tissue remains unchanged in wavelength. In contrast the light that is scattered from the moving blood cells undergoes a small change in wavelength, proportional to the speed of the erythrocytes, due to the