The analysis of microcirculation is based on specific alterations
Generally, to detect early capillaroscopic changes in connective auto-immune diseases, the following parameters are considered: presence of enlarged and giant capillaries, micro-haemorrhages, loss of capillaries, ramifed/bushy capillaries (angiogenesis) and disorganization of the vascular array.
Capillary loss and avascular areas
Capillary density is the number of capillaries per unit area of skin. Essentially it is measured by recording images from the capillary microscope and then counting the capillaries in a known area of skin.
Recordings of capillary density can estimate the functional density of capillaries under a given condition or the total density of vessels.
The extensive disappearance of capillaries can generate large avascular areas, which have a desert-like appearance in the nail-bed microvascular array. Loss of capillaries could be relevant in determining the severe tissue hypoxia that is involved in the development of digital skin ulcers.
Homogeneously enlarged microvascular loops (giant capillaries) are the earliest and most striking feature of secondary Raynaud phenomenon. The enlargements show a characteristic symmetrical shape.
The detection of even a single loop with a homogeneous increase in diameter to more than 50 µm at the level of the nailfold bed should be considered a potential marker of microangiopathy related to an early scleroderma-spectrum disorder if not systemic sclerosis itself.
Local microhemorrhages are also associated with early microvascular damage. The appearance of this feature, which arises from microvascular extravasation of red blood cells from the capillary loop, bridges the appearance of giant capillaries and the subsequent loss of capillaries.
Microhemorrhage is linked to the altered integrity of the microvessel wall and altered endothelial cell array.
Capillary loss induces local hypoxia and the consequent local production of vessel growth factors (such as vascular endothelial growth factor), which in turn stimulating the formation of new capillaries—neoangiogenesis.
The main morphological hallmark of angiogenesis is the clustering of twisted capillaries, with pronounced heterogeneity in shape and size, winding together with bushy capillaries. Highly convoluted and branched capillary loop clusters, surrounded by loss of normal capillary loops, are characteristic features of neoangiogenesis in advanced microangiopathy.
The healthy subject
In normal conditions, the microvascular pattern is characterized by a regular array of microvessels with large intra/interindividual variability. However, absolute absence of capillary loss and giant capillaries is expected in normal pattern.
Ageing is accompanied by a loss in dermal volume, a reduction in capillary density, shortened capillary loops, and rarefaction of larger microvessels.