To the Editor:
Two variants of laser Doppler monitoring exist to evaluate digital blood flow: laser Doppler imaging (LDI) and laser Doppler flowmetry (LDF). The first technique uses a scanning method with a distant light source and detector, while the second uses optical fibers to carry the light to and from the tissue. As a result, LDF measures the microcirculatory blood flow through a very small volume of tissue, whereas LDI scans a larger tissue area. LDF continuously measures skin blood perfusion; because of the scanning time, LDI cannot measure it continuously1.
We address the relationship between digital blood flow and capillaroscopic damage in patients with systemic sclerosis (SSc). Table 1 outlines the main studies.
Main studies of laser Doppler monitoring and capillaroscopy to evaluate microvascular damage in patients with systemic sclerosis (SSc).
Using LDI, Correa, et al found lower digital blood flow in 44 patients with SSc compared with healthy controls at baseline and after cold stimulus2. No correlation was found between functional and morphological microvascular abnormalities measured by LDI and nailfold capillaroscopy. The authors suggest that it is appropriate to consider LDI and nailfold capillaroscopy as complementary tools for evaluation of different aspects of SSc microangiopathy.
Using LDF, Cutolo, et al3,4 demonstrated a significantly lower fingertip blood perfusion in 34 patients with SSc compared with healthy controls. Fingertip blood perfusion correlated negatively with the extent of the nailfold microvascular damage, being lower in patients with SSc with the late nailfold videocapillaroscopy (NVC) pattern of microangiopathy. Interestingly, patients with SSc who showed the late NVC pattern of microangiopathy showed fingertip blood perfusion that was significantly lower than that of patients with the active or early NVC patterns.
Using LDI, Rosato, et al5 demonstrated a significantly lower skin perfusion of hands and fingers in 142 patients with SSc compared with healthy controls. Skin blood perfusion showed a negative correlation with microvascular damage scored according to NVC classification5.
In a previous study, we demonstrated that bosentan improved skin perfusion, evaluated by LDI, in patients with SSc, particularly in the skin region distal to the proximal interphalangeal joint and in patients with the early/active capillaroscopic patterns6. Additionally, we demonstrated a strong correlation between capillaroscopic patterns and LDI and pulsatility of digital arteries evaluated by photoplethysmography7.
Conversely, in 16 patients with SSc, Murray, et al8 demonstrated that nailfold capillaroscopy, LDI, and thermal imaging each independently provide good discrimination between patients with SSc and those with primary Raynaud’s phenomenon and healthy controls. LDI and thermal imaging give equivalent information on dynamic changes in the cutaneous microcirculation; however, these correspond only weakly to capillary morphology.
In 127 patients with SSc, Mugii, et al9 demonstrated that red blood velocity was significantly associated with NVC findings, including capillary ramification and capillary loss. Patients with the scleroderma active and late NVC pattern showed a lower red blood velocity compared with the scleroderma early pattern.
In 40 patients with SSc10, we demonstrated that thermoregulation of the skin of the fingers, evaluated by LDI after a cold test, is impaired early for the presence of both structural and anatomical changes of microcirculation, while in the dorsum of the hand, thermoregulation is impaired only in advanced stages of SSc microangiopathy.
The term commonly used to describe blood flow measurements by the laser Doppler technique is “flux.” Flux has been expressed by perfusion units, which are directly proportional to the product of the mean speed and the concentration of red blood cells. Since red blood velocity is compromised in microvascular disorders, the peripheral blood flow is also steadily reduced.
Skin blood flow depends not only on arterial inflow but also on local factors (blood gases, hormones, temperature). The nutritional skin flow is linked to the integrity of the microvasculature, which consists of a superficial subpapillary plexus and a profound cutaneous plexus. In some areas (e.g., digits, nose, lips, and ears) there are interconnections, often with vascular shunts, between these 2 plexuses. These so-called arteriovenous anastomoses (AVA) enable a fast increase or decrease in blood flow through the skin to regulate the body temperature. The AVA are innervated by the autonomous nervous system (a2c adrenergic receptors). In the fingers of patients with SSc, the absence of a correlation between skin perfusion and digital arterial flow is due to microvascular damage (giant capillaries, disorganization of the normal capillary array, avascular areas) and AVA shunts. In SSc, microvascular damage is responsible for the baseline digital blood flow reduction, while the AVA may be the main but not exclusive factor in controlling the digital blood flow during cold stimulus. Therefore, local factors are also involved in the regulation of digital blood flow during cold stimulus.
For these reasons, we can suppose that a relationship exists between structural and functional aspects of SSc microvasculature. We can conclude that capillaroscopy represents the best method to analyze microvascular damage in SSc. Therefore, capillaroscopy and LDI show a strong correlation in the definition of structural and functional microvascular damage.
