The lymphatic system is composed of a network of capillaries, vessels, trunks, ducts, and lymph nodes that collect fluid, macromolecules, proteins, and immune cells from the interstitial space, transport the fluid or lymph, and return it to the venous system [1
]. Unlike the hemovascular system, the lymphatics do not have a centralized pumping organ, and lymph is transported through the lymphatics, primarily by cyclic contractions of vessel subunits called lymphangions. Bounded by valves to ensure unidirectional flow, lymphangions are arranged in series and sequentially contract to propel the lymph through the lymphatics in a peristaltic manner [1
]. Although contractile function is crucial to the transport of lymph, the fundamental physiological mechanisms governing this phenomenon are not well understood, and there are no methods to evaluate contractile function noninvasively in humans.
Disruption of lymph transport, because of impaired lymphatic function, reduced numbers of lymphatic vessels, valvular insufficiencies, or exacerbated by combinations of the above results in the chronic condition of lymphedema (LE) and its sequelae of inflammatory response results [2–4
]. Treatment for LE is generally limited to compression bandaging and manual lymphatic drainage or massage to minimize swelling and encourage lymph drainage. There is no cure for lymphedema.
In Western countries, acquired LE is generally caused by trauma or surgery, typically after lymph node resection for cancer staging or therapy. It is estimated that, depending on the therapeutic approach taken, as many as 40% of all breast cancer patients who undergo axillary lymph node dissection [5–7
] and 64% of all cancer patients who undergo groin lymphnode dissection [8
] encounter acquired LE. LE may present weeks to years after trauma or surgery, often after a challenge to the immune system. Although there are no means to predict who will develop “secondary” LE and when, recent work suggests that causal or susceptibility genes may be involved [9
]. In contrast, hereditary or primary LE has been associated, thus far, with mutations in vascular endothelial growth factor receptor 3 [10,11
] (Milroy or Miege disease), SOX18 [11
], and FOXC2 [12–15
] (lymphedema-distichiasis), in which the latter disease is noted for valvular insufficiencies and has been characterized by lymphoscintigraphy as exhibiting lymph reflux in dilated lymphatic vessels of the leg [13
Currently, lymphoscintigraphy is the only means available to routinely image the lymphatics. Although gross lymph architecture such as main vessels and nodes are visualized in scintigrams, the long integration times associated with gamma cameras prevent imaging of real-time lymphatic contractile function and the spatial resolution limits visualization of fine lymphatic vasculature. Because the lymphatics provide little endogenous contrast, direct imaging of the lymphatics is difficult and lymphatic architecture and function generally cannot be probed directly with ultrasound, magnetic resonance, or computed tomography. In addition, the lymphatic vasculature is not readily accessible for administration of the milliliters of contrast agent needed for magnetic resonance or computed tomography angiography, making aberrant lymph architecture difficult to routinely assess (for reviews on lymphatic imaging, see Sharma et al. [16
], Szuba and Rockson [17
], and Lucarelli et al. [18
]). In an extensive review, Marshall et al. [19
] summarized the work of several groups who have developed optical imaging devices for human lymphatic imaging using near-infrared (NIR) fluorescence. In these studies, indocyanine green (ICG), a green dye used for hepatic clearance and ophthalmological indications, was used primarily to map the lymphatics and provide surgical guidance for nodal resection. Although these optical imaging devices are sensitive enough to image the lymphatics with milligram amounts of ICG, none have quantified contractile lymphatic function [19–23
]. Recently, we demonstrated the feasibility for noninvasive imaging and quantification of propulsive, contractile lymphatic function after intradermal administration of microgram
amounts of ICG [20,24
The objective of the phase 0 clinical feasibility study described was to evaluate the utility of NIR fluorescence to 1) image differences within the lymphatic architecture and its contractile function and 2) quantify the apparent propagation velocity and frequency or period of contractile transport in the arms or legs of healthy control subjects as well as of subjects clinically diagnosed with unilateral LE. The ability to noninvasively visualize lymphatic architecture and quantify its propulsive function within asymptomatic and symptomatic limbs of subjects with LE as well as within the limbs of healthy subjects provides the first opportunity to investigate changes in lymphatic function with human disease.