The lymphatic system plays a critical role in the process of reducing inflammation and edema, as well as a major role in the immune response. It moves fluid (lymph) from interstitial spaces in tissues via a network of collecting vessels to the lymph nodes, where it is mostly emptied into the venous system. Some fluid moves further along the lymphatic chain to other lymph nodes and eventually empties into the thoracic duct. PBM reduces swelling, and it seems likely that the lymphatic system is involved, although limited evidence exists to confirm this.

A PubMed search for a range of PBM terms (see Chapter 1) AND edema OR oedema OR swelling returns over 50 abstracts. Most of these abstracts are for post-mastectomy lymphedema, post-surgical swelling, acute soft-tissue injury, and arthritic joints.

The mechanism by which light affects edema has not been adequately explored, although it makes sense that it should because nitric oxide (NO) modulates lymphatic vessel contraction and subsequent lymph flow, and it has been established that PBM releases stores of NO8–10 and subsequently improves blood flow.

As with blood vessels, NO is produced by endothelial nitric oxide synthase (eNOS) in lymphatic endothelial cells, which in turn affects the dilation and force of lymph vessel contractions. Small amounts of NO are produced (by eNOS) when lymph vessels are stretched (by movement or a stretch/massage technique), and this leads to alternating periods of diastolic filling and systolic ejection of lymphangions (the tiny functional units of the collecting lymphatic system). Excessive amounts of NO produced by inducible nitric oxide synthase (iNOS) during inflammation can overwhelm the NO gradients produced by eNOS. The suppression of eNOS by iNOS disrupts the contraction/relaxation cycle in favor of excessive contraction and subsequent reduced flow.