Making Sense of Late Tissue Nodules Associated With Hyaluronic Acid Injections.

Background: The pathogenesis of delayed-onset tissue nodules (DTNs) due to hyaluronic acid (HA) injections is uncertain.

Objectives: To formulate a rational theory for DTN development and their avoidance and treatment.

Methods: A multidisciplinary and multicountry DTN consensus panel was established, with 20 questions posed and consensus sought. Consensus was set at 75% agreement.

Results: Consensus was reached in 16 of 20 questions regarding the pathogenesis of DTNs, forming the basis for a classification and treatment guide.

Conclusions: The group believes that filler, pathogens, and inflammation are all involved in DTNs and that DTNs most likely are infection initiated with a variable immune response. Injected filler may incorporate surface bacteria, either a commensal or a true pathogen, if the skin barrier is altered. The initially high molecular weight HA filler is degraded to low molecular weight HA (LMWHA) at the edge of the filler. Commensals positioned within the filler bolus may be well tolerated until the filler is degraded and the commensal becomes visible to the immune system. LMWHA is particularly inflammatory in the presence of any local bacteria. Commensals may still be tolerated unless the immune system is generally heightened by viremia or vaccination. Systemic pathogenic bacteremia may also interact with the filler peripheral LMWHA, activating Toll-like receptors that induce DTN formation. Given this scenario, attention to practitioner and patient hygiene and early systemic infection treatment deserve attention. Classification and treatment systems were devised by considering each of the 3 factors-filler, inflammation, and infection-separately.