A narrative review on fascial plane blocks - Part A: Anatomical foundations and mechanistic insights.

Fascial plane blocks (FPBs) have transformed perioperative analgesia by shifting attention from individual nerves to the complex architecture of fascia and its dynamic role in nociceptive modulation. Despite widespread use, their anatomical foundations, mechanisms of action, and factors influencing injectate spread remain incompletely understood, contributing to inconsistent clinical performance. This manuscript of our two-part review provides a comprehensive, anatomy-driven framework that clarifies the structural diversity of fascial planes, their intrinsic neural components, and the evolving consensus on terminology and classification. Drawing on cadaveric, radiological, and clinical evidence, we dissect the mechanisms that allow FPBs to achieve analgesia-ranging from local fascial modulation and paraneural diffusion to systemic effects-and outline the determinants that shape injectate distribution, including fascial barriers, tissue compliance, patient positioning, and hydrostatic forces. A detailed analysis of interregional nomenclature, unified classification schemes, and persistent knowledge gaps highlights the need for standardisation and mechanistic clarity. Together, these foundational concepts establish the anatomical and physiological bases upon which clinical applications are optimised, enabling a more precise, evidence-based selection and execution of FPBs across surgical and emergency settings.