Taxonomy of pain in SCD. The previously simple classification of pain in SCD assumed all pain was episodic and acute in nature. The current model includes the chronic pain that most patients with SCD are now known to experience, with potential contributions from multiple etiologies to varying degrees. Superimposed acute pain episodes can result from the classic vaso-occlusion or from exacerbations of chronic pain. Any one category of pain shown can likely indirectly contribute to the development of any other category, introducing even further complications.