According to the Merck Manual, pressure ulcers can develop and worsen across four stages, beginning with nonblanchable erythema (in which the ulcer may not yet be visible), then progressing to erosion (epidermal loss), full-thickness loss without bone or muscle exposure and finally leading to full-thickness loss with bone or muscle exposure. Pressure ulcers may not always develop following this exact pattern and may also be first seen as a deep and necrotic sore with full-thickness loss.
Also known as bed sores, pressure ulcers can be a breeding ground for antibiotic-resistant organisms acquired during a hospital stay. Tissue healing can be hampered by high bacteria counts within the wound. Inadequate treatment can be a cause of non-healing ulcers, but should also raise suspicions of possible complications, such as bacteremia, cellulitus or underlying osteomyelitus.