Some pressure sores develop in the tissue deep below the skin. This is called a deep tissue injury. The area may be dark purple or maroon. There may be a blood-filled blister under the skin.
That explains the part people find most confusing: the skin is not broken, so it doesn’t look like a wound. The injury is below the surface rather than on it.
Here’s what makes this worth acting on rather than watching. This type of skin injury can quickly become a stage III or IV pressure sore. Stage III is an open, sunken hole called a crater or ulcer, with the tissue below the skin damaged. Stage IV is a pressure ulcer so deep that there is damage to the muscle and bone, and sometimes to tendons and joints.
The heel is one of the places this is looked for. Pressure sores tend to form where skin covers bony areas.
Staging, or categories, describes how deep the pressure ulcer is. Early stages may be red skin without an open wound, and deeper stages mean deeper tissue damage.