A stage 4 pressure sore features a deep wound of the skin with exposed underlying tissue, such as muscle and bone, states WebMD. Tendons and other tissues may have incurred damage from the wound. Due to the extensive damage, the patient may experience little-to-no pain from the pressure sore.
The patient is at increased risk for serious complications, such as osteomyelitis and sepsis, in stage 3 and 4 cases, according to WebMD. The patient may exhibit signs of deep tissue damage without an open wound. In these cases, the flesh is often dark red or purple and may have a blood-filled blister. When thick layers of tissue and pus obscure the pressure sore, it may be difficult to classify its type.