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Bed sores - Causes of pressure ulcers

NHS Choices Medical Reference

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Pressure ulcers are caused by sustained pressure being placed on a particular part of the body.

This pressure interrupts the blood supply to the affected area of skin. Blood contains oxygen and other nutrients that are needed to help keep tissue healthy. Without a constant blood supply, tissue is damaged and will eventually die.

The lack of blood supply also means that the skin no longer receives infection-fighting white blood cells. Once an ulcer has developed, it can become infected by bacteria.

People with normal mobility do not develop pressure ulcers, as their body automatically makes hundreds of regular movements that prevent pressure building up on any part of their body.

For example, while you are asleep you may think you are lying still, but you may shift position up to 20 times a night.

Pressure ulcers can be caused by:

  • pressure from a hard surface, such as a bed or wheelchair
  • pressure that is placed on the skin through involuntary muscle movements, such as muscle spasms
  • moisture, which can break down the outer layer of the skin (epidermis)

The time it takes for a pressure ulcer to form will depend on:

  • the amount of pressure
  • how vulnerable a person's skin is to damage

Grade three or four pressure ulcers can develop over short time periods. For example, in susceptible people, a full-thickness pressure ulcer can sometimes develop in just one or two hours. However, in some cases the damage will only become apparent a few days after the injury has occurred. 

Increased risk

There are several factors that increase the risk of developing pressure ulcers. These include:

  • mobility problems - anything that affects your ability to move some or all of your body
  • poor nutrition - for your skin to remain healthy, it requires nutrients that can only be supplied by eating a nutritious diet
  • an underlying health condition that disrupts your blood supply or makes your skin more vulnerable to injury and damage
  • being over 70 years old
  • urinary incontinence  and/or bowel incontinence
  • serious mental health conditions

These are discussed in more detail below.

Mobility problems

Possible reasons for having a mobility problem are:

Poor nutrition

Reasons that your diet may lack nutrition include:

  • anorexia nervosa - a mental health condition where a person has an unhealthy obsession with maintaining a low body weight  
  • dehydration - you do not have enough fluids in your body
  • dysphagia - difficulty swallowing food

Health conditions

Health conditions that can make you more vulnerable to pressure ulcers include:

  • type 1 diabetes and type 2 diabetes - the high levels of blood sugar associated with diabetes can disrupt normal blood flow
  • peripheral arterial disease - the blood supply in the legs becomes restricted due to a build-up of fatty substances in the arteries of the legs
  • heart failure - previous damage to the heart means it is no longer able to pump enough blood around the body
  • kidney failure - the kidney loses most or all of its functions, which can lead to a build-up of dangerous toxins (poisons) in the blood that can cause tissue damage
  • chronic obstructive pulmonary disease (COPD) - a collection of lung diseases; the low levels of oxygen in the blood associated with COPD can make the skin more vulnerable to damage

Ageing skin

There are several reasons why ageing skin is more vulnerable to pressure ulcers. These include:

  • with age, the skin loses some of its elasticity (stretchiness), which makes it more vulnerable to damage
  • reduced blood flow to the skin because of the effects of ageing
  • the amount of fat under the skin tends to decrease as people get older


Both urinary incontinence (inability to control your bladder) and bowel incontinence (inability to control your bowels) can cause certain areas of the skin to become moist and vulnerable to infection. This can trigger pressure ulcers to form.

Mental health conditions

People with severe mental health conditions such as schizophrenia (a condition where people have problems telling the difference between reality and imagination) or severe depression have an increased risk of pressure ulcers for a number of reasons:

  • their diet tends to be poor
  • they often have other physical health conditions, such as diabetes or incontinence
  • they may neglect their personal hygiene, making their skin more vulnerable to injury and infection
Medical Review: September 08, 2012
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