Delayed wound healing – why does it happen?

Every wound, whether it be traumatic or surgical, will be exposed to bacteria from the surrounding environment. In most situations, the more damage to the tissues, the more delayed the process of healing will be. In this article we look at the process of healing and list 10 common reasons why a wound will fail to heal as expected.

Posted: 25 June 2020

Delayed wound healing – why does it happen?

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A. The process of delayed healing

The sequence of events that occur with delayed healing are;

  1. Contamination – most traumatic wounds will become contaminated with a variety of foreign material such as hair, saliva, plant matter and soil. In the early stages of a wound, bacteria may be present but won’t necessarily be replicating enough to affect wound healing.
  2. Bacterial colonisation – if a wound is open to the environment, then it’s normal for it to become exposed to environmental bacteria. Initially, the numbers of bacteria will be low and won’t affect healing. Whilst there may be no obvious signs of infection, the inflammatory phase of healing may be extended, and the wound may fail to progress to the proliferative phase.
  3. Critical phase of colonisation – eventually the bacterial load begins to overload the immune response, at which time the healing is likely to stop. Traumatic wounds may fail to respond to dressings, and the healing process may appear to be stuck. Wounds edges fail to join, but visually there may still be no obvious sign of infection.
  4. Local infection – inflammatory changes at the wound occurs as a response to the burden of bacteria thriving in the environment. Exudates will form and may become purulent. Erythema and swelling will be seen, and the wound will become sensitive and painful.
  5. Systemic infection – in some cases, bacteria can spread beyond the wound to the whole body. At this stage the level of inflammation becomes critical as it spreads to become a systemic process. Inflammatory signs may spread to the cardiovascular system, causing systemic illness, inactivity, inappetence, weakness and pyrexia. Sepsis is the eventual outcome.

See our article on Sepsis >

B. 10 common causes of delayed healing

As well as the events described above, there are a variety of reasons why wound healing may be delayed. The list is extensive, but we’ve included some of the more common ones you’re likely to encounter in veterinary medicine.

  1. Movement at the wound site – during healing, vascular tissues will be unable to anchor to wound margins if there is constant movement. Therefore, the placement of sutures needs to be planned carefully to ensure that the wound edges are apposed and secured.
  2. Necrotic tissue – dead and devitalised tissue will often delay healing. Look for a slough, which will appear as moist, loose, yellow tissue or an eschar, which appears as dry, thick, black and leathery tissue.
  3. Chronic disease – some debilitating conditions, such as neoplasia, diabetes mellitus and chronic renal failure, can compromise wound healing. For example, patients with cancer may have delayed wound healing resulting from tumour factors inhibiting wound closure.
  4. Toxins – occasionally we might try to help decontaminate a wound with an inappropriate antiseptic. For example, chlorhexidine is toxic to fibroblasts and can result in delayed healing.
  5. Poor blood supply – this will inevitably result in poor oxygenation and encourages necrosis.
  6. Dryness – if a wound is allowed to desiccate and become too dry, cells will dehydrate and die.  This will allow a scab or crust to form over the wound site, which slows healing.
  7. Wet wounds - if the skin is too wet there will be a risk of the wound becoming saturated and infected. However, moisture is important for healing, so getting the balance right is essential. Use appropriate dressings such as Allevyn to maintain an optimal level of moisture at the wound site.
  8. Excessive pressure at wound site – if this occurs, the blood supply to the capillary network and surrounding tissues can be disrupted, thereby delaying healing.
  9. Poor nutrition - the visual appearance of the patient or the wound is not a reliable indicator of whether the patient is receiving the best nutrition. Always check to ensure that their diet offers the appropriate requirements for their age and health.
  10. Drugs and medications – there are numerous drugs and therapeutics we use that can have an adverse effect on wound healing. Examples include;
    • Hydrocortisone – this inhibits capillary budding, fibroblast proliferation and reduces the rate of epithelialisation.
    • Chemotherapy – many of the drugs used are cytotoxic and will interfere with wound healing, so care must be used when adding chemotherapy after surgery. Cyclophosphamide is one such example.
    • NSAIDs – these have been shown to slow wound healing.
    • Vitamin E – this can reduce collagen production.
    • Zinc - excessive levels of zinc in the diet can inhibit macrophage function.

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