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Silver-containing dressings and diabetic foot ulcers

Duncan SW Stang
MChs SRch
Chief Podiatrist Foot Clinic
Department of Diabetes
Hairmyres Hospital
East Kilbride
E:duncan.stang@lanarkshire.scot.nhs.uk

The antimicrobicidal properties of silver were apparently known to the Ancient Egyptians, who placed silver coins in clay pots of drinking water to prevent waterborne infections.(2)
When silver is used in antimicrobial dressings in wound care, silver ions (Ag(+)) bind directly to proteins within the cell membrane of the bacteria, resulting in selective cell wall damage. In addition, Ag(+) ions block cellular respiration pathways within the bacteria, enhancing their microbicidal effects.(3)
In contrast, in tissue cells Ag+ ions induce the expression of two proteins, MT-1 and MT-2, which are beneficial in wound healing.(4) Dressings containing Ag+ ions may provide an ideal alternative to antibiotic treatment in suitable wounds, and reduce the potential for increasing antibiotic resistance among the common pathogens, causing foot infections in patients with diabetes.

Current practice
In the diabetic foot clinic at Hairmyres Hospital, the current policy is to limit the prescription of antibiotic therapy to those patients who present with clinical signs of infection. However, we do have a low threshold for prescription, in keeping with current expert guidance.(5)
This practice allows antibiotic therapy to be directed at those with the greatest likelihood of benefit, and minimises the risk of promoting bacterial resistance in those with a low bacterial load, which could be managed by alternative means.
Our reasons for not prescribing prophylactic anti­biotics are based on the following notions:

  • Overprescription of antibiotics has led to ­widespread bacterial resistance.(1)
  • Diabetic foot ulcers quite often have an ischaemic element, resulting in impaired microcirculation.(6) This may result in an impaired delivery of white blood cells to the site of infection and lowered ­tissue levels of antibiotics.
  • Many patients report side-effects during a course of antibiotics.(7) This can interrupt their eating habits and destabilise their diabetes control.

Modern wound care has advanced with the recent development of dressings incorporating potent anti-microbial agents. Dressings containing Ag(+) ions are being used increasingly, particularly as they possess many of the characteristics we seek in an ideal topical dressing.
When assessing the properties of a topical antimicrobial dressing, our team felt the following properties to be particularly important:

  • Provides a broad spectrum of sustained ­antimicrobial ­activity.
  • Has efficacy against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE).
  • Is able to establish a host-manageable bioburden.
  • Is able to contain bacteria within the wound ­dressing.
  • Provides a moist wound environment.
  • Manages wound exudate to prevent maceration of surrounding skin.
  • Is easy to apply and remove.
  • Is cost-effective when compared with antibiotic therapy.

As a result of these demands, we chose Aquacel Ag (Convatec) as we felt it met most of the criteria we were looking for. Studies have shown that Aquacel Ag provides broad-spectrum, sustained antimicrobial activity in vitro.(8) Additionally, a phase II noncomparative trial in superficial, mid-dermal and mixed partial-thickness burns rated Aquacel Ag positively in conformability and ease of use, and found that its speed of re-epithelialisation was satisfactory; the results appeared similar at least to those noted with silver sulphadiazine.(9)
Of more interest to our practice was a study of 134 nonischaemic diabetic foot ulcer patients comparing Aquacel Ag with a calcium alginate.(5) Preliminary results suggest that Aquacel Ag provides improvement in healing time, enhanced reduction in ulcer area and depth, total healing at eight weeks, and synergistic effects in patients treated with antibiotics.(5)

When do we use it?
Our choice of Aquacel Ag as a principal therapy for healing foot ulcers in patients with diabetes is based on patients meeting the following criteria:

  • Ulcers show no signs of clinical infection.
  • Chronic or slow-to-heal ulcers where critical ­colonisation is thought to be the cause of delayed healing.
  • Ulcers where there is a localised, nonspreading mild erythema.
  • In conjunction with antibiotic therapy, where ­clinical infection has been diagnosed but where local control of infection is thought likely to be ­adequate after a limited course of antibiotic ­therapy.

These criteria are used to select patients for Aquacel Ag dressing use, and are illustrated in the case study.

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Conclusion
Central to the management of healing in a patient with a diabetic foot ulcer is the need for a multidisciplinary foot team to address the multifactorial nature of the pathogenesis of the ulcer.(10)
In addition, it is essential to remember that if infection does occur in a diabetic foot ulcer, it can spread very rapidly. Therefore, when any patient is being managed without prophylactic antibiotic therapy, they must be monitored very closely.
Good lines of communication between all healthcare professionals who may be sharing the patient's care must be in place. Similarly, patients and their carers must be educated in recognising the signs of infection - swelling, spreading erythema, rise in blood sugars and feeling generally unwell, as well as having a clear plan of action if these factors occur.
Large randomised control trials are required to investigate the use of the prophylactic prescription of antibiotics versus the use of antimicrobial dressings in the management of nonclinically infected diabetic foot ulcers. Only by assessing these dressings in this way can we be confident that we can move away from prophylactic antibiotics. In our recent experience, the use of Aquacel Ag as a topical antimicrobial therapy has proved very successful and raises the possibility that not all patients with diabetic foot ulcers require antibiotic therapy, especially if there are no clinical signs of infection.
However, this mode of treatment can be carried out only with very close monitoring and education of everyone involved in the delivery of care.
The author would like to thank Dr Andrew Jamieson for his help in the preparation of this article.

References

  1. Van der Mee-Marquet N, et al, the Infection Survey study group of the Relais d'Hygiene du Centre. Virulence and antibiotic susceptibility of Staph aureus strains isolated from various origins. Pathol Biol (Paris)2004;52:579-83.
  2. Lansdown AB. A review of the use of silver in wound care: facts and fallacies. Br J Nurs 2004;13 Suppl 6:6-19.
  3. Dibrov P, et al. Chemiosmotic ­mechanism of antimicrobial activity of Ag(+) in Vibrio cholerae. Antimicrob Agents Chemother 2002;46:2668-70.
  4. Demling RH, DiSanti L. Effects of silver on wound management. Wounds 2001;13 Suppl A:5-15.
  5. Jude E. Non-ischemic diabetic foot ulcer: effects of Aquacel AG with hydrofibre versus alginate dressing. Presentation at the 2nd World Union of Wound Healing Societies' Meeting, Paris, 2004.
  6. Krentz AJ, et al. Peripheral arterial disease in diabetes: time for a ­co-­ordinated approach to management.Br J Diab Vasc Dis 2001;3:92-6.
  7. Dancer SJ. How antibiotics can make us sick: the less obvious adverse effects of antimicrobial chemotherapy. Lancet Infect Dis 2004;4:611-9.
  8.  Bowler P, et al. Infection control practices ­relating to chronic wounds. Poster ­presentation at the 8th National Conference NPUAP (National Pressure Ulcer Advisory Panel), New Orleans, 2003.
  9. Caruso DM, et al. Aquacel Ag in the management of partial thickness burns: results of a clinical trial. J Burn Care Rehabil 2004;25:89-97.
  10. Foster A, Edmonds M. An overview of foot disease in patients with diabetes. Nurs Standard 2001;16(12):45-52, quiz 54-55.