A number of recent systematic reviews and guidelines (NICE 2012, Lipsky 2011, 2012, Peters 2012) contribute significantly to treatment planning in diabetic foot infections. Hobizal (2012) and Robinson (2009) also provide useful overviews, the latter from the surgical point of view.
Infection in an ulcer can be difficult to treat because of:
- polymicrobial infection
- deficient immunity
- ischaemia
- spread of infection along deep tissue planes
- development of ostoemyelitis with dead bone
Diagnosis
The usual signs of swelling, rubor, heat and discharge may be obvious, but in an ischaemic foot with deficient neutrophil function the diagnosis may be difficult.
The "probe to bone test" was described by Grayson (1995). If a probe inserted into the ulcer goes directly down to bone, osteomyelitis is probably present. Further validation studies suggest the test is highly predictive in populations with a high prevalence of infection. However, wide variation in sensitivity (0.38-0.91) and the relatively low quality of studies led NICE (2011) to recommend that the test should not be used to exclude osteomyelitis.
Plain radiographs have a low sensitivity (Lipsky 2011, 2012, NICE 2011). Aragon-Sanchez (2011), however, found it combined well with the probe-to-bone-test to give high sensitivity and specificity.
MRI has high sensitivity and reasonable specificity, as does PET imaging, and is the most useful modality to identify occult infection (NICE 2011, Lipsky 2012).
Ultimately, surgical exploration and biopsy may be necessary to make the diagnosis. The gold standard is direct biopsy of bone in sterile conditions.
All standard imaging modalities can play a part in diagnosing infection, and normal pathways should be followed. There is a significant false negative rate. In the presence of Charcot arthropathy all imaging is particularly difficult to interpret.
Non-surgical treatment
An infected foot should be elevated and broad-spectrum antibiotics administered intravenously. The IWGDF review (Peters 2012, Lipsky 2011) found no evidence that any particular antibiotic regime gave superior results. Each unit needs to develop their own antibiotic protocol with the microbiologist according to local organisms and sensitivities. Initial treatment should cover Staph aureus and aerobic streptococci (Lipsky 2011). Improvement of the vascular inflow may improve the chance of limb salvage (Lipsky 2012).
Negative-pressure dressings may improve healing rates over conventional dressings, although the evidence is not yet strong (Evans et al 2001, Eginton et al 2003). The technique is useful in wounds which have failed other attempts at healing (Clare et al 2002).
The evidence remains conflicting on whether hyperbaric oxygen therapy is beneficial in treatment of diabetic foot infections (Peters 2012).
Granulocyte colony-stimulating-factor (G-CSF) has been reported in a number of small trials of limited quality. Meta-analysis of these trials suggest some benefit in reducing the amputation rate (Cruciani 2009, Peters 2012).
Surgical debridement and amputation
Established deep infection may require surgical debridement. Tan (1996) and Faglia (2007) found early surgical debridement reduced amputation rate, although both studies are subject to selection bias.
Conservative excision of metatarsal heads can yield excellent healing with a rate of persistent infection of 4.6% and major amputation 1.2% at 2 years (Aragon-Sanchez 2012, 2015). The poorest results are in 1st MTPJ infection.
Amputation may be required to for uncontrollable infection, and this may be required at various levels:
- Single or multiple toes
- Ray(s)
- Transverse amputations at the Lisfranc or midtarsal levels
- Hindfoot amputations such as Syme, Boyd or Pirogov
- Trans-tibial
- Trans-femoral
Distal amputations have a good healing rate overall and can maintain mobility in appropriate shoes and orthoses. Wounds can normally be closed primarily (Shaikh 2013), speeding recovery and avoiding the complexities of prolonged secondary wound management or VAC treatment.
However, losing one toe is a major risk factor for further amputation (Murdoch et al 1997, Griffin 2012). Loss of the great toe significantly increases pressure under the remaining forefoot (Armstrong and Lavery 1998) and there is a high incidence of subsequent lesser toe deformity (Quebedeaux et al 1996).
Midfoot amputations are probably best when a good plantar flap can be used for durable closure without tension. Amputations through the metatarsals allow preservation of the tibialis anteroir and peroneal muscle attachements so minimising the risk of subsequesnt deformity. Results from several series suggest a proximal re-amputation rate of about 15% and 3yr mortality of 5%, althought the latter seems remarkably low given the overall mortality of diabetic foot disease.
Midtarsal (Chopart) amputations require motor balancing by transfer of all available dorsiflexors into the talar neck to balance the Achilles tendon and prevent equinus. Hindfoot amputations have the advantage that they can usually be walked on a short distance without a prosthesis (useful for going to the toilet in the middle of the night) (Pinzur et al 2003). Overall, about 20% will require proximal re-amputation. (Schade 2010) Prosthetic fitting for outdoor mobility is more difficult than a below-knee stump. More than at most sites, the decision to do a hindfoot or BK amputation should be taken in close collaboration with the prosthetist.
Peters et al (2001) found that Sickness Impact Profile scores in diabetics with forefoot or midfoot amputations were no different from those without amputations, but diabetics with trans-tibial amputations scored significantly worse. It is worth trying to amputate distally.
Our own policy is to do forefoot procedures, up to the level of the Lisfranc joint, where possible. Wounds are left open and the patient mobilises in a Scotchcast boot, TCC or walker as soon as the wound has settled. Patients with wounds which cannot be closed at the Lisfranc joint or distal would usually be offered a proximal amputation, trans-tibial if possible.
A few resistant ulcers around the hindfoot can be closed by excision and grafting or flap closure in collaboration with a plastic surgeon (Attinger et al 2002, Musharrafieh et al 2003). However, ischaemia and infection limit what is feasible.