Infections in Diabetes Mellitus
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Direct answer
Infections in diabetes mellitus are more frequent and more severe because hyperglycaemia impairs neutrophil chemotaxis, phagocytosis and intracellular killing. Four infections are classically tested: malignant otitis externa, emphysematous pyelonephritis, mucormycosis and Fournier gangrene. Each has a distinctive organism and sign, and each calls for early, often surgical, intervention.
What you must remember
- Malignant otitis externa: Pseudomonas aeruginosa invades the skull base in elderly diabetics, causing severe otalgia, granulation tissue in the external canal and cranial nerve palsies, most often the facial nerve.
- Skull base imaging: CT or MRI detects osteomyelitis of the skull base, which determines the length of antipseudomonal therapy.
- Emphysematous pyelonephritis: gas in the renal parenchyma or collecting system, with Escherichia coli the commonest organism and Klebsiella the next most common.
- Emphysematous pyelonephritis treatment: prompt antibiotics plus urgent drainage, with nephrectomy reserved for selected cases such as unilateral disease with poor function.
- Mucormycosis: Rhizopus and other Mucorales thrive in ketoacidosis, and the rhino-orbital-cerebral form shows a black eschar, palatal necrosis and angioinvasion.
- Mucormycosis treatment: liposomal amphotericin B, urgent surgical debridement and correction of ketoacidosis; Mucorales are intrinsically resistant to voriconazole.
- Fournier gangrene: polymicrobial necrotising fasciitis of the perineum and genitalia, including anaerobes, spreading along fascial planes.
- Fournier management: urgent radical debridement and broad-spectrum antibiotics, with the diagnosis made clinically rather than by waiting for cultures.
Where the exam tries to catch you
These four infections are tested through their organisms and the drugs that fail against them.
- Trap: treating malignant otitis externa as simple otitis externa with ear drops Correct: it needs systemic antipseudomonal therapy for weeks, guided by imaging of the skull base.
- Trap: choosing voriconazole for mucormycosis Correct: Mucorales are intrinsically resistant to voriconazole, so liposomal amphotericin B is the first-line agent.
- Trap: operating on every emphysematous pyelonephritis Correct: most patients respond to antibiotics with drainage, and nephrectomy is kept for selected cases.
- Trap: waiting for culture before debriding Fournier gangrene Correct: the diagnosis is clinical, and surgery must not be delayed for laboratory results.
Frequently asked questions
What is the commonest organism in malignant otitis externa?
Pseudomonas aeruginosa is the commonest organism in malignant otitis externa, seen mainly in elderly diabetic patients. It spreads from the external auditory canal into the skull base, causing osteomyelitis and cranial nerve involvement.
Why does emphysematous pyelonephritis occur more often in diabetes?
Hyperglycaemia and glycosuria favour gas-forming bacteria in the renal parenchyma, and Escherichia coli is the commonest cause. Emphysematous pyelonephritis is diagnosed by CT, which shows gas in the kidney.
Which antifungal is first-line for mucormycosis?
Liposomal amphotericin B is the first-line antifungal for mucormycosis, given together with urgent surgical debridement. Correction of ketoacidosis and control of glucose are equally essential, while posaconazole is used as salvage therapy.
Why is Fournier gangrene considered a surgical emergency?
Fournier gangrene is a necrotising infection of the perineum and genitalia that spreads rapidly through fascial planes. Radical surgical debridement is required, and antibiotics alone are never sufficient.