Diabetic Bullae

Diabetic bullae are spontaneous, tense, non-inflammatory blisters on acral skin in people with diabetes, arising without trauma and healing without scarring within 2 to 6 weeks.

It is uncommon, affecting well under 1% of people with diabetes. Men are affected more often. Most have long standing diabetes with neuropathy, and it can be the presenting sign.

Blisters are intra-epidermal (non-scarring, most common) or sub-epidermal. Size ranges from under 1 cm to over 10 cm.

Pathophysiology.

Cause is unclear. Proposed factors are neuropathy, microangiopathy, unnoticed minor trauma, ultraviolet exposure, abnormal skin collagen and reduced skin adhesion.

Biopsy shows a split within or beneath the epidermis without inflammation. Direct immunofluorescence is negative.

• Sudden, painless, tense blisters with clear sterile fluid on a non-inflamed base, mostly on the dorsal or plantar feet and lower legs, less often the hands.
• Often bilateral and recurrent. Resolves without scarring over 2 to 6 weeks.

Red flags.

• Surrounding erythema, warmth, pus or spreading cellulitis.
• Underlying ulcer or ischaemic changes, absent pulses, systemic features.

• Clinical, in a person with diabetes and typical blisters without trauma or inflammation.
• Check glucose and HbA1c, and assess neuropathy and peripheral pulses.
• Swab and culture if infection is suspected (Staphylococcus aureus and Streptococcus pyogenes most often).
• Skin biopsy with direct immunofluorescence if inflammatory, widespread or atypical. Plasma or urine porphyrins for blisters on sun exposed skin.

Differential diagnosis.

• Bullous pemphigoid and epidermolysis bullosa acquisita (immunofluorescence positive).
• Porphyria cutanea tarda and pseudoporphyria (drugs such as non-steroidal anti-inflammatory drugs).
• Bullous impetigo, erysipelas, burns and friction blisters.
• Contact dermatitis and bullous drug eruption.

• Leave small intact blisters. Aspirate larger tense blisters with a sterile needle under aseptic conditions and keep the roof as a natural dressing.
• Cover with a non-adherent dressing and offload pressure; protective footwear.
• Do not de-roof unless ruptured. If ruptured, clean, trim loose skin and dress.
• Infection: flucloxacillin 500 mg to 1 g four times daily for 5 to 7 days (clarithromycin 500 mg twice daily if penicillin allergic), with culture directed changes.
• Optimise glycaemic control and check feet daily.

Monitoring. Review at 1 to 2 weeks until healed, then each diabetes review.

Referral and escalation. Same day diabetic foot protection service referral for any ulcer, infection or ischaemia. Dermatology if the diagnosis is uncertain or blisters are recurrent.