Friction Blister

A fluid filled blister formed by repeated shearing forces between the skin and an adjacent surface. The force separates keratinocytes in the stratum spinosum (the level of the spinous layer), and clear fluid fills the space between the layers.

• Common sites: the heels, toes, soles and the sides of the feet (new or ill fitting shoes, wet socks), and the palms and fingers (tools, sports equipment, rowing, gardening).
• Risk factors: ill fitting or new footwear, damp skin and moist socks (with heat and sweat), long distance walking or running, military marches, sport, and excessive loads.
• Dry, very wet skin, and very smooth skin lower the risk, whereas moderately moist skin increases friction the most.
• It is common in athletes and military recruits, with up to 50% of marathon participants and 20 to 40% of military recruits reporting blisters.
• Diabetic neuropathic feet, peripheral arterial disease and frail skin carry a risk of infection and ulceration.

• A tense or flaccid dome shaped vesicle or bulla, filled with clear fluid, on a red base, painful on pressure or walking. A blood filled blister indicates deeper injury.
• A hot spot, with redness and burning, precedes blister formation by hours.
• Rupture leaves a raw, painful base. Infection produces pus, spreading redness, warmth, lymphangitis and fever.
• Differential diagnosis: second degree burn, contact dermatitis, bullous impetigo, herpes simplex, tinea pedis (vesiculobullous), pompholyx, bullous pemphigoid, epidermolysis bullosa (inherited, from infancy), porphyria cutanea tarda (photo exposed sites), bullosis diabeticorum, arthropod bite, frostbite, and drug induced bullous eruption.
• Complications: infection (cellulitis, abscess), delayed healing, ulceration (in diabetes), pain and disability, and, in the immunocompromised, sepsis.

• Clinical, from history of friction and the characteristic site.
• Assess infection (pus, spreading erythema, fever), and examine the whole foot or hand, the shoe fit, and sock type.
• In diabetes or neuropathy, check sensation (10 g monofilament), pulses, capillary refill and glucose control, and measure the ankle brachial pressure index if pulses are reduced.
• Swab any pus. Consider X-ray if bone involvement is possible (diabetic foot).
• Unusual, recurrent or widespread blistering, or blistering at non-friction sites, requires biopsy and direct immunofluorescence.
• Check the tetanus immunisation status if the skin is broken.

• Small, intact, painless blisters: leave intact, because the roof is a sterile natural dressing. Cover with a hydrocolloid or foam dressing, and cushion with a donut shaped pad to offload. They absorb in 3 to 7 days.
• Large, tense or painful blisters (to allow walking):
  • Wash the hands and clean the blister with chlorhexidine 2% or povidone iodine 10%.
  • Puncture the edge with a sterile needle (a 21 gauge needle) at 2 or 3 points near the margin, and press the fluid out gently. Leave the roof in place to act as a biological dressing.
  • Cover with a non-adherent hydrocolloid or silicone dressing and secure with tape. Change the dressing every 2 to 3 days or when it lifts.
  • Do not remove the roof unless it is torn, dirty or infected. If the roof has come off, trim the loose edge, clean the base, and cover with a non-stick or hydrocolloid dressing.
• Blood filled blister: protect with a hydrocolloid or foam dressing and cushioning, and avoid draining if deep or if the patient has diabetes or a bleeding disorder.
• Analgesia: paracetamol 1 g orally 6 hourly (maximum 4 g per day), with ibuprofen 400 mg orally 8 hourly with food if no contraindication.
• Infected blister: drain, culture pus, and give oral therapy for 5 to 7 days:
  • Flucloxacillin 500 mg orally 6 hourly (child 12.5 to 25 mg/kg 6 hourly).
  • Cefalexin 500 mg orally 6 hourly.
  • Clindamycin 300 to 450 mg orally 8 hourly, or co-trimoxazole 160/800 mg 1 to 2 tablets 12 hourly if MRSA is likely.
  • In diabetic and vascular patients, start earlier, broaden cover, and arrange urgent specialist review.
• Prevention:
  • Properly fitting shoes, broken in gradually, with a thumb's width in front of the longest toe.
  • Moisture wicking synthetic or wool blend socks, double layer socks, and change socks when wet.
  • Lubricants or antiperspirants (aluminium chloride 20% solution at night), and taping or dressing hot spots early with hydrocolloid plasters or zinc oxide tape.
  • Gloves for manual work and sport.
  • Treat a hot spot at once by removing the cause and applying protection.
• Special groups:
  • Diabetes and neuropathy: do not self-drain, daily foot inspection, and review within 24 to 48 hours for any blister. Treat as a pre-ulcer.
  • Peripheral arterial disease and frail skin: avoid adhesive tape on the skin, use gentle dressings, and refer early.
  • Children: protect with dressings and reduce drainage. Examine for inherited blistering disease if blisters are recurrent.
  • Immunosuppression: treat infection early, with a longer course of antibiotics.
  • Pregnancy: the same measures. Flucloxacillin and cefalexin are suitable for infection.
• Follow up: review at 48 to 72 hours for infection, and weekly until healed. Refer to podiatry, a diabetic foot clinic, or dermatology if recurrent, widespread, slow to heal, or atypical.