Blisters (Vesicles and Bullae)

Blisters are skin swellings containing free fluid. Up to 5mm they are called vesicles, larger than 5mm they are called bullae. The fluid can be lymph, serum, extracellular fluid or blood. Some conditions cause both kinds of blister, but others mainly one or other type. Pustules are dealt with elsewhere (see ‘Purpura and petechiae’).

Published: 2nd August 2022 | Updated: 15th August 2022

Differential diagnosis

Common Diagnoses

  • Trauma: Skin Friction, Burns (Thermal and Chemical), Insect Bites
  • Herpes Simplex
  • Herpes Zoster
  • Childhood Viruses: Hand, Foot and Mouth Disease, Chickenpox
  • Eczema (Pompholyx and other Acute Eczemas)

Occasional Diagnoses

  • Pemphigus and Pemphigoid
  • Dermatitis Herpetiformis
  • Secondary to Leg Oedema (Various Causes)
  • Bullous Impetigo
  • Drug Reactions (e.g. Ace inhibitors, Penicillamine, Barbiturates)
  • Erythema Multiforme

Rare Diagnoses

  • Pemphigoid (Herpes) Gestationis
  • Porphyria
  • Toxic Epidermal Necrolysis (Lyell’s Syndrome)
  • Epidermolysis Bullosa
  • Allergic Vasculitis

Ready reckoner

Key distinguishing features of the most common diagnoses

TraumaHerpes SimplexHerpes ZosterChildhood VirusesEczema

Possible investigations

  • There are unlikely to be any investigations that will prove useful in primary care. Usually, the problem is either self-limiting or the cause obvious; in obscure cases, referral may result in skin biopsy to establish the diagnosis. Patch testing may also be useful to identify possible allergens in contact dermatitis, especially if occupational.

Top Tips

  • Herpes zoster involving the ophthalmic division of the trigeminal nerve will affect the eye in about 50% of cases. The likelihood is increased if there are blisters on the side of the nose. Ensure that the patient knows to seek help urgently if the eye becomes red or painful, or there is blurring of vision.
  • In uncomplicated herpes zoster, explain the natural history of the condition to the patient, resolving any worries (old wives’ tales abound) and warning about the possibility of postherpetic neuralgia.
  • Follow up unexplained rashes. The bullae of pemphigoid, for example, may be preceded by itching, erythema and urticaria by several weeks.

Red Flags

  • Herpes simplex and varicella zoster infections can become severe and disseminated in the immunosuppressed – admit. Similarly, herpes simplex can result in a serious reaction (Kaposi’s varicelliform eruption) in patients with atopic eczema.
  • Pregnant women with chickenpox are at significant risk of severe varicella pneumonia; there are also risks to the foetus. Follow the detailed guidance in the ‘Green Book’ (Immunisation against Infectious Disease, Her Majesty’s Stationery Office) when dealing with pregnant women who have been in contact with chickenpox.
  • Toxic epidermal necrolysis (scalded skin syndrome) can develop rapidly in infants and children, causing serious illness. Admit urgently if you suspect this diagnosis.
  • Pemphigus is a serious condition affecting a younger age group (usually middle-aged) than pemphigoid. Inpatient care is usually required.
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Website disclaimer

Pulse Reference is based on the best-selling book Symptom Sorter.

The experts behind Pulse Reference are Dr Keith Hopcroft who is the co-author of Symptom Sorter, a GP in Essex and Pulse’s editorial advisor and Dr Poppy Freeman, a GP in Camden and also a clinical advisor to Pulse.

This website is for clinical guidance only and cannot give definitive diagnostic information. Practitioners should work within the limits of their individual professional practice, seek guidance when necessary and refer appropriately.