Shared article


Definition, Classification and Description


A rash or eruption is a widespread skin condition consisting of multiple individual lesions. The lesions may be of a primary origin (due to the disease itself) or due to secondary responses to the disease such as itching or infection.[1]


Rashes are classified as either Endogenous or Exogenous:


  • Due to processes occurring within the body.
  • e.g. rashes seen in atopic eczema or drug reactions



    • Due to events occurring outside the body.
    • e.g. contact dermatitis or fungal infections


      The origin of the rash can be deduced by the distinctive differences displayed in three areas:[2]

      1. Symmetry
      2. Distribution
      3. Morphology



      Endogenous rashes are usually bilateral e.g. the rash of atopic eczema in children usually affects flexor surfaces of limbs on both sides.

      Exogenous rashes are often unilateral e.g. the rash of contact dermatitis seen in hairdressers may be confined to one hand. 



      Is generally specific to each condition, some have classic distributions e.g. scaly plaques on extensor surfaces of limbs in psoriasis and the malar flush of Lupus.



      General appearance of the rash helps identify the cause, differences in the margin and changes such as blisters or vesicles provide clues as to its origin.


      Below are some examples of classic rashes and their descriptions. It is important to remember that some conditions may present with an atypical rash.


      Atopic eczema

      Atopic eczema.[4]

      Eczema - in Greek means ''to boil over''

      Skin changes include: erythema, inflammation, scaling and blistering. Atopic eczema presents differently depending on age of patient:[1,2]

      • Babies – Itchy vesicular rash mainly on face and hands
      • Children – Itchy erythematous rash mainly in flexor regions of arms and legs also on neck, wrists and ankles
      • Adults - Lichenified, nodular rash mainly on hands



      Contact dermatitis-watch strap

      Contact dermatitis.[4]
      • Classic erythematous, itchy eruption in localised area.[2]
      • Common causative allergens include:[5]


      1. Nickel - watches, coins, keys, jewellery
      2. Chromates - cements, leather
      3. Lanolin - creams, cosmetics
      4. Rubber - foam in furniture



        • Raised erythematous plaques with well-demarcated edges and scaly surface, usually on extensor surfaces.[2]



        • Persistent erythematous eruption over forehead and cheeks
        • Prominent blood vessels
        • Pustules and papules often seen
        • Similar to butterfly rash of SLE[1,2]

        Charecteristic flushing of convex facial areas triggered by alcohol  and spicy food helps differentiate it from the rash of SLE.[5]


          Systemic Lupus Erythematosus

          Systemic Lupus Erythematosus.[4]
          • Malar erythematous eruption
          • Classic butterfly rash over nose and cheeks with sparing of the nasolabial folds[2]
          • Photosensitive eruption i.e often triggered by exposure to light 

          SLE is also associated with a Discoid rash which also demonstrates photosensative tendencies with a systemic distribution.[5]


            Erythema multiforme

            Erythema multiforme.[4]
            • Diffuse, erythematous eruption with symmetrical target lesions containing central blisters
            • Over time scales of skin get shredded
            • Often due to adverse drug reactions e.g. reaction to penicillin[3]


            Erythema nodosum

            Erythema nodosum.[4]
            • Painful, raised erythematous lesions
            • Mainly on front of shins
            • Sometimes on thigh or arms
            • Associated with sarcoidosis, Crohn’s, Ulcerative Colitis and some drug reactions[3]


            Pyoderma gangrenosum

            Pyoderma gangrenosum.[4]
            • Nodulo-pustular ulcers with tender, red, overhanging necrotic edge
            • Mainly seen on legs, abdomen or face
            • Associated with Crohn’s and Ulcerative Colitis[3]
            • Also associated with rhematological disorders including Rhematoid Arthritis, Bechets and sero-negative spondyloarthropathies
            • May be seen in Wegener's Granulomatosis[5]


            • White, patchy lesions with hyper pigmented boarders
            • Sun exposure results in increased itch
            • Associated with many autoimmune conditions[1]



            1. Gawkrodger D.J. Dermatology: An illustrated colour text. 4th ed. pp 14-54. Churchill Livingstone, 2007.
            2. Buxton P.K. ABC of dermatology. 4th ed. pp 8-82. BMJ publishing, 2003.
            3. Longmore M., Wilkinson I.B., Rajagopalan S. Oxford handbook of clinical medicine. 6th ed. pp 428-29. Oxford University press, 2005.
            4. Online Atlas of Dermatology Accessed 16/8/2011
            5. Collier J., Longmore M., Turmezei T., Mafi A.R. Oxford handbook of clinical specialties. 6th ed. pp584-603. Oxford University press, 2008.

            Fastbleep © 2019.