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The differential diagnosis of itchy skin

Author: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand, February 2016. 

The differential diagnosis of itchy skin — codes and concepts


Itch is defined by a desire to scratch.

An acute or chronic itchy rash is most often due to dermatitis/eczema. Dermatitis can be primary, or secondary to scratching.

Stages include:

  • Acute dermatitis: red, oozy, swollen skin
  • Subacute dermatitis: red, dry skin
  • Chronic dermatitis: skin coloured or dark, dry, thickened skin with prominent lines (lichenification)
  • Infected dermatitis: painful, swollen, pustules, crusting.
Stages of dermatitis
  • Dermatoscopic examination of hair shaft if scalp affected, possible burrows if hands affected
  • Swab for bacterial and viral culture if pustules, crusting
  • Skin biopsy for histopathology, and if available, direct immunofluorescence
  • If the itch is generalised and no primary skin rash observed, check blood count, iron studies, renal, liver and thyroid function, chest X-ray.

General treatments for itchy skin conditions may include:

Differential diagnosis of very itchy skin

  • Is the itch localised or generalised?
  • What is its distribution?
  • Is there a primary rash or not?
  • Erosions, crusting, bruising and infection can be due to excoriation and are of no help diagnostically.

Very itchy skin with localised rash

Contact dermatitis

Contact dermatitis

Head lice

  • Egg cases close to the scalp
  • Blood spots behind ears

Insect bites / papular urticaria

  • Crops of urticated papules
  • Central punctum or vesicle
  • Favour exposed sites, depending on the cause
Insect bites

Lichen planus

  • May be localised to any site
  • Grouped firm polygonal violaceous plaques
  • Biopsy confirmatory
Lichen planus

Lichen sclerosus

  • Vulva > penis > elsewhere
  • White dry skin
  • Sometimes, purpura, blisters, resorption, scarring
  • Biopsy confirmatory
Lichen sclerosus

Lichen simplex

  • Localised lichenification
  • Common sites: wrist, ankle, neck, scrotum or vulva
  • Sometimes bilateral
Lichen simplex

Pompholyx (dyshidrotic eczema)

  • Crops of vesicles along fingers, toes, palms, soles

Venous eczema

  • Affects one ankle initially then may spread to other leg and can generalise (autoeczematisation)
  • Signs of venous disease: hardened, narrowed ankle (lipodermatosclerosis), orange-brown discolouration (haemosiderin)
  • +/- Varicose veins
Venous eczema

Mildly itchy skin with localised rash

Asteatotic eczema

  • Crazy paving, red cracked patches
  • Mainly lower legs
Venous eczema


  • Itch is sometimes severe
  • Localised variant affects the scalp, elbows, knees; or palms and soles

Seborrhoeic dermatitis

  • In and around hair-bearing scalp, eyebrows, hairy chest
  • Skin folds behind ears, nasolabial fold, axilla
  • Salmon pink, flaky
Seborrhoeic dermatitis

Very itchy skin with generalised rash



Bullous pemphigoid

  • Elderly, especially with a brain injury
  • May start like eczema or urticaria
  • Large blisters
  • Biopsy confirmatory
Bullous pemphigoid

Dermatitis herpetiformis

  • Crops of tiny blisters quickly scratched
  • Biopsy confirmatory
Dermatitis herpetiformis

Atopic dermatitis (eczema)

  • Mainly flexural, symmetrical
  • May have dry skin
Atopic dermatitis (eczema)

Discoid eczema

  • Bilateral, not symmetrical
  • Roundish plaques
Discoid eczema



Lichen planus

  • Skin +/- mucosal surfaces
  • Grouped firm polygonal violaceous plaques on wrists, shins, lower back
  • Lacy white pattern in buccal mucosa
  • Painful erosions on tongue, vulva, vagina, penis
  • Biopsy confirmatory
Lichen planus

Mycosis fungoides (T-cell lymphoma)

  • Slowly evolving slightly scaly annular and roundish patches, plaques and sometimes nodules
  • Various morphologies
  • Itch is variable and can be absent, especially in the early stages
  • Buttocks, breasts common initial sites
  • Biopsy confirmatory
Mycosis fungoides


  • Multiple lichenified plaques

Nodular prurigo

Nodular prurigo


  • Burrows between fingers, wrist creases
  • May be secondarily infected
  • Papules in axillae, groin, penis
  • Polymorphous rash on trunk
  • Scale-crust between fingers, elbows, scalp in elderly or immune suppressed

Transient acantholytic dermatosis / Grover disease

  • Older male
  • Red crusted papules central trunk
  • May be precipitated by sweat
  • Symmetrical on scalp, shoulders, elbows, buttocks, knees
Transient acantholytic dermatosis


  • Acute < 6 weeks
  • Chronic > 6 weeks
  • Spontaneous or inducible weals
  • No blisters or dryness or scale
  • Scratch skin to elicit linear weal in dermographism

Mildly itchy skin with generalised rash


  • Itch is sometimes severe
  • Symmetrical well-circumscribed plaques with silvery scale
  • Generalised large or small plaques

Xerotic eczema

  • Generally dry skin
Xerosis (dry skin)

Localised itchy skin without rash

May have secondary lesions due to scratching: erosions, purpura, lichen simplex and secondary infection. Localised itch is often neuropathic/neurogenic. If scalp itchy, look carefully for head lice and their egg cases.

Localised neuropathic itch

Generalised itchy skin without rash

Examine carefully for scabietic burrows.

Pruritus/prurigo of pregnancy

  • Patient is pregnant

Systemic disease

Systemic associations with generalised pruritus

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