Differential diagnosis of very itchy skin

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


Introduction

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

If clinical diagnosis of an itchy skin problem is uncertain, consider performing the following tests:

  • 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 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

Consider:

  • 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 scalp
  • Blood spots behind ears

Insect bites / papular urticaria:

  • Crops of urticated papules
  • Central punctum or vesicle
  • Favour exposed sites, depending on 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, genitals
  • Sometimes bilateral
Lichen simplex

Pompholyx (dyshidrotic eczema):

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

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 itch skin with localised rash

Asteatotic eczema:

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

Psoriasis:

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

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

Autoeczematisation:

Autoeczematisation

Bullous pemphigoid:

  • Elderly, especially with 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

Erythroderma:

Erythroderma

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
  • Buttocks, breasts common initial sites
  • Biopsy confirmatory
Mycosis fungoides

Neurodermatitis:

  • Multiple lichenified plaques
Neurodermatitis

Nodular prurigo:

Nodular prurigo

Scabies:

  • 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
Scabies

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

Urticaria:

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

Mildly itchy skin with generalised rash

Psoriasis:

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

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.

Brachioradial pruritus:

  • Arms

Meralgia paraesthetica:

  • Lateral thighs

Notalgia paraesthetica:

  • Scapula

Vulval itch

Scrotal itch

Generalised itchy skin without rash

Examine carefully for scabetic burrows.

Pruritus/prurigo of pregnancy:

  • Patient is pregnant

Systemic disease:

  • Chronic renal insufficiency
  • Cholestasis
  • Iron deficiency
  • Polycythaemia vera
  • Hyperthyroidism
  • Lymphoma
  • Diabetic neuropathy
  • Drug-induced (eg opioid, vancomycin flushing)
  • Unknown origin

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