The differential diagnosis of itchy skin

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.

Acute dermatitis

Chronic dermatitis

Infected dermatitis

If the 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 the itch is generalised and no primary skin rash observed, check blood count, iron studies, renal, liver and thyroid function, chest X-ray.

Dermatoscopic examination of hair shaft

Dermatoscopic examination of possible burrows

Skin biopsy for histopathology

General treatments for itchy skin conditions may include:

• Topical emollients, hydrocortisone cream
• 1% menthol cream to cool localised itchy areas
• Oral antihistamines
• Tricyclic antidepressants such as amitriptyline.

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

• The site depends on the cause
• Irritant > allergic contact dermatitis
• Asymmetrical, odd shapes
• Often intermittent

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

Lichen planus

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

Lichen sclerosus

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

Lichen simplex

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

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

Mildly itchy skin with localised rash

Asteatotic eczema

• Crazy paving, red cracked patches
• Mainly lower legs

Psoriasis:

• 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

Very itchy skin with generalised rash

Autoeczematisation

• Nonspecific dermatitis
• The initial site often venous dermatitis
• Can also follow localised dermatophyte infection eg tinea pedis

Bullous pemphigoid

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

Dermatitis herpetiformis

• Crops of tiny blisters quickly scratched
• Biopsy confirmatory

Atopic dermatitis (eczema)

• Mainly flexural, symmetrical
• May have dry skin

Discoid eczema

• Bilateral, not symmetrical
• Roundish plaques

Erythroderma

• Whole body (>85%) involvement
• Preceding eczema, psoriasis or de novo
• Also consider pityriasis rubra pilaris, lymphoma, drug

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

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

Neurodermatitis

• Multiple lichenified plaques

Nodular prurigo

• Bilateral nodules on limbs resemble keratoacanthomas
• Papular variant

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

Transient acantholytic dermatosis / Grover disease

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

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

Spontaneous urticaria

Spontaneous urticaria

Dermographism

Mildly itchy skin with generalised rash

Psoriasis

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

Xerotic eczema

• Generally 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 — arm
• Meralgia paraesthetica — lateral thigh
• Notalgia paraesthetica — scapula
• Cheiralgia parasthetica — lateral aspect of the hand 
• Vulval itch
• Scrotal itch

Generalised itchy skin without rash

Examine carefully for scabietic 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

Renal pruritus

Breast cancer

Iron deficiency