The differential diagnosis of itchy skin — extra information
Introduction
Questions to consider
Very itchy skin with localised rash
Mildly itchy skin with localised rash
Very itchy skin with generalised rash
Mildly itchy skin with generalised rash
Localised itchy skin without rash
Generalised itchy skin without rash
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.
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:
- 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
- The site depends on the cause
- Irritant > allergic contact dermatitis
- Asymmetrical, odd shapes
- Often intermittent
- 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
- May be localised to any site
- Grouped firm polygonal violaceous plaques
- Biopsy confirmatory
- Vulva > penis > elsewhere
- White dry skin
- Sometimes, purpura, blisters, resorption, scarring
- Biopsy confirmatory
Pompholyx (dyshidrotic eczema)
- Crops of vesicles along fingers, toes, palms, soles
- 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
- Crazy paving, red cracked patches
- Mainly lower legs
- Itch is sometimes severe
- Localised variant affects the scalp, elbows, knees; or palms and soles
- 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
- Nonspecific dermatitis
- The initial site often venous dermatitis
- Can also follow localised dermatophyte infection eg tinea pedis
- Elderly, especially with a brain injury
- May start like eczema or urticaria
- Large blisters
- Biopsy confirmatory
- Crops of tiny blisters quickly scratched
- Biopsy confirmatory
- Mainly flexural, symmetrical
- May have dry skin
- Bilateral, not symmetrical
- Roundish plaques
- Whole body (>85%) involvement
- Preceding eczema, psoriasis or de novo
- Also consider pityriasis rubra pilaris, lymphoma, drug
- 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 (cutaneous 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 [see Mycosis fungoides pathology]
- Multiple lichenified plaques
- Bilateral nodules on limbs resemble keratoacanthomas
- Papular variant
- 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 and vesicles central trunk
- May be precipitated by sweating, fever, heat, sunlight, hospitalisation, and medications
- 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
- 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.
- Patient is pregnant
Systemic disease
- Chronic renal insufficiency
- Cholestasis
- Iron deficiency
- Polycythaemia vera
- Hyperthyroidism
- Lymphoma
- Diabetic neuropathy
- Drug-induced (eg, opioid, chloroquine, vancomycin flushing)
- Chronic pruritus of unknown origin
Systemic associations with generalised pruritus
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