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The differential diagnosis of itchy skin

Synonyms:

Causes of pruritus

Categories:

Diagnosis and testing

ICD-10:

R23.9, L29, F45.8

ICD-11:

EC90.0, EC90.1, EC90.3, EC90.6, EC90.Y, EC90.Z, ED54, JA65.11, ME65.1

SNOMED CT:

279333002, 239102001, 386403002, 276444007, 191960002, 403575000

Diagnosis and testing

The differential diagnosis of itchy skin

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

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

Acute dermatitis

Chronic dermatitis

Infected 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.

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

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

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 itchy skin with localised rash

Asteatotic eczema

Crazy paving, red cracked patches
Mainly lower legs

Venous eczema

Psoriasis:

Itch is sometimes severe
Localised variant affects the 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

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

Autoeczematisation

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

Discoid eczema

Bilateral, not symmetrical
Roundish plaques

Discoid eczema

Erythroderma

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

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 (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]

Mycosis fungoides

Neurodermatitis

Multiple lichenified plaques

Neurodermatitis

Nodular prurigo

Bilateral nodules on limbs resemble keratoacanthomas
Papular variant

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 and vesicles central trunk
May be precipitated by sweating, fever, heat, sunlight, hospitalisation, and medications

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

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

Urticaria

Xerotic eczema

Generally dry skin

Xerosis

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.