Eczema (eczematous inflammation) is the most common inflammatory disease of skin. These rashes are itchy and recognised by erythema, scale and vesicles, but can have secondary changes of infection, irritation or scratching. The term “dermatitis” is a broader, non-specific term which is not synonymous with eczema. There are three stages of evolution (acute, subacute and chronic) and numerous presentations depending on stage, age and aetiology. Histologically, the eczematous inflammatory processes have in common the spongiotic tissue reaction. Spongiosis refers to intra-epidermal oedema which resembles sponge.

Stages of eczema

Acute eczema

Clinical: Red, swollen, pebbly plaques. History of contact with specific allergen or chemicals. For the id reaction, the vesicles will occur at distant sites. Histology: Spongiosis, spongiotic vesiculation, intercellular oedema, perivascular dermal inflammation and occasional eosinophils.

Subacute eczema

Clinical: red, scaly lesions with indistinct borders, which may resemble psoriasis or fungal infections. Any allergic contact, asteatotic, atopic, nappy-related, chemical exposure, irritant contact, nummular, perioral-lick or stasis dermatitis may present this way. Histology: Less spongiosis and exocytosis (presence in the epidermis) of lymphocytes than acute form, and thickening (acanthosis) of the epidermis which may become psoriasiform. Parakeratosis, perivascular dermal inflammation and oedema are also present.

Chronic eczema

Clinical: Thick skin, skin lines accentuated (lichenified), fissures and excoriations. Caused by irritation of any subacute form, or appearing as lichen simplex chronicus. If the lichen simplex chronicus forms a local lump it is referred to as a prurigo nodularis. Histology: Hyperkeratosis, psoriasiform thickening of the epidermis, mild spongiosis, dermal mast cells and eosinophils. Lichen simplex chronicus will show marked hyperkeratosis, hypergranulosis, long epidermal ridges and vertical streaking of collagen in the dermis. Prurigo nodularis has thick, possibly excoriated, hyperkeratotic epidermis and marked dermal fibrosis and inflammation.

Specific types of eczema

Atopic dermatitis (atopic eczema)

Atopic dermatitis is an itchy, chronic relapsing skin disease which often start in childhood. There is a personal or family history of dry skin, eczema, hay fever, asthma and elevated serum IgE levels. There are essential and important clinical criteria, and the diagnosis requires exclusion of other conditions (e.g. scabies, contact dermatitis, psoriasis, photosensitive dermatosis). The pathogenesis and aetiology are not entirely clear and the disease is increasing in frequency.

Nummular dermatitis (discoid eczema)

Clinical: A chronic disorder of adults, of unknown aetiology, not related to atopy, but possibly to dry skin. Papules and papulovesicles coalesce to form nummular plaques 1-4 cm in diameter with oozing, crust, and scale. They are paler and les scaly than psoriasis. Most common sites of involvement are upper extremities, including the dorsal hands in women, and the lower extremities in men. Histology: Varies with duration. Spongiosis with mild acanthosis and exocytosis of inflammatory cells in earlier lesions. With time, the degree of acanthosis (thickening) increases. Additional features include scale-crust formation above the thickened epidermis and dermal perivascular inflammatory infiltrate.

Contact dermatitis

The substance could be an irritant or an allergen. Irritant (e.g. concentrated solvents, soaps) will cause a non-immunological reaction in any exposed person. Allergic reactions will occur in predisposed people on the basis of a delayed hypersensitivity reaction to a substance at low concentration and evolves rapidly at the site once sensitised. Occupational contact dermatitis is common and may be of irritant or allergenic (or both) types.

Irritant contact dermatitis

Commonly provoked by environmental substance e.g. contact with water, detergents and other chemicals where the epidermal barrier is compromised, and subsequently occurs most commonly on the hands, but any site where external stimuli could be suspected. Histology: Mild spongiosis, epidermal cell (keratinocyte) necrosis, and neutrophilic infiltration of the epidermis.

Allergic contact dermatitis

Clinical: Exposure to, and absorption of an antigen through skin. Most allergens are weak and there may be repeated exposure before sensitisation. The shape and location of the rash are the best clues. Histology: Subacute, chronic dermatitis or acute dermatitis may be seen. The dermal inflammatory infiltrate predominately contains lymphocytes and other mononuclear cells. Occasional atypical T-cell infiltrates may simulate mycosis fungoides.

Stasis dermatitis

Clinical: Occurring on the legs where venous drainage is impaired. However, most patients with venous insufficiency do not develop dermatitis. Unfortunately, topical medicines used in this situation seem to have many potential sensitising agents. Histology: Mild spongiosis, foci of parakeratosis and scale crust. Dermal changes are prominent with neovascularisation, haemosiderin deposition and varying degrees of fibrosis (depending on chronicity) and there is often ulceration.

Seborrhoeic dermatitis

Seborrhoeic dermatitis is a common and chronic disease which most commonly occurs on the scalp and face secondary to toxic substances produced by yeasts (malassezia), but with genetic and environmental factors contributing. Histology: Spongiosis at the side of a hair follicle, often with overlying scale crust, which may be acute, subacute or chronic depending on the lesion biopsied. Neutrophils within the epidermis or stratum corneum requires a search for yeast on a PAS stain. More chronic lesions show progressive psoriasiform hyperplasia of the epidermis with less spongiosis. Mild oedema of the papillary dermis with a mild superficial perivascular infiltrate of lymphocytes, histiocytes and neutrophils.

Asteatotic eczema

Asteototic eczema develops as the result of very dry skin. It is most common in the elderly and on the lower limbs. Histology: Usually a mild subacute spongiotic dermatitis. Compact and irregular stratum corneum. Id reaction Clinical: ‘Autoeczematisation’: generalised eczema in response to a localised dermatosis or infection at a distant site. Can be a pompholyx-like reaction affecting hands or more generalised papular eruption. Will resolve when the acute initiating process is controlled. Histology: Mimics that of the initial localised dermatosis or shows a spongiotic reaction with varied intensity. Mild dermal oedema and lymphocytic infiltration are seen. References: Thomas Habif, Clinical Dermatology 6th edition 2016, Elsevier, chapters 3-5 Weedon’s Skin Pathology, 4th edition, editor James W Paterson, Churchill Livingstone Elsevier A.Bernard Ackerman, Histological Diagnosis of Inflammatory Skin Diseases, 2nd edition, Williams & Wilkins Dermnet skin disease atlas at dermnet.com and Dermnet NZ online at dermnet.nz.org

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General Practice Pathology is a new regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs. The authors provide this editorial, free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.