Contact dermatitis is an allergic or irritant skin reaction caused by an external agent.

Patient may have a history of atopic dermatitis, previous episodes

There are two main types of contact dermatitis

• irritant contact dermatitis: common - non-allergic reaction due to weak acids or alkalis (e.g. detergents). Often seen on the hands. Erythema is typical, crusting and vesicles are rare
• allergic contact dermatitis: type IV hypersensitivity reaction. Uncommon - often seen on the head following hair dyes. Presents as an acute weeping eczema which predominately affects the margins of the hairline rather than the hairy scalp itself.

Cement is a frequent cause of contact dermatitis. The alkaline nature of cement may cause an irritant contact dermatitis whilst the dichromates in cement also can cause an allergic contact dermatitis

Pathophysiology

• Dermatitis triggers the disease process. The normal dermis has a small amount of lymphocytes and other immune cells but in skin with eczema there is a vast infiltrate visible.
• Keratinocytes in the epidermis start detaching from one another, becoming rounder and the intercellular spaces widening between them. If the eczema has come on acutely, this separation may be so severe that vesicles form. Under the microscope, this makes the epidermis look like a sponge, hence 'spongiotic' change.
• With chronic inflammation, histology shows:
  • Epidermal acanthosis: thickening of the epidermis due to hyperplasia.
  • Hyperkeratosis: thickening specifically of the stratum corneum.
  • Parakeratosis: retained nuclei in the stratum corneum indicating problems with the usual differentiation process.
• Often, the terms eczema and dermatitis are used interchangeably as the two are so closely linked. However, there are other instances where there is dermal inflammation without eczematous changes. Therefore, the two terms are not entirely the same.

PC

*Acute onset of Sx and sparing of unexposed skin

• pruritis
• burning
• erythema
• vesicles and bullae