GPAtlas🧭 When to suspect
Contact dermatitis is an eczematous (spongiotic) inflammation of the skin triggered by an external agent. It is one of the commonest skin complaints in general practice and the most frequent work-related skin disease in the UK. The single most useful diagnostic step is recognising that the pattern and distribution of the rash mirror the contactant – a band under a watch strap, the lobes from earrings, a periumbilical patch from a nickel jeans stud, the dorsum of the feet from footwear, or the eyelids from nail varnish transferred by touch.
There are two mechanisms, which frequently coexist. Irritant contact dermatitis (ICD) is direct, non-immunological damage to the skin barrier and will occur in anyone given sufficient exposure; it accounts for around 80% of occupational cases and most often affects the hands. Allergic contact dermatitis (ACD) is a type IV delayed cell-mediated hypersensitivity that develops only in a sensitised individual and typically appears 24–72 hours after re-exposure.
| Irritant contact dermatitis (ICD) | Allergic contact dermatitis (ACD) |
|---|---|
|
• Mechanism: direct, non-immune damage to the skin barrier – no prior sensitisation needed. |
• Mechanism: type IV delayed (cell-mediated) hypersensitivity – requires prior sensitisation. |
|
• Who: anyone exposed to enough irritant; around 80% of occupational cases. |
• Who: only the sensitised; develops after repeated exposure over months to years. |
|
• Onset: minutes to hours, or cumulative; often stings or burns more than it itches. |
• Onset: typically 24–72 hours after re-exposure; intensely itchy. |
|
• Distribution: sharply demarcated to the contact site, usually the hands. |
• Distribution: starts at the contact site but may spread or generalise. |
|
• Test: no confirmatory test – a clinical diagnosis. |
• Test: patch testing (specialist) confirms the allergen. |
Raise suspicion with a new, itchy, well-demarcated rash at a site of contact, a rash that improves away from work or on holiday (the classic occupational clue), or eczema that flares unexpectedly or reacts to a topical treatment (think ACD – including to the medicament itself). Higher-risk groups include those in wet-work occupations (hairdressers, nurses, caterers, cleaners, metalworkers, construction) and people with a history of atopy, who are particularly prone to ICD.
Source: DermNet
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