Atopic eczema, contact dermatitis, and seborrheic dermatitis look similar on a quick inspection (all three produce red, inflamed, sometimes scaly skin) but have different causes, different distributions on the body, and importantly, different treatments. Treating one as if it were another is a common reason that "eczema treatment isn't working". Atopic eczema is an internal inflammatory condition centred on barrier dysfunction; contact dermatitis is an external reaction to something the skin touched; seborrheic dermatitis is driven by yeast overgrowth in sebum-rich areas and responds to antifungal treatment, not to eczema treatment.
How to tell them apart by distribution
The single most useful clue is where on the body the rash appears.
Atopic eczema classically appears in flexural areas: the insides of elbows, behind the knees, the neck, the wrists, the ankles. In adults it also appears on the face (particularly eyelids), the hands, and sometimes the trunk. The pattern is typically symmetric, with similar areas affected on both sides of the body.
Contact dermatitis follows the shape of whatever touched the skin. A rectangular rash under a watchstrap. A stripe across the abdomen corresponding to a belt buckle. A rash on the earlobes and along the necklace line. A rash on the hands that stops exactly at the wrist. If the shape of the rash traces the shape of an object or a product application area, it's contact dermatitis until proven otherwise.
Seborrheic dermatitis appears in sebum-rich areas: the scalp, the eyebrows, the sides of the nose, behind the ears, the upper chest, occasionally the armpits and groin. It's almost never on the flexures, the forearms, or the lower legs, which would make atopic eczema the more likely diagnosis for rashes in those areas.
How they present differently
Atopic eczema is intensely itchy. The itch is the primary symptom and scratching drives much of the visible damage through the itch-scratch cycle. Skin is red, sometimes weepy in acute flares, thickened (lichenified) in chronic areas. Distribution is symmetric and flexural. On deeply pigmented skin the redness is the least dependable part of that description, and eczema on brown and black skin covers the papular, follicular and lichenified patterns to read instead.
Contact dermatitis varies. Irritant contact dermatitis (the more common form) produces redness, cracking, and burning sensations more than classic itch. Allergic contact dermatitis produces itch and sometimes small blisters, appearing 24 to 72 hours after exposure (this delay is why people struggle to identify the trigger). Shape follows exposure pattern.
Seborrheic dermatitis has a distinctive visual: fine greasy scale (often yellow-tinged) on a slightly red base, in the characteristic sebum-rich areas. Itch is usually mild to moderate, not the dominant symptom. It waxes and wanes with stress, tiredness, and seasonal changes; it usually worsens in winter.
What each one needs
Atopic eczema needs barrier repair with ceramide-containing emollients, systematic trigger identification, and short courses of topical steroid to break active flares. Treating it as a contact problem alone leaves the underlying barrier vulnerability unaddressed; treating it as a fungal problem with antifungals doesn't touch the relevant mechanism.
Contact dermatitis needs identification and avoidance of the trigger, a short course of topical steroid to calm the existing inflammation, and emollient support while the barrier repairs. Patch testing by a dermatologist is the specific diagnostic investigation if the trigger isn't obvious. Treating it as atopic eczema leads to perpetual flares because the offending product is still being used.
Seborrheic dermatitis responds to antifungal treatment (ketoconazole 2% shampoo or cream, ciclopirox, zinc pyrithione shampoo) used as directed for four to six weeks, sometimes with a brief topical steroid alongside for the most inflamed areas. Scalp seborrhoea responds to antifungal shampoos left on for five to ten minutes before rinsing. Treating seborrhoea with eczema emollients alone doesn't resolve it because the yeast driver is untouched.
The common overlaps
In real clinics, these conditions overlap in specific ways:
Someone with atopic eczema can develop contact dermatitis to a product they've been using for years, because the compromised barrier increases exposure to the ingredient. This is particularly common with fragrance, preservatives, and lanolin. A previously stable atopic eczema that suddenly worsens in a specific new pattern often has contact dermatitis layered on top.
Seborrheic dermatitis can overlap with atopic eczema on the face, particularly on eyelids and around the nose. The approach is usually to treat the seborrheic component (antifungal) and the atopic component (emollient, occasional low-potency steroid) in parallel rather than assuming one will resolve the other.
When to get a diagnosis
If a rash on your skin isn't responding to the treatment you're applying, the first question should be whether you have the diagnosis right. A dermatology opinion is worth getting when: the rash shape doesn't match the classic flexural distribution of atopic eczema; the rash is sharply demarcated in a way that suggests external exposure; the rash is in sebum-rich areas and has greasy scale; or when the condition hasn't responded to a reasonable trial of correctly-applied eczema treatment.
A short specialist consultation and, if appropriate, a patch test or a small skin biopsy often resolves the diagnostic question in a way that self-management cannot.