American Family Physician (AFP) Journal:

Seborrhoeic Dermatitis (SD) is a very common chronic and/or relapsing inflammatory skin disorder presenting with a papulosquamous morphology in areas rich in sebaceous glands, particularly the scalp, face, and body folds.

In adolescents and adults, SD clinical presentation may range from mild patches to diffuse scalp scaling. In infants, it mainly occurs on the scalp as yellowish, scaly patches (“cradle cap”).

Diagnosis of SD is usually easily made based on past history and typical clinical features.

Clinical presentations

Seborrheic dermatitis is a clinical diagnosis based on the location and appearance of the lesions.

The incidence of SD notably peaks in three age groups, in infancy between 2 weeks and 12 months of age, during adolescence, and between age 30 and 60 years during adulthood.

In infants, SD presents as “cradle cap”—yellowish, greasy and crusty skin on the scalp—that is usually self-limited.

In adolescents and adults, SD is a chronic or relapsing condition, often presents as flaky, greasy yellowish scales with erythematous skin in seborrhoeic areas such as the scalp, face (eyebrows, nasolabial folds, above the upper lip), ears, retro-auricular area, the upper chest and trunks.

The face, scalp, and chest are the sites most commonly involved in Adult SD, with around 88%, 70%, and 27% of cases developing lesions in these areas, respectively.

In addition to skin inflammation, SD can be associated with pruritus. 

Important Considerations in the Differential Diagnosis for Adult Seborrheic dermatitis (SD)

Scalp

• Psoriasis - usually nonpruritic and tends to affect the occipital and frontal regions, whereas SD tends to affect the vertex and parietal regions
• Eczema (contact) -due to the use of different shampoo and hair dye
• Darier’s disease - yellowish-brown clusters of rough dome-shaped papules in Seborrheic distribution; acanthosis

Face

• Psoriasis - rarely occurs in isolation; pitted nails
• Lupus erythematosus (LE) – discoid LE is associated with skin atrophy and scarring alopecia
• Rosacea – look for erythema and telangiectasia; it may cause Meibomianitis, along the posterior lid line
• Acne vulgaris – look for comedones, which are its hallmark
• Staphylococcal blepharitis (anterior lash line)
• Eczema (contact) - eyelids commonly involved (irritant - dry, scaly; or allergic - swollen, vesicular)
• Darier’s disease- Nail changes 

Trunk

• Psoriasis - sharply-defined red plaques with a loose, silvery lamella scale
• Pityriasis rosea - herald spot; collarette scale; Christmas tree distribution
• Pityriasis versicolor - not symmetrical; hypo/hyperpigmentation
• Subacute lupus erythematosus - photosensitive distribution
• Eczema (nummular) - intense pruritus
• Tinea corporis - raised leading edges and central clearing; uncommon in infants
• Erythema annulare centrifugum - recurrent polycyclic lesions that slowly expand and disappear
• Darier disease - Greasy wart-like papules and plaques
• Grover disease (transient acantholytic dermatosis) - acanthosis
• Drug reaction - drug history (neuroleptic; immunosuppressant; PUVA; lithium)
• Parapsoriasis - elderly; very slow growing; resistant to treatment
• Pemphigus foliaceus - fragile, painful blisters - Nikolsky sign is positive
• Secondary syphilis - lesions on the palms and soles; a history of chancre

Intertriginous Areas

• Psoriasis (inverse) - sharply-defined border
• Dermatitis (Contact) - itchy; vesicular
• Tinea cruris - advancing border; very uncommon in infants
• Erythrasma - coral-red fluorescence under Wood Lamp
• Candidiasis - satellite lesions; obesity; a history of immunodeficiency
• Hailey-Hailey disease (familial benign pemphigus) - acanthosis

Important Considerations in the Differential Diagnosis for Infantile (ISD)

Cradle Cap

• Tinea capitis - (look for broken hairs or “black dots”); very uncommon in adults
• Impetigo - yellow, honey-colored crusting

Diaper Region

• Irritant contact dermatitis – tends to spare the skin folds
• Candidiasis – either secondary or from colonization with fecal yeast; look for satellite lesions
• Infantile psoriasis – sharply-defined red plaques with silver scale
• Histiocytosis X (Langerhans cell histiocytosis) – tends to be confined to the skin folds with a purpuric rash on the body
• Acrodermatitis enteropathica – look for periorificial involvement and check zinc levels