Dermatitis Seboroik

01Pathophysiology

Seborrheic dermatitis is a chronic inflammatory skin condition driven by the host's immune response to Malassezia species, a genus of lipophilic dimorphic fungi that are part of the normal skin flora. These organisms preferentially colonize areas with a high density of sebaceous glands because they depend on lipids for nutrition. The yeast produces lipases that hydrolyze sebum triglycerides into oleic acid and other unsaturated fatty acids. In susceptible individuals, these metabolites penetrate the stratum corneum and trigger an aberrant inflammatory cascade characterized by complement activation, cytokine release (IL-1, IL-6, TNF-alpha), and recruitment of inflammatory cells. This explains the distribution pattern of the disease: it appears precisely where sebaceous activity is greatest, including the scalp, eyebrows, nasolabial folds, external ear canal, presternal chest, and intertriginous areas.

The reason the disease is so strongly linked to HIV/AIDS and immunosuppression is that T-cell dysfunction impairs the skin's ability to regulate the fungal burden and modulate the inflammatory response. In patients with advanced HIV (CD4 count below 200 cells/microL), seborrheic dermatitis can become explosive in severity and extent, covering large portions of the body. Similarly, the association with Parkinson disease and neurological conditions is believed to relate to autonomic dysfunction leading to increased sebum production, combined with immobility that may alter skin barrier function.

In infants, the condition manifests as "cradle cap" because neonatal sebaceous glands are transiently stimulated by residual maternal androgens, creating a lipid-rich environment favorable for Malassezia colonization. This is self-limited and typically resolves by 6 to 12 months of age as hormonal influence wanes.

The hallmark greasy, yellowish scale directly reflects the accelerated epidermal turnover and the lipid-rich inflammatory exudate on the skin surface. Pruritus, when present, is generally mild because the inflammatory infiltrate is superficial and does not deeply involve dermal nerve fibers.

02Classification and Clinical Manifestation

03Diagnostic Workup

Seborrheic dermatitis is overwhelmingly a clinical diagnosis. The combination of greasy, yellowish scaling in a characteristic distribution (scalp, nasolabial folds, eyebrows, ears, presternal chest) in a patient with a chronic, relapsing course is sufficient to make the diagnosis without any laboratory testing.

Biopsy is reserved for cases where the clinical picture overlaps with psoriasis, tinea, lupus, or other conditions and the distinction cannot be made on examination alone. The pathognomonic histological finding is "shoulder" parakeratosis, referring to mounds of parakeratotic cells that drape over the edges of dilated follicular openings, often containing neutrophils. This pattern differs from psoriasis, where parakeratosis is more confluent and associated with regular acanthosis and Munro microabscesses.

The most commonly tested clinical pearl in this section is the HIV screening trigger. When a vignette describes a young adult presenting with unusually severe, widespread, or treatment-resistant seborrheic dermatitis, the next best step is to offer HIV testing. This is because seborrheic dermatitis may be the first visible sign of underlying immunodeficiency, and its severity correlates inversely with CD4 count.

A KOH preparation is useful only to rule out dermatophyte infection (tinea capitis, tinea faciei) when the presentation is atypical, particularly in children with scalp scaling and hair loss.

04Management and Treatment

The first step for any patient presenting with seborrheic dermatitis is to determine severity and location. Mild scalp disease (simple dandruff) is managed with over-the-counter medicated shampoos. Ketoconazole 2% shampoo is the most evidence-based choice and targets the underlying Malassezia directly. The shampoo should be lathered and left in contact with the scalp for at least 5 minutes before rinsing to allow adequate antifungal penetration.

When scalp disease is more inflammatory (thick plaques, significant erythema, intense pruritus), a topical corticosteroid is added for rapid symptomatic relief. A mid-potency steroid solution or foam is appropriate for the scalp because the thick skin and hair coverage reduce the risk of steroid-induced atrophy. Limit use to 2 to 4 weeks and transition to maintenance antifungal shampoo alone.

For facial seborrheic dermatitis, the management approach shifts because facial skin is thin and vulnerable to corticosteroid side effects. Hydrocortisone 1% can be used briefly (1 to 2 weeks) to break a flare, but the preferred long-term strategy is a topical calcineurin inhibitor (tacrolimus 0.1% ointment or pimecrolimus 1% cream). These agents provide anti-inflammatory benefit without the risk of atrophy, telangiectasia, or perioral dermatitis that accompanies prolonged facial steroid use.

For infants with cradle cap, the management is conservative. Mineral oil softens the adherent scales, which are then gently removed. No aggressive scraping should be performed. Reassurance is a key component of management, as the condition is benign and self-resolving.

Contraindications and cautions to remember:

Prolonged use of mid- to high-potency topical corticosteroids on the face is contraindicated due to the risk of steroid rosacea, perioral dermatitis, and skin atrophy. Oral ketoconazole is generally avoided for seborrheic dermatitis because of the risk of hepatotoxicity; itraconazole or fluconazole are safer systemic alternatives when needed.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most tested trap with seborrheic dermatitis is the HIV association. When a vignette describes a young adult (often in their 20s or 30s) with an abrupt onset of severe, widespread, or treatment-refractory seborrheic dermatitis, the question is almost never about dermatology management. The correct answer is to screen for HIV. Students who focus on choosing the right shampoo or cream will miss the point entirely: the examiner is testing your ability to recognize seborrheic dermatitis as a sentinel marker of immunodeficiency.

A second common mistake involves confusing seborrheic dermatitis with psoriasis. Both can affect the scalp, and an overlap entity called "sebopsoriasis" exists. The discriminator that the exam will embed in the vignette is the quality and color of the scale (greasy and yellowish vs. silvery-white and micaceous) and the distribution (nasolabial folds and flexural creases vs. extensor surfaces and nails). Read the descriptive adjectives carefully.

A third pearl concerns the nasolabial fold. In the context of facial rashes, the nasolabial fold is a powerful anatomic clue. Seborrheic dermatitis involves the nasolabial folds; the lupus malar rash spares them. If a question stem describes facial erythema and mentions the nasolabial folds, pay close attention to whether the rash includes or excludes that area, as this single detail often determines the correct answer.

For management, remember that the exam will test whether you know to avoid prolonged use of potent topical corticosteroids on the face. If a vignette asks for the best long-term maintenance therapy for facial seborrheic dermatitis, the answer is a topical calcineurin inhibitor (tacrolimus or pimecrolimus), not a corticosteroid.

Finally, for infants, cradle cap is benign and self-limited. The examiner may try to create anxiety by describing a dramatic-looking scalp, but the correct answer is conservative management with emollients and gentle cleansing, not aggressive pharmacotherapy.