Pityriasis Rosea & Seborrhoeic Dermatitis
Dermatology · Papulosquamous · lean revision notes
Pityriasis Rosea & Seborrhoeic Dermatitis
Two common, benign papulosquamous disorders that are perennial NEET PG favourites. Pityriasis rosea (PR) is a self-limiting eruption classically heralded by a single "mother patch" and a viral (HHV-6/7) trigger; seborrhoeic dermatitis (SD) is a chronic relapsing inflammatory dermatosis of sebum-rich skin driven by Malassezia yeast. Both are clinical diagnoses — examiners test pattern recognition, the herald patch, the Christmas-tree distribution, the Malassezia link, and ketoconazole-based therapy.
Where they sit: the papulosquamous group
Papulosquamous disorders share papules + scaling as the unifying morphology. The classic NEET PG short-list:
| Disorder | Classic clue | Scale type | Course |
|---|---|---|---|
| Psoriasis | Auspitz sign, well-defined plaques on extensors | Silvery-white, micaceous | Chronic |
| Pityriasis rosea | Herald patch, Christmas-tree pattern | Fine, collarette (peripheral) | Self-limiting (6–8 wk) |
| Lichen planus | Violaceous, Wickham striae, 6 P's | Fine, adherent | Subacute–chronic |
| Seborrhoeic dermatitis | Greasy, sebum-rich sites, dandruff | Greasy yellow | Chronic relapsing |
| Secondary syphilis | Palms & soles, condyloma lata | Coppery, collarette (Biette) | Resolves with treatment |
| Pityriasis versicolor | Hypo/hyperpigmented macules, trunk | Fine, "coup d'ongle" scratch sign | Chronic, recurs |
High-yield: When a "papulosquamous on palms and soles" rash mimics PR but PR characteristically spares palms and soles, always exclude secondary syphilis (do VDRL/RPR). This is a repeatedly tested distractor.
Part A — Pityriasis Rosea
Definition & epidemiology
Pityriasis rosea is an acute, self-limiting papulosquamous eruption characterised by an initial herald patch (primary medallion / mother patch) followed 1–2 weeks later by a generalised secondary eruption in a "Christmas-tree" (fir-tree) distribution along skin cleavage (Langer) lines on the trunk.
- Peak age: 10–35 years (adolescents and young adults).
- Slight female predominance.
- Seasonal clustering (spring/autumn) supports an infectious aetiology.
Etiology & pathophysiology
- Strong association with reactivation of human herpesvirus 6 and 7 (HHV-6/HHV-7) — the single most tested fact. Viral DNA and active replication markers are demonstrable in plasma and lesional skin.
- Prodrome of malaise, headache, low-grade fever, and arthralgia in a minority supports a systemic viral process.
- Drug-induced PR ("pityriasis rosea-like eruption") is a key exam point — caused by ACE inhibitors (captopril), gold, NSAIDs, barbiturates, metronidazole, terbinafine, isotretinoin, and some vaccines/COVID-19 vaccination. Drug-induced cases lack a herald patch, are more extensive, and may show eosinophils on histology.
High-yield: HHV-6/HHV-7 reactivation = the accepted trigger for classic PR. A "PR-like eruption" without a herald patch + history of a new drug → think drug-induced PR.
Clinical features — stepwise evolution
Herald patch → 1–2 week interval → secondary generalised eruption → spontaneous resolution in 6–8 weeks
- Herald patch: a single 2–5 cm salmon-pink/erythematous oval plaque with a fine collarette of scale (scale attached at periphery, free centrally). Commonly on the trunk, neck, or proximal limb. Often mistaken for tinea corporis (KOH negative in PR).
- Secondary eruption: crops of smaller oval pink macules/plaques with peripheral (collarette) scale. Long axes of lesions run parallel to the lines of cleavage, producing the "Christmas-tree" appearance on the back.
- Distribution: trunk and proximal extremities; classically spares the face, palms and soles in adults.
- Symptoms: usually asymptomatic or mild itch; itch worsened by heat/sweating.
- Resolution may leave post-inflammatory hyper- or hypopigmentation, especially in darker (Indian) skin.
Important variants (frequently asked)
| Variant | Distinguishing feature |
|---|---|
| Inverse PR | Predominantly flexural/intertriginous (axillae, groin) + face; trunk relatively spared — common in children |
| Papular PR | Predominant small papules; more in children, darker skin |
| Vesicular PR | Vesicles, mimics varicella/dyshidrosis |
| Purpuric/haemorrhagic PR | Petechiae within lesions |
| PR gigantea (of Darier) | Few large plaques |
| Pityriasis circinata et marginata (Vidal) | Localised, large plaques in groin/axilla, chronic |
High-yield: Inverse pityriasis rosea affects flexures and the face and is more frequent in children — a classic single-best-answer trap.
Diagnosis & investigation of choice
- Clinical diagnosis. No confirmatory test is routinely needed.
- KOH mount of a scale: negative — used to exclude tinea corporis (the herald patch's main mimic).
- VDRL/RPR: order when palms/soles involved, in sexually active patients, or atypical morphology — to rule out secondary syphilis.
- Histopathology (only if atypical): focal parakeratosis (mounds), spongiosis, acanthosis, superficial perivascular lymphocytic infiltrate, and extravasated red cells — non-specific.
Management / drug of choice
Pityriasis rosea is self-limiting — reassurance is the cornerstone.
- Symptomatic: emollients, topical corticosteroids and oral antihistamines for pruritus.
- Drug of choice for symptomatic/extensive disease: oral aciclovir (high-dose, e.g. 800 mg 5×/day for 1 week) started early may hasten resolution and reduce itch — consistent with the HHV aetiology.
- UVB phototherapy for widespread, itchy, or persistent disease.
- Macrolides (erythromycin) have been used but evidence is weak/inconsistent.
- Counsel: lesions clear in 6–8 weeks; recurrence is uncommon (<3%); reassure regarding pigmentary change.
High-yield: PR in pregnancy, especially within the first 15 weeks, has been associated with increased risk of spontaneous abortion / adverse fetal outcome (linked to HHV-6 reactivation). Flag and counsel — a favourite "clinical scenario" question.
Complications & prognosis
- Post-inflammatory dyspigmentation (cosmetic, resolves over months).
- Rare obstetric risk in early pregnancy (above).
- Otherwise excellent prognosis; immunity tends to follow.
Key differentials of PR
- Tinea corporis (KOH +ve, active scaly border, central clearing).
- Secondary syphilis (palms/soles, lymphadenopathy, condyloma lata, VDRL +ve).
- Guttate psoriasis (post-streptococcal, droplet plaques, no herald patch, silvery scale).
- Nummular eczema, pityriasis lichenoides, drug eruptions.
Part B — Seborrhoeic Dermatitis
Definition & epidemiology
Seborrhoeic dermatitis is a chronic, relapsing inflammatory dermatosis affecting sebum-rich (seborrhoeic) areas — scalp, eyebrows, nasolabial folds, retroauricular region, central chest, and flexures — characterised by erythema with greasy yellowish scales.
- Bimodal age distribution: infants (first 3 months — "cradle cap") and adults (30–60 years).
- Male predominance (androgen/sebum influence).
- Dandruff (pityriasis simplex capillitii) is the mildest, non-inflammatory end of the SD spectrum.
Etiology & pathophysiology
The interplay tested in exams:
Sebum-rich skin → colonisation by lipophilic yeast Malassezia (formerly Pityrosporum) → lipase breaks sebum triglycerides → release of irritant free fatty acids (oleic acid) → inflammatory + hyperproliferative skin response in susceptible hosts
- Malassezia species (M. globosa, M. restricta) are the central driver — not a true infection but an abnormal host response to commensal yeast.
- Sebaceous gland activity (androgen-driven; hence infantile and post-pubertal peaks) is permissive.
- Host factors: stress, cold/dry winter, neurological disease, immunosuppression.
High-yield: Malassezia is the key organism in seborrhoeic dermatitis — this also explains why antifungal (ketoconazole) shampoo is first-line therapy.
Associations that make a question "non-easy"
| Association | Clinical relevance |
|---|---|
| HIV / AIDS | SD is severe, extensive, treatment-resistant; an early cutaneous marker — test for HIV in florid SD |
| Parkinson's disease & neuroleptics | Increased facial sebum (seborrhoea) → severe SD; improves with L-dopa |
| Depression, stroke, epilepsy | Higher prevalence |
| Down syndrome | Increased incidence |
High-yield: Sudden, severe, recalcitrant seborrhoeic dermatitis in a young adult → screen for HIV. Equally classic: SD as a feature of Parkinsonism.
Clinical features
Adult SD
- Scalp: ranges from fine dandruff to thick greasy yellow scales with erythema; itch common.
- Face: erythema and greasy scaling of eyebrows, glabella, nasolabial folds, beard; blepharitis (scaly lid margins).
- Trunk: "petaloid" or annular lesions on the presternal area; otitis externa, retroauricular fissuring.
- Flexures: moist, well-demarcated erythema in axillae, groin, inframammary folds (intertriginous SD).
Infantile SD ("cradle cap")
- Onset in first weeks–3 months; non-itchy, self-limiting.
- Thick, greasy, yellow-brown adherent scales on the vertex/scalp; may involve face, retroauricular area, and napkin (diaper) region.
- Generally resolves by 8–12 months; good prognosis, distinguishing it from atopic dermatitis (itchy, spares napkin area, later onset).
High-yield: Cradle cap = infantile seborrhoeic dermatitis; non-pruritic, self-limiting, and often affects the napkin area (helps differentiate from infantile atopic eczema, which is pruritic and spares the napkin area).
Diagnosis & investigation of choice
- Clinical diagnosis — distribution + greasy scale.
- KOH/culture not required (commensal yeast); skin biopsy only if atypical: shows "shoulder parakeratosis" around follicular ostia, spongiosis, and psoriasiform hyperplasia.
- Consider HIV testing when severe/atypical/refractory.
Management / drug of choice
Aim: reduce Malassezia, inflammation, and scale; control is the goal (chronic relapsing).
Adult scalp/dandruff → medicated shampoo (ketoconazole 2% / selenium sulphide / zinc pyrithione) twice weekly → add topical steroid lotion for flares → topical calcineurin inhibitor for maintenance on face
- Antifungal — drug of choice: topical ketoconazole (2% shampoo or cream); also selenium sulphide, zinc pyrithione, ciclopirox.
- Topical corticosteroids (mild, e.g. hydrocortisone) for short bursts to control inflammation/itch.
- Topical calcineurin inhibitors (tacrolimus, pimecrolimus) — steroid-sparing, ideal for face and long-term use.
- Keratolytics — salicylic acid, coal tar for thick scalp scale.
- Severe/refractory: short-course oral ketoconazole/itraconazole/fluconazole.
Infantile cradle cap
- Reassurance; emollient/white soft paraffin or olive oil to soften, then gentle removal with a soft brush; mild ketoconazole shampoo or 1% hydrocortisone for stubborn inflammation. Avoid potent steroids.
High-yield: Ketoconazole shampoo (antifungal targeting Malassezia) is the first-line and most-tested treatment for seborrhoeic dermatitis/dandruff.
Complications
- Secondary bacterial/candidal infection of fissured flexural lesions.
- Erythroderma (rare, severe).
- Psychosocial impact (visible facial/scalp disease).
- Persistent blepharitis.
Key differentials of SD
| Differential | How to distinguish from SD |
|---|---|
| Psoriasis ("sebopsoriasis" overlap) | Well-defined plaques, silvery (not greasy) scale, extensor/nail/scalp margin involvement, Auspitz sign |
| Atopic dermatitis (infants) | Pruritic, spares napkin area, later onset, flexural in older children |
| Rosacea | Central face papulopustules, telangiectasia, no greasy scale |
| Tinea capitis/corporis | KOH +ve, active border, hair loss with broken hairs |
| Contact dermatitis | Exposure history, vesicles, well-demarcated to contactant |
| SLE (malar) | Photosensitive, spares nasolabial folds, ANA +ve |
Pityriasis rosea vs seborrhoeic dermatitis — at a glance
| Feature | Pityriasis rosea | Seborrhoeic dermatitis |
|---|---|---|
| Trigger | HHV-6/7 reactivation | Malassezia yeast + sebum |
| Course | Self-limiting (6–8 wk) | Chronic, relapsing |
| Hallmark | Herald patch + Christmas-tree | Greasy yellow scale, sebum-rich sites |
| Scale | Fine, collarette (peripheral) | Greasy, yellowish |
| Itch | Mild/variable | Common |
| Sites | Trunk, proximal limbs (spares palms/soles) | Scalp, face folds, presternal, flexures |
| First-line Rx | Reassurance ± aciclovir/UVB | Ketoconazole shampoo + topical steroid |
| Key red flag | PR in early pregnancy | Severe SD → screen HIV/Parkinson's |
Mnemonics & eponyms
- PR herald patch — "the mother patch" (primary medallion / plaque of Brocq).
- Christmas-tree / fir-tree distribution along Langer's cleavage lines — picture lesions hanging off the spine like branches.
- Dandruff = pityriasis simplex; cradle cap = infantile SD ("Cradle cap is Crusty and not itchy").
- Malassezia mnemonic: "Malassezia Makes it greasy → Manage with ketoconazole."
- Secondary-syphilis trap rhyme: "PR spares the palms; syphilis loves them."
Recently asked / exam angle
- The herald patch precedes the generalised rash by ~1–2 weeks → name and identify it on an image.
- HHV-6/HHV-7 as the causal/triggering virus of PR (direct one-liner).
- PR spares palms and soles; palm/sole involvement → rule out secondary syphilis (VDRL).
- Inverse PR → flexures + face, common in children.
- PR in pregnancy (<15 weeks) → risk of fetal loss; counsel.
- Drug causing PR-like eruption → ACE inhibitors / NSAIDs / metronidazole / terbinafine.
- Malassezia as the organism in seborrhoeic dermatitis and dandruff.
- Ketoconazole shampoo = first-line for SD/dandruff.
- Cradle cap = infantile SD; non-itchy, self-limiting, may involve napkin area (vs atopic eczema).
- Severe, refractory SD in young adult → HIV; seborrhoea with Parkinson's disease.
- Differentiating greasy yellow scale (SD) from silvery scale (psoriasis) and collarette scale (PR/syphilis).
Rapid revision
- Pityriasis rosea = herald patch → Christmas-tree eruption along cleavage lines; self-limiting in 6–8 weeks.
- PR trigger = HHV-6/HHV-7 reactivation.
- PR collarette scale is attached peripherally, free centrally; spares palms/soles.
- Palms/soles + papulosquamous rash → exclude secondary syphilis (VDRL/RPR).
- Inverse PR = flexures + face, commoner in children.
- PR in early pregnancy → risk of miscarriage; counsel.
- Drug-induced PR-like rash: ACE inhibitors, NSAIDs, metronidazole, terbinafine, gold (no herald patch).
- PR management = reassurance ± oral aciclovir / UVB for symptoms.
- Seborrhoeic dermatitis = greasy yellow scale in sebum-rich sites; driven by Malassezia.
- SD first-line = ketoconazole shampoo + short topical steroid; tacrolimus on face for maintenance.
- Cradle cap = infantile SD: non-itchy, self-limiting, may involve napkin area.
- Severe/refractory SD → think HIV; seborrhoea + tremor/rigidity → Parkinson's disease.