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Lynch algorithm dermatology skin lesion rash classification

The Lynch Algorithm for Dermatologic Lesions

The Lynch Algorithm (originally described by PJ Lynch in 1984 in Ann Emerg Med) is a systematic, problem-oriented approach to diagnosing skin rashes and lesions through a branching decision tree. It was designed for non-dermatologists — particularly emergency physicians — and has since been modified into what is widely called the Modified Lynch Algorithm.

Core Concept

The algorithm classifies any skin presentation by morphology first — asking "What type of lesion is this?" — and then branches into a differential diagnosis based on clinical features. It is not a dermoscopy tool; it works with naked-eye morphology.

Step 1: Classify the Primary Lesion Type

The algorithm begins by placing the rash into one of six primary categories:
CategoryKey Features
MaculopapularFlat (macule) or raised (papule), non-fluid-filled; erythematous or pigmented
Petechial / PurpuricNon-blanching pinpoint hemorrhages (petechiae) or larger patches (purpura)
Diffuse ErythematousWidespread redness, often blanching; may be exfoliative (erythroderma)
Non-erythematousHypopigmented, white, or skin-colored lesions with no significant redness
VesiculobullousFluid-filled blisters — vesicles (<5 mm) or bullae (>5 mm)
PustularPus-filled lesions; may or may not be infectious

Step 2: Branch on Clinical Features

Within each category, the algorithm asks further branching questions:

Maculopapular Rash

  • Fever present? → Consider viral exanthem, drug reaction, Rocky Mountain Spotted Fever (RMSF), secondary syphilis, meningococcemia
  • No fever? → Drug rash, contact dermatitis, pityriasis rosea, psoriasis
  • Ill-appearing? → Toxic shock syndrome, staphylococcal scalded skin syndrome (SSSS), Kawasaki disease

Petechiae / Purpura

  • Palpable purpura? → Vasculitis (IgA vasculitis/Henoch-Schönlein, leukocytoclastic vasculitis)
  • Non-palpable, fever? → Meningococcemia, DIC, RMSF
  • Thrombocytopenic? → ITP, TTP, HUS
  • No fever, no thrombocytopenia? → Trauma, senile purpura, scurvy

Vesiculobullous

  • Dermatomal distribution? → Herpes zoster
  • Grouped vesicles on erythema? → Herpes simplex
  • Widespread, mucosal involvement? → Stevens-Johnson syndrome (SJS) / Toxic epidermal necrolysis (TEN) — emergency
  • Autoimmune (tense bullae)? → Bullous pemphigoid
  • Flaccid bullae? → Pemphigus vulgaris

Diffuse Erythematous (Erythroderma)

  • Exfoliating/scaling? → Exfoliative dermatitis, psoriasis, seborrheic dermatitis
  • Fever, toxic appearance? → SSSS, TEN, drug reaction (DRESS)

Non-Erythematous

Key differentials: syphilis (secondary — copper-colored), anthrax, vitiligo, tinea versicolor

Pustular

  • Follicular? → Folliculitis (bacterial, fungal)
  • Non-follicular, fever? → Acute generalized exanthematous pustulosis (AGEP), pustular psoriasis

Step 3: Identify "Red Flag" Features (Life-Threatening Rashes)

The algorithm specifically flags presentations requiring urgent action:
  • Ill appearance / vital sign instability
  • New-onset fever with rash
  • Mucosal or ocular involvement → SJS/TEN
  • Recent anticonvulsant or sulfa drug → DRESS syndrome
  • Skin pain (out of proportion to appearance) → Necrotizing fasciitis, TEN
  • Immunocompromised patient

Summary Flowchart Logic

Rash
 ├── Fluid-filled? → Vesiculobullous / Pustular branch
 ├── Non-blanching? → Petechiae/Purpura branch
 ├── Diffuse red, scaling? → Erythroderma branch
 ├── Hypopigmented / skin-colored? → Non-erythematous branch
 └── Flat/raised, blanching red? → Maculopapular branch
        ↓
   Apply red-flag checklist
        ↓
   Narrow differential by distribution, fever, exposure, timeline

Clinical Utility

The Lynch algorithm is particularly valuable in:
  • Emergency medicine: rapid triage of rashes in the ED
  • Primary care / internal medicine: first-pass evaluation by non-dermatologists
  • Medical education: teaching systematic dermatologic reasoning
The original 1984 paper by Lynch & Edminster (Ann Emerg Med 13(8):603–606) described it as a "problem-oriented system for the nondermatologist." Multiple modified versions have since been published for emergency medicine teaching, including the one used in the JETem mTBL module.
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