What is the initial approach to evaluating a patient with a rash, including history taking and physical examination (PE)?

Initial Approach to Evaluating a Patient with Rash

Begin by determining whether the rash is associated with systemic illness or represents a localized dermatologic condition, focusing first on life-threatening causes that require immediate intervention.

Essential History Components

Timing and Progression

• Date of rash onset and progression pattern—critical for distinguishing acute drug reactions from chronic conditions 1
• Index date determination: identify the first symptom (sore throat, skin pain, mucosal involvement) as this guides drug causality assessment 1
• Prodromal symptoms: fever, malaise, upper respiratory symptoms preceding rash suggest serious conditions like Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN) 1

Symptom Characterization

• Presence or absence of pruritus: distinguish between conditions with rash (atopic eruption, polymorphic eruption) versus pruritus without rash (intrahepatic cholestasis of pregnancy, systemic disease) 1
• Pain quality: painful rash, particularly on face and chest, raises concern for SJS/TEN 1
• Distribution pattern: seasonal/perennial/episodic symptoms help differentiate allergic from non-allergic causes 1
• Associated symptoms: sneezing, nasal congestion, ocular itching (allergic rhinitis) 1; versus cutaneous manifestations with airway obstruction, gastrointestinal symptoms, syncope (anaphylaxis) 1

Exposure History

• Medication history: document ALL drugs taken in the previous 2 months, including over-the-counter and complementary therapies, with exact start dates and any dose escalations 1
• Environmental exposures: specific allergen contact (animals, pollens, molds), irritants (smoke, fumes, chemicals) 1
• Tick exposure: particularly in endemic areas for rickettsial diseases 1
• Travel history: recent travel broadens differential to include tropical diseases 2
• Sexual history and IV drug use: risk factors for HIV and hepatitis 1

Associated Medical History

• Personal or family history of atopy (eczema, allergic rhinitis, asthma) 1
• Previous drug allergies with specific reaction details 1
• Recurrent herpes simplex virus infections (can trigger SJS/TEN) 1
• Pregnancy status: if pregnant, assess for pruritus location (palms/soles suggest intrahepatic cholestasis) 1

Red Flag Symptoms Requiring Urgent Evaluation

• Mucosal involvement (eyes, mouth, nose, genitalia) 1
• Respiratory symptoms: cough, dyspnea, bronchial hypersecretion, hemoptysis 1
• Gastrointestinal involvement: diarrhea, abdominal distension 1
• Systemic symptoms: fever with rash is a medical emergency until proven otherwise 3, 2
• Cutaneous manifestations with airway obstruction or syncope: anaphylaxis 1

Physical Examination Approach

Vital Signs and General Assessment

• Record vital signs and oxygen saturation with pulse oximetry 1
• Baseline body weight for fluid management if severe disease 1
• Mental status changes with fever and rash suggest encephalitis or sepsis 1

Rash Morphology Classification

Categorize rash into one of four morphologic patterns 3:

• Petechial/purpuric: non-blanching, suggests vasculitis or meningococcemia
• Erythematous: diffuse redness, consider drug reaction, viral exanthem, toxic shock
• Maculopapular: flat and raised lesions, most common pattern
• Vesiculobullous: blisters, consider SJS/TEN, pemphigus, herpes infections

Specific Lesion Characteristics 4, 3

• Color, size, shape, and scale of individual lesions
• Target lesions (particularly atypical targets) or purpuric macules suggest SJS/TEN 1
• Blanching: test by applying pressure to distinguish vascular from hemorrhagic lesions 4
• Koebner phenomenon: lesions appearing at sites of trauma 4

Distribution and Involvement Patterns

• Document extent of involvement using body surface area percentage (Lund and Browder chart for burns/SJS/TEN) 1
• Palms and soles: involvement suggests secondary syphilis, Rocky Mountain spotted fever, hand-foot-mouth disease 4
• Face involvement: initial presentation in SJS/TEN 1
• Sun-exposed areas versus covered areas 4
• Extensor versus flexor surfaces: psoriasis favors extensors, atopic dermatitis favors flexors 4

Critical Mucosal Examination

Examine ALL mucosal sites systematically 1:

• Eyes: conjunctivitis, purulent discharge, eyelid edema
• Mouth: mucositis, blisters, erosions
• Nose: nasal mucosa involvement
• Genitalia: erosions or blistering

Specific Physical Findings by Condition

• Excoriations without primary rash: consider intrahepatic cholestasis of pregnancy (pruritus without rash) 1
• Urticarial papules and plaques on abdomen: polymorphic eruption of pregnancy 1
• Eczematous rash on face, neck, flexural areas: atopic eruption 1
• Epidermal detachment with positive Nikolsky sign: SJS/TEN—this finding has prognostic value 1

Common Pitfalls to Avoid

• Do not dismiss absence of cutaneous symptoms in suspected anaphylaxis: while most cases include skin findings, their absence does not rule out anaphylaxis 1
• Do not confuse excoriations from scratching with primary rash: in intrahepatic cholestasis of pregnancy, the rash is secondary to scratching, not the primary pathology 1
• Do not overlook medication history: brand switches or medication errors can trigger reactions 1
• Do not assume viral illness without considering life-threatening causes: fever with rash requires systematic exclusion of meningococcemia, toxic shock syndrome, SJS/TEN, and Rocky Mountain spotted fever 3, 2
• Do not rely solely on laboratory tests for diagnosis: detailed history and physical examination are more valuable than non-specific laboratory findings 2

When to Obtain Additional Information from Witnesses

Obtain collateral history from family members or witnesses when evaluating suspected anaphylaxis or altered mental status, as patients may not recall the complete sequence of events 1. Review medical records including medication records to evaluate previous episodes 1.