About This Page
This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
The Bottom Line
- Painful blisters, mucosal erosions, fever, toxicity, rapid spread, ocular symptoms, or positive Nikolsky sign are emergencies
- Separate infectious vesicles from autoimmune bullae, drug reactions, contact dermatitis, burns, and metabolic blistering disorders
- SJS/TEN is primarily a clinical emergency: stop the culprit drug, admit, provide supportive care, and involve dermatology and ophthalmology
- Autoimmune blistering disease usually needs two biopsies: lesional skin for histology and perilesional normal-appearing skin for direct immunofluorescence
- Disseminated zoster, eczema herpeticum, neonatal HSV, and immunocompromised herpesvirus infection require urgent antiviral therapy
Approach to the Presentation
A vesiculobullous rash should be approached by blister size, depth, distribution, mucosal involvement, pain, itch, medication exposure, immune status, and systemic illness. Vesicles are small fluid-filled lesions; bullae are larger blisters. Tense bullae suggest subepidermal disease such as bullous pemphigoid, while flaccid bullae and erosions suggest intraepidermal disease such as pemphigus vulgaris or SJS/TEN. A localized linear itchy vesicular eruption after outdoor or occupational exposure suggests contact dermatitis, while dermatomal painful vesicles suggest zoster. In contrast, widespread painful blistering, mucosal disease, fever, or rapid detachment should be treated as a dermatologic emergency.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis | must-not-miss | Fever, malaise, skin pain, dusky macules, flaccid bullae, epidermal detachment, oral/ocular/genital mucosal erosions after high-risk medication | Clinical diagnosis; urgent skin biopsy may confirm full-thickness epidermal necrosis |
| Pemphigus Vulgaris | must-not-miss | Painful oral erosions often precede flaccid cutaneous bullae; positive Nikolsky sign; middle-aged or older adult | Skin biopsy + direct immunofluorescence showing intercellular IgG/C3; desmoglein antibodies |
| Disseminated HSV/VZV or Eczema Herpeticum | must-not-miss | Monomorphic punched-out vesicles/erosions, fever, pain, atopic dermatitis or immunosuppression; ocular involvement can threaten vision | HSV/VZV PCR from lesion swab; start acyclovir empirically if severe |
| Necrotizing Soft-Tissue Infection with Bullae | must-not-miss | Severe pain out of proportion, fever, toxicity, rapidly spreading erythema, haemorrhagic bullae, crepitus, anaesthesia, shock | Clinical diagnosis; urgent surgical exploration is definitive and should not be delayed for imaging |
| Bullous Pemphigoid | common | Elderly patient with intensely pruritic urticarial plaques and tense bullae; mucosal involvement uncommon | Biopsy with subepidermal blister + linear IgG/C3 along basement membrane on DIF |
| Allergic Contact Dermatitis | common | Very itchy vesicles or bullae in a sharply demarcated or linear pattern after exposure to plants, nickel, fragrances, rubber, or occupational agents | Clinical pattern; patch testing if recurrent or occupational |
| Herpes Zoster | common | Painful grouped vesicles in a unilateral dermatomal distribution; prodromal neuralgia; ophthalmic distribution is high risk | Clinical diagnosis; VZV PCR if atypical or immunocompromised |
| Bullous Impetigo | common | Flaccid superficial bullae, honey-coloured crust, usually children; caused by toxin-producing Staphylococcus aureus | Clinical diagnosis; bacterial culture if recurrent, outbreak, MRSA risk, or treatment failure |
| Dyshidrotic Eczema | common | Recurrent intensely itchy deep-seated tapioca-like vesicles on palms, sides of fingers, and soles; often triggered by sweating, irritants, or atopy | Clinical diagnosis; fungal scraping if unilateral or tinea suspected |
| Dermatitis Herpetiformis | less common | Symmetric intensely pruritic grouped vesicles and excoriations on elbows, knees, buttocks, scalp; associated with coeliac disease | Perilesional biopsy with granular IgA deposition in dermal papillae; coeliac serology |
| Porphyria Cutanea Tarda | rare | Fragile skin, bullae, erosions, milia, and hypertrichosis on sun-exposed dorsal hands; liver disease, hepatitis C, alcohol, estrogen, iron overload associations | Plasma/urine porphyrins, ferritin, liver tests, hepatitis C and HIV testing |
Red Flags & Key History
Symptoms
Skin pain, fever, malaise, mucosal erosions, eye pain, photophobia, or dysuria — severe cutaneous adverse reaction until proven otherwise
Rapidly spreading erythema with severe pain out of proportion — necrotizing soft-tissue infection
Immunosuppression, pregnancy, neonate, eczema with punched-out erosions, or disseminated vesicles — severe herpesvirus infection
New high-risk medication in previous 1-8 weeks — SJS/TEN, DRESS, or bullous drug eruption
Oral erosions before skin disease — pemphigus vulgaris is a key possibility
Intense itch with tense bullae in an older adult — bullous pemphigoid
Dermatomal pain followed by grouped vesicles — herpes zoster
Linear or sharply demarcated itchy vesicles after exposure — allergic contact dermatitis
Signs
Nikolsky sign or epidermal sheet-like detachment
Ocular involvement: conjunctivitis, corneal symptoms, eyelid erosions
Haemorrhagic bullae, crepitus, skin anaesthesia, or shock
Tense bullae on urticarial base in older adult
Grouped vesicles on erythematous base in dermatomal distribution
Honey-coloured crust or superficial flaccid bullae in children
Approach to Investigation
First-line
Full skin and mucosal examinationDocument distribution, tense vs flaccid bullae, mucosa, ocular involvement, palms/soles, BSA detached, Nikolsky sign, tenderness, and signs of infection or necrosis
Medication and immunosuppression reviewHigh-yield for SJS/TEN and bullous drug eruptions. Ask about anticonvulsants, allopurinol, sulfonamides, NSAIDs, antibiotics, antiretrovirals, immunotherapy, and recent dose changes
CBC, electrolytes, creatinine, liver enzymes, glucose if systemic or extensiveAssesses dehydration, secondary infection, organ involvement, and severity before systemic therapy or admission
Lesion swab for HSV/VZV PCR or bacterial culture when indicatedPCR is useful for atypical zoster/HSV, disseminated disease, immunocompromise, or eczema herpeticum; culture if impetigo, cellulitis, or MRSA concern
Second-line
Skin biopsy for histologyTake from a fresh intact blister edge or active lesion when autoimmune blistering disease, SJS/TEN, vasculitis, or diagnostic uncertainty exists
Direct immunofluorescenceSeparate perilesional biopsy in Michel medium for pemphigus, pemphigoid, dermatitis herpetiformis, linear IgA disease, or lupus blistering disease
Ophthalmology assessmentSame-day if SJS/TEN, ocular symptoms, herpes zoster ophthalmicus, or eyelid/conjunctival blistering
Specialist
Dermatology consultationUrgent for SJS/TEN, pemphigus, pemphigoid, extensive bullous disease, mucosal involvement, unclear diagnosis, or systemic therapy decisions
Surgical consultationImmediate if necrotizing soft-tissue infection is possible; imaging must not delay operative assessment
Management Principles
Canadian Dermatology Association resources + Canadian emergency practice principles1
Treat emergencies first
- SJS/TEN: stop culprit drug, admit urgently, manage fluids, temperature, pain, wounds, infection risk, nutrition, and ocular disease; involve dermatology and ophthalmology
- Necrotizing infection: broad-spectrum IV antibiotics and immediate surgical debridement
- Disseminated HSV/VZV or eczema herpeticum: start systemic acyclovir/valacyclovir promptly; IV therapy if severe, immunocompromised, ocular, or disseminated
2
Manage autoimmune blistering disease
- Confirm with biopsy and direct immunofluorescence where feasible, but do not delay escalation if severe mucosal disease or dehydration exists
- Pemphigus vulgaris often requires systemic corticosteroids and steroid-sparing immunosuppression/rituximab under dermatology care
- Bullous pemphigoid may be treated with potent topical corticosteroids for localized disease or systemic therapy for extensive disease
3
Manage common localized blistering disease
- Allergic contact dermatitis: remove trigger, topical corticosteroid potency matched to site/severity, oral antihistamines for sleep-disrupting itch
- Bullous impetigo: topical or oral anti-staphylococcal therapy depending on extent; consider culture in outbreaks or treatment failure
- Herpes zoster: antivirals within 72 hours or later if new lesions, ophthalmic involvement, severe pain, or immunocompromise
4
Patient counselling
- Explain medication avoidance and document severe drug reactions clearly
- Provide return precautions for fever, eye pain, mucosal lesions, spreading blistering, dehydration, or uncontrolled pain
- Review vaccination prevention where relevant, including varicella/zoster immunization eligibility
Complications & Pitfalls
- Not checking mucosa: Oral and ocular erosions separate benign blistering from potentially life-threatening disease.
- Waiting for biopsy in SJS/TEN: The next step is stopping the culprit drug and admitting; biopsy is supportive, not a prerequisite.
- Missing eczema herpeticum: Punched-out painful erosions in eczema require antiviral treatment, not just topical steroids.
- Using the wrong biopsy site: Autoimmune blistering disease needs lesional histology and perilesional direct immunofluorescence.
- Delaying surgery for necrotizing infection: Bullae plus pain out of proportion is a surgical emergency.
MCCQE1 Exam Tips
- 1Tense bullae in an elderly itchy patient = bullous pemphigoid; flaccid bullae plus oral erosions = pemphigus vulgaris
- 2Skin pain and mucosal erosions after a new medication should make SJS/TEN the working diagnosis
- 3Grouped vesicles in a dermatome = zoster; Hutchinson sign or eye symptoms = urgent ophthalmology
- 4Punched-out monomorphic erosions in atopic dermatitis = eczema herpeticum; give systemic antivirals
- 5Direct immunofluorescence is tested often: take perilesional skin, not the centre of an old eroded blister
- 6Bullae with severe pain out of proportion and systemic toxicity = necrotizing soft-tissue infection; surgery is the next best step
- 7Linear itchy vesicles after plant exposure = allergic contact dermatitis, not herpes zoster
practicetest your knowledge on rash — vesiculobullousApply what you've learnt with MCCQE1-style questions from the iatroX Q-Bank — dermatologic and beyond.
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