Tuberculids
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Direct answer
Tuberculids are cutaneous eruptions that arise from hypersensitivity to Mycobacterium tuberculosis at a distant focus, so the skin lesions contain very few or no bacilli. The three classical types are papulonecrotic tuberculid, lichen scrofulosorum and erythema induratum of Bazin, each with a distinctive morphology. Diagnosis rests on the clinical pattern, histology and evidence of a systemic tuberculous focus, and treating that focus usually clears the skin.
What you must remember
- Mechanism: a hypersensitivity reaction to tubercle bacilli or their products, not direct infection of the skin, so smears and cultures from lesions are usually negative.
- Papulonecrotic tuberculid: crops of crusted, necrotic papules, usually on the extremities, face and trunk, that heal with varioliform scarring.
- Lichen scrofulosorum: grouped, follicular, flesh-coloured or brownish papules on the trunk, seen most often in children, with follicular granulomas on histology.
- Erythema induratum of Bazin: tender, deep subcutaneous nodules on the posterior calves of young women, which recur, ulcerate and scar.
- Histology of Bazin's disease: lobular panniculitis with granulomas and vasculitis, distinguishing it from the acute, self-limiting erythema nodosum.
- Systemic evaluation: tuberculin testing or an interferon-gamma release assay, chest imaging and a search for a focus in the lymph nodes, bones or joints.
- Treatment: antitubercular therapy for the underlying disease usually clears the lesions; steroids may ease symptoms in selected cases.
- Differentials: lichen planus and sarcoidosis for lichen scrofulosorum, and erythema nodosum for erythema induratum of Bazin.
Three tuberculids compared
Tuberculids are classified by morphology and histology rather than by organism, and the table separates the three classical forms that candidates must recognise in a clinical stem. Site and pattern usually settle the diagnosis before histology is considered.
| Feature | Papulonecrotic | Lichen scrofulosorum | Erythema induratum of Bazin |
|---|---|---|---|
| Morphology | Crusted necrotic papules | Grouped follicular papules | Tender deep nodules that ulcerate |
| Typical site | Limbs, face and trunk | Trunk, often in children | Posterior calves |
| Histology | Wedge-shaped dermal necrosis with vasculitis, granulomas variable | Follicular granulomas | Lobular panniculitis with vasculitis |
| Course | Heals with varioliform scars | Resolves with treatment of the focus | Recurrent, with scarring |
Crusted papules on the limbs point to papulonecrotic tuberculid, grouped follicular papules on the trunk to lichen scrofulosorum, and tender calf nodules in a young woman to erythema induratum of Bazin. Each is a hypersensitivity phenomenon, so management aims at the systemic tuberculous focus rather than the skin.
Because the lesions contain few bacilli, a negative skin smear or culture never excludes a tuberculid. The skin biopsy is therefore read together with the tuberculin test and chest imaging.
Where the exam catches students
Most traps involve expecting bacteriological proof or treating the skin in isolation.
- Trap: expecting a tuberculid to be culture-positive Correct: the lesions contain very few bacilli, so the diagnosis is clinical and histological.
- Trap: treating erythema induratum of Bazin as simple erythema nodosum Correct: Bazin's disease is chronic, recurrent and ulcerating, and it is linked to tuberculosis rather than resolving in a few weeks.
- Trap: treating lichen scrofulosorum as lichen planus without a TB check Correct: it signals a tuberculous focus, so tuberculin testing and chest imaging are required.
- Trap: ignoring the systemic focus while treating only the skin Correct: tuberculids are hypersensitivity reactions, so management centres on the underlying tuberculosis.
Frequently asked questions
Why do tuberculids contain so few bacilli?
Tuberculids are hypersensitivity reactions to tubercle bacilli or their products carried from a distant focus, so the skin is not directly infected. This is why smears and cultures from the lesion are usually negative and why histology is read alongside the clinical picture.
What is lichen scrofulosorum?
Lichen scrofulosorum is a tuberculid of grouped, follicular, flesh-coloured or brownish papules, typically on the trunk and seen most often in children. Its histology shows follicular granulomas, and it resolves when the underlying tuberculosis is treated.
How does erythema induratum of Bazin differ from erythema nodosum?
Erythema induratum of Bazin is a chronic, recurrent tuberculid with tender deep nodules on the calves that may ulcerate and scar. Erythema nodosum is an acute, self-limiting panniculitis, and the two are separated by course, ulceration and histology.
How are tuberculids treated?
Tuberculids are treated with antitubercular therapy for the underlying tuberculosis, which often clears the skin lesions. Steroids may ease symptoms in selected cases, but they do not remove the source of the hypersensitivity reaction.