What are the cutaneous markers of tuberculosis, including examples of immune-mediated and direct inoculation (primary and secondary) types?

Cutaneous Markers of Tuberculosis

Cutaneous tuberculosis manifests through two distinct pathogenic mechanisms: immune-mediated reactions (tuberculids) that occur without viable bacilli in the skin, and direct inoculation forms (both primary and secondary) where viable Mycobacterium tuberculosis organisms are present in cutaneous tissue. 1, 2

(1) Immune-Mediated Cutaneous Markers (Tuberculids)

These represent hypersensitivity reactions to M. tuberculosis antigens in patients with moderate to high tuberculin sensitivity, occurring without viable organisms in the skin lesions 1, 2:

Erythema Induratum of Bazin

• Presents as nodules and abscesses, typically on the posterior lower legs 1
• Represents a panniculitis pattern with strong immune response 1
• May require longer treatment duration than standard TB therapy, with adjuvants including dapsone, potassium iodide, doxycycline, and corticosteroids to control inflammation 1
• Occurs in patients with high degree of tuberculin sensitivity 1

Papulonecrotic Tuberculid

• Manifests as macules and papules with necrotic centers 1
• Represents a necrotizing vasculitis pattern 1
• Typically appears on extensor surfaces of extremities 1

Lichen Scrofulosorum

• Presents as small, grouped macules and papules 1
• Often appears on trunk in children with underlying tuberculous focus 1
• Represents a lichenoid tissue reaction pattern 1

Key distinguishing feature: Tuberculids are paucibacillary (few to no organisms), making acid-fast bacilli staining and culture typically negative from skin lesions, though patients have evidence of TB elsewhere 1, 2, 3

(2) Direct Inoculation - Primary Cutaneous Tuberculosis

This occurs when M. tuberculosis is directly introduced into previously uninfected skin of a non-sensitized host 1, 2:

Tuberculous Chancre (Primary Inoculation TB)

• Presents as erosions and ulcers at the inoculation site 1
• Develops a painless ulcer with undermined edges and granular base 1, 4
• Associated with regional lymphadenopathy (forming the primary complex analogous to pulmonary Ghon complex) 2, 4
• Occurs 2-4 weeks after inoculation in non-immune individuals 2
• Multibacillary form with organisms readily demonstrable 2
• Can present as chronic, non-healing wounds that mimic other conditions 4

Clinical pitfall: TB chancre is frequently misdiagnosed as other chronic wounds, particularly in refugees or patients from endemic areas, leading to delayed recognition and treatment 4. The case presentation can be atypical, especially in immunocompromised patients 3.

(3) Direct Inoculation - Secondary Cutaneous Tuberculosis

This occurs through reactivation, contiguous spread, or hematogenous/lymphatic dissemination in previously sensitized individuals with established immunity 1, 2:

Lupus Vulgaris (Most Common Form)

• Presents as patches and plaques with apple-jelly appearance on diascopy 1, 5
• Multiple morphologic variants: plaque, ulcerative, hypertrophic, vegetative, papular, and nodular forms 5
• Occurs in previously sensitized individuals with high degree of tuberculin sensitivity 5
• Paucibacillary with organisms difficult to demonstrate 2, 3
• Can present as multifocal lesions even in immunocompetent individuals 5

Scrofuloderma

• Develops from contiguous spread from underlying tuberculous lymph nodes, bones, or joints 1, 3
• Presents as nodules progressing to abscesses, then ulcers with undermined edges and sinus tracts 1, 3
• Drainage produces caseous material 3
• Multibacillary with organisms usually demonstrable 2, 3
• Most commonly affects cervical, supraclavicular, and axillary regions 2

Tuberculosis Verrucosa Cutis (Warty TB)

• Results from exogenous reinoculation in previously sensitized individuals 1, 2
• Presents as verrucous (warty) patches and plaques 1
• Typically on hands, fingers, knees, or buttocks 2
• Paucibacillary due to strong cell-mediated immunity 2

Orificial Tuberculosis

• Occurs through autoinoculation in patients with advanced internal TB (pulmonary, intestinal, or genitourinary) 1
• Presents as painful erosions and ulcers around body orifices (mouth, anus, genitalia) 1
• Multibacillary with numerous organisms present 2
• Indicates severe underlying disease with poor prognosis if untreated 2

Tuberculous Gumma (Metastatic Tuberculous Abscess)

• Results from hematogenous spread in immunocompromised patients 1, 3
• Presents as subcutaneous nodules and abscesses that ulcerate 1, 3
• Multibacillary form 2
• Often associated with other organ involvement 3

Acute Miliary Tuberculosis of Skin

• Occurs via hematogenous dissemination in severely immunocompromised patients 1, 3
• Presents as widespread macules, papules, vesicles, or pustules 1
• Multibacillary with organisms readily found 2
• Indicates life-threatening disseminated disease 2

Critical Diagnostic Considerations

Approximately one-third of cutaneous TB cases have associated systemic involvement, necessitating thorough evaluation for pulmonary and extrapulmonary disease 2. All patients require chest radiography and systemic assessment 2, 4.

Culture for M. tuberculosis should be performed in all suspected cases, even when acid-fast bacilli stains are negative, as histopathology varies by type and organisms may be sparse in paucibacillary forms 3. Culture confirmation may take 6-8 weeks 4.

Treatment follows WHO recommendations for new pulmonary TB cases (RIPE therapy: rifampin, isoniazid, pyrazinamide, ethambutol) for both true cutaneous TB and tuberculids, with Erythema induratum of Bazin potentially requiring extended duration and anti-inflammatory adjuvants 1, 4.