Cutaneous Tuberculosis (Cutaneous TB) — Complete MD Exam Notes

1. Introduction

2. Pathogen

• Mycobacterium tuberculosis — aerobic, acid-fast bacillus (AFB)
• M. bovis — from infected animals/unpasteurized milk
• BCG strain — following BCG vaccination (rare, mostly in immunocompromised)

3. Pathogenesis / Routes of Infection

4. Classification

A. TRUE CUTANEOUS TB (Organisms Present in Skin)

1. Primary Inoculation Tuberculosis (Tuberculous Chancre)

• Occurs in: Previously uninfected individuals (primary infection)
• Route: Direct inoculation (cuts, abrasions — laboratory workers, pathologists, medical personnel)
• Features:
  • Chancre: Painless, indurated papule → ulcer at inoculation site (face, hands, feet)
  • After 3–8 weeks: Regional lymphadenopathy ("Primary complex" = chancre + lymphadenopathy)
  • Heals spontaneously in most, may disseminate
• Histology: Initially non-specific inflammation → later caseating granulomas
• AFB: Sometimes positive in lesion, usually positive in nodes

2. Tuberculosis Verrucosa Cutis (Warty TB)

• Occurs in: Previously sensitized individuals (re-infection)
• Route: Exogenous inoculation
• At risk: Pathologists, butchers, laboratory workers, children (playing barefoot in endemic areas)
• Features:
  • Begins as small, painless, hyperkeratotic papule
  • Evolves into verrucous (warty), irregular plaque with a "pebbly" or papillomatous surface
  • Site: dorsum of hands, fingers, feet, ankles
  • No lymphadenopathy (sensitized host)
  • Indolent; may persist for years
• Old names: Prosector's wart, Butcher's wart, Anatomist's wart
• Histology: Pseudoepitheliomatous hyperplasia, caseating granulomas in dermis, AFB rare
• Mantoux: Strongly positive

3. Lupus Vulgaris

• Most common form of CTB in India
• Occurs in: Previously sensitized, high-immunity individuals
• Route: Hematogenous spread; also direct extension from lymph nodes/bones; or post-primary inoculation
• Features:
  • Most commonly on head and neck (nose, cheek — "butterfly" distribution), also extremities
  • Begins as soft, reddish-brown papules → coalesce into plaques
  • Pathognomonic sign — Apple-jelly (Lupoid) nodules: Yellowish-brown translucent nodules visible on diascopy (pressing glass slide on skin)
  • Central scarring with active spreading periphery
  • Active edge + healing periphery can coexist (see image below)
  • Slow, relentlessly progressive — can persist for decades
  • Complications: Destruction of nasal cartilage, ectropion; squamous cell carcinoma can develop in scars (Marjolin's ulcer)
• Histology: Non-caseating or minimally caseating tuberculoid granulomas; Langerhans giant cells; AFB very rare
• Mantoux: Strongly positive

Large variegated plaque showing active erythematous border (double arrows) and healing/scarring periphery (single arrow) — characteristic of lupus vulgaris

4. Scrofuloderma

• Occurs in: Children and young adults
• Route: Contiguous spread from underlying infected lymph node (cervical most common), bone, or joint
• Features:
  • Initially: Firm, painless, non-tender subcutaneous nodule overlying infected node (usually cervical, axillary, or inguinal)
  • Nodule softens → skin becomes bluish-red, adherent, fluctuant
  • Breaks down to form irregular ulcers with undermined, "collar-stud" edges, and sinuses discharging watery pus/caseous material
  • Heals with cord-like, bridge scars (pathognomonic)
• Histology: Caseating necrosis, granulomas; AFB often found
• Mantoux: Usually positive
• DDx: Actinomycosis, atypical mycobacteriosis, syphilitic gumma, hidradenitis suppurativa

5. Orificial Tuberculosis (Tuberculosis Cutis Orificialis)

• Rarest and most severe form
• Occurs in: Severely immunocompromised with advanced internal TB
• Route: Autoinoculation — organisms from internal lesions (pulmonary, GI, genitourinary TB) shed into body orifices
• Features:
  • Sites: Around oral cavity (tongue, soft palate — from pulmonary TB), perianal region (from GI/GU TB), genitalia
  • Painful, punched-out ulcers with undermined edges, yellowish base; may have "soft palate perforations"
  • Poor prognosis; indicates terminal disease
• Histology: Caseating granulomas; AFB readily found
• Mantoux: May be negative (anergy due to severe immunosuppression)

6. Acute Miliary Tuberculosis Cutis

• Occurs in: Severely immunocompromised (infants, HIV, post-measles)
• Route: Hematogenous dissemination from primary focus
• Features: Multiple tiny papules, vesicles, or pustules — widespread over trunk; poor prognosis
• Histology: Non-specific initially; AFB may be found; granulomas form later

7. Metastatic Tuberculous Abscess (Tuberculous Gumma)

• Occurs in: Immunocompromised (poor host immunity)
• Route: Hematogenous spread from primary focus (often during latent TB reactivation)
• Features:
  • Cold, painless, fluctuant subcutaneous abscess without signs of acute inflammation
  • "Cold abscess" — may be single or multiple
  • Spontaneously ruptures → sinus formation
• Sites: Trunk, extremities
• AFB: Usually found in aspirate

B. TUBERCULIDS (Immunological / Hypersensitivity Reactions)

1. Lichen Scrofulosorum

• Most common tuberculid
• Age: Children and adolescents
• Features:
  • Asymptomatic, lichenoid, minute (1–3 mm) perifollicular or periductal papules
  • Grouped in discoid or annular plaques
  • Sites: Trunk (especially chest, abdomen)
  • Spontaneously resolves
• Histology: Superficial perifollicular/periductal granulomas; no caseation; no AFB
• Mantoux: Strongly positive
• Association: Usually with underlying lymph node or bone TB

2. Papulonecrotic Tuberculid

• Features:
  • Recurrent, symmetrical, necrotic papules and pustules
  • Heal with depressed, "punched-out" scars
  • Sites: Extensor surfaces of extremities (elbows, knees, buttocks)
• Histology: Obliterative vasculitis, wedge-shaped necrosis; no AFB
• Mantoux: Positive
• DDx: Pityriasis lichenoides, leukocytoclastic vasculitis

3. Erythema Induratum of Bazin

• Features:
  • Recurrent, tender, indurated erythematous nodules and plaques
  • Ulcerate and leave atrophic scars
  • Sites: Posterior calves (calves of legs — "stockings area")
  • Predominantly in middle-aged women
  • Associated with cold weather (cold panniculitis)
• Pathology: Lobular panniculitis with vasculitis and granulomas
• Note: When no TB link found → called "Nodular vasculitis"
• Mantoux: Positive; MTB-PCR may be positive in lesion

4. Rosai-Dorfman Disease (associated in some classifications)

5. Summary Comparison Table

6. Diagnosis

A. Clinical

• Site, morphology, epidemiology, contact history, BCG scar

B. Mantoux (Tuberculin Skin Test)

• Positive in most forms (strongly positive in tuberculids, lupus vulgaris, TB verrucosa)
• Negative (anergy) in orificial TB and miliary TB (immunocompromised)

C. Laboratory

D. Histopathology (Key for Exam)

• Caseating granuloma: Epithelioid cells + Langerhans giant cells + lymphocytes surrounding central caseous necrosis
• Non-caseating granuloma: Seen in lupus vulgaris, tuberculids (high immunity)
• "Apple-jelly" correlate: Tuberculoid granulomas in upper dermis

7. Differential Diagnosis

8. Treatment

Anti-Tubercular Therapy (ATT)

• In lupus vulgaris and severe/extensive disease: may extend continuation phase to 7 months (total 9 months)
• Tuberculids: Usually respond to 6-month regimen

Adjunctive / Surgical

• Surgical excision for localized TB verrucosa cutis (small lesions)
• Drainage for cold abscesses (scrofuloderma, tuberculous gumma)
• Reconstructive surgery for disfiguring scars (post-treatment)

Monitoring

• Clinical response in 4–8 weeks
• LFTs at baseline and monthly (rifampicin/INH hepatotoxicity)
• Visual acuity/color vision (ethambutol — optic neuritis)

9. Complications

10. High-Yield Points for MD Exam

1. Most common CTB in India = Lupus vulgaris
2. Pathognomonic sign of LV = Apple-jelly nodules on diascopy
3. Warty TB = TB verrucosa cutis (prosector's wart — occurs in sensitized)
4. Bridge-like/cord scars = Scrofuloderma
5. Mantoux negative in CTB = Orificial TB (anergy), Miliary TB
6. No organisms in skin = Tuberculids
7. Most severe form = Orificial TB (terminal disease)
8. Cold abscess = Scrofuloderma or Tuberculous gumma
9. Posterior calf ulcerating nodules in women = Erythema induratum of Bazin
10. PCR target gene = IS6110 of M. tuberculosis
11. Gold standard culture medium = Lowenstein-Jensen (L-J) medium
12. Complication of LV = SCC in scar
13. Treatment = 2HRZE + 4HR (6 months total)
14. IGRA not affected by BCG vaccination (unlike Mantoux)