Tuberculosis (TB) is a mycobacterial infection most commonly caused by Mycobacterium tuberculosis, but it can also be caused by Mycobacterium bovis and the bacillus Calmette-Guérin (BCG) vaccine (an attenuated M bovis strain). TB is distributed worldwide, and it is estimated that one-third to one-fourth of the world's population is infected (most without apparent disease). Most cases are reported from Southeast Asia (43%), Africa (25%), and the Western Pacific (18%), with over 50% of cases found in India, China, Indonesia, the Philippines, and Pakistan. Global incidence is on the decline since 2014, yet there are still an estimated 10 million new infections per year in 2020. TB remains a top-10 cause of death worldwide, accounting for 1.4 million deaths in 2020.

Risk factors for TB include poverty, being undomiciled, unemployment, crowding, and HIV infection. Approximately 8% of infections are seen in individuals infected with HIV; the risk of active TB in HIV-infected patients is 3%-8% per year with a lifetime risk of over 50%. Children are most susceptible to active TB with exposure, with 60%-80% developing active TB after exposure.

Cutaneous tuberculosis (CTB) is exceedingly rare, accounting for 1.5% -3% of extrapulmonary TB cases (8%-24% of TB infection is extrapulmonary). Approximately 1 out of 3 patients with CTB has systemic involvement with TB. CTB is more common in children, pregnant individuals, and HIV-infected persons.

CTB can be subclassified into 4 categories depending on route of infection: exogenous, endogenous, tuberculid, and vaccine-related.

Exogenous sources: Primary inoculation TB occurs at sites of trauma, abrasions, or wounds, or at mucous membranes.

• In patients with no immunity against TB, a scab or chancre forms at the wound site and can be associated with regional lymphadenopathy after several weeks. Lymph nodes can eventually suppurate, abscesses can form, and fever and pain can develop.
• In previously sensitized individuals, primary inoculation can lead to tuberculosis verrucosa cutis (warty tuberculosis) and is not usually associated with systemic symptoms.

Endogenous spread (hematogenous, lymphatic, or contiguous spread to skin):

• Scrofuloderma results from involvement of skin overlying a contiguous TB focus, usually a lymph node, bone, joint, or lacrimal gland. It is the most common form of CTB in children.
• Orificial TB is a rare mucosal autoinoculation TB from pulmonary, genitourinary, or intestinal disease shedding organisms that seed the adjacent orifices.
• Lupus vulgaris results from hematogenous spread and is a progressive form of CTB seen in individuals with prior sensitivity and low immunity.
• Tuberculous gumma (metastatic tuberculous abscess) is a rare form of hematogenous TB.
• Acute miliary TB is a rare but life-threatening form of hematogenous TB seen in patients with low immunity.

Tuberculids: Tuberculids are presumed to be a hypersensitivity or "id reaction" to the TB organism and occur in patients with high immunity. There are 3 forms:

• Micropapular (lichen scrofulosorum)
• Papular (papulonecrotic tuberculid)
• Nodular (erythema induratum) – Characterized by tender erythematous nodules in the posterior legs. Lesions are typically asymptomatic but may be painful. Localized adenopathy may occur.

Tuberculosis due to BCG vaccination: Lupus vulgaris, scrofuloderma, erythema induratum, papulonecrotic tuberculids, and lichen scrofulosorum have all been described.

All forms of CTB, including tuberculids, require systemic treatment. Without treatment, lesions can persist for months to years. The prognosis of miliary TB is often poor.

Cutaneous manifestations of TB can also be broadly categorized into multibacillary versus paucibacillary forms (based on bacillary load).

Paucibacillary forms have a sparse number of bacteria:

• Lupus vulgaris
• TB verrucous cutis
• Tuberculids

Multibacillary forms of cutaneous TB with high bacillary load include:

• Scrofuloderma
• Orificial TB
• Acute miliary TB
• TB gumma
• Tuberculosis chancre