Tuberculosis (TB) is an infection primarily caused by the bacterium Mycobacterium tuberculosis, which most commonly affects the lungs. The disease can spread beyond the respiratory system, manifesting in other organs, including the skin. Skin involvement, known as Cutaneous Tuberculosis (CTB), is considered a rare form of extrapulmonary TB, accounting for only about one to two percent of all cases. Because CTB presentation is highly varied, anyone observing suspicious or persistent skin changes should seek immediate consultation with a qualified healthcare professional for accurate diagnosis and care.
Visual Appearance of Cutaneous TB
The most common form of CTB in areas where the disease is not widely spread is Lupus Vulgaris, a chronic and slowly progressive condition. It typically presents as firm plaques with a characteristic reddish-brown to violaceous color. When examined under pressure with a glass slide (diascopy), the lesions show a distinct yellow-brown hue often described as the “apple jelly” sign. These plaques often have an advancing edge and a central area of atrophy and scarring, commonly appearing on the head, neck, and extremities.
Another distinct presentation is Scrofuloderma, which arises from an infection spreading outward from an underlying structure. It begins as firm, painless, subcutaneous nodules, sometimes referred to as cold abscesses, frequently located over infected lymph nodes, especially in the neck. The overlying skin becomes reddish-blue or violaceous before the nodules soften and rupture. This rupture creates ulcers and draining sinuses that release a watery, purulent, or thick, cheese-like material (caseous discharge).
Tuberculosis Chancre represents a primary infection. It appears two to four weeks after the bacteria enters the skin through a small wound, initially as a reddish-brown papule or nodule. This lesion rapidly enlarges and ulcerates, forming a shallow, well-defined sore with a granular base, and is typically painless. Regional lymph nodes usually become enlarged and firm, completing the primary complex.
A rare but serious form is Acute Miliary TB of the skin, which indicates a massive, widespread dissemination of the bacteria throughout the body. The skin lesions are small, numerous, and scattered, often presenting as tiny, reddish papules or vesiculopustules, usually measuring between one and five millimeters. This form is associated with significant systemic illness and is most often seen in individuals with severely compromised immune systems.
Infection Pathways to the Skin
The Mycobacterium tuberculosis bacterium reaches the skin through three main routes, determining the resulting clinical form of CTB. Exogenous inoculation occurs when the bacteria are directly introduced through a break in the skin barrier, such as a cut or abrasion. This direct entry typically results in the formation of a Tuberculous Chancre.
Endogenous spread, specifically by contiguous extension, involves the infection moving directly from an existing focus to the overlying skin. Scrofuloderma is the classic result, occurring when an infection within a deep structure (like a lymph node or bone) burrows outward to the skin surface. This continuous involvement leads to the characteristic breakdown and draining sinuses above the underlying abscess.
The third pathway is hematogenous dissemination, where the bacteria travel through the bloodstream from a distant internal focus, such as the lungs. This route deposits bacteria in the skin, causing conditions like Lupus Vulgaris and Miliary TB. The patient’s immune status and the number of bacteria disseminated influence which specific form develops.
Tuberculids: Non-Infectious Skin Reactions
A distinct category of skin presentations related to TB are the Tuberculids, which are not true infections of the skin. Tuberculids are considered a delayed hypersensitivity or allergic reaction to mycobacterial antigens that have spread through the bloodstream from an active but often hidden infection elsewhere in the body. Crucially, viable M. tuberculosis bacteria cannot be isolated from these skin lesions, meaning the lesions themselves are sterile.
One common example is Erythema Induratum of Bazin, which typically presents as tender, deep-seated, blue-red nodules or plaques on the calves, most often affecting young to middle-aged women. These nodules can sometimes ulcerate and may leave behind scarring.
Another form is Papulonecrotic Tuberculid, characterized by recurring crops of small, dusky-red papules and nodules that develop central necrosis. These papules form small ulcers that heal with characteristic depressed, pockmark-like scars, commonly appearing on the extremities. The presence of a Tuberculid strongly suggests an underlying systemic TB focus, usually in the lungs or lymph nodes. Treating the systemic infection with anti-TB medication is necessary for the skin lesions to resolve.
Confirming a Diagnosis and Treatment Protocol
Confirming a diagnosis of Cutaneous TB requires clinical suspicion combined with laboratory testing. A skin biopsy is the gold standard, providing tissue for histopathology to look for characteristic granulomas. Molecular testing, such as Polymerase Chain Reaction (PCR), detects M. tuberculosis DNA directly from the skin tissue, offering a faster result. Mycobacterial culture is important for growing live bacteria and performing drug susceptibility testing, which guides treatment. Immunological tests (TST or IGRA) indicate prior exposure, and a chest X-ray checks for concurrent pulmonary disease.
The treatment for Cutaneous TB is the same as for active systemic TB, requiring a long-term, multi-drug regimen to eradicate the infection. The standard initial phase involves a combination of four drugs: Isoniazid, Rifampicin, Pyrazinamide, and Ethambutol. This intensive phase is typically followed by a continuation phase of two drugs for several more months, with the total duration for drug-sensitive CTB often lasting six months. Strict adherence to the complete course of medication is paramount to ensure a cure and prevent the development of drug-resistant strains.