Lupus Vulgaris

01Pathophysiology

Lupus vulgaris is the most common form of cutaneous tuberculosis in previously sensitized (tuberculin-positive) individuals. It occurs in patients who have moderate to high cell-mediated immunity against M. tuberculosis. This immunological context is central to understanding both the clinical appearance and the diagnostic challenge of this disease.

The organism reaches the skin through several routes. Hematogenous dissemination from a pulmonary or extrapulmonary focus is the most frequent mechanism. Alternatively, the bacilli can arrive via lymphatic channels, by direct extension from underlying infected structures such as lymph nodes, bones, or joints, or by contiguous spread from a pre-existing cutaneous TB lesion such as scrofuloderma or a primary inoculation site. In rare cases, lupus vulgaris develops at the site of a BCG vaccination scar.

Because the host immune response is relatively robust, the bacillary load in the tissue is extremely low, making this a paucibacillary infection. The immune system mounts a vigorous granulomatous response characterized by epithelioid histiocytes, Langerhans multinucleated giant cells, and a peripheral cuff of lymphocytes. Central caseous necrosis may be present but is often minimal. This granulomatous infiltration causes the characteristic dermal nodules and the progressive, destructive course of the disease.

The reason the patient develops the classic brownish-red, soft plaques is the dermal granulomatous infiltration itself. When you compress the overlying vasculature with a glass slide (diascopy), the reddish hue blanches and the underlying granulomas become visible as translucent brownish-yellow "apple jelly" nodules. The softness of the granulomatous tissue also explains why a blunt probe can be pushed through the lesion with minimal resistance, producing the positive probe sign.

The chronic, smoldering nature of the inflammation leads to progressive tissue destruction over years, particularly of cartilage (nose, ears). This same chronic inflammation also places the patient at risk for squamous cell carcinoma arising within a long-standing lupus vulgaris lesion, a concept analogous to malignant transformation in any chronic wound or scar (Marjolin-type ulceration).

02Classification and Clinical Manifestation

2A. Classification of Cutaneous TB (Contextual Placement)

Understanding where lupus vulgaris sits within the broader classification of cutaneous tuberculosis is tested frequently. The table below shows the full spectrum:

The key concept: lupus vulgaris occupies a high-immunity, endogenous position. This distinguishes it from orificial TB and metastatic TB abscess (both low immunity) and from tuberculosis verrucosa cutis (also high immunity but exogenous inoculation).

2B. Morphological Variants of Lupus Vulgaris

The plaque form on the face is the presentation most commonly described in vignettes. The ulcerative and mutilating form is the variant that test writers use to emphasize the progressive, destructive nature and the risk of complications such as secondary SCC.

03Diagnostic Workup

The best initial step in any exam vignette is recognition of the clinical presentation. A slowly enlarging, painless, reddish-brown plaque on the face of a patient from a TB-endemic area should immediately prompt you to perform diascopy. The visualization of apple jelly nodules at the bedside is essentially pathognomonic and is what the vignette will describe or expect you to identify.

However, the most accurate (confirmatory) test is a skin biopsy with histopathological examination. The biopsy reveals well-formed granulomas composed of epithelioid cells, Langerhans multinucleated giant cells, and a variable amount of caseous necrosis, surrounded by a lymphocytic infiltrate. It is critical to understand that AFB staining of the biopsy specimen is usually negative because lupus vulgaris is paucibacillary. A negative AFB stain does NOT rule out the diagnosis. This is a frequently tested concept.

The Mantoux test is strongly positive and supports the diagnosis, but a positive Mantoux alone does not confirm lupus vulgaris because it only indicates prior sensitization to M. tuberculosis, which is common in endemic populations.

Mycobacterial culture is the gold standard for organism identification in tuberculosis generally, but in lupus vulgaris its sensitivity is low due to the very small number of organisms in the tissue. Culture results take 6 to 8 weeks and are often negative. PCR for M. tuberculosis DNA is increasingly valuable because it offers rapid confirmation even in paucibacillary disease and is the best molecular adjunct when smear and culture fail.

A chest radiograph should be obtained in every patient to evaluate for concurrent pulmonary tuberculosis, even if the patient is asymptomatic.

04Management and Treatment

The treatment of lupus vulgaris follows the standard WHO anti-tuberculosis regimen, identical to the treatment of pulmonary TB. The regimen is abbreviated as 2HRZE / 4HR, meaning four drugs for the first 2 months (intensive phase) followed by two drugs for the remaining 4 months (continuation phase).

During the intensive phase, all four drugs work synergistically. Isoniazid is the most potent bactericidal agent against actively multiplying bacilli. Rifampicin provides sterilizing activity by killing semi-dormant "persisters." Pyrazinamide targets intracellular bacilli within acidic macrophage environments. Ethambutol serves as a companion drug that prevents the emergence of resistance, particularly to Isoniazid.

During the continuation phase, Isoniazid and Rifampicin are continued for 4 additional months to eliminate remaining bacilli and reduce the risk of relapse.

Treatment response in cutaneous TB is monitored clinically. The lesion should show progressive flattening, softening, and reduction in size over weeks to months. Some authorities recommend extending treatment to 9 to 12 months if clinical response is slow or in cases with extensive tissue involvement, though 6 months remains the standard recommendation per WHO guidelines.

Contraindications and precautions to be aware of:

• Pregnancy: Pyrazinamide is generally avoided (safety data limited); Streptomycin is contraindicated (ototoxicity to fetus). The regimen in pregnancy typically consists of Isoniazid + Rifampicin + Ethambutol for 9 months.

• Hepatic disease: All three first-line agents (H, R, Z) carry hepatotoxic potential. In pre-existing liver disease, Pyrazinamide is the first drug dropped, followed by Isoniazid if necessary. Close LFT monitoring is mandatory.

• Renal impairment: Ethambutol and Pyrazinamide require dose adjustment. Isoniazid and Rifampicin are hepatically metabolized and are generally safe.

• Rifampicin interactions: Rifampicin is a potent CYP450 inducer. It reduces the efficacy of oral contraceptives, warfarin, antiretrovirals (protease inhibitors), and many other drugs. This is a commonly tested pharmacology point.

The next best step logic: if a vignette presents a patient with confirmed lupus vulgaris, the answer is to start standard four-drug anti-TB therapy. Do not wait for culture results. If the vignette presents a patient already on treatment who develops jaundice, the next step is to stop all hepatotoxic drugs, check LFTs, and reintroduce drugs sequentially once liver function normalizes (Rifampicin first, then Isoniazid, then Pyrazinamide last).

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with lupus vulgaris questions is the negative AFB stain. Students see a granulomatous facial lesion in a patient from a TB-endemic area, the biopsy shows caseating granulomas, but the AFB stain comes back negative. Many students hesitate to diagnose cutaneous TB when the stain is negative, or they pivot incorrectly to sarcoidosis or a fungal infection. Remember: lupus vulgaris is paucibacillary, so a negative AFB stain is expected, not a reason to abandon the diagnosis. The combination of caseating granulomas, strongly positive Mantoux, and characteristic clinical findings (apple jelly nodules) is sufficient.

A second trap involves confusing lupus vulgaris with sarcoidosis (lupus pernio). Both cause chronic granulomatous facial plaques, and both names contain the word "lupus," which adds to the confusion. The discriminator is the type of granuloma: caseating points to TB, non-caseating points to sarcoidosis. Also, sarcoidosis typically shows Mantoux anergy (negative tuberculin test due to sequestration of T-cells in granulomas), whereas lupus vulgaris shows a strongly positive result.

A third trap involves the malignant transformation potential. If a vignette describes a patient with a decades-old lupus vulgaris lesion that suddenly develops a rapidly growing, hard nodule within the plaque, the answer is biopsy to evaluate for squamous cell carcinoma. Students sometimes miss this because they remain anchored on the TB diagnosis.

The core competency being tested across these questions is the ability to recognize a chronic granulomatous facial lesion in the correct epidemiological context, correctly interpret a negative AFB stain in the setting of caseating granulomas, distinguish lupus vulgaris from sarcoidosis and other facial granulomatous diseases, and initiate appropriate anti-TB therapy without waiting for culture confirmation.