Cutaneous tuberculosis: A great imitator
Introduction
Tuberculosis (TB) is an ancient disease that is caused by Mycobacterium tuberculosis and mainly affects human beings. With the pandemic of HIV infection and the emergence of antimicrobial resistance against M tuberculosis, TB has reemerged in recent decades and represents a growing threat to public health and economic growth.1 According to the World Health Organization (WHO), TB is the ninth leading cause of death worldwide and the leading cause as a single infectious agent, ranking above HIV and AIDS.2 The incidence rates of TB vary widely among countries. Developing countries have the greatest burden of this disease, whereas China, India, and Indonesia alone accounted for 45% of global cases in the newly issued global TB report.2 Pulmonary TB is the most common type, accounting for more than 80% TB cases. About 20% of TB cases are extrapulmonary, including TB lymphadenitis, pleural TB, TB meningitis, osteoarticular TB, genitourinary TB, abdominal TB, cutaneous TB (CTB), ocular TB, TB pericarditis, and breast TB.[1], [3] The incidence of extrapulmonary TB has a much stronger relationship with HIV infection, emergence of multidrug-resistant TB, and immunosuppressive treatment.4 Only 1% to 2% of all extrapulmonary TB cases shows cutaneous involvement.[5], [6]
CTB is a relatively uncommon disease. The disease’s prevalence increased dramatically during the 17th and 18th centuries and declined in the 19th and early 20th centuries, as Bacillus Calmette-Guerin (BCG) vaccine and improved therapeutic approaches effectively reduced the incidence of TB worldwide. TB has become a major health concern again in recent decades, with CTB being a threat, especially in areas where TB prevalence, HIV infection, and multidrug-resistant TB remain high.
Section snippets
Pathogenesis
CTB was first described by Theophile Laennec (1781-1826) in 1826, before M tuberculosis bacillus was discovered and isolated by Robert Koch (1843-1910) in 1882.5 CTB is mainly caused by M tuberculosis bacillus, and rarely by M bovis or BCG.7 The main components of M tuberculosis are proteins, polysaccharides, and lipoids. The acute skin reaction to the tuberculin is caused by polysaccharides. Proteins are the most important antigens of M tuberculosis and can induce the T-cell immune response
Classification and manifestations
Classification of CTB is complex and inconsistent. Two primary categories are most widely accepted: true CTB and tuberculids (Table 1).[6], [15], [16], [17] If M tuberculosis can be found at the lesional sites with common testing tools such as smear, culture, or polymerase chain reaction (PCR) examination, it is defined as true CTB. True CTB can be further divided into:
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exogenous TB caused by inoculation (tuberculous chancre and TB verrucosa cutis)
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endogenous TB due to direct spread or
Treatment and prognosis
The antituberculous drugs isoniazid, rifamycin, pyrazinamide, and ethambutol are the first-line treatment for all types of CTB,68 following the standard regimen of WHO recommendation for the treatment of new cases of pulmonary TB.5 For true CTB, the intensive phase begins and lasts for 2 months, followed by the maintenance phase for 4 months. Drug doses used in the intensive phase for adults and teenagers are rifampicin (450 mg/day for <50 kg body weight and 600 mg/day for >50 kg body weight),
Conclusions
Among the classic great imitators, including syphilis and leprosy, CTB is becoming the foremost challenge to dermatologists in daily practice, not only due to the diversifying manifestations, multiple transmission pathways, perplexing pathogenesis, ambiguity in diagnosis, prolonged treatment course, and emergence of multiple drug resistance, but also due to the global migration and increasing use of immune targeting treatment in patients with malignancies and autoimmune or chronic inflammatory
Acknowledgments
Baiyu Zhong, MD, Zhifang Zhai, MD, and Yi You, MD (Department of Dermatology and Venereology, Southwest Hospital, Army Medical University, Chongqing) assisted in collecting clinical data and providing patient pictures.
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