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In tropical regions, cutaneous tuberculosis is an endemic condition due to the high prevalence of TB in many developing countries. Tuberculosis in Western (c.q low endemic) countries is gradually declining and the WHO developed new guidelines for these countries towards elimination (http://www.who.int/tb/publications/elimination_framework/en/ )
The decline of tuberculosis in Western countries is influenced by immigration of persons from high endemic countries, HIV infection and the amount of MDR/XDR strains, although immigration is the most important contributing fact [1, 2]. Cutaneous tuberculosis remains rare in the West, representing 1.5 to 2.1 percent of all tuberculosis cases and ranks fifth on the list of extrapulmonary manifestations [3, 4].
Cutaneous TB is a skin infection usually due to bacteria of the Mycobacterium tuberculosis complex (including the four species M. tuberculosis, M. bovis, M. africanum, and M. microti)
All mentioned varieties are acid-fast bacilli (AFB)which indicates that they are not discoloured by alcohol after stained red with fuschin. The following determination methods are used:
– Direct light microscopic detection of AFB in specimens;
– Culture on Lowenstein-Jensen medium requiring at least 28 days;
– More recently, Nucleic Acid Amplification Tests (NAAT) have been developed with promising sensitivity and specificity, although large studies on the diagnostic value for cutaneous tuberculosis are currently lacking .
– Still experimental are mass spectrometry techniques to diagnose TB .
The advantage of the newer techniques like NAAT and mass spectrometry are their fast turn-around time (a few hours for NAAT, and in theory minutes for mass spectrometry) and the potential high specificity in pauci-bacillary TB forms.
– Cutaneous histology shows a characteristic inflammatory infiltrate with epithelioid cells and giant cell granuloma, sometimes with a caseating center, although this histopathological picture is not pathognomonic.
– The positivity of the tuberculin skin test (PPD-H) can form an indication for disease in a patient with symptoms suggesting TB.
– Interferon gamma release assay (IGRA) whole blood test measures a person’s immune reactivity to M. tuberculosis. White blood cells from most persons that have been infected with M. tuberculosis will release interferon-gamma (IFN-g) when mixed with antigens (substances that can produce an immune response) derived from M. tuberculosis.
Cave: A negative test (PPD-H/IGRA) does not exclude the disease.
The clinical forms of cutaneous tuberculosis are many and varied. Sub-classification is build upon 1) the bacterial load i.e. the number of bacteria infecting the patient (paucibacillary low load infection as opposed to a multibacillary high load infection), and 2) the mode of inoculation, extension and transmission, i.e.: by direct inoculation, continuous infection, or hematogenous dissemination (Table I). Clinically distinguishable entities are:
– Primary inoculation TB: seen in naïve patients.
– Scrofuloderma: gums, fistula in the skin originating from a suppurating underlying infected organ, i.e. a lymph node bone, joint, or hematogenous spread of a lung (seen in 5.8% of cutaneous TB cases in industrialised countries). In developing countries the prevalence is much higher (seen 72,5% of cutaneous TB cases in Morocco) [3, 4].
– Tuberculosis Periorificialis: by extension to the skin of an active infection or auto-inoculation of a pulmonary, digestive, genital or urinary focus.
– Acute Miliary Tuberculosis: seen in severely immunocompromised patients.
– Tuberculosis Verrucosa Cutis: also known as "lupus verrucosus" or "prosector’s wart", and "warty tuberculosis", expressing a re-inoculation by self-inoculation by external contact, (seen in 8.9% of cutaneous TB cases in industrialised countries).
– Lupus vulgaris: linked to the resurgence of latent tuberculous focus, characterized by a slowly progressive polycyclic skin eruption, "lupomes" located predominantly in the head and neck region. This is the most common form diagnosed in industrialized settings [7, 8]. By contrast in developing countries it occupies only a minority of cutaneous TB cases (12.9% in Morocco) .
In contrast to the true cutaneous tuberculosis mentioned above, the tuberculides should be noted and are different in aetiology . Tuberculids are sterile cutaneous reactions to a TB focus elsewhere in the body (4% of cutaneous TB cases in Morocco, as opposed to 95% in Hong Kong). They can be subdivided in the following forms: papular tuberculid, papulo-necrotic tuberculid or lichen scrofulosorum, Bazin erythema induratum (characterised by chronic nodules progressing to ulcerations on the lower limbs) . Finally, BCG vaccination can lead to local or generalized severe, sometimes fatal, septic complications.
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