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  1. Microbiology
    1. It is an acid and alcohol fast, obligate, intracellular, non cultivable mycobacterium, having special affinity for skin, nerves and muscle tissue. When stained with Ziehl- Nielsen’s method, the AFB appear as slender rod shaped bacteria 1.0-8.0 μm in length and 0.2-0.5 μm in width, with parallel sides and rounded ends placed singly or in masses (globi).
    2. The average percentage of morphologically normal (viable) i.e. solid staining or deeply staining bacilli, in smears is termed as "morphological Index" (MI).(becomes 0 in 6 weeks of therapy)
    3. Lepromin test was first described by Mitsuda and Hayashi and involves intradermal injection of autoclaved emulsified preparation of lepromatous tissue standardized according to its content of M. lepae. Usual concentration of AFB in integral lepromin is between 4x107 and 1.6 x 108 AFB/ml.
  2. Immunology
    1. Cell mediated immunity has an important role to play in the aetiopathogenesis of leprosy. Various possible defects in CMI in leprosy are proposed which include lack of circulating lymphocytes, induction of suppressor T-Cell (O K T8 + Phenotype) in LL; distribution of Helper-Suppressor cells in the lesion as M-leprae reactive T cells are more.
    2. Decrease in macrophage activity due to lysosomal defects, oxygen radicals, defective processing, presentation, defective interaction and aberrations in IL-2, interferon-g and transfer factor functions.
  3. Clinical Features
    1. Indeterminate Leprosy
      1. This represents the patients where leprosy is in very early stages of evolution.
      2. Macules in children are found on the trunk, buttocks, arms and thighs. Facial lesions are common in Africa.
      3. Hypopigmented ill defined macules are less than 5 cm in diameter, surface is smooth, normal hair growth, no change in texture, impaired sweat function, normal or blunted sensation, no nerve thickening are salient features. Skin smears are negative.
    2. Tuberculoid Leprosy (TT) (saucer right way up)
      1. Lesions may present as macules or plaques.
      2. Macular lesions may be single or few, well defined, asymmetrically distributed, found on face and dorsal aspects of extremities and trunk, hypopigmented to erythematous with dry surface due to anhidrosis and complete appendageal loss and anaesthesia. Nerve to the patch is enlarged. Lepromin test is strongly positive (3+).
    3. Borderline Tuberculoid Leprosy (BT)(satellite)
      1. Skin lesions morphologically resemble tuberculoid leprosy but the borders are indistinct.
      2. The lesions are 3-10 in number ,hypopigmented or erythematous with satellite lesions at the periphery, with sparsening of appendages, asymmetrically distributed hypo aesthetic patches. 
      3. Annular lesions may be present. Peripheral nerve enlargement is asymmetrical. Lepromin response is weakly positive (2+-1+).
    4. Borderline Leprosy (BB) (Annular lesions)
      1. Few patients fall in this category due to extremely unstable immunological status, and any enhancement or decline in cellular immune status results in down grading or reversal to BL or BT respectively. Lesions are pleomorphic consisting of macules, papules, plaques or a combination of them.
      2. They are multiple and bilateral yet asymmetrical with variable degree of hypoasthesia.
      3. Annular punched out, inverted saucer shaped lesions are characteristically present.
      4. In a patient down grading from BT, nerve enlargement is asymmetrical, whereas in a patient upgrading from BL, most peripheral trunks are affected.
      5. Slit skin smears are moderately (BI of 3+) and a lepromin test is negative.
    5.  Borderline Lepromatous Leprosy (BL)
      The lesions are numerous (more than 30) small, smooth and shiny, hypoaesthetic with normal appendages and tend to be asymmetrical though wide spread. Thickened peripheral nerve trunks tend to be symmetrical. Lepromin response is most often negative. Skin smear positivity with a BI of over 3 confirms diagnosis
    6. Lepromatous Leprosy (LL)
      1. Lepromatous leprosy is a multisystemic disease. There may be numerous, small (less than 2 cm), bilaterally symmetrical, smooth, shiny, ill defined macules, flesh coloured papules, plaques and nodules which are hypopigmented or slightly erythematous in colour that may coalesce. In dark skinned patients they appear coppery, which are better seen with oblique lighting.
      2. There is no perceptible sensory loss. Diffuse infiltration of the face leading to appearance of leonine facies can occur.
      3. Ear lobules are mostly infiltrated and skin becomes shiny and velvety. Lateral madarosis is present.
      4. In advanced disease, infiltration may involve the axillae, eyelids, midline of back, palms, groins, perineum and hairy scalp. There is symmetric peripheral nerve trunk involvement leading to glove and stocking anaesthesia.
      5. Lepromin tests is negative and slit skin smear may yield a BI of 6+.
      6. Histoid Leprosy It is a form lepromatous leprosy. The lesions are clinically flesh coloured to translucent dome shaped discrete succulent nodules present on normal looking skin. Pink nodules without infiltration on pinna, alae nasi, eyebrow, dorsum of fingers, either side of tendo-achlles are seen.
Signs of activity in leprosy
On skin patch Nerve Skin smear
•  Erythema
•  Increase in size of lesions
•  Increase in number of lesions
•  Appearance of new lesion
•  Satellite lesions
•  Thickening with tenderness
•  Increase in level of anaesthesia
•  Increase in number of nerve involvement
•  Increase in muscle weakness
•  New muscle paralysis
Positive for AFB
  1. Pure Neuritic Leprosy (PN)
    1. In this type no skin lesions are seen and peripheral nerves are thickened.  Majority of the cases are mononeuritic. Ulnar, lateral popliteal and median nerve are commonly involved. About 25% of patients complain of paraesthesia.
    2. Nerve biopsy is diagnostic which slows presence of AFB with lympho histocytic infiltrate. These lesions do not undergo ENL reaction. Neural histoid variant along the course of nerves has been reported.
Nerve Level of involvement
i) Ulnar
ii) Median
iii) Dorsal cutaneous branch of ulnar
iv) Common peroneal (lateral popliteal)
v) Superficial peroneal
vi) Posterior tibial
Elbow (Posteriorly)
Around fibular neck
Front of ankle
Just below medial malleoli

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