Keywords

Introduction

The “gold standard” for the diagnosis of an infectious condition is demonstration and culture of the microorganism. M. leprae is usually not identified in paucibacillary cases and is not cultivable. The search for another diagnostic tool is fulfilled by histopathology which not only provides enough diagnostic clues but also helps in classifying the disease as per five-part Ridley-Jopling classification. Slit skin smear is a fact technique to demonstrate acid-fast bacilli (AFB), but its usefulness is limited in clinical practice.

Slit Skin Smear (SSS) [1, 2]

  • Tissue fluid from the skin lesion and sites with expected high bacterial load (e.g., earlobe) is taken on a glass slide and is dried and stained with Ziehl-Neelsen stain or Fite stain (Fig. 3.1a, b).

  • The number of bacteria is counted under light microscopy at high magnification with oil immersion, and the bacterial index is calculated as per Ridley’s logarithmic scale.

  • The sensitivity and, hence, diagnostic usefulness depend on the bacterial load and are low in paucibacillary cases.

  • Hence, the clinical utility of SSS is limited, as SSS will be positive in multibacillary cases where clinical diagnosis itself is obvious, but it would not help in paucibacillary cases where clinical diagnosis is doubtful and requires diagnostic support.

Fig. 3.1
A set of two microscopic images named A and B. Image A is the microscopic image of a slide with M leprae. The organisms can be observed as rod-shaped structures which are densely stained. Image B is the microscopic image of a slide with clumps of M leprae. It has a lobular appearance and is darkly stained.

(a) M. leprae is seen as rod-shaped, bright red colored bacilli (Ziehl-Neelsen, ×1000). (b) Clumps of M. leprae (globi) (Ziehl-Neelsen, ×1000)

Full size image

Histopathology

For diagnostic purposes, deep biopsy including the complete dermis from the most active part of lesion should be obtained. Tuberculoid leprosy is characterized by well-formed granulomas, often perineural in location and assuming a serpentine shape. On the other hand, in the lepromatous spectrum, there is a diffuse infiltration of histiocytes laden with bacilli throughout the dermis, sparing the upper papillary dermis (grenz zone). However, in several cases, the clinical features do not correlate with the histological findings. Clinically, tuberculoid lesions may show lepromatous features histologically, and vice versa. Further the histological features and bacteriological index may vary from lesion to lesion, even within the same patient. Clinicopathological correlation is essential. The salient features of different poles of leprosy are summarized in Table 3.1 and are discussed below. While H&E stain provides details of infiltration and architectural changes, bacilli are visualized on Wade-Fite stain or Fite-Faraco stain [3, 4].

Table 3.1 Histopathological classification of leprosy
Full size table

The salient histopathological features of leprosy are discussed below [5, 6, 7].

Tuberculoid Leprosy (TT)

  • Multiple, well-formed, noncaseating granulomas in the dermis rimmed by lymphocytes.

  • Granulomas are composed of epithelioid histiocytes and some multinucleate Langhans giant cells (Fig. 3.2a–c).

  • Caseous necrosis is rare but may be seen in granulomas involving the nerve.

  • Granulomas originate in perineural locations and expand to involve other areas.

  • No grenz zone. Granulomas may erode the undersurface of the epidermis.

  • AFB not seen.

  • When visualization of nerves on H&E staining is difficult, S-100 immunostaining would help.

  • Granulomas may extend to and damage arrector pili muscles, hair follicles, and sweat glands.

Fig. 3.2
A set of three microscopic images named A, B, and C. Image A depicts the skin section, with the outer epidermal layer thinned out, a granuloma can also be observed inside the layers. B is the image of a densely stained epithelioid granuloma. C is the image with Langhans giant cell histocytes and epithelioid cells stained.

(a) Epidermal thinning with well-formed epithelioid granuloma (H&E ×100). (b) Well-formed, elongated epithelioid granuloma (H&E ×400). (c) Langhans giant cell, histiocytes, and epithelioid cells in granuloma (H&E ×400)

Full size image

Borderline Tuberculoid Leprosy

  • Grenz zone seen (Fig. 3.3a–c).

  • Well-formed granulomas.

  • Lymphocytes are less than those in TT.

  • Swollen nerve bundle may be identified within the granuloma.

  • BI is 0–2+.

Fig. 3.3
A set of three microscopic images named A, B, and C. A is the image of skin section, the epidermal layer thinned, a granuloma can also be observed within. B is the image of a granuloma which is oblong shaped, cells can be observed in the margin. C is the image of a well defined granuloma, cells can be observed at the center, and along the margin.

(a) Epidermal thinning, grenz zone, and well-formed granuloma reaching up to deeper dermis (H&E ×100). (b) Oblong-shaped well-formed granuloma with lymphocytes at the margin (H&E ×400). (c) Well-formed granuloma with histiocytes and epithelioid cells in the center and lymphocytes at the margin (H&E ×400)

Full size image

Borderline Leprosy

  • Epithelioid cells present in a diffuse manner.

  • Lymphocytes are present diffusely.

  • Langhans giant cells usually not present.

  • Nerve bundles identified—Schwann cell proliferation. Perineural fibrosis may be noted.

  • BI is 2–3+.

Borderline Lepromatous Leprosy

  • Foamy macrophages with granular cytoplasm diffusely present (Fig. 3.4a, b).

  • Lymphocytes variable—present around the nerves.

  • Grenz zone.

  • BI is 2–4+.

Fig. 3.4
A set of two microscopic images named A and B. A is the image of macrophages traversing vertically through the dermal layer. B is the mage of a skin section, with cells within.

(a) Foamy macrophages infiltrating the dermis (H&E ×100). (b) Sheets of foamy macrophages (H&E ×400)

Full size image

Lepromatous Leprosy

  • Grenz zone.

  • Sheets of foamy histiocytes containing numerous AFB. Globi seen. Presence of numerous bacilli gives a grayish tinge to the cytoplasm on H&E stain (Fig. 3.5a–c).

  • Nerve bundles lack cellular infiltrate but may show damage.

  • BI 5+.

Fig. 3.5
A set of five microscopic images named from A to E. A is the section of the Grenz zone with cells in the top and mid regions. B is the image of macrophages lining the surface. C is the image with a higher magnification. Image D is the skin section with rod-shaped organisms within. And image E is the section with higher magnification.

(a) Grenz zone and foamy macrophages in the upper and mid dermis (H&E ×40). (b) Foamy macrophages with interspersed lymphocytes (H&E ×100). (c) Higher magnification (H&E ×400). (d) Rarely, acid-fast lepra bacilli seen as blue-gray intracytoplasmic mass (H&E ×400). (e) Higher magnification (H&E ×400)

Full size image

Indeterminate Leprosy

  • Sparsely perivascular, perineural, and periadnexal infiltrate of lymphocytes and a few histiocytes.

  • Bacilli are usually absent. AFB may be found when a dermal nerve is followed on serial sections.

Histoid Leprosy

  • Atrophic epidermis.

  • Grenz zone may be seen.

  • Circumscribed nodular mass in the dermis consisting of spindle cells filled with bacilli. Cells are arranged in a storiform pattern. Central necrosis may be seen.

  • Pseudocapsule may be noted.

Pure Neural Leprosy

  • Epithelioid granulomas, mononuclear cell infiltrate, perineural/subperineural edema, fibrosis, and a decrease in myelinated fibers.

  • Immunohistochemistry using antibodies against lipoarabinomannan and phenolic glycolipid 1 (PGL-1).

Type 1 Reaction

Upgrading Reaction

  • Granulomatous destruction of the nerves and dermal edema.

  • Increased granulomatous organization, increased lymphocytes, and an increased number of multinucleated giant cells.

Downgrading Reaction

  • Macrophages replace lymphocytes and epithelioid cells.

Type 2 Reaction (Erythema Nodosum Leprosum)

  • Superimposed upon the chronic inflammation of lepromatous leprosy.

  • Dermal neutrophils with vasculitis.

  • In older lesions, neutrophils may not be found.

  • Predominantly lobular panniculitis may be noted.

Resolution [8]

Resolution of histopathological changes in leprosy is seen after treatment or, sometimes, even without treatment. The fate however differs in tuberculoid and lepromatous poles.

  • In tuberculoid poles, the granuloma usually disappears within a year. Nonspecific lymphocytic infiltration may be the only residual change.

  • In lepromatous lesions, degenerative changes cause lipid accumulation, reflecting as foamy changes. These foamy residues resolve very slowly and may persist for years.