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PA19.1-6 | Tuberculous Lymphadenitis — Pathology & Specimen — Part 2

Microscopic Features: Reading the Slide

A four-panel pathology diagram shows low, intermediate, and high-power microscopic features of tuberculous lymphadenitis, including caseous necrosis, epithelioid granulomas, Langhans giant cells, and lymphocytic cuffing. — **Panel A:** Low-power lymph node section with effaced architecture, absent/blurred follicles and sinuses, pale caseous necrosis, granulomatous cellular zones, and an inset showing partial architecture preservation in early disease.. **Panel B:** Intermediate-power confluent caseating epithelioid granuloma with central caseous necrosis, epithelioid histiocyte mantle, Langhans giant cells, lymphocytic cuff, and fused adjacent granulomas.. **Panel C:** High-power diagnostic zone showing epithelioid histiocytes with pale eosinophilic cytoplasm, elongated folded kidney-shaped vesicular nuclei, indistinct borders, syncytial sheets, and Langhans giant cell with peripheral horseshoe nuclei.. **Panel D:** Comparison of Langhans giant cell with peripheral horseshoe or wreath nuclear arrangement versus foreign-body giant cell with central randomly distributed nuclei..

This is the core of PA19.5. When you examine a lymph node section for suspected TB, look systematically at four zones — low power first, then high power.

Low power (4× or 10×):
You will see the normal lymph node architecture effaced — the usual follicles and sinuses are replaced by pale, amorphous areas (caseous necrosis) surrounded by cellular zones. In early disease, architecture may be partially preserved.

Intermediate power (10×):
Around each pale necrotic zone, a cellular mantle of epithelioid histiocytes and giant cells forms the granuloma wall. In lymph nodes, multiple adjacent granulomas typically fuse, forming confluent caseating epithelioid granulomas rather than discrete compact follicles (the latter are more typical of early pulmonary TB).

High power (40×) — the diagnostic zone:

Epithelioid histiocytes: Large cells with abundant, pale eosinophilic cytoplasm. Nuclei are elongated, folded, or kidney-shaped (vesicular chromatin — open, not condensed). Cells lie in sheets, borders indistinct (syncytial arrangement).

Langhans giant cells: Multinucleate cells with 5–50 nuclei arranged in a horseshoe (peripheral/wreath) pattern at the cell margin. Cytoplasm is abundant and pale. Do not confuse with foreign-body giant cells (nuclei central and randomly distributed).

Caseous necrosis centre: Pink, granular, structureless material — no nuclei, no cell outlines. May show dystrophic calcification in older lesions.

Lymphocytic cuff: CD4+ T lymphocytes ring the outer margin of the granuloma (small, dark nuclei).

Fibrous reaction: In older lesions, a fibrous capsule or collar encircles the granuloma; may show hyalinisation.

High-power microscopic view showing Langhans giant cell with horseshoe nucleus arrangement, surrounded by epithelioid histiocytes and lymphocytic cuff in tuberculous lymphadenitis — **Panel A:** Langhans giant cell with horseshoe nucleus arrangement, multinucleated giant cell cytoplasm, nuclear peripheralization. **Panel B:** Epithelioid histiocytes with elongated nuclei, pale pink cytoplasm, cell borders, histiocyte clustering pattern. **Panel C:** Lymphocytic cuff with small lymphocytes, caseous necrosis zone, granuloma-necrosis interface, inflammatory cell infiltrate.

SELF-CHECK

On a high-power H&E section of a lymph node, you see a large multinucleate cell with nuclei arranged in a peripheral horseshoe pattern. The surrounding cells have folded kidney-shaped nuclei and pale syncytial cytoplasm. Centralmost is pink structureless material. This picture is MOST consistent with:

A. Suppurative inflammation with central necrosis

B. Sarcoidosis — non-caseating granuloma with Schaumann bodies

C. Tuberculous lymphadenitis — caseating epithelioid granuloma

D. Diffuse large B-cell lymphoma with geographic necrosis

Reveal Answer

Answer: C. Tuberculous lymphadenitis — caseating epithelioid granuloma

The triad — Langhans giant cell (horseshoe nuclei peripherally), epithelioid histiocytes (kidney-shaped folded nuclei, pale syncytial cytoplasm), and central caseous (structureless pink) necrosis — is pathognomonic for TB. Sarcoidosis produces tight epithelioid granulomas but without caseous necrosis (non-caseating). DLBCL shows large lymphoid cells with prominent nucleoli, not granuloma formation. Suppurative inflammation features neutrophils, not epithelioid cells.

Ziehl-Neelsen Stain and Demonstrating the Bacillus

A four-panel medical diagram shows Ziehl-Neelsen stained acid-fast bacilli as sparse magenta rods in blue caseous lymph node material, with the staining principle, reporting phrase, and diagnostic follow-up pathway. — **Panel A:** Ziehl-Neelsen stained lymph node caseum showing blue methylene blue counterstain, pale caseous material, and sparse slender magenta-red acid-fast bacilli with identification arrows.. **Panel B:** Mycobacterium tuberculosis bacillus with mycolic acid-rich waxy cell wall retaining hot carbol fuchsin and resisting acid-alcohol decolourisation.. **Panel C:** Microscope finding and reporting phrase: 'AFB seen - consistent with Mycobacterium sp.', with caution that ZN stain does not distinguish M. tuberculosis from NTM.. **Panel D:** TB lymphadenitis diagnostic approach: granulomatous caseation to ZN stain, AFB seen report Mycobacterium sp., AFB not seen but suspected TB proceeds to culture or GeneXpert..

Histological morphology alone is strongly suggestive, but the definitive morphological confirmation of TB requires demonstration of acid-fast bacilli (AFB).

Ziehl-Neelsen (ZN) stain principle: M. tuberculosis has a thick, waxy cell wall rich in mycolic acids that binds hot carbol fuchsin (red dye) and resists decolourisation with acid-alcohol — hence acid-fast. Background and non-AFB organisms are counterstained blue (methylene blue).

What you see: Slender, slightly curved, red (magenta) rods, 1–4 µm long, set against a blue background. They may appear individually or in small clusters. They are often sparse in lymph node caseum — you may need to scan multiple high-power fields.

Reporting: 'AFB seen — consistent with Mycobacterium sp.' Note: ZN does not distinguish M. tuberculosis from non-tuberculous mycobacteria (NTM) — culture or GeneXpert is required for species identification.

Why AFB may be scanty in nodes: The host's immune response is effective at limiting bacillary load in lymph nodes (unlike post-primary pulmonary TB cavities, which are teeming with organisms). A negative ZN stain does not exclude TB.

Ziehl-Neelsen stained microscopy showing red acid-fast bacilli against blue background, alongside TB lymphadenitis diagnostic algorithm flowchart. — **Panel A:** Ziehl-Neelsen stained section (1000× oil immersion) showing red acid-fast bacilli, blue methylene blue counterstain background, caseous material, individual bacillus identification arrows. **Panel B:** TB lymphadenitis diagnostic hierarchy flowchart showing FNAC, ZN stain, excision biopsy, culture methods, GeneXpert MTB/RIF with sensitivity rates and timeframes.

CLINICAL PEARL

The diagnostic hierarchy for TB lymphadenitis — remember this sequence:

FNAC (Fine Needle Aspiration Cytology): First-line, minimally invasive. Smear shows epithelioid cell clusters, Langhans giant cells, caseous background. Sensitivity ~70–80% in India. ZN stain on FNAC material detects AFB in ~40–60% of confirmed cases.

Excision biopsy / core biopsy: Gold standard for histology. Provides tissue architecture — confluent granulomas, caseous necrosis, ZN stain.

Culture (Löwenstein-Jensen or BACTEC liquid medium): Definitive species ID and drug-sensitivity testing (DST). Takes 2–8 weeks. Required for drug-resistant TB.

GeneXpert MTB/RIF (CBNAAT): Molecular rapid test from FNAC material, pus, or biopsy tissue. Detects M. tuberculosis DNA AND rifampicin resistance within 2 hours. WHO-endorsed first-line test where available (AYUSH/NTEP protocol in India).

Mantoux / IGRA: Adjuncts. Positive = exposure, not active disease (especially in BCG-vaccinated individuals). Not diagnostic alone.

Key practical rule: In India, treat as TB if FNAC shows caseating granulomas + clinical context, even with negative ZN, while culture is pending. Do not wait 8 weeks for culture before starting treatment in a confirmed granulomatous picture.

Differential Diagnosis: Granulomatous Lymphadenitis

⚑ AI image — pending faculty review (auto-QA score 7/10; best of 3 attempts)

A comparison diagram shows how caseation, granuloma pattern, giant cells, organisms, and special stains distinguish tuberculosis, sarcoidosis, histoplasmosis, cat-scratch disease, and tuberculoid leprosy in granulomatous lymphadenitis. — **Panel A:** Diagnostic framework: caseation present, non-caseating naked granulomas, stellate suppurative granuloma, perineural granuloma, TB, fungal infection, sarcoidosis, cat-scratch disease, tuberculoid leprosy. **Panel B:** Tuberculosis: caseating epithelioid granuloma, central caseous necrosis, epithelioid histiocytes, Langhans giant cell, AFB on Ziehl-Neelsen stain. **Panel C:** Sarcoidosis: non-caseating naked granuloma, sparse lymphocytes, epithelioid histiocytes, multinucleated giant cell, Schaumann body, asteroid body, no organisms. **Panel D:** Histoplasmosis: caseating or non-caseating granuloma, macrophages, intracellular oval yeasts, PAS/GMS positive organisms. **Panel E:** Cat-scratch disease: stellate suppurative granuloma, central neutrophilic abscess, palisading histiocytes, surrounding granulomatous inflammation. **Panel F:** Tuberculoid leprosy: epithelioid granuloma, perineural inflammation, peripheral nerve bundle, Langhans giant cell, scant organisms.

Not every granuloma is TB. The caseation is your most important discriminator. Construct your differential systematically:

Feature	TB	Sarcoidosis	Fungal (Histoplasma)	Cat-Scratch Disease	Leprosy (tuberculoid)
Granuloma type	Caseating epithelioid	Non-caseating, tight (naked granuloma)	Caseating or non-caseating	Stellate/suppurative + granuloma	Epithelioid, perineural
Giant cells	Langhans type	Schaumann/asteroid bodies	Langhans type	Rare	Langhans type
Organisms	AFB (ZN +)	None	PAS/GMS + yeast	Gram-neg rod (Warthin-Starry)	AFB (modified ZN)
Caseous necrosis	Yes, central	Absent (key!)	Variable	Central stellate necrosis	Absent/focal
Special feature	Merging granulomas	Fibrotic capsule, hyaline	Capsule-positive yeast (narrow bud)	Bartonella serology	Nerve infiltration

Sarcoidosis is the most commonly confused entity in India. The absence of caseous necrosis is the defining distinguishing feature — sarcoid granulomas are sometimes called 'naked' because they lack the surrounding lymphocytic cuff as well. Schaumann bodies (laminated calcium-phosphate inclusions) and asteroid bodies (star-shaped inclusions in giant cells) support sarcoidosis but are not pathognomonic.

Practical tip: When a granuloma lacks caseous necrosis, always request special stains — PAS and GMS (for fungi), Gram stain (for bacteria), Fite-Faraco stain (for leprosy) — before calling it sarcoidosis.

SELF-CHECK

A 28-year-old presents with bilateral hilar lymphadenopathy and cervical lymph nodes. Biopsy shows tight, well-formed epithelioid granulomas WITHOUT caseous necrosis. Giant cells contain laminated calcific inclusions. ZN stain is negative. The MOST likely diagnosis is:

A. Tuberculous lymphadenitis

B. Cat-scratch disease

C. Sarcoidosis

D. Cryptococcal lymphadenitis

Reveal Answer

Answer: C. Sarcoidosis

The combination of bilateral hilar nodes, tight non-caseating epithelioid granulomas, Schaumann bodies (laminated calcific inclusions in giant cells), and negative ZN stain is characteristic of sarcoidosis. TB is excluded by the absence of caseous necrosis and negative AFB. Cat-scratch disease shows stellate/suppurative necrosis (not granulomatous without necrosis). Cryptococcal infection would show capsulate yeast on mucin stains (mucicarmine positive).