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Cryptic Miliary Tuberculosis — Complete Review

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Definition

• The classic miliary pattern on chest X-ray is absent or undetectable (hence "cryptic" = hidden)
• The tuberculin skin test (TST) / Mantoux is negative (anergic)
• The clinical presentation is atypical, insidious, and non-specific
• Diagnosis is typically made late, often at autopsy or by biopsy

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Historical Context

• Described by Simmonds (1949) in elderly patients at autopsy — widespread miliary TB with no ante-mortem diagnosis
• Term "cryptic" coined to distinguish from classic miliary TB with overt radiological and clinical features
• Also called "non-reactive" miliary TB when there is minimal granuloma formation (necrosis without palisading histiocytes)

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Pathogenesis

Primary / Reactivation TB focus
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Erosion into blood vessel (artery or vein)
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Hematogenous dissemination
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Seeding of multiple organs (lung, liver, spleen, bone marrow, kidney, adrenal, brain, eye)
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Formation of miliary granulomas (1–2 mm)
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In cryptic form: immune anergy / old age → poor granuloma formation,
no recognizable X-ray pattern, negative TST

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Classification of Miliary TB

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Clinical Features

General Features (Nonspecific — classic clue for exams)

• Prolonged, low-grade fever — weeks to months, not responding to broad-spectrum antibiotics
• Night sweats
• Significant weight loss and anorexia
• Progressive weakness and malaise
• No localizing signs — this is the hallmark

Organ-Specific Features

Exam Pearl: Choroidal tubercles on fundoscopy in a patient with PUO strongly suggest miliary TB.

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Investigations

1. Hemogram

• Pancytopenia — bone marrow involvement
• Leukopenia or leukocytosis
• Monocytosis — characteristic
• Anemia (normocytic normochromic)
• Leukaemoid reaction (rare but documented)

2. ESR

• Markedly elevated (often >100 mm/hr)

3. LFTs

• Elevated ALP and GGT >> ALT/AST (granulomatous hepatitis pattern)

4. Mantoux / TST

• Negative in cryptic miliary TB (anergic) — this is key
• Negative TST does NOT exclude TB (Harrison's, p. 684)
• Repeat TST may turn positive with treatment

5. IGRA (QuantiFERON-TB Gold)

• Sensitivity similar to TST — also false-negative in miliary TB and immunosuppression (Harrison's, p. 684)
• A negative IGRA does NOT rule out miliary TB

6. Chest X-Ray

• Normal in cryptic miliary TB — the defining feature
• May show subtle bilateral fine nodular shadows (1–2 mm) only in advanced disease
• Classic "snowstorm" appearance — absent in cryptic form

7. HRCT Thorax

• More sensitive than CXR — may show miliary nodules not visible on plain film
• Findings: diffuse 1–3 mm nodules in random distribution, ground-glass opacities, tree-in-bud pattern, interlobular septal thickening

8. Sputum Smear & Culture

• AFB smear: low sensitivity in miliary TB
• Culture (LJ medium / BACTEC): gold standard but takes 2–8 weeks
• GeneXpert MTB/RIF: rapid, detects MTB and rifampicin resistance (preferred if sputum available)

9. Biopsy — KEY DIAGNOSTIC TOOL

"If miliary TB is suspected, liver biopsy for acid-fast smear, culture, and PCR probably still has the highest diagnostic yield" — Harrison's, p. 684

Granulomatous disease in liver or bone marrow biopsy should ALWAYS prompt reconsideration of miliary TB (Harrison's, p. 684)

10. Fundoscopy

• Choroidal tubercles — pale/yellowish lesions near optic disc
• Pathognomonic of miliary TB — present in ~10–30% cases
• Should be done in every suspected case

11. CSF Analysis

• If meningism present: elevated protein, lymphocytosis, low glucose, AFB/culture/PCR

12. Urine

• Sterile pyuria → early morning urine for AFB culture

13. Serum Adenosine Deaminase (ADA)

• Elevated in TB (non-specific but supportive)

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Diagnosis — Summary Algorithm

PUO + weight loss + night sweats
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CXR (may be NORMAL in cryptic TB)
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HRCT Thorax
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TST/IGRA (often NEGATIVE — do not rely on)
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Fundoscopy → choroidal tubercles?
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Blood counts, LFTs, bone marrow aspirate
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LIVER BIOPSY / BONE MARROW BIOPSY
(granulomas → AFB smear, culture, PCR)
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DIAGNOSIS CONFIRMED

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Differential Diagnosis of Cryptic Miliary TB

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Treatment

Regimen

• Drug doses: H = 5 mg/kg, R = 10 mg/kg, Z = 25 mg/kg, E = 15 mg/kg

CNS Involvement

• If TB meningitis co-exists: 9–12 month regimen with high-dose Rifampicin; follow TBM treatment protocol

Corticosteroids

• Prednisolone may be used as adjunct in:
  • Severe respiratory failure / ARDS
  • Addison's disease (adrenal insufficiency)
  • TBM co-existing with miliary TB
• Role remains unclear/controversial in miliary TB without these indications (Treatment of Drug-Susceptible TB, p. 32)

Pyridoxine (Vitamin B6)

• 25 mg/day with Isoniazid to prevent peripheral neuropathy

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Complications

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Prognosis

• Mortality: 25–30% even with treatment (higher without it)
• Worse prognosis with:
  • Delayed diagnosis
  • ARDS
  • CNS involvement
  • Immunosuppression (HIV, malignancy)
  • Drug-resistant TB
• Early diagnosis and prompt treatment are key to survival

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Exam High-Yield Points Summary

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