Tuberculosis

TB is a high-yield topic for UK finals, particularly its diagnosis, management, and public health implications. Prevalence in the UK is highest in urban areas and among vulnerable populations (migrants, homeless, immunosuppressed). *M. tuberculosis* is an acid-fast bacillus (AFB) spread via respiratory droplets.

**Latent TB** occurs when the immune system contains the infection, making the individual non-infectious. **Active TB** presents with symptoms and is infectious. Pulmonary TB (60% of cases) typically presents with chronic cough, night sweats, weight loss, and haemoptysis. Extrapulmonary TB can affect lymph nodes (scrofula), CNS (meningitis), bones (Pott’s disease), or disseminate (miliary TB).

Diagnosis involves sputum samples for AFB smear and culture, with rapid PCR (GeneXpert) for early detection and resistance testing. Screening for latent TB uses Mantoux or IGRA. Treatment is a prolonged course of 'RIPE' antibiotics, with mandatory contact tracing and public health notification.

*Mycobacterium tuberculosis* is an obligate aerobe with a unique waxy cell wall (mycolic acids) that confers resistance to many disinfectants and antibiotics, and prevents Gram staining.

Upon inhalation, bacilli are phagocytosed by alveolar macrophages. If not cleared, a **Ghon focus** (granulomatous inflammation) forms. Spread to hilar lymph nodes creates a **Ghon complex**. Most primary infections are contained by T-cell mediated immunity, forming **caseating granulomas** (central necrosis surrounded by epithelioid macrophages and Langhans giant cells).

Reactivation typically occurs in areas of high oxygen tension (e.g., lung apices), leading to tissue destruction and cavitation. **Miliary TB** results from haematogenous dissemination, often seen as diffuse 'millet seed' opacities on CXR.

Finals often test recognition of TB presentations, especially in high-risk groups.

**Systemic symptoms**: Persistent cough (>3 weeks, dry or productive), haemoptysis, fever, drenching night sweats, significant weight loss, fatigue.

**Physical signs**: Often minimal in early disease. May include apical crackles, signs of pleural effusion, or consolidation in advanced pulmonary TB.

**Extrapulmonary signs**:
- **Lymphadenopathy**: Painless, firm, often cervical ('scrofula').
- **Skeletal (Pott's disease)**: Back pain, kyphosis, neurological deficits (cord compression).
- **CNS (Meningitis)**: Headache, fever, confusion, cranial nerve palsies. High mortality if untreated.
- **Miliary TB**: Multi-organ involvement, severe systemic symptoms.

**High-risk factors**: HIV infection (TB is a leading cause of death in HIV patients), immunosuppression (e.g., TNF-alpha inhibitors – *always screen before starting!*), diabetes, chronic kidney disease, silicosis, homelessness, close contacts of active TB cases, migrants from high-incidence countries.

Investigations aim for rapid diagnosis, resistance testing, and identifying extent of disease.

1. **Microbiology**:
- **Sputum**: 3x samples (including one early morning) for **AFB smear** (Ziehl-Neelsen or Auramine stain) and **culture** (Lowenstein-Jensen or MGIT – gold standard, but takes weeks).
- **PCR (GeneXpert/NAAT)**: Rapidly detects *M. tuberculosis* DNA and Rifampicin resistance within hours – crucial for early management.
- Other samples: Bronchoalveolar lavage, gastric aspirate (children), lymph node biopsy, CSF, urine, tissue biopsies.
2. **Imaging**:
- **Chest X-ray**: Upper lobe infiltrates, cavitation, hilar lymphadenopathy, pleural effusion, or diffuse 'miliary' pattern. May be normal in early disease or extrapulmonary TB.
- CT/MRI: For extrapulmonary disease (e.g., CNS, spine).
3. **Latent TB Screening**:
- **Mantoux test (Tuberculin Skin Test)**: Inject PPD intradermally; measure induration at 48-72 hours. Positive indicates exposure, but can be false positive with BCG vaccination.
- **Interferon-Gamma Release Assay (IGRA)**: Blood test, preferred if BCG vaccinated or for contact tracing, as it's more specific for *M. tuberculosis* exposure.
4. **Routine tests**:
- **HIV test**: Mandatory for all TB patients due to co-infection risk and impact on treatment.
- **Liver function tests (LFTs)**: Baseline and regular monitoring due to hepatotoxicity of RIPE drugs.
- **Renal function tests**: For drug dosing.
- **Visual acuity/colour vision**: Baseline for Ethambutol.

Management is complex and guided by specialist TB teams and public health.

**Active Pulmonary TB (Standard)**:
1. **Initial Phase (2 months)**: **R**ifampicin + **I**soniazid + **P**yrazinamide + **E**thambutol (RIPE).
2. **Continuation Phase (4 months)**: **R**ifampicin + **I**soniazid.
- Total 6 months. Directly Observed Therapy (DOT) may be used to ensure adherence.

**Extrapulmonary TB (e.g., Meningitis, Bone/Joint TB)**:
- Continuation phase often extended to 10 months (total 12 months).
- Corticosteroids (e.g., Dexamethasone) are often co-administered for TB meningitis or pericarditis to reduce inflammation and complications.

**Latent TB Treatment**:
- Options include: 3 months Rifampicin + Isoniazid, OR 6 months Isoniazid alone.

**Monitoring for Side Effects (Crucial for SBAs!)**:
- **Rifampicin**: Hepatotoxicity, orange secretions, P450 induction (reduces efficacy of oral contraceptives, warfarin).
- **Isoniazid**: Hepatotoxicity, peripheral neuropathy (prevent with Pyridoxine/Vit B6).
- **Pyrazinamide**: Hepatotoxicity, hyperuricaemia/gout, arthralgia.
- **Ethambutol**: Optic neuritis (red-green colour blindness, reduced visual acuity). *Requires urgent ophthalmology review if symptoms occur*.

**Public Health & Contact Tracing**:
- TB is a **notifiable disease** in the UK. Public health teams manage contact tracing, screening close contacts with Mantoux/IGRA, and offering prophylactic treatment if indicated.