Tuberculosis

Tuberculosis is the second topic of Sivakumar and Pelly's chapter on tropical diseases in Oh's Manual. It had spent years in obscurity, never being interrogated in the SAQs directly until Question 12 from the second paper of 2020, but it comes up as a differential fairly often.

A good resource is this clinical review article from Critical Care (2013). Additionally, specifically to address the questions asked in 2020, the 2000 American Thoracic Society (ATS) and CDC diagnostic standards statement is what the CDC quote on their website.  If you're going to complain that those references haven't been updated in ten years, here's a more recent paper by Ryu (2015)

Predisposing factors:

• HIV infection
• Pneumoconiosis
• NIDDM
• Chronic renal failure/haemodialysis,
• Malnutrition
• Solid organ transplant (or bone marrow transplant),
• Gastrectomy (prevents chemical destruction of the swallowed organsim)
• Jejunoileal bypass
• Injecting and inhalational drug abuse
• Alcoholism
• COPD
• Prolonged steroid use
• Institutional living conditions (nursing homes, homeless shelters, prisons)
• Urban dwelling
• Poverty, crowding, and lack of access to health care

 

Clinical manifestations

Pulmonary manifestations

• Apical lung lesions
• Cough, dyspnoea, haemoptysis
• Hilar lymphadenopathy
• Cavitation
• Pneumothorax
• Bronchopleural fistula
• Pleural effusion

Central nervous system disease

• A "thick gelatinous exudate around the sylvian fissures, basal cisterns, brainstem and
  cerebellum" develops
• Prodrome lasts 2-8 weeks
• Cranial nerve palsies occur in 20–25% of patients
• Choroidal tubercles may be present (pathognomonic)
• Mortality approaches 50%

ICU emergencies involving tuberculosis

This is present in the Manual as a straight-up point form list, suggesting that it has SAQ importance; eg. List six life-treatening complications of tuberculosis in the ICU patient.

• Massive haemoptysis
• Respiratory failure
• Pericardial tamponade
• Small intestinal obstruction
• Tuberculous meningitis
• Status epilepsy due to tuberculomas

Diagnosis of tuberculosis

Investigation	Advantages	Limitations
Culture	Gold standard Also provides drug susceptibility information Only 100 or so organisms are required for a culture	Takes an extremely long time
AFB identification (ZN stain)	Next best thing to gold standard Relatively fast compared to culture	Induced sputum or bronchoscopy speciments are required for maximum sensitivity  Requires a high pathogen load: there must be 5,000 to 10,000 bacilli per ml
Nucleic acid amplification (NAA)	Very rapid (couple of hours) Can be performed on many different specimen types Requires as few as 10 bacilli High specificity: if its positive, its positive. Effective at discriminating M.tuberculosis from other mycobacteria, which AFB may not.	Sensitivity is poor: may return a negative result in ~50% of patients with culture-positive tuberculosis Does not replace AFB smear and culture Remains positive for months after TB is successfully treated
Skin tuberculin test	Widely available, including resource-poor environments Does not require a dedicated laboratory High positive predictive value in areas of high TB community prevalence	Slow Poor sensitivity and specificity in areas of low TB prevalence Unclear what to do with intermediate results
Line probe assay	A rapid test for drug sensitivity which can detect gene markers associated with drug resistance	Cannot replace conventional culture and sensitivity testing Only appropriate for patients already tested positive
Interferon-γ release assays	Quick and convenient screening tool for exposure to TB	Cannot discriminate between latent and active TB Does not give drug sensitivity data
Xray or CT imaging	Presence of cavitating lesions strongly raises suspicion of TB Guide for bronchoscopy or imaging-guided aspiration of more diagnostic material Convenient mechanism of monitoring disease activity when the diagnosis is already established	Not diagnostic of TB per se, only raises suspicion Requires patient transport and radiation exposure Requires radiology staff to be exposed to TB patient
Histology	Pathological diagnosis of a caseating granuloma in an anatomical specimen is diagnosit Tissue can be cultured and tested with NAA	Highly invasive Requires a good quality specimen Not widely avalable

Infection control for pulmonary tuberculosis

Infection control precautions listed here have been pulled directly from the CDC document, where most people get their infection control guidelines from:

• Airborne transmission precautions:
  • Should be commenced as soon as a patient is suspected of having TB
  • N95 masks for staff attending directly to patient care, and any visitors
  • Surgical mask for the TB-positive patient when in transport or in waiting areas
  • Single room
  • Airflow of six or more air changes per hour (ideally, 12)
  • HEPA filtration of any recirculated air
  • Negative pressure system is desirable
  • Private bathroom is desirable if the pt. is conscious
  • Minimise aerosol-generating procedures;
  • Minimise staff involved in any necessary aerosol-generating procedures
• Clearing of the infectious status:
  • Patients can be considered noninfectious when they meet all of the following three criteria:
    • They have three consecutive negative AFB sputum smears collected in 8- to 24-hour intervals (at least one being an early morning specimen);
    • Their symptoms have improved clinically (for example, they are coughing less and they no longer have a fever);
    • They are compliant with an adequate treatment regimen for 2 weeks or longer.

In this brief summary, one does not even attempt to discuss the management, as it is a complex topic best left to ID professionals. Suffice to say all the drugs you will use will interact with everything, cause organ toxicity, and likely involve some combination of isoniazid, rifampicin, pyrazinamide and ethambutol. 

Outcomes

• In-hospital nortality for active TB requiring mechanical ventilation is 33 to 67%.