Respiratory System (RS) Module — 3rd Year MBBS

📚 Study Tip

This chapter follows the supplied KMU learning outcomes and builds tuberculosis from the organism and disease mechanism to diagnosis, treatment and prevention. First understand the disease process and drug logic; then use the AIM High-Yield Review for rapid revision.

3rd Year MBBS KMU Curriculum AIM Learning Cycle

📖 AIM Learning Material

Pulmonary Tuberculosis and Granulomatous Lung Diseases

An integrated approach to pulmonary tuberculosis, Mycobacterium tuberculosis, antituberculous pharmacotherapy, drug resistance, sarcoidosis, hypersensitivity pneumonitis, pulmonary eosinophilia and tuberculosis prevention and control.

Topic Introduction

Pulmonary tuberculosis (TB) is a chronic infectious disease caused mainly by Mycobacterium tuberculosis. It is especially important because the organism can survive inside macrophages, provoke a granulomatous immune response, remain latent for years and later reactivate. Understanding TB therefore requires linking microbiology with immune-mediated tissue injury, characteristic pathological lesions, clinical and radiological findings, laboratory diagnosis and multidrug treatment.

This topic also includes important non-tuberculous disorders that may produce granulomatous or inflammatory lung disease, particularly sarcoidosis, hypersensitivity pneumonitis and pulmonary eosinophilia. Finally, because TB spreads between people and treatment failure can create drug resistance, prevention, contact tracing, treatment adherence and public-health control form an essential part of the topic. The supplied curriculum specifically integrates Pathology, Pharmacology, Microbiology, Community Medicine, Medicine and Family Medicine.

A. Mycobacterium tuberculosis — Properties, Transmission and Pathogenesis

Mycobacterium tuberculosis is a slender, aerobic, non-spore-forming bacillus with a lipid-rich cell wall containing large amounts of mycolic acids. This unusual wall makes the organism relatively resistant to ordinary staining and allows it to retain certain dyes even after treatment with acid-alcohol; therefore, it is described as an acid-fast bacillus. The lipid-rich wall also contributes to slow growth and helps the organism resist destruction within host phagocytic cells.

Important microbiological properties

Acid fast: demonstrated by acid-fast staining techniques.
Obligate aerobe: this helps explain the preference of reactivation TB for well-oxygenated regions such as the lung apices.
Slow growing: conventional culture requires substantially more time than culture of ordinary bacteria.
Intracellular survival: bacilli can persist within macrophages.
Cell-mediated immunity is central: both protection and much of the tissue injury in TB arise from the host T-cell response.

Transmission and entry

Pulmonary TB is transmitted mainly through airborne droplet nuclei generated by a person with infectious pulmonary or laryngeal disease. Very small particles can remain suspended in air and reach the terminal airways and alveoli when inhaled. Transmission is favored by prolonged close contact, poor ventilation and a high bacillary burden in the source patient.

Pathogenesis

After inhalation, bacilli reach alveoli and are phagocytosed by alveolar macrophages. Instead of being immediately destroyed, some organisms survive and multiply intracellularly. Infected macrophages may carry organisms through lymphatics to regional lymph nodes. During the first few weeks, the host develops a T-cell-mediated immune response against mycobacterial antigens.

Core mechanism

Inhaled bacilli → macrophage infection and intracellular survival → activation of T lymphocytes → macrophage activation and granuloma formation → containment of bacilli with tissue injury → latent infection, progressive disease or later reactivation.

Activated CD4+ T cells produce cytokines, particularly interferon-gamma, that increase the ability of macrophages to kill intracellular organisms. Macrophages transform into epithelioid cells and may fuse to form multinucleated giant cells. Aggregates of these cells surrounded by lymphocytes form a granuloma. In TB, the centre commonly undergoes caseous necrosis.

This immune response is beneficial because it limits spread, but it is also responsible for tissue destruction. Thus, the classic tuberculous lesion is produced not simply by direct bacterial damage but by the interaction between persistent bacilli and the host cell-mediated immune response.

AIM VISUAL 01 — Mycobacterium tuberculosis: Entry to Granuloma Formation

B. Pulmonary Tuberculosis — Types, Pathology and Morphology

The pathological appearance of pulmonary TB depends on whether infection is occurring for the first time, whether previously contained infection has reactivated, and how effective the host immune response is. The two classic patterns are primary tuberculosis and secondary or reactivation tuberculosis.

Primary pulmonary tuberculosis

Primary TB develops in a person who has not previously mounted an effective immune response to M. tuberculosis. The initial pulmonary lesion is usually a small area of gray-white inflammatory consolidation called a Ghon focus, classically located in the lower part of the upper lobe or upper part of the lower lobe near the pleura.

Bacilli drain through lymphatics to hilar lymph nodes. The combination of a Ghon focus and involved draining hilar lymph nodes is called the Ghon complex. Healing may occur by fibrosis and calcification. A healed and calcified primary complex is sometimes termed a Ranke complex.

Secondary or reactivation pulmonary tuberculosis

Secondary TB usually develops in a previously sensitized person, most commonly because dormant bacilli reactivate when immune control weakens, although reinfection can also occur. Because the host is already sensitized, the inflammatory and tissue-destructive response is generally more marked.

The classic lesion is located near the apex of one or both upper lobes. The high oxygen tension of the apical lung favors growth of this aerobic organism. Caseation and tissue destruction may progress to form an irregular pulmonary cavity.

Gross morphology

Areas of gray-white or yellow-white consolidation.
Central soft, cheesy caseous necrosis.
Fibrosis in healing lesions.
Cavitation in progressive secondary pulmonary TB.
Pleural involvement may occur when lesions extend toward the lung surface.

Microscopic morphology

The characteristic lesion is the caseating granuloma. It consists of central amorphous granular necrotic material surrounded by epithelioid macrophages, multinucleated giant cells and lymphocytes. Langhans-type giant cells may show nuclei arranged toward the peripheral part of the cell, although this appearance is not specific for TB.

Progressive and disseminated disease

If immune containment fails, bacilli may spread through airways, lymphatics or blood. Bronchogenic spread produces additional pulmonary lesions. Hematogenous dissemination can produce numerous tiny lesions in multiple organs, creating miliary tuberculosis. Progressive pulmonary destruction may also cause fibrosis, cavitation, hemoptysis and impairment of respiratory function.

AIM VISUAL 02 — Primary versus Secondary Pulmonary Tuberculosis

C. Pulmonary Tuberculosis — Clinical Features, Radiology and Complications

Clinical presentation depends on the extent and activity of disease. Primary infection may be asymptomatic or produce mild illness, whereas active secondary pulmonary TB more often produces constitutional symptoms together with respiratory complaints.

Clinical features

Persistent cough, with or without sputum production.
Fever, often low-grade and prolonged.
Night sweats.
Loss of appetite and weight loss.
Fatigue and general weakness.
Chest discomfort or pleuritic pain when pleura is involved.
Hemoptysis, particularly when cavitary disease damages pulmonary vessels.

The constitutional features arise largely from sustained inflammation and cytokine activity. Hemoptysis occurs when inflammatory destruction, cavitation or erosion of vessels allows blood to enter the airways.

Radiological features

Imaging findings should always be interpreted with the clinical and microbiological picture because no single radiological pattern proves TB.

Primary disease: parenchymal focus with hilar or mediastinal lymph-node enlargement may be seen.
Secondary disease: upper-lobe or apical infiltrates are characteristic.
Cavitation: suggests destructive active disease and often corresponds to a high bacillary burden.
Miliary pattern: numerous tiny nodules distributed through both lungs indicate hematogenous dissemination.
Healing: fibrosis and calcification may remain as residual evidence of previous infection.

Important complications

Massive or recurrent hemoptysis.
Pleural effusion or pleural disease.
Extensive fibrosis and permanent lung damage.
Bronchiectatic change in damaged lung.
Disseminated or miliary TB.
Spread to meninges, bones, kidneys and other organs.
Secondary colonization of a residual cavity by fungi such as Aspergillus.

Diagnostic clue: A prolonged cough associated with fever, night sweats, weight loss and upper-zone cavitary disease should strongly raise suspicion of pulmonary TB, but bacteriological confirmation and assessment of drug resistance are still required.

AIM VISUAL 03 — Pulmonary TB: Pathology to Clinical and Radiological Findings

D. Laboratory Diagnosis, Investigations and Differential Diagnosis

Diagnosis of active pulmonary TB requires integration of symptoms, imaging and direct evidence of the organism whenever possible. A patient may have a strongly suggestive clinical picture, but microbiological testing is particularly important because it confirms disease and helps detect drug resistance.

Specimen selection

For pulmonary disease, sputum is the preferred specimen when the patient can produce it. Other respiratory samples may be required when sputum cannot be obtained or when results remain inconclusive despite strong clinical suspicion.

Major laboratory approaches

Acid-fast microscopy: demonstrates acid-fast bacilli and can rapidly support a diagnosis, but it cannot reliably distinguish M. tuberculosis from all other mycobacteria.
Rapid molecular testing: detects mycobacterial genetic material and can identify important resistance markers, especially rifampicin resistance, much faster than conventional culture.
Culture: provides definitive microbiological isolation and enables more complete drug-susceptibility testing, although growth is slow.
Drug-susceptibility testing: determines whether an isolate is susceptible or resistant to important antituberculous drugs and is essential when resistant TB is suspected.

Immunological tests

Tuberculin skin testing and interferon-gamma release assays indicate immune sensitization to mycobacterial antigens. They may support identification of TB infection, but they do not by themselves prove active pulmonary disease. Active TB must be evaluated with clinical, radiological and microbiological evidence.

Histopathology

Tissue biopsy may show caseating granulomatous inflammation. However, granulomas are not unique to TB, so morphology must be correlated with microbiological evidence and the clinical setting.

Differential diagnosis of granulomatous inflammation

Granulomatous inflammation in the lung has both infectious and non-infectious causes. Important considerations include:

Tuberculosis and other mycobacterial infections.
Deep fungal infections.
Sarcoidosis.
Hypersensitivity pneumonitis.
Exposure-related granulomatous disorders such as chronic beryllium disease.
Certain granulomatous vasculitic disorders.

The presence of necrosis increases suspicion for infection, particularly TB, but is not absolutely specific. Conversely, non-necrotizing granulomas suggest sarcoidosis in the correct clinical setting, but infection still needs to be excluded. Modern diagnostic approaches to sarcoidosis therefore require a compatible presentation, granulomatous pathology when tissue confirmation is needed, and exclusion of alternative causes. :contentReference[oaicite:1]{index=1}

AIM VISUAL 04 — Diagnostic Pathway for Suspected Pulmonary Tuberculosis

E. First-Line Antituberculous Drugs

The major first-line drugs used for drug-susceptible TB are isoniazid, rifampicin, pyrazinamide and ethambutol. Their mechanisms differ, which is precisely why combining them is effective: different drugs attack different bacterial targets and reduce the probability that naturally resistant bacilli will survive treatment.

Drug	Main mechanism	Important role	Characteristic adverse effects
Isoniazid (H)	Prodrug activated within the bacillus; inhibits synthesis of mycolic acids required for the mycobacterial cell wall.	Strong bactericidal activity against actively multiplying bacilli.	Hepatotoxicity; peripheral neuropathy from functional pyridoxine deficiency.
Rifampicin (R)	Inhibits bacterial DNA-dependent RNA polymerase and therefore RNA synthesis.	Powerful bactericidal and sterilizing activity.	Hepatotoxicity, orange-red discoloration of body fluids and major drug interactions from hepatic enzyme induction.
Pyrazinamide (Z)	Prodrug converted to pyrazinoic acid; particularly active against bacilli in acidic intracellular or inflammatory environments.	Important sterilizing action during the initial phase of therapy.	Hepatotoxicity and hyperuricemia; arthralgia may occur.
Ethambutol (E)	Inhibits arabinosyl transferases involved in synthesis of the mycobacterial cell wall.	Helps protect the regimen against emergence of resistance while susceptibility is being established.	Optic neuritis with reduced visual acuity and impaired red-green colour discrimination.

Isoniazid: important pharmacological points

Isoniazid enters mycobacteria and is activated by a bacterial catalase-peroxidase enzyme. Activated metabolites interfere with enzymes involved in mycolic-acid synthesis. Because mycolic acids are essential components of the mycobacterial cell wall, inhibition rapidly affects actively dividing bacilli.

Isoniazid may interfere with pyridoxine metabolism, producing peripheral neuropathy, particularly in susceptible patients. Pyridoxine supplementation is therefore important in groups at increased risk, including pregnancy and certain patients with nutritional, renal or neurological vulnerability.

Rifampicin: important pharmacological points

Rifampicin blocks transcription by binding bacterial DNA-dependent RNA polymerase. It is particularly important because of its bactericidal and sterilizing activity. A major pharmacological issue is enzyme induction, which can reduce concentrations of many co-administered drugs. This makes interaction checking essential.

Pyrazinamide and ethambutol

Pyrazinamide contributes strongly during the early phase of therapy by acting in environments where some persistent bacilli remain metabolically active. Ethambutol has a different cell-wall target and is especially useful in preventing selection of resistant organisms before full susceptibility information is available.

Therapeutic logic: Do not memorize HRZE merely as four letters. Each drug attacks the bacillus differently, so the combination improves bacterial killing while making emergence of resistance much less likely.

AIM VISUAL 05 — First-Line Antituberculous Drugs

F. Drug Resistance, Second-Line Drugs and MDR/RR-TB

M. tuberculosis develops clinically important resistance mainly through spontaneous chromosomal mutations followed by selection of resistant organisms. Resistance is not produced because the bacillus deliberately adapts to a drug. Instead, resistant mutants may already exist within a large bacterial population. If treatment is inadequate, interrupted or effectively reduced to a single active drug, susceptible organisms are killed while resistant organisms survive and multiply.

Resistance sequence

Naturally occurring resistant mutant → inadequate effective multidrug exposure → susceptible bacilli eliminated → resistant mutant survives → resistant population expands → treatment failure and transmissible drug-resistant TB.

Important resistance terminology

RR-TB: tuberculosis resistant to rifampicin.
MDR-TB: resistance to at least both rifampicin and isoniazid.
Pre-XDR-TB: rifampicin-resistant/MDR disease with additional resistance to a fluoroquinolone.
XDR-TB: more extensive resistance involving rifampicin, a fluoroquinolone and additional key drugs used for resistant disease.

Current WHO terminology and treatment selection are based on the actual drug-resistance pattern rather than the old practice of assigning all patients to fixed numerical retreatment categories. WHO currently defines MDR-TB as resistance to both rifampicin and isoniazid and prioritizes modern all-oral regimens for eligible patients with MDR/RR-TB. :contentReference[oaicite:2]{index=2}

Important drugs used in resistant TB

Fluoroquinolones — levofloxacin and moxifloxacin: inhibit bacterial DNA gyrase and interfere with DNA replication. They are highly important components of many resistant-TB regimens.

Bedaquiline: inhibits mycobacterial ATP synthase, reducing production of cellular energy. It is an important modern drug for MDR/RR-TB. QT-interval prolongation is an important safety concern.

Linezolid: binds the 50S ribosomal subunit and inhibits bacterial protein synthesis. Important toxicities include bone-marrow suppression, peripheral neuropathy and optic neuropathy, particularly with prolonged exposure.

Pretomanid: is a nitroimidazole-class antituberculous drug used as part of specific multidrug regimens for resistant TB rather than as monotherapy.

Clofazimine: has antimycobacterial activity and is incorporated into several resistant-TB regimens. Skin discoloration and gastrointestinal effects are notable adverse effects.

Cycloserine/terizidone: interferes with peptidoglycan synthesis. Neuropsychiatric toxicity and seizures are clinically important cautions.

Delamanid: is another newer agent used in selected drug-resistant TB regimens; QT prolongation is an important concern.

Other agents: ethionamide/prothionamide, para-aminosalicylic acid and, in selected circumstances, amikacin may be considered when designing regimens according to susceptibility, previous exposure, toxicity and available effective oral drugs.

Exam distinction: “Second-line drug” does not mean that all such drugs are weak. Modern MDR/RR-TB therapy includes highly important agents such as bedaquiline, linezolid and fluoroquinolones; the regimen is selected according to resistance and eligibility rather than simply adding one drug to a failing first-line regimen.

WHO’s 2025 treatment framework prioritizes all-oral regimens for drug-resistant TB. The main duration categories are approximately 6-month regimens, 9-month regimens in appropriate patients, and longer individualized regimens when shorter options cannot be used. :contentReference[oaicite:3]{index=3}

AIM VISUAL 06 — How Drug Resistance Develops and How MDR-TB Therapy Responds

G. Treatment of Pulmonary TB, Multidrug Therapy and Special Situations

The aim of TB treatment is not only to improve symptoms. Effective therapy must rapidly reduce the number of viable organisms, eliminate persistent bacilli that could cause relapse, prevent emergence of drug resistance and interrupt transmission. For these reasons, active TB is treated with a combination of effective drugs for an adequate duration.

Why multidrug therapy is essential

A pulmonary cavity can contain a very large bacterial population. Within this population, a small number of bacilli may possess spontaneous resistance to one drug. Monotherapy would remove susceptible organisms and allow the resistant subpopulation to dominate. The chance that one organism is naturally resistant to several unrelated drugs simultaneously is far lower. Combining drugs therefore makes resistance much less likely while also allowing drugs with different activity against rapidly multiplying and more persistent bacilli to work together.

Newly diagnosed drug-susceptible pulmonary TB

A well-established WHO regimen for drug-susceptible TB is 6 months: an initial 2-month phase with isoniazid, rifampicin, pyrazinamide and ethambutol followed by a 4-month continuation phase with isoniazid and rifampicin, written as 2HRZE/4HR. Daily administration is used. :contentReference[oaicite:4]{index=4}

WHO also recognizes a shorter 4-month rifapentine–moxifloxacin-containing regimen for eligible patients, but the traditional 2HRZE/4HR regimen remains a core undergraduate regimen and is widely used. :contentReference[oaicite:5]{index=5}

Management principles in MDR/RR-TB

MDR/RR-TB should be managed according to rapid resistance testing, prior drug exposure and susceptibility to key agents. It should never be managed simply by adding one new drug to a failing regimen. Current WHO guidance prioritizes eligible patients for short all-oral regimens. A major 6-month option is BPaLM—bedaquiline, pretomanid, linezolid and moxifloxacin. Other 6-month all-oral options are also available, while approximately 9-month regimens or longer individualized regimens are used when appropriate. :contentReference[oaicite:6]{index=6}

Pregnancy

Active TB during pregnancy requires treatment because untreated maternal disease presents substantial risk to both mother and fetus. WHO states that the standard 6-month drug-susceptible TB regimen can be used during pregnancy. Pyridoxine supplementation is recommended when isoniazid is used. Drug-resistant TB in pregnancy needs specialist selection of drugs because the safety profile of individual agents must be considered carefully. :contentReference[oaicite:7]{index=7}

Hepatic insufficiency

Isoniazid, rifampicin and especially pyrazinamide can cause hepatotoxicity. Pre-existing liver disease therefore requires careful assessment of baseline hepatic function, the severity of liver disease and the number of hepatotoxic drugs that can safely be used. Significant symptoms or biochemical evidence of drug-induced liver injury during treatment requires prompt clinical review rather than routine continuation without assessment.

Renal insufficiency

Isoniazid and rifampicin are mainly handled hepatically and generally present fewer dosing problems in renal impairment than drugs that depend substantially on renal elimination. Ethambutol and pyrazinamide require particular attention in significant renal impairment, and the dosing interval or regimen may need adjustment according to renal function and specialist guidance. Pyridoxine is also important in patients receiving isoniazid when renal failure increases the risk of neuropathy.

Monitoring response

Clinical improvement, microbiological response, adherence and adverse effects should all be monitored. Failure to improve should trigger assessment for poor adherence, incorrect diagnosis, malabsorption, major drug interaction or drug resistance rather than automatic extension or random alteration of treatment. WHO emphasizes rapid testing for resistance when treatment failure is suspected. :contentReference[oaicite:8]{index=8}

AIM VISUAL 07 — Treatment Logic of Pulmonary Tuberculosis

H. Sarcoidosis

Sarcoidosis is a multisystem inflammatory disorder characterized pathologically by non-necrotizing granulomas. The exact initiating cause is not established. The disease appears to result from an exaggerated cell-mediated immune response to one or more unidentified antigens in genetically susceptible individuals.