SN5 SHORT NOTE 5M ★ High-yield

Classify pleural effusion. Describe malignant mesothelioma and add a note on pneumothorax.

Pathology · 2nd Professional — Para-Clinical

Pleural effusion is the accumulation of fluid in the pleural cavity, which normally contains only 10 to 20 mL of lubricating fluid.

Type	Nature	Causes
Hydrothorax (transudate)	Clear, protein under 3 g/dL	Cardiac failure (the commonest overall), nephrotic syndrome, cirrhosis, hypoalbuminaemia
Exudate	Turbid, protein over 3 g/dL	Infection (parapneumonic, tuberculosis), malignancy, pulmonary infarction, connective tissue disease, pancreatitis
Empyema	Frank pus	Bacterial pneumonia, lung abscess, penetrating trauma, oesophageal rupture
Haemothorax	Blood	Trauma, ruptured aortic aneurysm, malignancy
Chylothorax	Milky lymph rich in triglycerides	Thoracic duct obstruction — lymphoma, trauma, filariasis

Light criteria for an exudate — any one suffices

Pleural fluid protein to serum protein ratio above 0.5
Pleural fluid LDH to serum LDH ratio above 0.6
Pleural fluid LDH above two-thirds of the upper limit of normal serum LDH
Additional pointers — adenosine deaminase (ADA) and lymphocyte predominance suggest tuberculosis (important in India); low glucose and pH suggest empyema or rheumatoid disease; cytology and pleural biopsy for malignancy

Type	Cause
Spontaneous — primary	Rupture of subpleural bullae; tall thin young men; smoking
Spontaneous — secondary	COPD, tuberculosis, asthma, cystic fibrosis, Pneumocystis pneumonia
Traumatic and iatrogenic	Rib fracture, central line insertion, pleural aspiration, ventilation
Tension pneumothorax	A one-way valve lets air in but not out; pressure rises, the mediastinum shifts away, venous return falls — a life-threatening emergency requiring immediate needle decompression, not a radiograph

A malignant tumour of mesothelial cells, arising from pleura, peritoneum, pericardium or tunica vaginalis
Asbestos exposure in about 80%, with a latency of 25 to 45 years; even brief or indirect exposure suffices, including household contact with workers’ clothing
Not related to smoking — unlike asbestos-related bronchogenic carcinoma, whose risk smoking multiplies
Morphology — a thick, white, firm rind encasing and compressing the lung; three histological patterns — epithelioid (commonest, best prognosis), sarcomatoid, and biphasic
Immunohistochemistry is essential to distinguish it from metastatic adenocarcinoma — mesothelioma is calretinin, WT1 and CK5/6 positive and CEA and TTF-1 negative; adenocarcinoma is the reverse
Presents with breathlessness, chest pain and a recurrent bloody pleural effusion; prognosis is poor, with median survival about 12 months

CLINICAL PEARL

Asbestos causes both carcinoma and mesothelioma, and they behave quite differently.
Bronchogenic carcinoma is the commoner asbestos malignancy and its risk is multiplied by smoking.
Mesothelioma is rarer, is not affected by smoking, and needs far less exposure. Pleural plaques mark exposure but are themselves benign and do not become malignant.

Always aspirate an undiagnosed effusion and apply Light criteria; the transudate/exudate distinction directs everything that follows
In India a lymphocytic exudate with a high ADA is tuberculous until proved otherwise, and is often treated on that basis
Empyema requires drainage, not antibiotics alone; a loculated empyema may need fibrinolytics or surgical decortication
Tension pneumothorax is a clinical diagnosis — treat immediately with a needle in the second intercostal space in the midclavicular line; waiting for a radiograph kills patients
Mesothelioma is compensable industrial disease in many jurisdictions, and an occupational history must always be taken and recorded

All questions in Respiratory Pathology