Understanding Cough Mechanisms and Causes

Study Notes

Cough is a reflex mechanism to clear the airways of foreign bodies, retained secretions, and aspirated material.
An acute cough lasts for less than 3 weeks, while a chronic cough lasts for 8 weeks or more.
Cough receptors are located within the respiratory mucosa and lower esophagus.
Stimulation of cough receptors sends signals via the vagus nerve to the medulla oblongata, which in turn sends impulses to respiratory muscles via the phrenic nerve and spinal nerves.

Acute cough is often caused by viral infections and is usually self-limiting.
Chronic cough can be caused by debilitating illnesses like malignancy, tuberculosis, or other inflammatory conditions.

Pharynx: Post-nasal drip, infection, GERD, cigarette smoking, and allergies are common causes.
Larynx: Acute laryngitis is usually caused by viral infections (influenza, parainfluenza, RSV, adenovirus) or bacterial infections (Haemophilus influenzae, diphtheria). Chronic laryngitis is commonly caused by cigarette smoking, GERD, TB, and fungal infections in immunocompromised patients.
Trachea: Viral tracheitis is a common cause characterized by retrosternal pain and a usually dry cough.
Bronchi: Acute bronchitis often presents with a dry or productive cough, worse in the morning. Chronic bronchitis, commonly associated with long-standing cigarette smoking, leads to productive cough, also worse in the morning. Cough in asthma is usually worse at night.
Lung parenchyma: Cough in parenchymal lung diseases is often accompanied by hemoptysis (coughing up blood), as seen in pneumonia or cancer. Tuberculous cough is commonly associated with night sweats and fever, with or without hemoptysis.

The cough reflex can be weakened by weak respiratory muscles, chest wall deformities, abnormal airway secretions, and suppression of the respiratory center (due to drugs, encephalitis, or stroke).

Possible causes include ACE inhibitors, GERD, sinus disease with post-nasal drip, and cough-variant asthma (90% of cases).

Dyspnea refers to a subjective awareness of increased respiratory effort.
The onset of dyspnea can be sudden (minutes) or gradual (hours, days, weeks, months, or years).

Systemic causes: Anemia, obesity, psychogenic factors, thyrotoxicosis, metabolic acidosis
Cardiac causes: Pericarditis, pericardial effusion, cardiomyopathy, ischemic heart disease, mitral valve disease
Respiratory causes:
- Airway: Laryngeal tumor, foreign body, asthma, COPD, bronchiectasis, bronchogenic cancer, bronchopneumonia
- Parenchyma: Pneumoconiosis, interstitial lung disease (ILD), sarcoidosis, TB, lobar pneumonia, metastatic tumor
- Pulmonary circulation: Pulmonary embolism, primary pulmonary hypertension
- Pleura: Pneumothorax, pleural effusion, empyema
- Chest wall: Kyphosis, kyphoscoliosis, Guillain-Barré syndrome, myasthenia gravis

Dyspnea on flat position:
- Left-sided heart failure
- Ascites
- Pregnancy
- Large pleural effusion
- Respiratory muscle weakness
Dyspnea on sitting position:
- Right-to-left shunt due to patent foramen ovale or atrial septal defect (ASD)
Dyspnea on one side:
- Usually indicates the healthier lung is preferentially used for breathing due to disease on the other side.