The differential diagnosis of cough is best developed with the use of anatomy. Cough may arise from an irritative focus anywhere along the respiratory tract. The irritation may be intrinsic, in which case it is usually inflammatory, neoplastic, or toxic, or it may be extrinsic, in which case it is often neoplastic or vascular (Table 20).
Intrinsic irritation
Pharyngitis, whether due to virus, streptococcus, or diphtheria, is a common cause of cough. Hypertrophied tonsils or adenoids may also initiate the cough reflex. Other pharyngeal causes are angioneurotic edema, leukemia, and agranulocytosis. The esophagus is an extrinsic cause of cough in most cases, but a tracheoesophageal fistula from esophageal carcinoma or reflux esophagitis with repeated aspiration of hydrochloric acid (HCl) may cause a chronic cough. Diverticula of the esophagus may press on the trachea and cause a cough.
In the larynx, the numerous infections of the pharynx discussed above may irritate the cough centers but, in addition, laryngeal polyps, tuberculosis, and trauma from overuse are important causes. The more common causes of cough, especially a nonproductive cough, are in the tracheobronchial area. Numerous viruses cause tracheobronchitis, especially influenza, but bacterial causes such as whooping cough should always be considered. Tuberculosis and carcinoma are important here, as are toxic gases such as chlorine and cigarette smoke. Bronchiectasis, whether congenital or acquired, and the associated postnasal drip from chronic sinusitis must not be forgotten. A search for asthma is important in areas with high pollen counts.
In the alveoli, in addition to pneumonia, tuberculosis, and carcinoma (particularly metastatic), several new etiologies are added. Thus, pulmonary embolism, parasites, fungi (such as actinomycosis), pneumoconiosis, reticuloendothelioses, and autoimmune diseases (i.e., Wegener granuloma) should be included.
Extrinsic irritation
The extrinsic causes are mainly from the structures of the mediastinum, especially the heart. A large heart from CHF or a single chamber enlargement (as in mitral stenosis) may compress the bronchus and recurrent laryngeal nerve and cause a cough. Pericarditis, aortic aneurysms, and rings are other cardiovascular causes. Finally, other structures in the mediastinum such as a substernal thyroid, a large lymph node from Hodgkin lymphoma, and occasionally a dermatoid cyst must be considered. Trauma can lead to a cough whether it hits the lung, mediastinum, or pericardium.
Approach to the Diagnosis
Clinically, exposure to dust, smoke, and various gases should be looked for in the patient presenting with a cough. Postnasal drip from chronic sinusitis should be ruled out. An allergic history (e.g., hay fever) is important. Cardiovascular disease should be carefully excluded, especially when sputum is negative for routine cultures, tuberculosis, fungi, and Papanicolaou smears and chest x-rays, bronchoscopy, and bronchography are normal. Hysterical cough should be considered, however, as well as reflux esophagitis and hiatal hernia. A sputum and nasal smear for eosinophils should be done to rule out asthma. A trial of therapy may be indicated.
Other Useful Tests
1. CBC and C-reactive protein (CRP) (pneumonia)
2. Sedimentation rate (infection)
3. Sputum smear and culture (pneumonia)
4. Sputum volume study (bronchiectasis)
5. Sputum for eosinophils (asthma)
6. Arterial blood gases (chronic pulmonary disease)
7. Sputum cytology (neoplasm)
8. Sputum for AFB smear and culture (tuberculosis)
9. Sputum for fungal smear and culture
10. Tuberculin test
11. Histoplasmin skin test
12. Coccidioidin skin test
13. Blastomycin skin test
14. Sweat test (fibrocystic disease)
15. α1-Antitrypsin assay (pulmonary disease due to α1-antitrypsin deficiency)
16. Pulmonary function testing (CHF, chronic pulmonary disease)
17. Barium swallow (hiatal hernia with reflux esophagitis)
18. Cold agglutinins (mycoplasma pneumonia)
19. Serologic tests (Legionnaires' disease, mycoplasma pneumonia)
20. X-ray of sinuses (sinusitis)