Cough Reflex: Anatomy, Causes & Approach - Full 10-Mark MD Exam Answer

I. Definition

II. Anatomy of the Cough Reflex Arc

A. Cough Receptors (Sensory Transducers)

• Most sensitive sites: Larynx (especially the posterior aspect), carina, branching points of main bronchi
• Tracheobronchial tree (main bronchi and large airways - richly innervated)
• Lower oropharynx and hypopharynx
• External auditory meatus and tympanic membrane (Arnold's nerve - auricular branch of vagus) - explains ear/syringing-induced cough
• Esophagus and stomach (explains GERD-induced cough)
• Pleura, pericardium, diaphragm (mechanical receptors only)
• Paranasal sinuses

Important: It is difficult/impossible to initiate cough from small airways and alveoli - teleologically sound because even vigorous cough cannot generate sufficient airflow to clear distal airways.

B. Afferent Pathway

• Primary nerve: Vagus nerve (CN X) - carries signals from all the above sites
• Other afferents:
  • Trigeminal nerve (CN V) - from nasal mucosa
  • Glossopharyngeal nerve (CN IX) - from pharynx
  • Phrenic nerve - from diaphragm and pleura
• Cell bodies lie in the inferior vagal ganglion (nodose ganglion)
• Afferent impulses travel to the medulla oblongata diffusely
• Synapse in the Nucleus Tractus Solitarius (NTS)

C. Central Pathway (Cough Center)

• Located in the upper brainstem and pons (medulla oblongata)
• Nucleus Tractus Solitarius (NTS) receives all afferent input
• Second-order neurons from NTS project to respiratory-related regions of the pons, medulla, and spinal cord
• A cough pattern generator in the brainstem coordinates the sequential motor output
• Higher cortical centers (cerebrum) modulate the reflex:
  • Sensory processing: Urge-to-cough sensation, stimulus localization, intensity discrimination
  • Motor modulation: Voluntary cough induction, effort modulation, voluntary suppression of cough
  • Affective processing: Unpleasantness, emotional response to cough

D. Efferent Pathway

• Recurrent laryngeal nerve (branch of vagus) - to laryngeal muscles
• Phrenic nerve (C3, C4, C5) - to diaphragm
• Spinal motor nerves (T1-L2) - to intercostal, abdominal, and accessory muscles

E. Effector Organs (Expiratory Muscles)

• Diaphragm
• Internal intercostal muscles
• Abdominal wall muscles (especially rectus abdominis, external oblique)
• Laryngeal muscles (for glottis closure and sudden opening)
• Accessory muscles of respiration

III. Phases of the Cough Reflex (Mechanics)

• Guyton & Hall, Medical Physiology, p.501

IV. Causes of Cough

Classification by Duration

Common Causes (Murray & Nadel Table 37.1):

• Viral URTI / Tracheobronchitis
• Bronchopneumonia, Viral pneumonia
• Pertussis ("whooping cough")
• Acute-on-chronic bronchitis

• Pulmonary tuberculosis (TB)
• Cystic fibrosis
• Bronchiectasis

• Asthma (including cough-variant asthma)
• Non-asthmatic eosinophilic bronchitis
• Chronic bronchitis / COPD
• Upper Airway Cough Syndrome (UACS) / Chronic postnasal drip

• Interstitial pulmonary fibrosis
• Emphysema
• Sarcoidosis

• Lung cancer (most important to exclude)
• Benign airway tumors
• Mediastinal tumors

• Left ventricular failure / pulmonary edema
• Pulmonary infarction (PE)
• Aortic aneurysm

• Gastroesophageal reflux disease (GERD) / Laryngopharyngeal reflux
• Recurrent microaspiration
• ACE inhibitor use (bradykinin accumulation - classic drug cause)
• Aspiration of foreign bodies (especially in children)
• Middle ear pathology (Arnold nerve stimulation)
• Psychogenic/habit cough
• Endobronchial lesions

• GERD
• Asthma (including cough-variant)
• ACE inhibitors
• Rhinitis/UACS (upper airway cough syndrome)
• Drugs/Diffuse parenchymal lung disease

V. Approach to a Patient with Cough

Step 1: History

• Duration: Acute vs. subacute vs. chronic
• Character: Dry/non-productive vs. productive (purulent, mucoid, hemoptysis)
• Timing: Nocturnal (LVF, asthma, GERD), postprandial (GERD/aspiration), positional
• Associated features: Fever, wheeze, dyspnea, weight loss, heartburn, postnasal drip
• Drug history: ACE inhibitors (must stop and re-evaluate after 4 weeks)
• Smoking history
• Occupational/exposure history
• Features of enhanced cough reflex: Triggered by laughing, cold air, deep breath, prolonged talking

Step 2: Physical Examination

• General: Fever, weight loss, clubbing, lymphadenopathy
• ENT: Nasal polyps, sinusitis, pharyngeal cobblestoning (postnasal drip)
• Ear: External auditory canal (Arnold nerve)
• Chest: Wheeze (asthma), crepitations (LVF, fibrosis, pneumonia), signs of consolidation
• Abdomen: Hepatomegaly (RHF)

Step 3: Investigations

• Chest X-ray (CXR) - exclude malignancy, TB, pneumonia, LVF
• Spirometry - obstructive (asthma/COPD) vs restrictive pattern
• Stop ACE inhibitor if on it; reassess at 4 weeks

• For UACS: ENT review, CT sinuses
• For Asthma: Peak flow monitoring, bronchodilator reversibility, methacholine challenge
• For GERD: 24-hour pH monitoring, empirical PPI trial
• For Eosinophilic bronchitis: Induced sputum eosinophil count (>3%)
• For TB: Sputum AFB smear/culture, Mantoux, IGRA
• For malignancy: CT chest, bronchoscopy + biopsy
• For ILD: HRCT thorax, BAL, lung biopsy
• For LVF: Echocardiogram, BNP
• For aspiration: Modified barium swallow, pH-impedance study

Step 4: Algorithmic Management of Chronic Cough

1. Stop ACE inhibitor, stop smoking
2. Chest X-ray to exclude malignancy and serious intrathoracic pathology
3. Treat most likely cause systematically (GERD → PPI; Asthma → inhaled steroids ± bronchodilator; UACS → intranasal corticosteroids, antihistamines; Eosinophilic bronchitis → inhaled corticosteroids)
4. If all common causes excluded, refer to specialized cough clinic
5. Consider neuromodulators (gabapentin, pregabalin) for refractory chronic cough with sensitized cough reflex

VI. Special Points for MD Exam

• Arnold's nerve reflex cough (auricular branch of vagus): Stimulation of external auditory canal (ear syringing, foreign body) triggers cough - clinically important
• ACE inhibitors cause cough in ~10-15% patients (more in women, Asian populations) via bradykinin/substance P accumulation - switch to ARB
• Cough syncope - prolonged coughing raises intrathoracic pressure, reduces venous return, causes transient cerebral hypoperfusion
• The triad of chronic cough: UACS + Asthma + GERD accounts for >90% of chronic cough cases in non-smokers not on ACE inhibitors with normal CXR
• C fibers vs. Aδ fibers: C fibers (nociceptors) - slow, unmyelinated, chemical stimuli; Aδ fibers (mechanoreceptors/RAR) - fast, myelinated, mechanical stimuli - both converge on NTS
• Chronic cough sensitization (central and peripheral) is the mechanism behind refractory chronic cough

• Guyton and Hall Textbook of Medical Physiology, 14e, p.501 (Cough Reflex - sequence of events, pressures, velocities)
• Murray & Nadel's Textbook of Respiratory Medicine, 7e, Chapter 37 (Sensory receptors, receptor types, approach, causes table)
• Polverino et al. "Anatomy and Neuro-Pathophysiology of the Cough Reflex Arc." Multidisciplinary Respiratory Medicine (PMC3415124)