Absence of Cough Reflex in Posterior Circulation Stroke
Posterior circulation strokes can impair the cough reflex because they damage brainstem structures—particularly the medulla—that contain the neural pathways essential for both the afferent sensory input and efferent motor coordination of the cough reflex. 1
Neuroanatomical Basis
The cough reflex depends on intact brainstem circuitry, specifically:
The medulla contains the cough center and houses critical cranial nerve nuclei (glossopharyngeal and vagus nerves) that mediate both the sensory detection of laryngeal irritation and the motor response required for effective coughing 2
Lateral medullary infarctions (Wallenberg syndrome) are particularly associated with impaired cough reflex and silent aspiration, as documented in case reports of posterior circulation stroke presenting with new continuous cough due to aspiration from dysphagia 1
The superior laryngeal nerve and glossopharyngeal nerve provide the afferent sensory limb of the cough reflex through TRPV1 receptors, and these pathways are vulnerable to posterior circulation ischemia 2
Clinical Implications and Risk Stratification
Reduction in laryngeal sensation and impaired cough reflex sensitivity are well-documented in stroke patients, placing them at substantially increased aspiration risk 2
Key clinical considerations:
Posterior circulation strokes, particularly brainstem lesions, delay or abolish the cough response to tussigenic challenges, even when anterior circulation strokes may preserve some reflex capacity 2
The prevalence of impaired cough reflex testing in post-stroke patients is relatively low (5.8%), but when present—especially in brainstem strokes—it indicates severe neurological compromise 3
Hemorrhagic strokes, wide circulation infarctions (TACI), and specifically brainstem strokes significantly delay the cough response to citric acid challenge testing 3
Contrast with Anterior Circulation Strokes
An important distinction exists between stroke locations:
Middle cerebral artery strokes can cause "cough apraxia" (inability to cough on command), particularly with left-sided lesions, but this represents a cortical motor planning deficit rather than true reflex absence 2
Bilateral strokes show weak or absent voluntary cough in 84% of aspirators, but this differs mechanistically from the brainstem-mediated reflex cough impairment seen in posterior circulation strokes 2
Clinical Assessment and Management
Alert patients at high risk for aspiration should be observed drinking small amounts of water (3 oz), and if they cough or show clinical signs of aspiration, they should be referred for detailed swallowing evaluation 2
Critical assessment points:
Patients with reduced consciousness are at extremely high risk for aspiration and should not be fed orally until consciousness improves 2
The subjective assessment of voluntary cough as the sole predictor of aspiration has uncertain value due to poor reliability, though objective aerodynamic measures show all voluntary cough parameters are abnormal in stroke patients compared to controls 2
Cough reflex testing to inhaled irritants (citric acid, capsaicin) shows impaired sensitivity in stroke patients, with bedridden tube-fed patients showing the highest pneumonia rates (64.3%) when they cannot cough even at maximum citric acid concentrations 2
Prognostic Significance
Oropharyngeal dysphagia has a much greater impact on clinical outcomes than impaired cough reflex alone, with the poorest prognosis occurring when both protective mechanisms are compromised 3
Outcome data demonstrate:
The first episode of pneumonia in dysphagic stroke patients occurs within the first month after stroke onset in all cases, with pneumonia associated with absent reflex cough, COPD, severe impairment of consciousness, and poor functional outcome 4
Implementation of structured dysphagia protocols incorporating cough reflex testing reduced aspiration pneumonia rates from 28% to 10% in acute stroke patients, with 81% returning to normal diet by 3 months 5
Subacute post-stroke oropharyngeal dysphagia prevalence (40.4%) far exceeds impaired cough reflex testing prevalence (5.8%), suggesting swallowing receives stronger cortical control than the cough reflex 3