Silent Aspiration: Definition, prevalence, risk factors
Aspiration is when secretions, food, liquid or gastric contents enter into the trachea, past the vocal folds and possibly into the lungs. When aspiration occurs, the normal response is to cough or throat clear to attempt to remove the aspirate from the airway.
The cough reflex has both sensory (afferent) and motor (efferent) components. Pulmonary irritant receptors (cough receptors) in the epithelium of the respiratory tract are sensitive to both mechanical and chemical stimuli. Stimulation of the cough receptors by dust or other foreign particles produces a cough, which is necessary to remove the foreign material from the respiratory tract before it reaches the lungs.
The cough receptors are located mainly on the posterior wall of the trachea, pharynx, and at the main carina, the point where the trachea branches into the main bronchi. The receptors are less abundant in the distal airways, and absent beyond the respiratory bronchioles. When triggered, impulses travel via the internal laryngeal nerve, a branch of the superior laryngeal nerve which stems from the vagus (CN X), to the medulla of the brain. This is the afferent neural pathway.
The efferent neural pathway then follows, with relevant signals transmitted back from the cerebral cortex and medulla via the vagus and superior laryngeal nerves to the glottis, external intercostals, diaphragm, and other major inspiratory and expiratory muscles. The mechanism of a cough is as follows:
- Diaphragm (innervated by phrenic nerve) and external intercostal muscles (innervated by segmental intercostal muscles) contract, creating a negative pressure around the lung.
- Air rushes into the lungs in order to equalise the pressure.
- The glottis closes (muscles innervated by recurrent laryngeal nerve) and the vocal cords contract to shut the larynx.
- The abdominal muscles contract to accentuate the action of the relaxing diaphragm; simultaneously, the other expiratory muscles contract. These actions increase the pressure of air within the lungs.
- The vocal cords relax and the glottis opens, releasing air at over 100 mph.
- The bronchi and non-cartilaginous portions of the trachea collapse to form slits through which the air is forced, which clears out any irritants attached to the respiratory lining.
Respiratory muscle weakness, tracheostomy, or vocal cord pathology (including paralysis or anesthesia) may prevent effective clearing of the airways. The reflex is impaired in the person whose abdominals and respiratory muscles are weak. This problem can be caused by disease condition that lead to muscle weakness or paralysis, by prolonged inactivity, or as outcome of surgery involving these muscles. Bed rest interferes with the expansion of the chest and limits the amount of air that can be taken into the lungs in preparation for coughing, making the cough weak and ineffective. This reflex may also be impaired by damage to the internal branch of the superior laryngeal nerve which relays the afferent branch of the reflex arc.
Definition of Silent Aspiration:
Patients who have impaired coughing are at increased risk of silent aspiration. Silent aspiration is when there is no outward signs of swallowing difficulty. Therefore, secretions, food, or liquid enter into the airway, past the vocal folds, and no cough, throat clear or distress occurs.
Mechanisms associated with silent aspiration may include central or local weakness/incoordination of the pharyngeal musculature, reduced laryngopharyngeal sensation, impaired ability to produce a reflexive cough, and low substance P or dopamine levels. In terms of prognosis, silent aspiration has been associated with increased morbidity and mortality in many but not all studies (Ramsey, D., Smithard, D., & Karla, L., 2005).
Aspiration without an accompanying cough or obvious distress has been demonstrated on videofluroscopy (VF) and fiberoptic endoscopic evaluation(FEES) of swallowing. If the patient does not cough or show signs of distress during aspiration, a bedside swallow assessment cannot detect it. Instead it requires an objective test- VF or FEES.
However, many researchers argue that silent aspiration is volume dependent. The 3-ounce water swallow challenge is when a patient drinks 3-ounces of water without interruption. The individual fails if he stops, coughs or chokes during or immediately after completion. Studies have shown that a larger volume will elicit a cough reflex in individuals who previously aspirated smaller volumes (Leder et. al, 2011).
Prevalence of silent aspiration:
Studies have shown as many as 25-30% of dysphagic patients undergoing VF aspirating silently. Silent aspiration occured in stroke patients 28-38% of the time. Patients with tracheostomy tubes have increased aspiration rates. Elpern et al. found that 50% of stable ventilator dependents aspirated and of those patients, 77% were silent aspiration (Ramsey, D., Smithard, D., & Karla, L., 2005).
Risk factors of silent aspiration:
Risk factors for silent aspiration are older age, previous cerebrovascular disease, insulin-dependent diabetes mellitus, previous myocardial infarction and chronic obstructive pulmonary disease. Other conditions are head/neck malignancy, trach/vent patients, Parkinson’s disease, myasthenia gravis and Down syndrome(Ramsey, D., Smithard, D., & Karla, L., 2005)..
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