Causes of Stridor and Wheeze (Breathing Sounds)

stridor vs wheezing

Stridor and wheezing are two terms for different breathing sounds that are often used synonymously although there is a difference. Wheezing refers to the high-pitched or whistling sound, which is most prominent when breathing out (expiration). A wheeze is usually heard clearly upon auscultation (use of a stethoscope) although at times it may be audible, especially to the patient, without any need for a stethoscope. A stridor is also high pitched but ‘rough’ sound sometimes described as crowing. It  easily audible without the use of a stethoscope (auscultation) and while it is present when breathing in (inspiration) or out (expiration), it is usually more prominent on inspiration.

Anatomy of the Respiratory System

Tracheobronchial tree leading to the lungs

The respiratory system is made up of the organs that allow ventilation, which is the entrance and exit of gas or air passages, and the lungs, which is responsible for gas exchange between the air and the blood. Starting from the nose, air travels through the nasal cavity, into the pharyng, larynx and trachea (air or wind pipe). Here the air will diverge into the two bronchi, each of which divide multiple times to form bronchioles that empties air into the air sacs (alveoli) of the lungs. The tracheobronchial tree is a series of ‘tubes’ that lead to the lungs. These tubes are thin walled, lined with membranes that secrete mucus, and contain smooth muscle in the wall. Under certain conditions, the membranes may produce more mucus, the smooth muscle may contract or the wall of the tubes may become inflamed resulting in swelling. All these mechanisms will cause constriction (narrowing) of the tubes to varying degrees thereby affecting the flow of air. However any mass within the airways, whether a foreign body, abscess or tumor will also affect the passage of the air. The disrupted airflow is often the reason for abnormal breathing sounds like a wheeze or stridor, similar to the changes in the flow of air through a flute.

Abnormal Breathing or Chest Sounds

Wheezing is a result of irregular narrowing of the lower airways, often seen in asthma, while a stridor is a mechanical (physical abnormalities) or functional (impaired functioning) of the air passages, including the throat.A stridor is more often seen in respiratory infections, disorders of the vocal cords, tumors or allergic reactions. Irrespective of the type of sound, any abnormal chest sounds heard upon breathing should be investigated by a medical doctor to verify the extent to which the air flow is affected. Subsequently the cause of the wheeze or stridor has to be identified and treated accordingly.

It is difficult to always ascertain whether a sound is a stridor or a wheeze, and a stridor is often described as a wheeze by most people. There are a number of causes of wheezing and a stridor and some of the causes may overlap. It is therefore important to take note of other signs and symptoms or the results of clinical tests and diagnostic techniques to make a final diagnosis.

Causes of Wheezing and Stridor

There are various causes of wheezing and stridor, all of which affect the flow of air through the respiratory passages to some degree. Some of these causes include the following conditions.

  • Asthma is probably the most well known cause of a wheeze. There is usually a history of asthma and it is triggered by certain stimuli, like inhaled allergens (dust, pollen, etc) or ingested allergens (dairy, nuts, etc), environmental conditions like cold or wet weather, or strenuous physical activity (exercise induced asthma).
  • Acute bronchitis is usually the result of a respiratory tract infection and fever, pain and persistent coughing along with a stridor may be noticed. The cough is often productive, meaning that mucus (phlegm) is expectorated upon coughing.
  • Allergic reaction or anaphylaxis occurs after exposure to an allergen. Examples include hypersensitivity to shellfish, certain drugs, insect bites like a bee sting. The reaction causes narrowing of the airways shortly after exposure to the allergen and requires immediate medical attention to restore airway integrity.
  • Bronchiectasis is a lung disease that results in excessive mucus accumulation due to widening of the airways. The mucus build up causes an irregular air passage, affecting the flow of air and resulting in abnormal breathing sounds like a wheeze or stridor.
  • Common childhood respiratory infections that cause stridor include bronchiolitisand croup. Bronchiolitis is a common childhood infection of the respiratory system, which is common in the “cold and flu” season. Bronchiolitis is usually caused by the respiratory synctial virus (RSV) and presents with typical symptoms of a “chest infection”, along with rapid, shallow breathing. Croup is a respiratory tract that causes swelling of the trachea (wind pipe) and larynx and may be accompanied by stridor. It mainly affects children and has a characteristic barking-like cough.
  • Chronic obstructive pulmonary disease (COPD), like emphysema or chronic bronchitis, is progressive lung disease that typically occurs as a result of long term exposure to toxins that irritate the respiratory passages. This is often seen in cigarette smokers and may also be present in those with long term exposure to noxious fumes, air pollution, airbone particles and dust. Excessive mucus secretion, a “tight chest”, wheezing or stridor and difficulty in breathing are the characteristic symptoms of COPD.
  • Drugs like non-steroidal antiinflammatories, including aspirin and certain anti-hypertensive medicines may cause wheezing. This may be a result of the drug side effects or due to an allergic reaction to the active ingredient, filler (or bulking) substances in the drug or preservatives and colorants. In certain cases, drug interactions may occur when more than one drug is used simultaneously and can cause various signs and symptoms.
  • Tumors affecting the airway or lung may be the cause of a stridor. Endobronchial tumors or laryngeal tumors are more likely to be the cause of a stridor as it affects the passage of air to the lungs. However lung cancer will also cause abnormal breathing sounds, especially if the mass is at multiple sites or the tumor has grown to a substantial size.
  • Foreign object(s) that lodge in the airway passages are more likely to occur in children and will cause pain, along with wheezing and depending on the size of the object, difficulty breathing may also be present. In the cases of larger objects, the child may be in respiratory distress, turn “blue” (cyanosis) due to a lack of oxygen and urgent medical attention is required.
  • Gastroesophageal reflux disease (GERD) may cause aspiration, where gastric acid enters and irritates the airway lining, often causing spasm of the larynx. This is more likely to occur in chronic cases of GERD and develops slowly if the GERD is untreated. Typical symptoms include heartburn and/or chest pain.
  • Epiglottitis is a serious condition that results from a infection like strep throat. The epiglottis is a small flap that prevents food and drink from entering the lungs during swallowing. In a case of epiglottitis, the epiglottis becomes inflamed and swollen thereby impairing normal air flow. It may also occur as complication to chickenpox and in fungal infections of the throat and respiratory passages, often seen in HIV/AIDS.
  • Airborne irritants like smoke and fumes from certain chemicals can cause constriction of the respiratory passages when inhaled. This affects the airflow, resulting in a wheeze or stridor.
  • Most respiratory tract infections may cause a wheeze or stridor due to narrowing of the air passages as a result of inflammation or excess mucus production. Cases of a retropharyngeal abscess or peritonsillar abscess (quinsy), which are more likely in children, are uncommon and may require immediate medical attention as a retropharyngeal abscess can cause almost total obstruction of the airway and even result in death.
  • Vocal cord paralysis or dysfunction will result in a stridor along with changes in voice quality or even a loss of the voice. There are numerous causes from viral infections, to tumours, neck injury or neurological disorders like Parkinson’s disease.
  • Congenital (birth) anomalies of the tracheobronchial tree will result in stridor. These defects requires specialist medical attention as episodes may cause a total collapse or obstruction of the airway and can be fatal. Laryngomalacia is one of the more common anomalies in infants, where the airway collapses during inspiration due to impaired airway development and stridor is one of the defining features along with feeding difficulties.
  • Congestive heart failure is a slow progressing cardiac condition and a wheeze or stridor may be present for a long period of time. Other signs and symptoms of congestive heart failure include swelling of the legs, fluid in the lungs, shortness of breath and fatigue.
  • Swelling and spasm of the larynx may occur after intubation or even a bronchoscope and s stridor may be audible. This gradually settles and further treatment may not be necessary.
  • Pulmonary embolism is the result of blockage of one or more arteries of the lung. It causes sudden shortness of breath, severe chest pain and a cough. A wheeze or stridor may also be present.

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