Causes and mechanisms of colds
What causes colds?
Colds are usually caused by viral infections. One of the major symptoms of a cold is nasal congestion, which can also be the result of several other causes.1 Here we take a closer look at the causes of colds and their mechanisms.
What are the functions of a healthy nose?
A healthy nose:2,3
- Warms (just under normal body temperature 31-33oC), moistens (close to 98%), and filters inspired air before it enters the lungs, to ensure that the delicate lung tissue doesn’t become damaged by cold or dry air, or irritated by small particles (e.g. dust).
- Reduces infections caused by viruses.
- Detects and destroys foreign substances before they enter the rest of the body (nasal membranes bring air in contact with immunoglobin, a protein that detects foreign substances such as viruses, bacteria, and allergens, triggering an immune response to destroy them before they enter the rest of the body).
- Allows us to smell the air as it flows through the nose, which contributes towards our sense of taste.
- Allows the air resonating in the nose to help give our voice its particular sound.
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Causes of nasal congestion4,5
Nasal congestion
Nasal congestion is caused by swelling of the mucus membrane:6
- It is not caused by too much mucus, as is often thought, although excessive mucus can exacerbate nasal congestion6
- It is one of the oldest and most common human complaints7- a ‘stuffy’ nose is among the most troublesome symptoms of viral infections of the nose6
- It leads to difficulties in breathing, hearing and sleeping; and can also cause voice changes that may be socially embarrassing8
- It could also lead to impairment in the ability to smell
1- Viral infection
Common cold and influenza
2- Allergies
Pollens, animal dander, certain foods, insect venom, latex and various medicines.
3- Inhaled irritants/pollutants
Gases, metals, organic chemicals and biological material surrounding a carbon core.
4- Structural abnormalities
Septal deviation, concha bullosa and adenoid hypertrophy
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Nasal congestion: Viral infections
Colds and influenza are major causes of nasal congestion
The common cold and influenza are both classed as upper respiratory tract viral infections. They share similar symptoms, but there are some notable differences:
The common cold is the most prevalent acute illness in the world:1
- Referred to as a single illness, but caused by numerous viruses from different families1
- Rhinoviruses are the most common cause, accounting for >80% of cases in the peak season1
- Usually self-limiting and confined to the upper respiratory tract1
- Nasal congestion and rhinorrhoea (runny nose) are the main clinical symptoms, with sore throat and cough also common1,9
- The average adult experiences 2 to 4 colds annually10
- Children experience 6 to 8 episodes per year10
Influenza:
- Primarily caused by either of two categories of virus: Influenza A or B11
- Symptoms vary from person to person
- Additional symptoms could be non-nasal, including fever, myalgia and fatigue, and can lead to serious complications12,13
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Mechanisms of viral nasal congestion
How colds and flu cause nasal symptoms14-16
Colds and flu cause nasal congestion in a similar way, by triggering the immune response and stimulating the inflammatory cascade to release pro-inflammatory mediators such as cytokines, bradykinins and prostaglandins. These dilate the blood vessels in the nasal epithelium, leading to congestion in the nasal passages.
The body’s natural defences against viral infection cause the familiar symptoms of the common cold and flu:17-19
- Rhinovirus infection triggers the release of pro-inflammatory and vasoactive mediators (cytokines, bradykinins, prostaglandin) from nasal epithelial cells14-16
- Vasoactive mediators cause dilation of large capacitance veins resulting in increased vascular permeability and leakage of plasma exudate into the submucosal space14-16
Excess fluid and inflammatory cells in the submucosal space cause swelling of the nasal mucosa, blocking the nasal passageway.14-16
