Also what can we learn from the common cold about psychological and psychiatric conditions? (or conditions like psychological fatigue and chronic fatigue)
Reading notes from the article: “Mechanisms of symptoms of common cold and flu” by Ronald Eccles | Common Cold Centre, Cardiff School of Biosciences, Cardiff University
From the book: “Common Cold” ed. by R. Eccles and O. Weber © 2009 Birkhäuser Verlag
What causes the symptoms?
The symptoms are caused by the immune response.
The major cell type monitoring the host for viral infection is the macrophage.
The surface of the macrophage has Toll-like receptors that recognize components of the virus (or bacteria in the case of bacterial infections) and induce the production of cytokines.
Cytokines recruit other immune cells and trigger inflammation as a local response or circulate in the blood stream and generate systemic responses/symptoms such as fever.
The inflammatory mediator bradykinin is thought to be a most important factor that causes the local symptoms of URTI, such as sore throat and nasal congestion.
What causes a sore throat?
Bradykinin stimulates pain nerve endings in the airway to cause the sensation of sore throat pain and this is enhanced by the presence of prostaglandins.
The implicated nerves are the cranial nerves that innervate the nasopharynx and pharynx.
Paracetamol and aspirin that inhibit prostaglandin synthesis provide relief for sore throat.
What causes a “runny nose”?
Early symptoms consist of a watery nasal secretion from the nasal glands. This can be controlled by anticholinergic treatments that act on the glands.
In the later phase the nasal discharge is a complex mix from nasal and lacrimal glands, goblet cells, plasma cells and plasma exudates from capillaries. This cannot be inhibited by anticholinergics.
The nasal discharge and sputum may be colorless, yellow or green. Neutrophils and pro-inflammatory monocytes have azurophil granules that contain the green protein myeloperoxidase. If the nasal discharge contains few leukocytes it is clear or white, if it has a significant number of leukocytes it is yellow and if it has a very large number it becomes green.
What causes a blocked nose or “nasal congestion”?
Vein swelling which is the dilation of large capacitance veins that are sometimes called “erectile tissue” as they can swell and block the nose by limiting the nasal airflow.
These are controlled by sympathetic vasoconstrictor nerves that release the neurotransmitter noradrenaline (norepinephrine), which is a constrictor of blood vessels.
Topical or oral administration of sympathomimetics such as pseudoephedrine causes constriction of nasal erectile tissue and nose decongestion.
The nasal veins are five times more sensitive to adrenaline than the heart. Therefore it possible to use low doses of the above substances without side effects on the heart.
What causes cough?
The cough reflex is due to mechanical stimulation of the larynx.
If the larynx is inflamed and hyperreactive, cough may be spontaneous or may be linked to stimulii like cold air or airway vibration.
If infection and inflammation spreads to lower airways it can lead to mucus production and expectoration. Cough linked to expectoration is called productive.
What causes headache?
Could be related to cytokines (e.g. TNF of interferons-IFNs). Cytokines increase the levels of prostaglandin E2 (PGE2) in the brain and CSF and it is likely that prostaglandins are involved in headache as their synthesis inhibitors (aspirin, paracetamol, ibuprofen) are effective against headache.
What causes fever?
Cytokines released from macrophages and other leukocytes, most importantly IL-2 and IL-6. They cross the blood-brain barrier or interact with the vagus nerve to signal to the hypothalamus to increase temperature. In the hypothalamus (ventromedial preoptic area abbr. VPMO) they induce cyclooxygenase (COX)-2-dependent prostaglandin synthesis which results in shivering, constriction of skin blood vessels and chilliness.
What causes mood changes, psychological effects and fatigue?
Cytokines released from leukocytes.
Exogenous administration of cytokine IFN-α is used as therapy for chronic viral diseases such as hepatitis B and C, and therapy is associated with flu-like side effects similar to those observed in respiratory track infection, such as mood changes and fatigue.
Psychiatric side effects such as depression, irritability, lack of motivation, impaired concentrations, psychoses and confusional states have been reported in patients with IFN-α treatment.
Cytokine-induced alterations in serotonin metabolism and dopamine in basal ganglia play important role in depression and fatigue and this may be the mechanism implicated in respiratory track infection.
Seema Sarin, MD, Bradley Undem, PhD, Alvin Sanico, MD, Alkis Togias