Scientists may have got more of a grasp on the somewhat mysterious inflammatory syndrome that can occur in children with the coronavirus.
While youngsters rarely become seriously ill with the infection itself, NHS doctors were told to look out for signs of "multi-system inflammation" after intensive care units in London saw eight children with unusual symptoms, some of whom tested positive for the coronavirus.
The mysterious inflammation has been likened to atypical Kawasaki disease; a rare condition that usually affects children under five and causes blood vessels to become inflamed, leading to heart complications in around a quarter (25%) of patients. Left untreated, it can be fatal in 2% to 3% of cases.
To learn more, scientists from the US Centers for Disease Control and Prevention (CDC) analysed over 1,700 people aged under 21 who "met the definition" for multi-system inflammation.
Results suggest cases peaked between two and five weeks after coronavirus incidences reached a high, suggesting the mysterious inflammation "results from delayed immunologic responses" to the infection.
Watch: Coronavirus can cause inflammatory syndrome in children
The participants reported to the CDC showed signs of multi-system inflammation between March 2020 and January 2021.
This was defined as having a fever, at least two "organ system involvement", "laboratory evidence" of inflammation and a confirmed coronavirus infection. Patients were excluded if there was another "plausible explanation" for their illness.
They were then divided into groups according to the onset of their symptoms – before or after 1 July 2020, in line with the US' "two major" multi-system inflammation peaks.
Everything from gastrointestinal symptoms and rashes to low blood pressure and pneumonia were reported.
"Nationally, the first two MIS-C [multiple inflammatory syndrome in children] peaks followed the COVID-19 [the disease caused by the coronavirus] peaks by two to five weeks," the scientists wrote in the journal JAMA Pediatrics.
"The geographic and temporal association of MIS-C with the COVID-19 pandemic suggested MIS-C resulted from delayed immunologic responses to [the coronavirus] infection."
With a third coronavirus wave threatening the US, the scientists have urged medics maintain a "high index of suspicion" when presented with a potential multi-system inflammation case.
The Royal College of Paediatrics and Child Health (RCPCH) has previously said "paediatric inflammatory multi-system syndrome temporally associated with coronavirus" can occur alongside or a few weeks after a positive or negative test for the infection, providing any other "microbial cause" is ruled out.
While cases of the inflammatory syndrome have spiked during the pandemic, not everyone with the condition swabs positive for the coronavirus, which may come down to a false-negative result.
Scientists from the New York University Grossman School of Medicine have previously noted "significant clinical overlap exists" between Kawasaki and the coronavirus-related inflammatory syndrome.
The latter, however, "has been characterised by more widespread systemic inflammation and higher rates of acute complications, including cardiogenic shock"; when the heart suddenly cannot pump enough blood to meet the body's needs.
The inflammatory disorder has also been likened to toxic shock syndrome (TSS).
Often associated with tampon use, TSS is a medical emergency that can lead to fever, flu-like symptoms, nausea, vomiting and difficulty breathing.
Experts have warned these symptoms are a sign the body is overwhelmed as it tries to fight an infection.
While the inflammatory syndrome may sound alarming, the vast majority of children are said to recover with hospital treatment, with experts repeatedly stressing there is no need to panic.
The RCPCH previously advised parents call 999 or take their child to A&E if they:
Become pale, mottled or abnormally cold to touch
Pause in their breathing, have an irregular breathing pattern or grunt
Have severe breathing difficulties, while becoming agitated or unresponsive
Go blue around the lips
Have a seizure
Become extremely distressed, confused, lethargic or unresponsive
Develop a rash that does not disappear with pressure, like when pressed under a glass
Have testicular pain, especially teenagers
Great Ormond Street Hospital has also flagged fatigue, red or cracked lips, swelling and peeling of the hands and feet, headaches, swollen lymph nodes in the neck, and unexplained irritability as symptoms of coronavirus-related inflammation.
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