Sequela

Sequela (pronounced si-kwel-uh or si-kwee-luh)

(1) Any abnormal bodily condition or disease related to or arising from a pre-existing disease

(2) Any complication of a disease (technically wrong but widely used).

(3) In general, non-medical usage, the terms sequela and sequelae mean consequence and consequences (rare except in academic writing).

1793: From the Latin sequēla (that which follows, consequence) from sequi (present active infinitive of sequor) (to follow); the plural is sequelae and the original use was in pathology.  From the same root, Middle English gained sequence, a borrowing from the French sequence (a sequence of cards, answering verses).  In Latin the related forms were sequentia (a following) and sequens (following); in English, the most common relation is sequel.

Strangely, in oral use, regardless of the details being described, sequela is used usually in the plural.  A sequela is a pathological condition caused by some previous disease, injury, therapy, or other trauma and, typically but not exclusively, is a chronic condition caused by an earlier and more acute condition.  It is different from, but is a consequence of, the first condition.  In medicine, to be defined as a sequela (rather than what is called “late effect”), the onset of the second condition must happen within a relatively (varies with the nature of the first) short time; an interval of decades would see the second dubbed a “late effect”.  Some conditions may be diagnosed retrospectively from their sequelae; an example is pleurisy and the range of symptoms now collectively referred to in medical slang as “Long COVID” is more correctly described as the “Post-acute Sequelae of COVID-19 (PASC)”.  As nouns, the difference between sequela and complication is that sequela is a condition which is caused by an earlier disease or problem.  A complication is something going wrong; an unfavorable result of a disease, health condition, or treatment.

The Sequelae of COVID-19

The novel virus SARS-CoV-2 caused the COVIS-19 pandemic.  Around the world, a subset of patients who sustain an acute SARS-CoV-2 infection subsequently develop a wide range of persistent symptoms that do not resolve, even over many months; the diagnosis is Post-acute sequelae of COVID-19 (PASC), the verbal shorthand for which is the more digestible Long COVID.  Thus far, all findings about PASC are inconclusive but it appears individual patients may have one or more underlying biological factors driving their symptoms, none of which need be mutually exclusive.  Potential contributors to PASC include injury to one or multiple organs, persistent reservoirs of SARS-CoV-2 in certain tissues, the induced re-activation of pathogens such as herpesviruses, SARS-CoV-2 interactions with host microbiome communities, clotting or coagulation issues, dysfunctional brainstem or nerve signaling, ongoing activity of primed immune cells and autoimmunity due to molecular mimicry between pathogen and host proteins.

Logo of the Sequela Foundation.

Different PASC symptoms in different patients does imply different therapeutic approaches will likely be required and the identification of individual human genome variants that may augment environment-driven contributions to PASC will contribute to strengthening the effort.  The individualized treatment of PASC will stimulate what was already a growing interest in personalized predictive & preventative medicine.