Insidious onset means that a condition or its symptoms began gradually and were difficult to notice at first. It describes how the problem started, rather than naming its cause or saying how serious it is.
The word alone does not tell you that. It does not mean that a condition was necessarily severe by the time it was noticed, or that there were no symptoms at all. Early changes may simply have been hard to recognise.
It can help to describe the earliest changes you remember and how they developed. The phrase does not provide an exact start date. It is one part of the history, which is considered together with the symptoms and examination findings.

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