Please answer.
Here is the sentence (article) :
Frequently, derealization occurs
in the context of constant worrying or '
http://en.wikipedia.org/wiki/Intrusive_thoughts' that people find hard to switch off.
(In such cases it can build unnoticed along with the underlying
http://en.wikipedia.org/wiki/Anxiety attached to these disturbing thoughts, and be recognized only in the aftermath of a realization of crisis, often a
http://en.wikipedia.org/wiki/Panic_attack, subsequently seeming difficult or impossible to ignore. This type of anxiety can be crippling to the affected and may lead to avoidant behaviour. Those who experience this phenomenon may feel concern over the cause of their derealization. It is often difficult to accept that such a disturbing symptom is simply a result of anxiety, and the individual may often think that the cause must be something more serious. This can, in turn, cause more anxiety and worsen the derealization.
WHAT is mean this first sentence?