This was published in a Danish research journal, but I was able to find this pre-publication version in English ..... the author refers to "extended" withdrawal as "enduring" withdrawal ...
SEROTONIN AND SSRI DISCONTINUATION SYNDROME (2013)
The first part of the article is pure science, and can be skipped ... however, the last 2/3 of the article is the discussion, and is worth reading I think ... for example ....
From the article:
We don’t know what causes enduring antidepressant withdrawal. It may be nothing to do with serotonin. Similar problems happen on antipsychotics and benzodiazepines, too.
In fact, benzodiazepines, another antidepressant class of drugs, are involved in a sort of withdrawal symptoms like SSRI, so there is no evidence of difference between their side effects.
Researchers also noticed that there are, actually, no differences between benzodiazepine and SSRI action.
What is the difference between dependence and withdrawal reactions? A comparison of benzodiazepines and selective serotonin re-uptake inhibitors.2012
SSRI withdrawal varies among individuals: may not be a problem for someone, while for others can last years. All the enduring problems can follow either abrupt or tapered discontinuation of treatment.
Also nowdays we don’t know how common these states are: many people presenting to their doctors are told they have a recurrent mood disorder and are put back on an antidepressant because the problems look “depressive”, and most doctors don’t think that problems of this sort could persist this long.