Premenstrual Dysphoric Disorder (PMDD)
PMDD is basically PMS (premenstrual syndrome) so severe that it has an official label as a psychiatric diagnosis. Huh? Look at it this way: PMS is a term most english-speaking adults are familiar with, but it's not a "diagnosis". To offer women a "treatment", in our medical system, a "diagnosis" is basically required. Insurance companies make a big fuss over doctor visits for which there is no "diagnosis". Some doctors fear giving a treatment without a formal "diagnosis". For these purposes, "premenstrual syndrome" just didn't do the job -- so a new name was coined to meet these needs. This doesn't change the symptoms!
The other reason for a new name is to allow researchers to study severe PMS and separate it from milder forms. As you may know by now, there is a "rule book" for making diagnoses in psychiatry (other medical specialties use it too) called the DSM, the Diagnostic and Statistical Manual. This is used by researchers to classify and separate symptom patterns. The newest DSM version uses the term PMDD. This is actually an improvement -- it was "late luteal phase dysphoric disorder" in the last version!
However, for the purposes of anyone looking for help for PMS symptoms, these terms don't end up mattering much. You can call it PMS, or PMDD, and your doctor will know what you're talking about. What does matter, though, is that the symptoms you have only occur in the second half of your cycle.
If they happen even a little in the first half of your cycle, then whatever you have, it isn't just PMS. You may be seeing a (more medical jargon here) "premenstrual exacerbation of an existing condition", meaning that your symptoms get worse near your menstrual period, but are also there at other times.
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