Hi

... welcome to the board.
You are not alone in your plight, as you have probably noted by reading through the other posts here.
I don't know that I can give you any advice other than what is contained in this board and the corresponding board for women with PMDD. More than likely, your wife realizes that she has a problem, but she might truly have difficulty reconciling with the idea of actually admitting it to herself, and to you (which is what it sounds like by the way you describe things). There are many reasons why your wife may be reluctant to admit (or make a conscious realization) that she needs help. These include:
1. PRIDE, she might think of herself as a very capable and strong woman who doesn't have any weaknesses or need any help. You might compare this feeling to how you might think and feel on a deeply personal level if you were to be stricken by impotence from undiagnosed medical causes, and you did not know that it had a medical cause and could be treated. You would undoubtedly feel like less of a man than you truly are. I hope I am not being too personal here, but this is the best way I can describe this point in order for it to be fully understood.
2. FEAR that she might indeed be losing her mind and will be marked as a 'crazy' person needing life-long medication or institutionalization (not as far-fetched as you think

)
3. FEAR that if she did openly admit that she has a problem that she will lose you and the children (a very reasonable fear for a woman, although it may be illogical to a man's way of thinking

)
4. FEAR that she may not be taken seriously and be marked forever with the STIGMA of being a hysterical woman on PMS, and being told to 'get over it' (a very harmful and old-fashioned notion about the female reproductive cycle and its related issues).
4. LACK OF SUFFICIENT KNOWLEDGE about womens medical issues, i.e. thinking that PMS is normal and that all women have it. Not all women have it, and yes it is a medical problem, and yes it can be treated very successfully if diagnosed and treated correctly.
Unfortunately not much can be done about this situation until she herself reaches the point of no-return, where she will be forced one way or another to admit that she needs help. It is very sad, but very true.
BUT that is not to say that you should give up pointing out to her that she has a medical problem that needs attention. Honestly, you might be a tad overly tolerant of the continued and stubborn refusal on her part to do something about an issue which is causing a great deal of suffering, not only for her, but for you and the children too.
A good dose of laying on the guilt-trip (or putting your foot down) is not always such a bad idea. But I warn you, the truth always hurts and there is a chance this approach may not work. ALTERNATIVELY, it may have a great chance of working, but before it does, you may have to be prepared for an initial blow-up and some unpleasantness until reality sets in and she can see things for what they are.
Is it better to be stung once by a bee, which then dies, or to receive multiple stings from a wasp, which then continues on its merry way after it has done its damage?
But if this sounds too drastic, you might get some good advice from your family doctor, only, DON'T let your wife know you have spoken with him/her about this problem. (At least wait until well after your wife has freely admitted that she has a problem and has gone to seek medical help out of her own volition).
And there is nothing wrong with not sharing this kind of information with your wife right away. If your motivations are pure and out of an act of true love to save her, you, your relationship and your family, it is a noble act. You might ask the doctor to keep it confidential for the time being too, and he/she should respect your wishes otherwise he/she is not worth their salt, or very wise or knowledgeable about the true effects of this disorder.
You might also want to contact one of the doctors I've listed
http://www.network54.com/Forum/message?forumid=241962&messageid=1088981722 and ask for advice about this situation from them.
All the best,
Anthea