If your cat has "megacolon" (enlarged flacid colon) due to "obstipation" (severe intractable constipation) as the primary problem, then daily use of laxatives may be helpful, however this is a lifelong problem. On the other hand, I have been very pleased with the results of subtotal colectomy over the years, which involves removing the majority of the diseased colon in cases of megacolon/obstipation. While the cats often have looser stools initially, usually that improves to a formed state within 3 to 4 months as the end part of the small intestine takes on the water reabsorption function that the colon formerly had. These cats then dont need further "clean-outs" and have a good quality of life. While I have managed many such cats medically, the results are frustrating for owners and the repeated procedures costly.
I operate them earlier now and it works well for most people.
The diarrhea you had seen was likely "slipping past" the rock hard stool accumulation in the colon.
Good luck with your cat!
You can find a little more information about Manx cats at:
http://www.petplace.com/cats/choosing-a-manx/page1.aspx
There are medications to help with constipation which may help your cat. If her colon is full again, she will need to have it clean out again by your veterinarian. Then you can talk to your vet about starting a laxative, such as lactulose or miralax, and cisapride which stimulates the muscle of the colon to help move the stool out. Don't start these until her colon is empty though or she may have uncomfortable cramping.
You should also feed a highly digestible canned food. Pumpkin can be great for bowel movements but it may bulk the stool which you wouldn't want in this situation.
This is likely a problem your cat will continue to have but hopefully you can work with your vet to manage the problem successfully and keep your cat comfortable. Good luck.
Judy Karnia, DVM
Scottsdale Cat Clinic
I've heard that getting your cat to eat pure pumpkin, in a can, not spiced pie filling, helps regulate bowel movements. I've never been able to get my cats to eat it though.