#megacolon

Possible causes include: 

Dehydration (increased water loss, decreased water intake)

Painful defecation (including orthopedic disease)

Reluctance to defecate (fear, undesirable litter box)

Mechanical obstruction 

Functional obstruction (megacolon, ileus)

Metabolic disease

A thorough history will help differentiate between possible causes. Inquire about water intake, litter box habits, interaction with other animals in the household, and expression of pain in other aspects of their life.

Diagnostics: First episode - may not pursue further diagnostics, treatment (hydration) may be adequate. Recurrent - CBC, biochem T4, UA. Digital rectal exam. Depending on what other clinical signs are being displayed, rads, US, neuro exam etc. may also be performed.

Five steps are involved in treatment:

Rehydrate  - IV/SQ fluids

Feces Removal - enema or manual extraction - In my experience, this is not usually pursued for a first-time offender.

Diet - High fiber, wet-food diet 

Laxative

Promotility drug - Consider after other therapies have been tried and failed. - Cisapride is a good option.

Bonus step - enhance the environment. - ensure duplicates of water, food, and litter boxes. - make sure litter boxes are large, uncovered, clean and easy to access. - make sure water sources are fresh daily; possibly try a water fountain to encourage water intake.