Overflow incontinence happens when the normal flow of urine is blocked and the bladder cannot empty completely. Overflow incontinence can be due to a number of conditions:

• A partial obstruction. In this case the urine cannot flow completely out of the bladder, so it never fully empties.
• An inactive bladder muscle. In contrast to urge incontinence, the bladder is less active than normal, not more. It cannot empty properly and so becomes distended, or swells. Eventually this distention stretches the internal sphincter until it opens partially and leakage occurs.

The causes of the conditions leading to overflow incontinence include:

• Tumors
• Certain medications (anticholinergics, antidepressants, antipsychotics, sedatives, narcotics, alpha-adrenergic agonists, beta-adrenergic agonists, calcium channel blockers)
• Benign prostatic hyperplasia (enlarged prostate)
• Scar tissue
• Nerve damage. In such cases, nerves in the bladder are damaged so that the body cannot feel when the bladder is full, and the bladder does not contract. Such damage can be caused by spinal cord injuries, previous surgery in the colon or rectum, and pelvic fractures. Diabetes, multiple sclerosis, and shingles also can cause this problem.

Patients with functional incontinence have mental or physical disabilities that keep them from urinating, although the urinary system itself is normal. Conditions that can lead to function incontinence include:

• Parkinson's disease
• Alzheimer's disease and other forms of dementia. Mental confusion may prevent both recognition of the need to void and locating a bathroom.
• Severe depression. In such cases, people may become incontinent because they are indifferent to self-control.