An enlarged bladder is a condition where the organ holds an abnormally large volume of urine because it cannot empty completely. This chronic incomplete emptying, known as urinary retention, stretches the bladder wall over time, leading to significant enlargement. The causes generally fall into three categories: physical blockages preventing outflow, disrupted nerve signals, and failure of the bladder muscle itself. This chronic distension poses various health risks.
Physical Blockages
A physical obstruction that prevents urine from exiting the bladder through the urethra is the most direct cause of enlargement. This mechanical barrier forces the detrusor muscle to work against increased resistance. Initially, the muscle compensates by thickening and becoming stronger.
The most common example is Benign Prostatic Hyperplasia (BPH) in men, where the prostate gland compresses the urethra. Other obstructions include urethral strictures, bladder stones, or tumors blocking the bladder neck. Over an extended period, the sustained effort against the obstruction weakens the detrusor muscle, eventually causing it to fail to empty completely.
Nerve Signal Disruption
A functioning bladder relies on communication between the detrusor muscle and the central nervous system; disruption causes a neurogenic bladder. Nerves signal the brain when the bladder is full and relay the command for the detrusor to contract and sphincters to relax. Damage to the brain, spinal cord, or peripheral nerves interrupts this process.
Chronic conditions like diabetes can cause peripheral neuropathy, damaging the nerves that supply the bladder and impairing the ability to sense fullness. Spinal cord injuries, strokes, and diseases such as Multiple Sclerosis (MS) also disrupt control pathways. Depending on the damage, the bladder may become flaccid (failing to contract) or spastic (contracting inefficiently), resulting in chronic urinary retention and enlargement.
Bladder Muscle Failure
Bladder muscle failure is a state where the muscle responsible for expelling urine loses its contractile strength. This condition is often the end result of chronic overstretching caused by long-term retention. Persistent distension causes micro-damage and stretching between the detrusor muscle cells, impairing the muscle’s ability to propagate the electrical signals needed for contraction.
The muscle tissue may eventually undergo fibrotic change, replacing functional muscle fibers with non-contractile scar tissue. Even if the original blockage or nerve issue is resolved, the detrusor muscle may be permanently damaged and unable to generate the necessary force to empty the bladder. In some instances, the cause of this muscle weakness is unknown, referred to as idiopathic detrusor atony, but the bladder remains chronically full and enlarged.
Consequences of Chronic Enlargement
A chronically enlarged bladder holding residual urine leads to several health complications. The most frequent consequence is an increased risk of recurrent Urinary Tract Infections (UTIs), because retained urine provides an environment for bacteria to multiply.
Stagnant urine also increases the likelihood of bladder stone formation due to the precipitation of mineral salts. Elevated pressure can cause backflow (vesicoureteral reflux) up the ureters toward the kidneys, causing the collecting system to swell (hydronephrosis). If this pressure is not relieved, it can damage the kidney’s filtering structures, potentially leading to irreversible kidney failure.