USMLE® Step 1 style questions USMLE
A 65-year-old man comes to clinic because of urinary incontinence for the past 2 weeks. He describes having difficulty initiating urination or telling when or if his bladder is full or empty. He denies frequency, urgency, or bowel dysfunction. Past medical history is significant for stroke 1 year ago in addition to hypertension and coronary artery disease. Past surgical history is significant for lower back surgery 1 month ago after a car accident. Physical examination shows decreased sphincter tone and 110 ml of post-void residual urine. Diagnostic tests show normal bladder capacity, no sign of infection, normal prostate-specific antigen levels, and normal creatinine. Which of the following is the most likely diagnosis?
Content Reviewers:Rishi Desai, MD, MPH
With neurogenic bladder, neurogenic means arising from the nervous system, so neurogenic bladder is typically some difficulty emptying the bladder normally, as a result of either damage to the peripheral nerves, brain, or spinal cord.
As urine flows from the kidney, through the ureters and into the bladder, the bladder starts to expand into the abdomen. The bladder is able to expand and contract because it’s wrapped in a muscular layer, called the detrusor muscle, and within that, lining the bladder itself is a layer of transitional epithelium containing “umbrella cells”. These umbrella cells get their name because they physically stretch out as the bladder fills, just like an umbrella opening in slow-motion.
In a grown adult, the bladder can expand to hold about 750ml, slightly less in women than men because the uterus takes up space which crowds out the bladder a bit.
Okay - so when the urine is collecting in the bladder, there are basically two “doors” that are shut, holding that urine in. The first door is the internal sphincter muscle, which is made of smooth muscle and is under involuntary control, meaning that it opens and closes automatically. Typically, the internal sphincter muscle opens up when the bladder is about half full.
Now the second door is the external sphincter muscle, and it’s made of skeletal muscle and is under voluntary control, meaning that it opens and closes when a person wants it to. This is the reason that it’s possible to stop urine mid-stream by tightening up that muscle, which is called doing kegel exercises. Once urine has passed through the external sphincter muscle, it exits the body, in women the exit is immediate and in men the urine flows through the penis before it exits.
So, when specialized nerves called stretch receptors in the bladder wall sense that the bladder is about half full, they send impulses to the spinal cord, specifically the sacral spinal cord at levels S2 and S3, known as the micturition center, and the brain, specifically two locations in the pons—the pontine storage center and pontine micturition center.
The spinal cord response is part of the micturition reflex. And it causes an increase in parasympathetic stimulation and decrease in sympathetic stimulation which makes the detrusor muscle contract and the internal sphincter relax. It also decreases motor nerve stimulation to the external sphincter allowing it to relax as well.
At this point, urination would occur at this point, if not for the pons. The pons is the region of the brain that we train to voluntarily control urination.
If we want to delay urination, or hold it in, the pontine storage center overrides the micturition reflex, and when we want to urinate, the pontine micturition center allows for the micturition reflex to happen.
Now with neurogenic bladder - the exact pattern of symptoms depends on the nerve that is damaged.
In diabetes mellitus, excess glucose levels in the blood attaches to various proteins - a process called glycation. This process can damage sensory nerve fibers in the bladder wall, in the pelvic nerve, or in dorsal nerve roots entering the spinal cord, all of which interferes with the initial stretch signal that gets sent out as the bladder fills.
Also, with herpes, the virus takes up a home in the dorsal nerve roots for months to years and it can also disrupt the sensory fibers that they carry within them.
This all means that as the bladder fills to capacity and stretches, that sensory information is not received, and the bladder starts to overflow, drop by drop, out of the urethra. This is called overflow incontinence.