In this weekly series, you get to think like a scientist as we dive into one or two medical journal articles to gain insights into a common health problem. Links to the journal articles are provided.
Many people blame their frequent visits to the bathroom every night on an overactive bladder (OAB) or, for men, an enlarged prostate. While these may be frequent causes of nocturia (getting up more than one time per night), an often overlooked cause is obstructive sleep apnea. If you wonder how sleep apnea could cause you wake up multiple times at night to urinate, an excellent study by Miyauchi et al confirms both the relationship between nocturia and sleep apnea as well as the improvement seen with treatment.
Too much urine at night
We begin by noting that some patients who get up frequently at night do so because they make too much urine at night. The bladder is simply filling up more quickly than it should. Normally for young people, no more than 20% of your daily urine production should come while you are asleep. In older patients, less than 33% is more normal. Patients who exceed this are said to have nocturnal polyuria, meaning they make too much urine at night. This is easily diagnosed by having patients keep a diary for several nights to measure how much urine they make during the night compared to the day.
While there are several causes of nocturnal polyuria, an often overlooked cause is obstructive sleep apnea. Blockage of your airway in sleep apnea results in a lot of negative pressure in the chest as you try to breathe (try closing your throat and taking a breath). This causes more blood to return to the heart (right atrium). When this part of the heart expands from the extra blood, it releases a hormone (atrial natriuretic peptide) that makes you make more urine. It’s as if the body thinks there is too much blood volume (maybe you drank a lot of fluid) and tries to get rid of the excess fluid. So, patients with obstructive sleep apnea make too much urine at night.
The encouraging news is that treatment of the sleep apnea such as with a mask providing continuous airway pressure (CPAP) not only treats the sleep apnea but can also reduce urine production at night and nocturia. So next time you find yourself getting up that 3rd or 4th time at night, consider having an evaluation of whether you are making too much urine at night. If so, a test for sleep apnea may be in order.