Having to get up at night to pass water is an annoying symptom that many people assume is related to drinking too much fluid during the evening. While that may be the case, it’s a symptom you should not ignore. Frequent urination at night can be a sign of several different medical conditions, including uncontrolled high blood pressure, or a side effect of your antihypertensive treatment. Your doctor may be able to change your medication, or the time you take them, so you don’t need to get up at night quite so often.
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Waking at night to visit the bathroom and pass water is known as nocturia. This annoying symptom can lead to daytime fatigue for both you and your bed partner, and affects your quality of life – especially if it happens two or more times a night.
Nocturia is often overlooked as a symptom of high blood pressure, yet studies show that people with nocturia are more likely to have high blood pressure than those who don’t experience passing water at night.
In one study involving 610 older people with a raised systolic blood pressure (upper reading, average blood pressure 173/86 mmHg), nocturia was the most frequent complaint, affecting 68% of men and women alike. In another study involving 5,257 men aged 20 and over, having hypertension increased the chance of nocturia by 40% – a similar increase to that seen for older men with benign prostate enlargement.
Why high blood pressure cause frequent urination at night
The relationship between uncontrolled hypertension and nocturia is partly due to the way urine output is regulated, and partly an attempt by your body to filter excess fluid and sodium from the circulation to bring your blood pressure down.
When you have uncontrolled high blood pressure, your kidneys have to work harder to excrete the same amount of sodium as people with normal blood pressure. This leads to a tendency to retain both salt and fluid. This retained fluid may stay within the circulation, causing blood pressure to increase even further.
Having high blood pressure also damages the tiny filters in the kidneys so they are less able to filter fluid from your circulation, so a vicious cycle builds up. This is one reason why your doctor will suggest regular blood tests to check your kidney function.
Excess fluid tends to pool in the long veins of the lower limbs under the effects of gravity. In some people, this excess fluid may lead to swelling of the ankles and calves especially towards the end of the day, and if you stand for prolonged periods of time without exercising (which helps to pump the pooled fluid back towards your heart).
When you go to bed, and lie flat at night, the effects of gravity change, and any excess fluid in your body is more easily redistributed and reabsorbed into your circulation for your kidneys to filter out.
Normally at night your pituitary gland secretes a hormone called vasopressin, or antidiuretic hormone (ADH), which stimulates reabsorption of fluid in the kidneys. This allows you to sleep throughout the night without having to answer a call of nature.
Having a raised systolic blood pressure (upper reading) suppresses the secretion of ADH, however, so the kidneys continue to filter excess fluid into the bladder.
This is a natural feedback system that’s designed to help reduce a raised blood pressure that’s linked to fluid retention. In some people with hypertension, the production of ADH drops to very low or undetectable levels at night, so that urine output continues as it normally would during the day. You may even pass more urine at night than during your waking hours, although this is unusual.
Another factor is that your heart has to work harder to pump blood out against a high arterial blood pressure, which can stretch the upper chambers of the heart (atria). These respond by releasing a hormone called atrial natriuretic peptide which causes the kidneys to filter more fluid, in an attempt to lower your blood pressure.
Nocturia and blood pressure medications
In some people, frequent urination at night is linked to their blood pressure treatment, especially diuretics (known as water tablets) and calcium channel blockers. In women under the age of 55, for example, taking a calcium channel blocker antihypertensive can more than double the risk of nocturia.
Researchers have found that being on any diuretic can increase the chance of one nightly episode of nocturia by over a third (38%).
A Swedish study of 10,216 older people found that taking a diuretic doubled the prevalence of nocturia in both men and women, and there was a strong association between diuretics and have to pass water more than three times a night. In males, 12.5% of men taking a diuretic had up to 2 episodes of nocturia per night, while 21.1% had three or more episodes. Among females, 18.4% of women taking a diuretic had up to two episodes per night, while 35.1% had three or more episodes.
When you are prescribed a diuretic, you are usually told to take it in the morning to avoid nocturia. However, in some cases, this may actually contribute to nocturia especially in older people.
Some urologists have found that taking a particular diuretic 6 hours before going to bed helped a group of elderly males to pass water during the evening, which resulted in a substantial reduction in nocturia. Only try this under the supervision of your own doctor.
Do see your doctor if you have nocturia
If you are troubled by frequent urination at night, do see your doctor to find out why. If you have uncontrolled high blood pressure, then starting treatment may solve the problem. If you are on antihypertensive medication, then your doctor may be able to change your medication, or advise whether you could try taking it at a different time.
Nocturia also occurs with other health conditions, and does not necessarily mean you have a high blood pressure. Your doctor will screen you for other possible causes, such as bladder or kidney problems, diabetes, congestive heart failure, obstructive sleep apnoea or, in men, an enlarged prostate gland.
It will also help to cut back on excessive drinking of fluids in the evening, and to avoid alcohol.
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