The urinary system is responsible for the production, storage and expulsion of urine from the body. Urine is a combination of waste products, excess fluid and electrolytes, as well as toxins and other metabolites. The kidneys therefore serve to filter the blood of these substances and maintain homeostasis. In the process, blood volume, salt-water levels and blood pressure is regulated. The urinary system comprises the two kidneys and ureters on either side, the bladder and urethra. Sometimes the diluted substances within the urine precipitate and accumulate to form a hard mass known as a stone. It can then become lodged in any part of the urinary system or as is often the case, passed out with either no symptoms or after causing significant discomfort.
What is a urinary stone?
A urinary stone (urolith) is an accumulation of salts that form a hard mass within the urinary tract. Most urinary stones originate in the kidneys and is therefore known as a kidney stone (renal calculus). Less frequently a urinary stone may form in the bladder where it is known as a bladder stone (vesical calculus). Some bladder stones may be a kidney stone that has traveled down the ureter and is trapped within the bladder where it may grow further in size. However, stones almost never form in the ureters or urethra but may be lodged in these areas where it is referred to as the ureteral calculus or urethral calculus. It can sometimes grow within these tracts, particularly in the ureters. Nephrolithiasis is the term for the formation of stones in the kidney while urolithiasis refers to the formation of stones anywhere in the urinary tract.
Types of Urinary Stones
There are several types of urinary stones, which vary depending on where it forms. Most kidney stones contain calcium with other compounds and the majority of these are calcium oxalate (sometimes calcium phosphate) with the second most common being magnesium ammonium phsophate. The other common types of kidney stones, which only comprise a minority compared to the calcium stones, are urate or uric acid stones. Rarely, a kidney stone may be hydroxyapatite, brushite, cystine or mixed stones. With bladder stones, the most common type is a uric acid stone. Less common types of bladder stones include calcium oxalate, calcium phosphate, ammonium urate, cysteine, or magnesium ammonium phosphate stones.
Causes of Urinary Stones
The reasons for the formation of a urinary stone depends on the location where it originates. Nevertheless, there are some common factors. Stones may form when the urine is supersaturated with certain compounds like calcium, oxalate, uric acid and urea. This may be due to higher than normal amounts of these compounds being passed out due to metabolic disturbances, excessive intake of foods high in these compounds or lower fluid volume in the urine related to causes like dehydration.
Two other factors are important considerations in the formation of urinary stones. First is a nidus, which is a small particle that can cause other particles to accumulate around it. The second is urinary stasis which means that urinary flow is restricted or stagnant possibly due to an obstruction in the tract. However, stones may spontaneously form in the absence of known risk factors and are therefore termed idiopathic since the cause is not known or understood.
The causes and risk factors associated with urinary stones include :
- Low fluid intake
- Certain foods
- Family history
- Kidney disease
- Metabolic disorders
- Bladder outlet obstruction
- Neurogenic bladder
- Bladder diverticula
- Non-infectious bladder inflammation (cystitis)
- Foreign bodies
- Medical devices such as urethral catheters and stents
- Surgery to the bladder and/or urethra
Signs and Symptoms of Urinary Stones
The clinical presentation of urinary stones depends on multiple factors including :
- size of the stone
- location of the stone
- underlying urinary tract disease
Renal colic is the term for kidney stone pain which tends to appear in episodes of severe pain. The pain associated with kidney stones is more prominent when the stone enters the ureter. It typically starts in the flanks and may extend downwards to the pubis as the stone passes through the ureter. However, a stone that remains in the kidney, particularly when there is an infection, when it is large or causes an obstruction, will cause persistent pain in the mid back region (read more on kidney pain location).
Bladder stones tend to present with suprapubic pain. However, it is not uncommon for there to be no pain until the stone causes an obstruction of urine outflow from the bladder and/or if there is an infection. Read more on bladder pain location. It is important to note that stones tend to cause more pain when in the ureter or urethra. Pain may only be present during urination (dysuria) or the pain is aggravated during urination.
Apart from dysuria, there are a host of other symptoms of urinary stones. This include frequent urination, waking at night to urinate (nocturia) and/or urinary retention. Hematuria (blood in urine), pyuria (pus in urine) and proteinuria (protein in the urine) may also be present and will appear as dark urine, cloudy urine or foamy urine respectively. Although these changes in the urine may sometimes not be clearly evident despite the presence of blood, pus and/or protein in the urine. Children may also present with bedwetting.
Other clinical features depends largely on the cause of the urinary stone and underlying infections. It may include symptoms like fever and chills, nausea, vomiting, pain during defecation and/or pain with an erection (men).