Stone Nutrition Examination Survey (NHANES 2007-2010) report that the

Stone disease: An
introduction

Worldwide kidney
stone is a common problem and its prevalence has increased over the past 20
years. The National Health and Nutrition Examination Survey (NHANES 2007-2010)
report that the prevalence of kidney stones among American adults is 8.8%:
10.6% among men and 7.1% among women, and it is anticipated that there will be
an increase in kidney stones in the future due to global warming, lifestyle changes,
diet and obesity.1 In the
Middle East, the lifetime risk of kidney stone appears to be even higher. The
awareness of renal stone disease in children is high and recurrence rates of
50% after 10 years and 75% after 20 years have been reported.2

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Kidney stones
are characterised by sudden onset of lumbar pain that may be accompanied by
nausea and vomiting, gross or microscopic haematuria. The location of the pain
depends on the location of stone in the urinary tract.2 Apart from being costly due to both medical
treatment and time lost from work, kidney stones are also associated with
increased rates of chronic kidney disease, hypertension and myocardial
infarction.  The recurrence rate of
kidney stones is high. After an initial stone, there is a 50% chance of forming
a second stone within 7 years if left untreated. Since most patients with stone disease have
identifiable risk factors, it is advisable to evaluate the underlying causes of
stone formation.1

 

Roughly 80% of
kidney stones contain calcium, and the majority of them are composed mainly of
calcium oxalate. Most of calcium oxalate stones contain some calcium phosphate,
while 5% have hydroxyapatite or brushite as their main constituent and 10%
contain some uric acid. Pure uric acid, cystine and infection stones are less common.
Although composition of each stone correlates with supersaturation values in
the urine, calculi are seldom found without an admixture of many salts and not
every passed stone can be retrieved for chemical analysis.2

Pathophysiology

Calcium stone formation
involves different phases of increasing accumulation of CaOx and calcium
phosphate (CaP) – nucleation, crystal growth, crystal aggregation and crystal
retention.3

Nucleation

Nucleation is
the process by which free ions in solution associate into microscopic
particles. Crystallization can occur in solution micro-environments, such as
may be present in certain points in the nephron, as well as on surfaces, such
as those of cells and on extracellular matrix. There is considerable dispute
about the importance of free solution crystallization versus crystallization at
other sites, in renal tubules or on bladder walls, on normal or damaged cells,
on areas denuded of cells by certain forms of injury, or at interstitial sites.3

Crystal growth

Crystal growth
is the next major step of stone formation after nucleation. Reduction in the
potential energy of the atoms or molecules when they form bonds to each other
is the driving force for crystallisation. In a supersaturated liquid several
atoms or molecules start forming clusters; the bulk free energy of the cluster
is less than that of the liquid.3

The total free
energy of the cluster is increased by the surface energy (surface tension). Molecular
size and shape of the molecule, the physical properties of the material, SS
levels, pH, and defects that may form in the crystal’s structure determine the
crystal growth. Crystal growth is one of the prerequisites for particle
formation. Using the powerful atomic-force microscope (AFM), Laboratory researchers
are discovering complex growth mechanisms and three-dimensional structures of solution-based
crystals.3

Crystal aggregation
(crystal agglomeration)

In crystal aggregation,
crystals in the solution stick together and form larger particles.  Aggregation of particles in solution is
determined by a balance of forces, some with aggregating effects and some with
disaggregating effects. A small interparticle distance increases attractive
force and favours particle aggregation. In addition, Tamm-Horsfall glycoprotein
and other molecule may act as glue and increase viscous binding.3 

Furthermore,
aggregate may be stabilised by solid bridges formed by crystalline material connecting
two particles. The main force that inhibits aggregation is the repulsive
electrostatic surface charge, known as Zeta potential. In various steps of
stone formation, crystal aggregation is a more important factor than nucleation
and growth because aggregation occurs within seconds.3

Crystal retention

Urolithiasis
requires formation of crystals followed by their retention and accumulation in
the kidney. Crystal retention can be caused by the association of crystals with
the epithelial cells lining the renal tubules. Crystal formation predominantly depends
on the composition of the tubular fluid, while crystal retention might depend
on the composition of the renal tubular epithelial cell surface. A non-adherent
surface of the distal tubules, collecting ducts, ureters, bladder, and the
urethra may provide a natural defence mechanism against crystal retention, and
may become defective when the anti-adherence properties are compromised. In a
cell culture model, Verhulst et al.,observed
upregulated cell surface expression of hyaluronic acid, osteopontin, and their
receptor CD44, as well as the formation of a hyaluronic acid-dependent cell
coat, and suggested that it may play a crucial role in the process of crystal retention.3

Risk factors

Professional chefs and
taxi drivers who often try to minimise their fluid intake to avoid too many
‘toilet stops’ are at higher risk of getting stone disease. Apart from
environmental and lifestyle factors diet-related factors are also known to
increase stone risk. Diet-related factors that are known to increase stone risk
are listed in Table 1.4

Table
1: Dietary risk factors associated with increased stone risk

Tea or coffee
(particularly instant coffee) without milk has been shown to increase oxalate excretion,
although this effect is probably offset by their diuretic action. Dietary
calcium has a biphasic risk curve: stone risk is greater in those on a high or
low calcium diet. The link between vitamin D intake and renal stones is less
clear: while excessive active (1,25-OH) vitamin D supplementation, increases
the risk of stone formation, there is no evidence that correction of native
(25-OH) vitamin D deficiency has the same effect; moreover, correction, especially
if there is secondary hyperparathyroidism, is likely to be a health benefit. Excess
of vitamin C could also increase the risk of calcium oxalate stone formation, but
in practice this is rarely encountered. High dietary intake of potassium or
magnesium is inversely related to stone formation because potassium promotes urinary
citrate excretion, and both citrate and magnesium inhibit crystal formation.
However, the impact of low urinary magnesium on stone risk is at best modest.4

Sites of stone
growth

Randall’s plaques

Supersaturation of
calcium phosphate may exist in the loop of Henle even though urine is not
generally supersaturated with it. This may lead to precipitation of calcium
phosphate in interstitial sites in the inner medulla. These deposits often
become extensive enough to be visible macroscopically in the form of Randall’s
Plaques. Some studies have demonstrated stones that appear to have been
attached directly to the Randall’s plaque which has eroded through the
overlying uroepithelium at the surface of a renal papilla. Although Randall’s
plaques appear to be a risk factor for stone formation, it is still unclear
whether they are necessary in every stone that is formed, since intratubular
crystals as well and  prominent
crystalluria are features of stone disease.5

Calcium oxalate
“receptors” in collecting duct epithelium

Crystalluria is
commonly observed in hyperoxaluric patients, as well as in patients with the
usual sort of calcium oxalate nephrolithiasis where intratubular crystals of
calcium oxalate have also been demonstrated.5

Cells internalise the
crystals, where they fate may undergo dissolution or transcytosis through the
epithelial layer. This process may have consequences for cell function;
initiating mitogenesis, activation of arachidonic acid and other signaling
pathways. The interaction of stone crystals of various types with primary
cultures of inner medullary collecting duct cells demonstrates saturability and
inhibition to some degree of one crystal type by others. Thus, it seems like
there are some receptor-like features of cells to which stone crystals adhere.5

Loss of cell
polarity increases adherence of calcium oxalate crystals. This suggests that
the basolateral membranes of tubule cells have components to which the crystals
can attach. Enrichment of cell membranes with phosphatidylserine also leads to
enhanced calcium oxalate adherence. Also, proteins and glycosaminoglycan
expressed at the cell surface have also been implicated as attachment sites for
calcium oxalate at least. These include hyaluronan, nucleolin, annexin II, and
osteopontin. It is likely that a number of different structure or molecular
components are responsible for crystal attachment. As noted above, these may
include phosphatidylserine component of the lipid bilayer, the acidic side
chains of proteins (carboxyl groups of amino acids or sialic acid-containing
glycosidic side chains). Atomic force microscopy has been used to measure the
force of attraction of carboxyl and amidinium groups to the surface of
crystals.5

 

Clinical feature of
stone disease

Stone passage

Stones that are
nonobstructing produce no symptoms or signs apart from haematuria. Stone
passage produces renal colic that usually begins as a mild discomfort and
progresses to a plateau of extreme severity over 30–60 minutes. In case of obstruction
in the uretero-pelvic junction, pain localizes to the flank, and as the stone
moves down the ureter the pain moves downward and anterior. Stones at the
uretero-vesicular junction often cause dysuria and urinary frequency mistaken
for infection. Colic is free of body position or motion and is described as a
boring or burning sensation associated with nausea and vomiting.6

Urological management of
stones

Sound waves are
used to break the stone into small pieces that can more easily pass into the bladder.
 Extracorporeal shock wave lithotripsy
(ESWL) is widely used and valuable for small stones. Modern instruments permit
local stone disruption with high-powered lasers by facilitating passage of
endoscopes up the ureter into the kidney pelvis. Percutaneous stone removal via
instruments introduced into the kidney through a small flank incision permits
disruption and removal of even very large stones.6

Renal function reduction
in stone forming people

The National Health and
Nutrition Examination Survey III data set states that subjects with a BMI
greater than or equal to 27 who had kidney stones had lower estimated
glomerular filtration rates than non–stone formers (non-SFs) matched for age,
sex, race, and BMI. SFs also have higher blood pressures than non-SFs. Urinary
tract obstruction, sequelae of urological interventions, and the processes that
cause stone formation may all injure renal tissue, reduce renal function and
raise blood pressure.6

Stone inhibitors

Stone inhibitors
are defined as molecules that increase the supersaturation (SS) required to
initiate nucleation, decrease crystal growth rate and aggregation, and inhibit
secondary nucleation. Inhibitors of calcium stone formation act by preventing crystal
growth and aggregation by coating the surface of growing calcium crystals or by
complexing with calcium and oxalate.3

Citrate

Citrate, a
tricarboxylic acid, circulates in the blood and is derived from endogenous
oxidative metabolism. It is complexed to calcium, magnesium and sodium at
physiological pH of 7.4. Citrate has been widely studied for its stone inhibiting
action in urine and it has been found to be particularly effective against the
calcium oxalate and phosphate stones.  Citrate appears to alter both calcium oxalate
monohydrate and calcium phosphate crystallisation. It reduces the concentration
of calcium oxalate by complexing with calcium and this is possibly due to
direct effects on the crystal surface rather than to an alteration of the availability
of free calcium.3

Pyrophosphates

Pyrophosphate at
low concentration of 16 ?M, inhibits CaOx monohydrate (COM) crystal growth by
50%. The urinary pyrophosphate levels are (20–40 mM)
enough to inhibit CaOx and CAP crystallisation. Pyrophosphate and diphosphate
inhibits the precipitation of CaP, while diphosphates inhibit the growth of
apatite crystals.3

 

Pyrophosphate
reduces the absorption of calcium in the intestine and this action is probably
mediated by formation of 1,25 (OH)2 – vitamin D. A study by Sharma et al., reported low 24-hour urinary
excretion of pyrophosphate in stone formers (50.67-2.16 mmol/24 h) as compared
to normal subjects (71.46-5.46 mmol/24 h) (p

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