are stones that form in the kidneys, so they are commonly known as kidney stones,
though the medical term is nephrolithiasis. In over 90% of cases, the
stones that form are calcium oxalate stones, i.e. formed from a combination of
calcium and oxalate. The stones may calcify, though some
cats may develop calcified blood clots rather than actual stones. The
presence of kidney stones increases the risk of kidney infections
and may cause CKD.
stones are inactive so they do not cause any problems.
A major problem arises if the cat's body attempts to pass the stones.
although painful, may be successfully passed, but larger stones may lodge in the ureter
(the tube connecting the kidney to the bladder). This is called
obstructive nephropathy or a blockage. This is very serious because it
prevents waste products that would normally be excreted via the bladder
from being excreted, so they build up in the kidneys. This can cause
may move into the ureter, then back into the kidney. This is less serious
but may still cause kidney damage.
may form if there is reduced urination for some reason, or if the urine
contains more of the substances contained in the stones (in over 90% of cases,
calcium and oxalate) than usual.
Urine that is more acidic increases the
risk of developing calcium oxalate stones. The recent trend for acidifying
cat food (which is done to prevent the opposite problem of struvite
crystals, which develop in urine that is too alkaline) is thought to be a
factor in some cases.
Many cats with
calcium oxalate stones also have idiopathic
calcium levels in the blood with no obvious cause). It is not yet known
which comes first, the hypercalcaemia or the kidney stones.
Occasionally, calcium oxalate stones may be caused by a
Vitamin B6 deficiency.
Calcium in food
does not contribute to the development of kidney stones. In fact, calcium
in foods can bind with oxalate and therefore help prevent the formation of
kidney stones. However, calcium supplements may contribute to the problem.
Studies indicate that certain breeds
appear to be more likely to develop calcium oxalate stones, including
Persian, British Shorthair, Ragdolls and Scottish Fold cats.
In some cases,
there may be no obvious signs, since cats instinctively try to hide pain. In other cases
the cat may be subdued and lethargic and not want to eat. Some cats will
blood in the urine (haematuria) or frequent
urinate more, but others exhibit reduced urination. If your cat is
unable to urinate, this is a medical emergency and you need to get to a
vet as quickly as possible.
Your vet will
probably run blood tests. If a cat's
suddenly becomes extremely high, a kidney stone blocking the ureter may be
the cause. The cat may also have elevated phosphorus and potassium levels,
and be anaemic.
Your vet will
also palpate (feel) your cat's kidneys, which may be tender to the touch.
The vet may also be able to feel a
difference in size, in which case an ultrasound
will usually be performed. X-rays
may also be performed to assess the number and size of the stones.
The ultrasound may show one small kidney and one enlarged kidney.
What tends to happen in such cases is
that a stone moves into one ureter, blocking it, but the cat does not
exhibit any symptoms. This kidney gradually ceases to function and
shrinks. The remaining kidney has to take over some of its work, so it
grows in size. If a stone eventually also moves from that kidney into the
ureter, the cat is then in crisis, because the one working kidney is no
longer able to function. This is sometimes referred to as bilateral ureteral obstruction. It is commonly known as big kidney little
Some cats with
big kidney little kidney syndrome will be unable to urinate, which is a
medical emergency and you need to get to a vet as quickly as possible. See below for treatment options.
Inactive kidney stones are not normally treated in
cats. The cat should be monitored via urinalysis (to make sure the cat has
and x-rays every few months (to see if the stones have moved).
oxalate stones cannot be dissolved through a change in
If you are
acidified food (these foods are often labelled "for urinary tract health"),
stop immediately. These foods are intended to treat the opposite problem
to calcium oxalate stones, and can therefore make the problem worse by
making the urine even more acidic. You should not give your cat anything containing cranberries
for the same reason. Acidified foods are not normally appropriate for CKD
cats in any event.
As it happens,
foods formulated for CKD, such as Hill's k/d, are also suitable for cats
with calcium oxalate stones, whether or not they have CKD.
Pet food safety: sodium in pet foods
(2008) Chandler ML Topics in Companion Animal Medicine
23(3) pp148-53 states that "Increased dietary
sodium increases urine output and may decrease the risk of forming calcium
oxalate uroliths due to the decrease in relative supersaturation of solutes.
However, caution should be used in increasing the sodium intake of patients
with renal disease as increased dietary sodium may have a negative effect on
the kidneys independent of any effect on blood pressure." Do not
increase your cat's
sodium intake without your vet's approval.
If the cat has developed obstructive nephropathy, there are a number of
possible treatments, which are listed below in order of invasiveness.
If your cat does need more proactive treatment, do not
give up hope. One study,
Rate and frequency of recurrence of uroliths after
an initial ammonium urate, calcium oxalate, or struvite urolith in cats(2009) Albasan A, Osborne CA, Lulich JP, Lekcharoensuk C, Koehler
LA, Ulrich LK & Swanson LL Journal of the American Veterinary Medical
Association 236(12) pp1450-55 found that
only 7% of cats developed calcium oxaatale stones again following
treatment, and the average grace period before this happened was over two
years. The authors believe the actual recurrence rate may be higher than
this, but it does appear that following successful treatment, the chance
of recurrence is relatively low.
This means that
the vet tries to flush out the stones. The cat will be hospitalised for
2-3 days on intravenous
fluids. How successful this is depends upon
a variety of factors, including the size of the stone and how long it has
been in the ureter. Calcified blood clots
can often be dislodged, but fewer than 10% of kidney
stones are passed.
such as Lasix or mannitol are used to increase the flushing effect and
help force the stone out.
Medications may also be given to relax the ureter in
the hope that this will help the stone to pass through. Commonly used
medications for this purpose are amitriptyline and prazosin, a muscle
relaxant. One cat who was given prazosin for six weeks managed to pass the
stones and lived for a further 20 months, see
Extracorporeal shock wave
lithotripsy uses shock waves to smash the stones so they can be passed.
This works well in humans, but can be difficult to perform in cats because of their small size. However, a small number of facilities in the USA do now offer this
treatment. It is only suitable if the stones are large enough to be seen
In one or two centres in
the USA, a stent (a tube which by-passes the ureter) can be inserted.
The stones can then be passed through the stent. The success rate is
approximately 94%, and the stent can remain in place for years.
In about 5% of
cases, the stent moves, but it can usually be repositioned.
This is a
relatively new microsurgery which was pioneered at the Animal Medical
Center in New York City. It consists of a tube within a tube surrounded by
mesh. Although the tube may fill up with kidney stones, the urine should
still be able to flow through the mesh. The tube also contains an access
point in the abdomen which can be easily accessed later under sedation
only. If the cat survives the initial surgery to fit the tube, the prognosis is good.
The surgery is
now available at a small number of centres in the USA, Canada, UK, France,
Switzerland, Italy and France. One member of
Tanya's CKD Support Group
had this surgery performed on her cat in the USA in early 2012. She paid US$2000 for
the tube. Surgery costs are an additional US$4000 - 6000.
A number of
possible surgeries are available, such as opening up the kidney and
removing the stones. This is very expensive
(it costs several thousand dollars) and invasive. It has a relatively high mortality rate,
and the stones often recur. In almost all cases, a stent or a subcutaneous
ureteral bypass would be a better choice.
is a dietary supplement for humans made from activated couch grass
(Agropyron repens). It is said to help dissolve calcium oxalate stones. It
has apparently undergone trials in humans in Cuba and Romania. It is not
approved for cats, though I did hear from one lady whose vet decided to
use it as a last ditch attempt to help her cat. As always, do not use this
product without your vet's approval.
Some cats have the opposite problem of struvite
crystals, which are formed when the cat's urine is too alkaline. Harpsie's
Website has some information about this
condition, which is often known as Feline Lower Urinary Tract Disease or