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THE RENAL TRACT 177
Xanthogranulomatous pyelonephritis (XGP)
This condition (which gets its name from the yellow
colour of the kidney) is the result of renal
obstruction by calculi in the pelvicalyceal system.
Frequently, a staghorn calculus is responsible.
The kidney becomes chronically infected and
the calyces enlarge and become filled with infected
debris. The cortex may be eroded and thin (Fig.
7.20).
On ultrasound, these appearances are similar to
TB or to a pyonephrosis. The latter is usually
accompanied by a more severe, acute pain and
fever whereas XGP or TB has a lower-grade,
chronic pain.
CT may differentiate TB from XGP and is also
more sensitive to extrarenal spread of disease.
Hydatid cysts
The Echinococcus parasite spends part of its life
cycle in dogs. The larvae may be transmitted to
humans through contact with dog faeces, finding
their way to the lungs, liver and, less frequently, the
kidneys.
The parasite forms a cyst which has a thickened
wall, often with smaller, peripheral daughter cysts.
Frequently the main cyst contains echoes.
Figure 7.20 Xanthogranulomatous pyelonephritis.
Numerous stones have obstructed the PCS, which
contains debris. The renal cortex is thin, and the
architecture of the kidney is difficult to recognize.
The condition is rare in the UK, but may be
diagnosed when small, grape-like cysts are passed
in the urine.
DIFFUSE RENAL DISEASE AND RENAL
FAILURE
Most diffuse medical renal conditions have non-specific
appearances on ultrasound, the kidneys often
appearing normal in the early stages of disease.
Renal failure may be acute or chronic and its causes
are numerous. If acute, an increase in overall renal
size may be observed and there may be a diffuse
alteration in the renal echogenicity, however this can
be either hypo-or hyperechoic compared with normal.
Either increased or decreased corticomedullary
differentiation may also be observed (Fig. 7.21).
Although ultrasound is successful in detecting renal
parenchymal disease, the acoustic changes are not
specific and the cause must usually be diagnosed histologically,
19 ultrasound being invaluable in directing
the biopsy procedure.
In chronic renal failure the kidneys shrink and
the cortex thins. The end-stage kidney can be quite
tiny and hyperechoic and may be difficult to differentiate
from the surrounding tissues (Fig.
7.21C).
Depending on the cause, either one but generally
both of the kidneys are affected.
Acute tubular necrosis
Acute tubular necrosis is the result of ischaemia,
which destroys the tubules of the kidney, resulting
in acute renal failure. It occurs when there is a sudden
decrease in renal perfusion as a result of a
severely hypotensive episode, for example, cardiac
arrest, massive haemorrhage, drug toxicity or septicaemia.
Patients are treated temporarily by dialysis.
Tubular damage is capable of regeneration once the
blood supply and perfusion pressure return to normal,
reversing the renal failure. If suspected, it is
useful to perform a biopsy to determine the cause of
renal failure, in order to plan further management.
On ultrasound the kidneys are normal in size
or slightly enlarged. They may be completely normal
in appearance, a not uncommon finding,
although in some cases the echogenicity is altered,