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THE RENAL TRACT 189
vascular rejection or acute tubular necrosis can be
present under such circumstances. 34
Vascular complications can include arterial stenosis
or thrombosis, venous stenosis or thrombosis,
pseudoaneurysms and arteriovenous fistulae. 33
Renal artery stenosis
This generally occurs at the site of the anastomosis
close to the iliac artery but can also occur along the
length of the artery or even affect the intrarenal
branches. The patient may present with severe,
difficult-to-control hypertension, graft dysfunction,
or both. Alternatively the patient’s renal function
may deteriorate following angiotensin-converting
enzyme inhibitor therapy and this is also an indication
of a possible underlying RAS. Careful Doppler
examination is now the accepted first-line investigation
in the diagnosis of RAS.
In most cases it is possible to trace the artery
back to its anastomosis with the iliac artery, using
colour Doppler. If the site of the stenosis is identified,
spectral Doppler will demonstrate an increase
in peak systolic velocity at the lesion, followed by
poststenotic turbulence (Fig. 7.28, B,C). This can
be difficult to pinpoint with MRA, especially if
bowel is overlying the vessel.
A delayed systolic rise (the parvus tardus waveform)
can be identified in the intrarenal spectral
Doppler waveforms, as for the native kidney (see
above). The diagnosis however is primarily made
on the peak systolic velocity within the renal artery.
A value of < 2.5 m/s is normal while > 2.5 m/s
constitutes RAS. If the stenosis is severe, it may be
difficult to identify colour flow in the kidney and
the waveform may be reduced in velocity with a
tiny, damped trace in the main vessel.
A stenosis affecting an interlobar artery may
result in focal, segmental changes in the kidney.
In general, contrast angiography is only used to
grade and treat stenoses after a positive ultrasound
scan, or when a high index of clinical suspicion persists,
despite a negative ultrasound. 33
Renal vein thrombosis
The occlusion may be partial or complete and the
venous Doppler spectrum may therefore be absent
(Fig. 7.29).
If venous thrombosis is partial, the arterial spectral
waveform becomes very pulsatile, with reverse end
diastolic flow; in the clinical setting of an oliguric
patient with a tender graft in the early postoperative
period, this is almost pathognomonic for RVT.
During the early stages, when thrombosis is
incomplete, venous flow may be seen in the kidney,
but the artery is of reduced velocity. 25
The ultrasound findings of renal vein thrombosis
may be indistinguishable from severe rejection; however
venous flow is generally unaffected in the latter.
Thrombosis is rare, occurring typically in the
immediate postoperative period. 33 It may be associated
with a faulty venous anastomosis, secondary
to compression of the vein, for example by a large,
perivenous collection, or the patient may have an
increased thrombotic tendency for a number of
reasons.
Pseudoaneurysms and arteriovenous fistulae
These may sometimes form as a result of vascular
damage during biopsy procedures. They are usually
not significant and tend to resolve spontaneously
(Fig. 7.30A).
An arteriovenous fistula shows an irregular knot
of vessels on colour or power Doppler with a pulsatile
venous waveform and high peak and end diastolic
velocity in the feeding artery. A large draining
vein may also be seen.
A pseudoaneurysm may appear cystic on the
grey-scale image, but will demonstrate filling on
colour Doppler with a pulsatile flow velocity waveform
(Fig. 7.30B, C). Careful biopsy technique
helps to avoid such lesions (see Chapter 11).
Infection
This is characterized by swelling of the uroepithelium,
especially with fungal infections. Fungal balls
may be visible as relatively hyperechoic structures
within the PCS (Fig. 7.31).
Acute tubular necrosis
This may demonstrate prominent medullary pyramids
on ultrasound, with low end diastolic flow.
Reverse end diastolic flow is uncommon but recognized.
A biopsy is required for confirmation.