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higher rates of blood transfusion and arterial embolization
in complex stone situations
Identifying the right indications remains key in estab-
lishing the optimal use of miniaturized systems, but
published outcomes are heterogeneous in terms of SFR
and complication rates reported. Miniaturized PNL seems to
be more effective for smaller rather than larger renal stones
>20 mm . The question arises whether miniaturized
PNL may compete more with SWL and ureteroscopy (URS)
than with conventional PNL. While most published series so
far reported an advantage for percutaneous techniques over
URS , De et al recently published a meta-analysis
comparing percutaneous stone removal with URS . Al-
though the overall SFR was in favor of PNL, subgroup
analysis indicated that URS provided a significantly higher
SFR rate than miniaturized systems. However, the same
limitations as for the present analysis apply to their
systematic review, with one of the major weaknesses being
the inclusion of different tract sizes. De et al concluded that
URS should be recommended over minimally invasive PNL
for stones sized <20 mm because of the generally lower
morbidity of retrograde access. When compared to SWL,
miniaturized PNL demonstrates higher SFR and a lower rate
of auxiliary measures, although most data come from
pediatric series that were not included in this systematic
review On the basis of the current literature and
experiences, downsized instruments may be used for stones
in all locations accessible for standard PNL. The best
indications seem to be medium stones of up to 20 mm,
although there are no data to support an upper limit. In
general, instruments should be adapted to the anatomy.
Patients with tiny collecting systems may especially benefit
from the use of smaller systems. Another potential
indication might be stones located within calyceal divertic-
ula. Future research should evaluate such issues.
Hemodynamic, electrolyte, and metabolic changes have
been prospectively evaluated when comparing miniaturized
with standard PNL Interestingly, a trend towards
metabolic acidosis was observed in the mPNL group, possibly
associated with prolonged operating time and higher
intrarenal irrigation pressure. Tepeler et al measured
intrarenal pelvic pressure during PNL procedures using 4.8 Fr
nephroscopes in comparison to conventional PNL. Intrarenal
pressure was significantly lower in the conventional group
during all steps of the procedure. Even though there was no
difference in outcome in their series, surgeons should be
aware of higher pressure for downsized systems Place-
ment of a ureteral catheter may be helpful for irrigation
outflow to allow intermittent flushing .
It is sobering that even the simplest parameter, SFR, is
generally difficult to compare because of different defini-
tions of SFR with regard to the time until stone-free status is
achieved and whether or not residual fragments are
accepted, as well as the maximum fragment size allowable
to justify classification as ‘‘clinically insignificant’’. Further-
more, the imaging modality used to assess stone-free status
varies, with most series using ultrasound or kidney/ureter/
bladder X-ray (KUB), although CT is more sensitive for small
residual fragments .
3.7.3. Implications for research
The concept of reducing the morbidity of PNL by downsizing
the access tract seems convincing. The findings that success
rates are high with miniaturized instruments demonstrate
that the concept does not negatively impact the outcome.
However, well-designed studies are missing and no conclu-
sion can be drawn in terms of potentially lower morbidity.
3.7.4. Limitations
This review has several limitations. Many of the studies
included may be affected by selection bias, outcome-
reporting bias, and the use of different tract sizes. More
than half of the studies were single-armcase series. For five of
the studies, only abstracts were published, severely limiting
the information available and its quality. Furthermore, the
studies were heterogeneous in design, with differences in the
size of tracts used in both the interventional and control arms
(when available) and in the size and location of stones
treated.
Many of the studies included also suffered from other
important methodological limitations. Importantly, most of
the studies on mPNL used single-step dilatation, whereas
conventional PNL procedures were performed with step or
balloon dilatation, entailing different complication rates for
bleeding, for example . Moreover, most mPNL proce-
dures were performed with low intrapelvic pressure, while
standard PNL operations were much more frequently
performed at high pressure, with implications for postop-
erative fever and sepsis, for example . In addition,
assessment of the postoperative SFR was obfuscated by the
use of both less sensitive standard radiology (KUB) and
significantly more sensitive CT . Finally, many studies
did not report statistical calculations of differences between
interventional groups for the main outcome measures.
4. Conclusions
The available evidence indicates that mPNL is at least as
efficacious and safe as standard PNL for the removal of renal
calculi, with a limited risk of significant (Clavien grade 2)
complications. However, the quality of the evidence was
poor and drawn mainly from small studies, the majority of
which were single-arm case series and NRCSs, and only two
of which were RCTs. Hence, the risks of bias and
confounding were high. Furthermore, the tract sizes used
and the types of stones treated were heterogeneous. Thus,
more reliable data from well-designed and adequately
sampled and powered RCTs are warranted.
Author contributions: Thomas Knoll had full access to all the data in the
study and takes responsibility for the integrity of the data and the
accuracy of the data analysis.
Study concept and design: Knoll, Tu¨ rk, Ruhayel, Tepeler.
Acquisition of data: Knoll, Ruhayel, Tepeler, Yuan.
Analysis and interpretation of data: Knoll, Tu¨ rk, Ruhayel, Tepeler.
Drafting of the manuscript: Knoll, Ruhayel, Tepeler, MacLennan, Yuan.
Critical revision of the manuscript for important intellectual content: Knoll,
Tu¨ rk, Ruhayel, Tepeler, Dabestani, MacLennan, Petrˇı´k, Skolarikos, Seitz,
Straub, Yuan.
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