Synopsis
Acute ischemic stroke is a heterogeneous disease, with
major stroke caused by proximal artery occlusions representing the stroke
subtype with the most devastating outcomes. With the recent success of the endovascular clot retrieval trials for major stroke, the primary role of the
radiologist to identify the patients suitable
for treatment rapidly and accurately. The most common way to get this
information is with non contrast CT, and CT angiography. In present clinical
practice, time is dominant (<6hr) over physiology for decisions related to
implementing therapy. There is evidence from the recent trials that it may be
possible to extend the selection
criteria to include a larger group of patients that will still benefit from IA
therapy. However, this will require clinical trials to demonstrate that advanced imaging techniques to select these patients
for treatment, leads to improved outcomes.Abstract
Intravenous thrombolysis is the mainstay of
acute ischemic stroke management for any patient presenting with disabling
deficits within 4.5 hrs of treatment onset. This is based on the results of the National Institute of Neurological Disorders
(NINDS) (1) study that showed a 16%
increase in favourable outcome in patients receiving IV tpa within 3 hr and a subsequent
trial Haacke et al (2) that
showed increase favourable
outcome by 10% in the 3 - 4.5 hr window
. A recent meta- analysis (3 ) showed a benefit for IV tpa, irrespective of age and stroke severity with
earlier treatment (<3hrs) being
associated with bigger proportional gains. However, after IV tpa alone, only 10
– 15% of internal carotid artery occlusions and 25 to 50% of middle cerebral
artery occlusions recanalise. Proximal artery occlusion and thrombi greater than
8mm are relatively resistant to IV tpa alone (4,5). Proximal artery occlusion
is the cause of occlusion in up to a
third of cases. As such methods aiming at improving recanalization and reperfusion
have been the basis of large clinical trials.
The results of the recent trials of
endovascular clot retrieval in acute ischemia (6-10) have proven the
effectiveness of endovascular therapy in
improved outcome in patients with intracranial large vessel occlusion when
compared to those that receive IV treatment only. Systematic review (11) of the
recent endovascular trials show between a 10% - 33% benefit in favourable outcome for
patients treated with combined IA/IV
compared to IV alone. Comparison of the trials shows better outcomes are associated with use of
stent retriever catheter than either intrarterial thrombolysis or earlier coil
retrievers. Stent retrievers were associated with good reperfusion in between
80 – 94% of patients compared with much lower historical reperfusion rates.
Comparison of trial data also suggests that both
deceased time to reperfusion and better achieved reperfusion (TICI 2b (50%) or
3(100%) reperfusion) both result in more favourable outcome (11). Although the
trials were all slightly different in inclusion criteria the results all
endorse the recently adopted American
Stroke Association guidelines for endovascular clot retrieval in the following patients: Patients >18years who were
previously independent, were treated with IV tpa within 4.5hts of stroke onset,
had an NIHSS score of >6, had internal carotid or middle cerebral artery M1
segment occlusion and had an ASPECTS > 6 and were treated <6hrs.
The role of the radiologist in the treatment
and assessment of acute stroke is:
1.
Diagnosis of acute ischemia
Non contrast CT (NCCT)is
known to be unreliable for the diagnosis
of acute ischemia with agreement between skilled reviewers of only (12) (50 –
70% agreement). This is improved if a systemised approach of scan review using
ASPECTS is employed (13) ( 75% better for dichotomised , >6) and further if
Aspects is applied to multiphase CT Angiography (CTA) data (>85%) (14).
Diagnosis of ischemia is additionally improved with the use of CTPerfusion(CTP)
( 91% sensitive ,100% specific)(15). Renal
disease, contrast nephropathy radiation (extra 4 mS, additional 2 – 3 minutes
scan time and 5 – 15 min post processing) are no longer barriers to routine
implementation of CTP. DWI MRI scanning
remains the gold standard for identification of ischemic lesions in the brain.
2.
Assessment for IV therapy-<4.5hrs
The exclusion of haemorrhage
is easily done by NCCT or MR. The exclusion of a large infarct ( > 1/3 MCA
territory (~100ml) is variably performed by NCCT, NCCT + ASPECTS ( ~<6 ~100ml,
<7~70ml ,~ 75%sens, 75% sensitivity)
(16), CTA source images, CTP, CTP
Aspects, CBF( Aspects/vol), CBV ( Aspects /vol) or DWI vol, DWI Aspects (85%sens,
80%specificity) (16)
3.
Assessment for IA therapy-<6hrs
Requires identification of
a blocked vessel. Although clot may be seen well on thin slice NCCT, minimum imaging now requires CTA, MRA before
proceeding to endovascular clot retrieval. There is still controversy
about the place of CT or MR penumbral imaging in the selection of patients for IA therapy. The trials that used penumbral imaging for
patient selection had the best patient outcomes but were also the trials that
had the shortest time to reperfusion and
had the highest revascularisation rates.
Assessment of core (<70 ml) :
NCCCT, Aspects, CTA aspects, Aspects/vol , CBV, CBF, DWI.
4.
Endovascular clot retrieval.
Should be performed within 6 hr of stroke
onset by a skilled operator
5.
Access 24 hr availability.
This is best facilitated
with dedicated stroke centres – networks, expedited access to scanners,
neurointerventionalists availability 24/7 and potentially stroke
ambulances
6.
Research
Current research is focussed
on expanding the eligibility pool of patients that can receive endovascular
therapy . Trials are underway and needed
to assess the benefit to the elderly, patients with mild ischemia (all
in trials had NIHSS>5), wake up
strokes (mismatch imaging may help), time >6hr (patient profile and mismatch
imaging may help), treatment of large infarcts especially in early time window (MR Clean aspects 0 -4 no benefit) and
development of better techniques to assess salvageable tissue: sodium ,amide,
DTI, DKI, DSI.
Acknowledgements
No acknowledgement found.References
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