Subtypes classification and prediction of outcome in acute cerebral ischemia using spect imaging
(1997)
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- Authors
- Supervisors
- Laterre, Emile-Christian
- Abstract
- Cerebrovascular diseases represent the third leading cause of death in the industrialized countries and lead to major disability in many patients. Our knowledge of stroke risk factors, diagnosis, and treatment of ischemic stroke has grown substantially during the last decade. New acute treatments mainly based in the ischemic penumbra concept are now investigated in the setting of stroke care units. <BR> Early therapeutic interventions aim to reverse the ischemic process, thereby salvaging the ischemic brain tissue before its transformation into irreversible infarct. Standard treatment include general measures, such as maintaining adequate blood pressure and cardiac outflow, increase of cerebral perfusion with hypercolemic hemodilution, and treatment of cerebral edema and intracranial hypertension. However, the main challenging therapeutic avenues are reperfusion treatment with thrombolytic agents, antithrombotic therapy, neuroprotective agents, growth factor, and therapy modulating the early gene expression. Because it is not likely that one treatment will be effective for all patients, the pathological mechanism underlying ischemia should be clarified in the acute stage using appropriate investigations. This requires individual evaluation of the location and extent of the ischemic lesion, arterial occlusion site, residual blood flow in the infarct core, extent of the penumbral zone with persistent viable tissue, critical duration of ischemia, and the anastomotic collateral systems. The time window for acute therapeutic interventions is universally accepted to be 6 hours. However, this time interval remains controversial. It may be shorter or longer in some patients according to the above pathophysiological factors. Thus, clinical investigations should help define which treatment to give, which type of brain ischemia to treat, which time interval for successful intervention, and which outcome may be expected. <BR> The currently available imaging techniques in acute ischemic stroke are PET, SPECT, brain CT, MRI, Mr-angiography, utlrasonography, and arterial angiography. Each has advantages and limitations and some of them only give partial informations on the pathophysiological determinants. SPECT is a non-invasive functional imaging technique, which is readily available in emergency conditions to assess the brain tissue perfusion. The present work demonstrates that SPECT can depict the ischemic lesion with a higher sensitivity than CT in the acute phase. With its high sensitivity, SPECT can be used for early characterization of stroke subtypes. It can also determine the extent of ischemia, the residual perfusion in the ischemic core, the cerebral tissue viability, and the collateral supply in the peri-infarct area. SPECT can add useful additional informations to ultrasonography on the arterial occlusion site and the related hemodynamic consequences. The present work also shows that SPECT indices such as the degree and size of hypoperfusion and clinical indices can reliably predict outcome. However, functional status and mortality are better predicted with the size of hypoperfusion and the Canadian Neurological Score, respectively. Thus, different clinical and SPECT indices with threshold values should be appropriately chosen according to the outcome end point. Early thrombolytic therapy can improve functional status. However, despite stringent exclusion criteria, fibrinolysis increases mortality and the rate of symptomatic hemorrhagic transformation of the cerebral infarct. Thus, additional criteria should allow more refined selection of patients likely to benefit from this therapy with a lower risk of hemorrhage. In this respect, SPECT can add informations on stroke subtypes, extent of ischemia, collateral supply, and remaining perfusion, which have been shown to be correlated with outcome after thrombolysis. In the future, some PET tracers fit for high resolution gamma cameras should allow most hospitals to monitor the stroke evolution o the clinical setting. <BR> Patients with transient ischemic attack have different vascular risk factors, etiology, and accordingly different risk for early or late stroke recurrence. Similarly to TIA patients with cerebral infarct, categorization of patients with persistent hypoperfusion may have important therapeutic impact. However, the present work shows no significant differences in vascular risk and etiological factors and risk for stroke recurrence between the patients with or without focal prolonged hypoperfusion. Thus, SPECT does not seem to be useful in the management of TIA patients, but further studies, using high resolution cameras and vasoreactivity tests, should reevaluate its potential role in larger populations of patients. Measurements of the cerebrovascular reserve capacity may be helpful in patients with TIA or minor stroke due to carotid occlusive disease. Early identification of exhausted cerebrovascular reserve is needed to plan acute medical interventions and carotid surgery. This can be achieved with SPECT studies using acetazolamide challenge. >BR> The encouraging results described above do not mean that SPECT is the single investigation to perform in ischemic cerebral diseases. Indeed, it does not give informations about the arterial occlusion site and the ischemic or hemorrhagic nature of stroke. Thus, in spite of improvement in SPECT or PET technologies, they must be added to brain CT or MRI to exclude cerebral hemorrhage and techniques imaging the vascular tree, such as ultrasonography, angiography, or MR-angiography, to localize the occlusion site. Because performing consecutively several techniques is time consuming, the main limitations of SPECT or PET is the potential delay for an early therapeutic intervention within the 6-hour limit. <BR> the new MRI techniques, such as diffusion-weighted and perfusion imaging and MR-spectroscopy, are promising to enhance the capacity for detecting ischemic stroke in the very early phase, assessing the brain tissue viability, and evaluating the metabolic evolution of cerebral ischemia. In combination with MR-angiography, this very sensitive technique is likely to become the single investigation to perform in the acute phase, which will allow to save time for early therapeutic interventions such as thrombolysis. However, this will also imply rapid access to MRI in emergency and medical expertise at any time of the day. <BR> In conclusion, the SPECT findings can guide the therapeutic strategy, even when performed beyond the 6-hour therapeutic window. However, the place of this functional cerebral imaging in cerebrovascular diseases should be reevaluated once the new MRI technologies will be available and validated in routine clinical practice. Indeed, T1- and T2-weighted imaging, will be probably the first –choice, single examination to perform to get all the needed informations about the ischemic process. On the other hand, if MRI is not available or cannot be performed due to the patient’s status, SPECT should be then performed in addition to the CT scan and ultrasonography
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Citations
Laloux, P. (1997). Subtypes classification and prediction of outcome in acute cerebral ischemia using spect imaging. https://hdl.handle.net/2078.5/111405