Prognostic value of ultrasound in acute stroke

During recent years, ultrasound has become an important non-invasive imaging technique for bedside monitoring of acute stroke therapy and prognosis. By providing valuable information on temporal patterns of recanalization, ultrasound monitoring may assist in the selection of patients for additional pharmacological or interventional treatment. Ultrasound also has an important prognostic role in acute stroke. A prospective, multicenter, randomized study confirmed that a normal MCA finding is predictive of a good functional outcome in more than two-thirds of subjects. After adjustment for age, neurological deficit on admission, CT scan results, and preexisting risk factors, ultrasound findings remained the only independent predictor of outcomes [23].

The analysis of flow signal changes during throm-bolysis acquired by TCD further confirmed the prognostic value of transcranial ultrasound. Acute arterial occlusion is a dynamic process since thrombus can propagate and break up, thereby changing the degree of arterial obstruction and affecting the correlation between TCD and angiography.

A complete occlusion should not produce any detectable flow signals. However, in reality, some residual flow around the thrombus is often present. The Thrombolysis in Brain Ischemia (TIBI) flow-grading system was developed to evaluate residual flow non-invasively and monitor thrombus dissolution in real time [24]:

(TIBI 0 and 1 refer to proximal occlusion, TIBI 2 and 3 to distal occlusion and TIBI 4 to recanalization.)

Applying these criteria in acute stroke the TIBI classification correlates with initial stroke severity, clinical recovery and mortality in patients treated with recombinant tissue plasminogen activator (rt-PA). The grading system can be used also to analyze reca-nalization patterns.

The waveform changes (0 ! 5) correlate well with clinical improvement and a rapid arterial recanalization is associated with better short-term improvement, whereas slow flow improvement and dampened flow signals are less favorable prognostic signs [24].

Even incomplete or minimal recanalization determined 24 h after stroke onset results in more favorable outcome compared with persistent occlusion [25].

Reperfusion is important for prognosis. Both partial and full early reperfusion led to a lesser extent of neurological deficits irrespective of whether this occurred early or in the 6- to 24-hour interval.

Progressive deterioration after stroke due to cerebral edema, thrombus propagation, or hemodynamic impairment is closely linked to extra- and intracranial occlusive disease. Transcranial color-coded duplex is also useful for the evaluation of combined i.v.-intraarterial (i.a.) thrombolysis. Patients receiving combined i.v.-i.a. thrombolysis show greater improvement in flow signal and higher incidence of complete MCA recanalization compared with those receiving i.v. thrombolysis, especially when the MCA was occluded or had only minimal flow [26].

Patients with distal middle cerebral artery occlusion are twice as likely to have a good long-term outcome as patients with proximal middle cerebral occlusion. Patients with no detectable residual flow signals as well as those with terminal internal carotid artery occlusions are least likely to respond early or long term. The distal MCA occlusions are more likely to recanalize with i.v. rt-PA therapy; terminal ICA occlusions were the least likely to recanalize or have clinical recovery with i.v. rt-PA compared with other occlusion locations [27].

Alexandrov et al. [28] described the patterns of the speed of clot dissolution during continuous TCD monitoring: sudden recanalization (abrupt normalization of flow velocity in a few seconds), stepwise recanalization as a progressive improvement in flow velocity lasting less than 30 min, and slow recanalization as a progressive improvement in flow velocity lasting more than 30 min. Sudden recanalization reflects rapid and complete restoration of flow, while stepwise and slow recanalization indicate proximal clot fragmentation, downstream embolization and continued clot migration. Sudden recanalization was associated with a higher degree of neurological improvement and better long-term outcome than stepwise or slow recanalization.

A tandem internal carotid artery/middle cerebral artery occlusion independently predicted a poor response to thrombolysis in patients with a proximal MCA clot, but not in those with a distal MCA clot [29].

Ultrasound has an important prognostic role in acute stroke and can be used to monitor thrombus dissolution during thrombolysis.

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