NBIC REVIEW PAPER PRESENTATION

COLLATERAL BLOOD FLOW DYNAMICS IN STROKE

Shrut Kirti Saksena 8th Semester

CONTENTS• Introduction• Anatomy Of Collateral Circulation• Imaging Of Collateral Vessels• Digital Subtraction Angiography (DSA)• CT Angiography• Magnetic Resonance Angiography (MRA)• Trans-cranial Doppler (TCD)• Augmentation Of Cerebral Blood Flow In Acute

Stroke• Conclusion

INTRODUCTIONStroke continues to impose an overwhelming burden on global health.It is the second most common cause of death.

Main causes of stroke are: -• Ischemia, or restricted blood flow; typically due to occlusion of a

cerebral arteries • hypertension • diabetes

Collateral flow i.e. perfusion via alternative, indirect pathways, might off set potential injury to the brain.

Imaging of the brain and vessels has shown that collateral flow can sustain brain tissue for hours after the occlusion of major arteries to the brain, and the augmentation or maintenance of collateral flow is therefore a potential therapeutic target.

ANATOMY OF COLLATERAL CIRCULATION

Three principal anatomical features underlie collateral perfusion to the brain:-

• The first consists of large-artery communications between the extra-cranial and intra-cranial circulations

• Second, four major arteries coalesce to form an equalising distributor, the circle of Willis, which can redistribute blood flow in the event of a sudden occlusion of a parent vessel i.e. the anterior and posterior communicating arteries of the Circle of Willis -PRIMARY COLLATERAL PATHWAY.

• Third, leptomeningeal anastomoses potentially provide arterial blood to the cortical surface i.e. SECONDARY COLLATERAL PATHWAY

Factors that stimulate collateral recruitment : -

• Systemic Blood Pressure

• Systolic Hypertension

• Atherosclerosis, an intracranial disease, also results in vessel stiffening and could inhibit blood flow.

• The Pace of Occlusion - Gradual Chronic Occlusion e.g. progressive atherosclerotic internal carotid artery stenosis at the bulb, or neovascularisation.

IMAGING OF COLLATERAL VESSELS

Although no ideal or specific imaging modality is available for demonstration and accurate measurement of the collateral circulation, several techniques can provide insight into collateral flow in patients with ischemic stroke .

These are:-• Digital Substraction Angiography (DSA)• CT Angiography• Magnetic Resonance Angiography (MRA)• Trans-Cranial Doppler(TCS)

DIGITAL SUBTRACTION ANGIOGRAPHY(DSA)

Conventional digital subtraction angiography is referred to as the gold standard against which all other methods are compared.

It allows assessment of all three major collateral routes:

• Extracranial–Intracranial Anastomoses • Willisian • Leptomeningeal collaterals.

• Retrograde filling of three or more branches of the Middle Cerebral Artery (MCA) up to the M2 segment is said to be evidence of good collaterals, whereas anything less was rated as poor.

• A five-point scale is also used to study collaterals that was based on a score endorsed by the American Society of Interventional and Therapeutic Neuroradiology and the Society of Interventional Radiology.

• The drawback of DSA is that it is an invasive technique.

CT ANGIOGRAPHY• The non-invasive nature of CT angiography and its

rapid availability for patients with acute stroke makes it ideal for study of collateral status.

• Three categories for collateral status: Good, Moderate and Poor collaterals.

• The National Institutes of Health Stroke Scale (NIHSS) score was significantly lower in patients with good collaterals than in patients in the other two groups.

• More routinely used than DSA but has less spatial resolution.

MAGNETIC RESONANCE ANGIOGRAPHY (MRA)

• This has been used to grade collateral status and its relation to outcome.

• Hyper-intense proximal intracranial vessels on MRI obtained with fluid-attenuated inversion recovery (FLAIR) in patients with acute stroke are indicative of intraluminal thrombus.

• Distal hyper-intense vessels have a serpentine appearance, and might be an indicator of slow retrograde collateral flow.

• Drawback-The high sensitivity of MRA restrict it to detect leptomeningeal collaterals.

TRANS-CRANIAL DOPPLER (TCD)

• The researchers used TCD to judge collateral status within 24 h of a stroke secondary to carotid dissection and showed how this non-invasive technique could help to establish the long-term prognosis in such patients.

• Flow velocity was systematically measured within the ophthalmic, anterior, and posterior communicating arteries.

• TCD’s drawback is that it provides very little information about collaterals and only Circle of Willis.

AUGMENTATION OF CEREBRALBLOOD FLOW IN ACUTE STROKE

The therapeutic modalities used to increase blood flow to the brain are:

• Plasma Expanders: Dextran and Hydroxyethyl Starch were used as plasma expanders.

• No improvement in neurological outcome or reduction in mortality was recorded

• Vasodilators: Drugs (methylxanthine derivatives) that cause cerebral arterial vasodilation could potentially increase blood flow to ischemic tissue through collateral channels

• Induced hypertension: A rise in systemic blood pressure could improve blood flow to the brain, possibly through increased collateral flow.

CONCLUSIONIn acute stroke, the severity of ischemia determines how fast brain tissue might sustain irreversible damage.

• Pial collaterals, if well developed, might allow protracted tissue survival in the event of a proximal occlusion of a large intracranial blood vessel.

• Imaging of collateral blood flow is challenging, but multimodal CT and MRI techniques (perfusion combined with vessel imaging) seem to be the most promising methods for the routine assessment.

REFERENCES

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