Decreased Infarct Volume and Intracranial Hemorrhage Associated with Intra-Arterial Nonionic Iso-Osmolar Contrast Material in an MCA Occlusion/Reperfusion Model

Discussion

Both IV and IA IRCM injection may increase the permeability of the blood-brain barrier under normal conditions in animals.^14⇓–16 Theoretically, it is possible that large IV or IA doses of IRCM (as used in therapeutic revascularization procedures) may contribute to a BBB opening, edema, and HT.^16,17 Following ischemia in humans, additional leakage of IRCM across the damaged BBB has been reported under a variety of circumstances. Contrast deposition during CT perfusion in the acute ischemic setting has been noted to be a marker of subsequent ICH.^1⇓–3 It remains unclear whether this is both cause and effect. Lummel et al⁴ recently constructed a nexus between hyperattenuated cerebral lesions and IRCM deposition immediately after mechanical revascularization preceded by CTA/CTP, suggesting that early hyperattenuated lesions were IRCM-volume related with a higher risk of secondary HT, but they did not find them necessarily predictive of reduced clinical functional outcome.

A variety of IRCM has been used clinically in human stroke evaluation and therapy, and IRCM differs in its effects on and potential for passage across the injured BBB. Variable passage across the BBB may modulate infarct edema and the risk of ICH. Ionic hyperosmolar IRCM (∼1200 mOsm) has been abandoned for IA use due to increased neurotoxicity compared with the low-osmolar IRCM (∼700 mOsm). Doerfler et al¹⁸ showed decreased infarct volume following IV nonionic low-osmolar IRCM (770 mOsm) versus ionic hyperosmolar iothalamate (2520 mOsm) in a rodent permanent occlusion stroke model, and they suggested that osmolarity, at least in part, explained the effect. Newer iso-osmolar IRCM is theoretically less cytotoxic due to lower osmolarity (similar to plasma, ∼300 mOsm), compared with low-osmolar IRCM.^19,20 However, complex mechanisms other than osmolarity alone are acting as well.

IRCM effects on infarct volume and ICH may be due to osmotic, direct neurotoxic, hydrostatic, or vascular occlusive effects. The belief that IRCM hyperosmolarity is responsible for several adverse effects of IRCM via toxic ionic load conflicts with data suggesting a beneficial osmotic tonicity effect of intravascular IRCM on the extravascular, extracellular space based on molecular size.^21⇓–23 The molecule size of dimeric iodixanol (1000 d, approximately 6 times larger than mannitol) is approximately twice as large as monomeric iopamidol (500 d); it may be less able to traverse early damage to the blood-brain barrier to exert adverse osmotic or direct toxic effects in the extracellular space. Conversely, iopamidol may ultimately more easily traverse the damaged membrane to increase edema, rather than decrease it, by osmotic effect.

Endothelial cell morphology and function may be directly affected variably by IRCM injection, contributing to IRCM and fluid passage beyond the arterial lumen.²⁴ There is conflicting evidence regarding increased neurotoxicity once the IRCM has crossed the BBB in animals and humans.^{17,25⇓⇓⇓–29} Heinrich et al³⁰ compared the direct cytotoxic effects of dimeric iso-osmolar and monomeric iso-osmolar IRCM on renal tubular cells in vitro and found that dimeric IRCM have stronger direct cytotoxic effects, postulating a mechanism independent from osmolarity.³⁰

Molecular chemotoxicity, related to the chemical structure of the molecule, decreases as the number of carboxyl groups decreases and the number of hydroxyl groups increases. Increasing the number of hydroxyl groups also increases the solubility of the agent, thus reducing its tendency to bind to tissues and proteins.³¹ Agents with no carboxyl groups and a number of hydroxyl groups evenly distributed around the main molecule have a reduced propensity for neurotoxicity.³² Iodixanol has an increased number of hydroxyl groups compared with low-osmolar IRCM.

Hydrolysis of iodixanol in vitro can produce a derivative of propylene glycol (2,3-dihydroxy-1-propylamine HOCH₂-CH(OH)-CH₂-NH₂). Propylene glycol when injected intra-arterially in an ischemic brain model of rats has been found to decrease BBB dysfunction by a “sealing” effect, with subsequent decreased permeability and infarct size.³³ Although iodixanol has no biotransformation after intravenous injection, a similar “sealing” effect in the BBB is possible.^34,35

Clot formation and coagulation interactions by IRCM also have impact on infarct volume and ICH.³⁶ Mechanisms and effects may be paradoxical, with equilibrium among thrombotic, antiplatelet, and fibrinolytic properties resulting in a dominant state, which varies among different IRCM. Specifically, iodixanol has been found to have a less thrombotic effect than iopamidol.³⁷ However, no Doppler-measured CBF differences were identified postreperfusion in our animals that could reasonably explain an effect of persistent reduced blood flow on infarct volume in the iodixanol group. Even in the absence of coagulation-mediated oligemic effects, increased hemoglobin binding attributed to IRCM may exert a circulatory hypoxic effect, promoting neurotoxicity via reduced oxygen delivery to tissue.³⁸

Physical properties such as viscosity differ among IRCM and can exert a hydrostatic effect.³⁹ Iodixanol has increased viscosity (11.8 cP) compared with iopamidol (4.7 cP) at 37°C. The reduced hydrostatic effect across an injured BBB may account for the reduced infarct edema volume measured compared with saline or a less viscous low-osmolar contrast. Differences in infusion duration were not encountered in our study. Higher viscosity fluid may reduce antegrade reperfusion and reduce hydrostatic infarct edema. If so, reduced viscosity-related antegrade reperfusion may represent a new therapeutic option for further exploration.

Changes in the environment, such as decreased pH or increased temperature in the hypoxic brain tissue, can cause changes in the physicochemical properties of IRCM as well. The role these differences play in our particular results is difficult to postulate.

We observed a trend toward increased mortality and worse neurologic score in the saline group compared with iodixanol and iopamidol. It is likely that significant damage of the BBB had already occurred by the time of reperfusion 5 hours after ischemia. Reperfusion itself caused an inflammatory cytotoxic effect, with increased edema and ICH. Reperfusion increases levels of reactive oxygen species, including the superoxide radical and nitric oxide.^40,41 Oxidative radicals trigger activation of metalloproteinases, which disrupt permeability barriers and, in turn, potentiate injury to microvasculature and neural cells during ischemia and reperfusion.^42,43 Larger infarcts and increased HT occur in the reperfusion of MCA occlusion compared with permanent occlusion, and metalloproteinases are particularly increased after reperfusion compared with models of permanent occlusion.¹²

Aquaporin-4 has been linked to the severity of infarct volume but not with HT.⁴⁴ Therefore, the mechanism of HT, though linked to mechanisms affecting reperfusion, infarct volume, and edema previously discussed, might have a separate and different pathophysiologic pathway after reperfusion in acute ischemic stroke in humans and animals.⁴⁵

The contributions of hemorrhagic changes, typically petechial in the rat, might also increase differences in apparent, measurable secondary infarct edema. However, on the basis of our pilot work, more hemorrhagic changes were anticipated in the IRCM group, as opposed to lesser areas with iodixanol. In contrast to our pilot work, in which 37% of saline-infused and 75% of iohexol-infused rats exhibited HT, all animals in the current study exhibited HT, most frequent and voluminous in the saline and iopamidol groups. The higher percentage of ICH in all groups can be attributed to the additional burden of postreperfusion transportation, anesthesia, MR imaging, recovery, and delay before sacrifice, likely contributing to stress-induced infarct volume and HT. Nevertheless, we identified decreased HT in the iodixanol group. Thus, we might indeed be identifying a direct or indirect effect of iodixanol, whose exact mechanisms and significance remain to be clarified.

There are a number of limitations of our methods, analyses, findings, and hypotheses regarding the IRCM effect on infarct volume and ICH. First, our data for infarct volume as measured by DWI are inconclusive at 24 hours, mainly due to field inhomogeneity artifacts precluding 4 early cases from adequate evaluation. The role of ADC for prediction of HT in our population is yet to be explored further.^46⇓–48 The 5-hour time window is later than typically used in ischemic rat models. Knight et al⁴⁹ reported no T2 signal changes early after reperfusion in a model of 2–4 hours of transient ischemia. Thus, the late time was chosen to guarantee maximal ischemic potential.

The large volume and concentration of prolonged IA IRCM injection was chosen not only to avoid overlooking any potential adverse effect not obvious with lower volumes but also to more closely simulate the effect of frequent, repetitive injections in the setting of prolonged occlusion, as is commonly used in human stroke intervention, in the timeframe when intervention is most typically applied and when benefits of revascularization are expected to be diminishing.

Another limitation is the lack of images before reperfusion. Any proposed protective effect of iodixanol might have been demonstrated by different rates of increase, or reduction, of infarct edema volume between prereperfusion and postreperfusion scans. With surgery performed remote from the scanner site, imaging during occlusion was impractical at this stage of investigation.

Differences in infarct volume and ICH might be attributed to inadequate MCA occlusion or reduced reperfusion effect in the iodixanol group. However, Doppler CBF showed no statistical differences among groups, with overall no indication of inadequate occlusion or differences in reperfusion. In addition, we have found spontaneous hyperthermia useful in estimating successful occlusion of the MCA territory in our model, with direct correlation between the body temperature and infarct size.⁵⁰ All animals showed hyperthermia with similar average body temperatures before reperfusion in all 3 groups. Therein, we have no evidence that the infarct development before reperfusion was different in all 3 groups.

Nevertheless, further investigation with a larger number of animals could improve the consistency of late DWI measurements; if possible, MR imaging before reperfusion could help to confirm our results.