Transient ischemic attacks due to increased platelet aggregation and adhesiveness. Ultrastructural and functional correlation

Genetic Modifiers of Stroke in Patients with Sickle Cell Disease-A Scoping Review

Sickle cell disease (SCD) clinically manifests itself with a myriad of complications. Stroke, both ischemic and hemorrhagic, as well as silent white matter changes, occurs at a relatively high prevalence. Understanding why and in whom stroke is most likely to occur is critical to the effective prevention and treatment of individuals with SCD. Genetic studies, including genome- and exome-wide association studies (GWAS and EWAS), have found several key modifiers associated with increased stroke/stroke risk in SCD via mechanisms including Hemoglobin F (HbF) modulation, inflammation, cellular adhesion, endothelial disruption, and hemolysis. We present a review on the modifiers that have most clearly demonstrated an association to date. More studies are needed to validate other potential polymorphisms and identify new ones. Incorporating gene-focused screenings in clinical care could provide avenues for more targeted, more effective, and less toxic prevention of stroke in this population. The data from this review will be used to inform the initial GWAS performed by the International Hemoglobinopathy Research Network (INHERENT) consortium.

Do we need more guidance on thrombophilia testing? Challenges and special considerations

INTRODUCTION

Thrombophilia testing (TT) is a laboratory procedure designed to detect the risk factors involved in the pathogenesis of vascular occlusions. The role of TT is also controversial because it has a limited impact on the choice and duration of antithrombotic treatments.

AREAS COVERED

We reviewed, by examining MEDLINE up to October 2023. Accepted and not accepted thrombophilia markers are discussed along with the appropriateness or not of prescribing TT in several conditions such as: provoked and unprovoked venous thromboembolism (VTE), women who are planning a pregnancy whose relatives had VTE or have a hereditary thrombophilia, before assumption of estro-progestins, after multiple pregnant loss, arterial thrombosis, retinal vein occlusion, and splanchnic vein thrombosis.

EXPERT OPINION

TT is not essential in the management of VTE, but it may be useful for limiting adverse events in case of thrombophilia. We expose our criticism of items afforded by other guidelines by presenting our opinion based on both the scientific evidence and clinical practice. We also deal with common mistakes in prescribing and interpretations of TT hoping to purpose an educational approach on this topic. Finally, we emphasize the creation of the expert in hemostasis and thrombosis who should be present in every hospital.

Role of the redox state of the Pirin-bound cofactor on interaction with the master regulators of inflammation and other pathways

Persistent cellular stress induced perpetuation and uncontrolled amplification of inflammatory response results in a shift from tissue repair toward collateral damage, significant alterations of tissue functions, and derangements of homeostasis which in turn can lead to a large number of acute and chronic pathological conditions, such as chronic heart failure, atherosclerosis, myocardial infarction, neurodegenerative diseases, diabetes, rheumatoid arthritis, and cancer. Keeping the vital role of balanced inflammation in maintaining tissue integrity in mind, the way to combating inflammatory diseases may be through identification and characterization of mediators of inflammation that can be targeted without hampering normal body function. Pirin (PIR) is a non-heme iron containing protein having two different conformations depending on the oxidation state of the iron. Through exploration of the Pirin interactome and using molecular docking approaches, we identified that the Fe2+-bound Pirin directly interacts with BCL3, NFKBIA, NFIX and SMAD9 with more resemblance to the native binding pose and higher affinity than the Fe3+-bound form. In addition, Pirin appears to have a function in the regulation of inflammation, the transition between the canonical and non-canonical NF-κB pathways, and the remodeling of the actin cytoskeleton. Moreover, Pirin signaling appears to have a critical role in tumor invasion and metastasis, as well as metabolic and neuro-pathological complications. There are regulatory variants in PIR that can influence expression of not only PIR but also other genes, including VEGFD and ACE2. Disparity exists between South Asian and European populations in the frequencies of variant alleles at some of these regulatory loci that may lead to differential occurrence of Pirin-mediated pathogenic conditions.

A new decade awaits sticky platelet syndrome: where are we now, how do we manage and what are the complications?

INTRODUCTION

Sticky platelet syndrome is a less known platelet function disorder with a familiar occurrence and likely genetic background. Clinically, it is characterized by an increased risk of venous and arterial thromboembolic events and obstetric placenta-mediated complications. The increased aggregation after low-dose ADP and/or epinephrine is its distinctive laboratory feature. Though described for almost 40 years, several issues regarding its etiology, involved pathomechanisms, genetic background, optimal diagnostic and treatment approach remain controversial.

AREAS COVERED

The work aims to summarize published studies, the actual definition of the syndrome, and point out its drawbacks. A literature search on Medline, Embase, and archives from EHA congresses was performed (terms: 'sticky platelet syndrome' - 'platelet hyperreactivity' - 'platelet hyperaggregability'). The authors added in their unpublished data. The introductory overview of the present understanding is followed by the discussion of the pathophysiologic, diagnostic, and therapeutic problems.

EXPERT OPINION

Despite the growing evidence provided by case reports and series, the lack of robust studies limits the decision-making on diagnostics and management. The diagnostic issues, particularly the standardization of light transmission aggregometry, represent the crucial problem for the broader acceptance of the syndrome.

Platelet hyperaggregability and venous thrombosis risk: results from the RETROVE project

The aim of the study was to determine whether platelet hyperaggregability correlates with short closure times (PFA-100) and if hyperaggregability is associated with the risk of venous thrombosis in a Spanish population. Case--control study (RETROVE project) involving 400 patients with venous thrombosis and 400 healthy controls. We determined platelet aggregation in platelet-rich plasma (PRP) by light transmission aggregometry. Various concentrations of two aggregation agonists [ADP and epinephrine (EPI)] were tested to determine the percentage of maximal aggregation and the percentage area under the curve (AUC). Venous thrombosis risk associated with platelet hyperaggregability was calculated by logistic regression. We estimated the crude and adjusted (by sex and age) odds ratios (OR) for venous thrombosis risk. An agonist concentration of 0.5 μmol/l differentiated between hypo-responders and hyper-responders at the following AUC cut-off values: EPI: the 50th percentile for aggregation with 0.5 μmol/l of EPI (EPI_AUC) was 22.53% (>22.53% = hyper-EPI); the crude risk for venous thrombosis was statistically significant (OR = 1.37; 95% CI 1.03-1.82); ADP: the 75th percentile for aggregation with 0.5 μmol/l of ADP (ADP_AUC) was 29.6% (>29.6% = hyper-ADP), with a significant crude risk for venous thrombosis (OR = 1.44; 95% CI 1.05-1.98). However, after adjustment for confounders (age), the ORs for EPI or ADP aggregation were no longer significant. EPI_AUC and PFA-100 values with the EPI agonist were significantly correlated (R = -0.342, P < 0.01). Only 12% of the PFA-100 values were explained by platelet aggregation. In this case--control study, platelet hyperaggregability was not associated with the risk of developing venous thrombosis.

High Prevalence of Sticky Platelet Syndrome in Patients with Infertility and Pregnancy Loss

Platelet hyperaggregability, known as sticky platelet syndrome (SPS), is a prothrombotic disorder that has been increasingly associated with pregnancy loss. In this retrospective study, we aimed to investigate the clinical and diagnostic relevance of SPS in 208 patients with infertility and unexplained pregnancy loss history. We studied 208 patients that had been referred to undergo a dose-dependent platelet aggregation response to adenosine diphosphate and epinephrine using light transmission aggregometry modified by Mammen during an 11-year period. Patients’ platelet aggregation response was compared with platelet function in 29 female healthy controls of fertile age with no previous history of pregnancy loss. We found a prevalence of SPS type II (33.2%) in 208 female patients with infertility and pregnancy loss. ∆-epinephrine-induced platelet aggregation in patients with SPS was significantly decreased (median 7% and range −21 to 43%) compared to patients without SPS (median 59%, range 7–88% and p < 0.0001) and healthy controls (median 57%, range 8–106% and p < 0.0001). The optimum SPS-diagnostic cutoff value for ∆-epinephrine aggregation was ≤32% (sensitivity 95.7%, specificity 95.2%). SPS patients with low-dose acetylsalicylic acid (ASA) therapy (n = 56) showed improved pregnancy outcome (32 pregnancies; live births n = 18 (56%)) compared to SPS patients without low-dose ASA (n = 13) (3 pregnancies; live births n = 1 (33%)). Our study demonstrates the clinical and diagnostic relevance of platelet hyperaggregation in women with infertility and pregnancy loss history. Further studies should investigate the potential of SPS as a novel decisional tool with both diagnostic and clinical implications in infertility and pregnancy loss.

Ten Years' Experience with the "Sticky Platelet Syndrome"

We describe a 10-year experience with a con genital platelet abnormality characterized by hyperaggre gability with ADP and/or epinephrine. The syndrome has so far been identified in >200 patients and their families. Clinical symptoms are transient or permanent arterial oc clusions, thrombotic in nature, affecting especially the coronary and cerebral vasculature, including ophthalmic vessels. The patients have no identifiable risk factors and are usually young (5-45 years), and other hypercoagula ble states are excluded. In many cases the vascular acci dent is precipitated by severe stressful events. Occasion ally the syndrome is found in patients with recurrent ve nous thromboembolism while they are on optimal oral anticoagulant therapy. In the laboratory the patients' platelets are hyperaggregable with decreasing ADP and/ or epinephrine concentrations added to platelet-rich plasma. Two forms are identified: Type I is marked by hyperaggregability with ADP and epinephrine, while Type II evidences hyperaggregability only with epineph rine. No abnormalities are found when other aggregation stimulators are used. By electron microscopy the plate lets are more readily activated by surface contact, with a greater tendency toward aggregate formation. Plasma lev els of platelet release proteins, platelet factor 4, and β-thromboglobulin are not elevated. The syndrome is treated with low-dose aspirin, which reverses the hyper aggregability and improves clinical symptoms. We hy pothesize that in vivo adrenaline release leads to in creased platelet clumping, which can transiently or per manently occlude small vessels. It is conceivable that altered surface receptors on the platelets might be respon sible for this congenital problem.

Sticky platelet syndrome: an important cause of life-threatening thrombotic complications

Sticky platelet syndrome (SPS) is a prothrombotic thrombocytopathy with familial occurrence, characterized by hyperaggregability of platelets in response to adenosine diphosphate (ADP), epinephrine (EPI) or both. The syndrome has been identified in approximately 21% of unexplained arterial thrombotic episodes, regarded to be the most common thrombophilia in arterial thrombosis and 13.2% of unexplained venous thromboembolism (VTE). The relatively young age at the first manifestation, relation to fertility and pregnancy, seriousness of the symptoms, easy and effective management of the disorder indicate to the necessity to take it into account in the differential diagnosis of the underlying cause of the thrombotic event. As the various localizations of the thrombosis in SPS have been reported, its management often requires a multidisciplinary approach. This review deals with the clinical aspects of thrombophilia, its etiopathogenesis, diagnosis as well as novel advances in the treatment and outlines the challenges for the further research.

Extensive thrombosis and first-trimester pregnancy loss caused by sticky platelet syndrome

BACKGROUND

Sticky platelet syndrome is an autosomal-dominant thrombophilia characterized by platelet hyperaggregability in the presence of adenosine diphosphate or epinephrine. The result clinically can be widespread thromboses, often arterial, in patients without apparent risk factors for thrombotic disease. Limited data exist regarding its role in adverse pregnancy outcomes.

CASE

A gravid woman with two previous first-trimester miscarriages presented at 11 weeks of gestation with a deep venous thrombosis. Despite anticoagulation, she developed extensive and progressive arterial and venous thromboses and suffered a fetal demise. A thrombophilia panel was unremarkable, but platelet aggregometry demonstrated hyperactive platelets in the presence of adenosine diphosphate and epinephrine consistent with sticky platelet syndrome.

CONCLUSION

Sticky platelet syndrome causes arterial thromboses and may be an underappreciated etiology for adverse pregnancy outcomes.

Hematologic disorders and ischemic stroke. A selective review

More than a dozen primary hematologic disorders have been associated with ischemic stroke. Inherited deficiencies of antithrombin III, protein C, and protein S have been linked with stroke in case reports; optimal screening requires functional as well as antigenic assays. Antiphospholipid antibodies and lupus anticoagulants are the most frequently identified acquired states associated with ischemic stroke. Polycythemia vera, sickle cell anemia, sickle-C disease, and essential thrombocythemia are the major disorders of formed blood elements causing stroke. Special, step-wise screening for occult prothrombotic entities in stroke patients is recommended for young persons with stroke of uncertain cause, for those with prior venous thrombosis, for those with a family history of unusual thrombosis, and for those with no other explanation for recurrent stroke. Acquired, perhaps transient, abnormalities of platelets, coagulation inhibition, and fibrinolysis may contribute importantly to brain ischemia in synergy with other mechanisms, but at present these remain ill-defined. The contribution of prothrombotic diatheses to stroke is probably underrecognized and warrants further investigation.

Outcome of TIAs of unknown aetiology following non-invasive evaluation

A proportion of patients with apparently classical transient ischaemic attacks (TIA), will have no cause for their symptoms discovered when investigated. We have reviewed 74 patients with TIA and a further 28 with amaurosis fugax whose symptoms remained unexplained following non-invasive carotid evaluation using pulsed Doppler imaging with spectrum analysis. After a mean follow-up of 31.7 months, 48.6% of TIA patients were symptom free, 36.5% had experienced recurrent symptoms and 8.1% had suffered a stroke. Among the patients with amaurosis fugax the equivalent figures were 32.1% symptom free, 42.7% recurrent symptoms, and 10.7% stroke. These figures suggest that unexplained TIAs do not necessarily have a benign prognosis and more extensive or repeated investigation of these patients is warranted.

Etiology, prognosis, and hemostatic function after cerebral infarction in young adults

We retrospectively evaluated 66 patients younger than 40 years of age who presented with acute nonhemorrhagic cerebral infarction (n = 63) or transient ischemic attacks (n = 3) to determine the possible etiology and long-term outcome at a mean follow-up interval of 3 years after initial presentation. A probable cause for the stroke was identified in 24 patients (36%); this group included one woman with a history of recurrent spontaneous abortions and a positive test for the presence of the lupus anticoagulant. We performed detailed hemostatic investigations at follow-up in 38 (90%) of the remaining 42 patients in whom the cause of the stroke was unknown or uncertain; results of the basic hemostatic screening tests (including that for fibrinogen) were uniformly normal. All 38 patients demonstrated a normal fibrinolytic response as measured by tissue plasminogen activator release to a standard venous occlusion stress test; concentration of the inhibitor of tissue plasminogen activator was not increased. No abnormalities in the concentrations of the inhibitory proteins C or S or antithrombin III were identified, and none of the 38 patients had evidence of a lupus anticoagulant. Neurologic recovery was complete or the residual disability mild in 46 of 59 (78%) patients. Overall prognosis was excellent and independent of whether a precipitating factor for the stroke could be identified.

Transient ischemic attacks and normal cerebral angiograms: a follow-up study

To determine the outcome of patients with carotid transient ischemic attacks (TIAs) and normal cerebral angiograms, we assessed 68 patients (40 men, 28 women) aged 24-72 (mean 53.5) years for recurrent TIAs and strokes and for the development of cardiac disease over 2-6 (mean 4.4) years. All but one patient had a follow-up interview in early 1987; that patient had died of an unrelated cause (lung cancer) 18 months after the presenting TIA. The diagnosis was changed at the follow-up interview in three patients (multiple sclerosis, meningioma, migraine). Among the 64 remaining patients, at admission cranial computed tomography had shown cerebral infarction in 11 of 64, two-dimensional echocardiography had been abnormal in nine of 61, Holter monitoring had been abnormal in eight of 45, and twelve-lead electrocardiography had been abnormal in three of 64. Two patients had abnormalities on both echocardiography and Holter monitoring. At the follow-up interview of the 64 remaining patients, TIAs had recurred in nine and three had developed a completed stroke; cardiac disease (angina in seven, myocardial infarction in four) was noted in 11 patients. Findings from cardiac investigations on admission in the nine patients with recurrent TIAs had been abnormal in six and normal in three; all three patients who developed a stroke had had abnormal cardiac findings. Overall, further neurologic or cardiac events occurred in 12 of 46 patients (26%) with normal and in 10 of 18 patients (55.5%) with abnormal findings on admission (p less than 0.01). In the presence of normal angiograms, extensive cardiac investigations may help predict the outcome of patients with TIAs.

Atypical progressive stroke syndrome associated with oral contraceptives and cigarette use

We report a case of progressive multifocal intracranial distal arterial branch occlusive disease in a young adult who smoked heavily and used oral contraceptives. The distribution of the angiographic lesions and the associated telangiectasias and rete mirabile resemble Moya-Moya disease except for clearly absent proximal carotid arterial disease. This type of distal multifocal cerebral arterial occlusive pattern appears to be a very rare manifestation of cigarette and oral contraceptive use. The patient has remained without symptoms for greater than 3 years off oral contraceptives and cigarettes and on antiplatelet agents.

Hemostatic Balance Index in TIA patients: sex-related changes

In order to evaluate the occurrence of hemostatic disorders, 37 patients with transient ischemic attacks (TIAs) and 50 control subjects were studied by means of the Hemostatic Balance Index (H.B.I.) derived from Raby's Thrombodynamic Potential Index (T.P.I.) and Fearnley's Whole Blood Diluted Lysis Time (W.B.D.L.T.). Results showed a significant increase in T.P.I. and a tendency to a decrease in fibrinolytic activity in the TIA group: H.B.I. was shown to be significantly increased, thus indicating a pro-thrombotic imbalance in these patients. The occurrence of similar changes in TIA females when compared to male patients marks the importance of plasmatic factors in the mechanism of thrombotic disorders in females with cerebrovascular disease.

Norepinephrine in cochlear microcirculation of guinea pigs

The blood flow in the radiating arteriole through a small cochlear fenestra was recorded with motion pictures in anesthetized guinea pigs, before and after norepinephrine injection into the ipsilateral carotid artery. The flow velocity was determined by measuring the displacement of plasma space running through the radiating arteriole per second. Norepinephrine injection of 0.01 and 0.15 mg/kg increased both flow velocity and arterial blood pressure. The flow velocity increase related directly to the increase in arterial blood pressure. However, a larger norepinephrine dose (1.2 and 2.5 mg/kg) led to transient cessation in flow, which was independent of the blood pressure increase. Dilatation of vessel diameter was always observed with the rise in blood pressure, irrespective of norepinephrine doses. When blood flow in the radiating arteriole stopped after a large norepinephrine injection, the arteriole's vascular lumen was completely obstructed by the aggregated red blood cells. These results suggested that cochlear microcirculation is disturbed by microemboli formed by norepinephrine-induced platelet hyperaggregation.

Primary thrombocythemia in a patient with cerebellar infarction

A patient suffering from essential thrombocythemia presented manifestations of digital thromboses and two cerebral ischemic strokes. Anomalies of platelet function are discussed in relation to published data and the efficacy of antiaggregant treatment is stressed.

Effects of low dose aspirin on platelet function in patients with recent cerebral ischemia

We tested the antiplatelet effects of low-dose aspirin in patients with occlusive cerebrovascular disease, because conventional dosage aspirin inhibits vascular synthesis of prostacyclin at the same time that it inhibits platelets. The effects on platelet function and thromboxane A2 synthesis of 40 mg of aspirin daily or 40 mg aspirin plus dipyridamole were measured in 23 patients starting within a week after the onset of cerebral ischemia. All patients had normal baseline platelet aggregation responses to four stimuli: arachidonate, epinephrine, adenosine diphosphate and collagen. The generation of thromboxane A2 by platelets, measured as serum thromboxane B2, was also normal. After 3 to 7 days of low dose aspirin therapy, platelet aggregation responses were suppressed to the extent observed with higher dosage aspirin. Serotonin release during platelet aggregation was inhibited by more than 95% and thromboxane B2 levels in clotted blood fell by more than 95%. Responses to aspirin treatment were similar in patients with transient ischemic attacks and in those with stroke and were also similar in both sexes. No differences in platelet responses were observed between patients receiving aspirin alone and aspirin plus dipyridamole. Thus 40 mg aspirin daily inhibited platelet responses as effectively as higher doses of aspirin in patients who had recent cerebral ischemia and showed a cumulative antiplatelet effect.

Platelet-vessel wall interactions: implication of 5-hydroxytryptamine. A review

The evidence for an impact of platelet-derived 5-hydroxytryptamine (5-HT) on local tissue perfusion is reviewed. By interacting with 5-HT2 serotonergic receptors, 5-HT, directly or through amplification, activates platelets, endothelial and vascular smooth muscle cells producing platelet aggregation, vascular permeability increase and large vessel constriction. Pharmacodissection in experimental animals with selective serotonergic 5-HT2 receptor antagonists, e.g. ketanserin, shows that 5-HT largely contributes to the platelet-mediated increase in vascular permeability, to platelet-vessel wall interaction during hemostasis, to cardiopulmonary dysfunction provoked by thromboembolism and to the platelet-mediated inhibition of peripheral collateral circulation. Clinical results obtained with ketanserin further substantiate an involvement of platelet-derived 5-HT in the pathogenesis of impaired tissue perfusion in some cardiovascular conditions.

Platelet aggregability in cerebral thrombosis--analyzed for vessel stenosis

Platelet aggregation in the post-acute phase of 48 patients with cerebral thrombosis was measured to see if any specific type of cerebral infarction is associated with enhanced platelet aggregation. All patients were examined with cerebral angiography and computed tomography (CT). Stenotic lesions in major cranial arteries were analyzed by measuring the apparent diameter. Severe stenosis was defined as 75 per cent constriction or more. Enhanced aggregation of platelets (secondary aggregation at 1 microM ADP or less) was present in 5 of 25 patients (20%) who had severe vessel stenosis or occlusion. CT examination frequently revealed both cortical and deep involvement. On the other hand, 13 of 23 patients (57%) with less stenotic lesions showed enhanced aggregation and that was statistically significant (p less than 0.05). Many patients of this group had persistent hypertension and small deep infarctions. Platelet aggregation was also measured in 20 hypertensive control subjects without stroke. Four of them (20%) showed enhanced aggregation. These findings suggest that a combination of enhanced platelet aggregation and hypertension increases the risk of small deep infarctions accompanied by mild stenotic changes of the major cranial arteries.

A study of hemostasis in ischemic cerebrovascular disease. II. Abnormalities in platelet ADP release, platelet cyclooxygenase regeneration time and platelet factor 3 availability

Most earlier studies of platelet function in stroke patients have been performed in the acute phase and are hampered by diagnostic insecurity. A sample of totally 67 young adults below the age of 55, with ischemic cerebrovascular disease (TIA and minor stroke) were investigated at a late stage after acute disease and compared to 20 healthy controls. Patients with atherosclerotic signs at cerebral angiography had significantly (p less than 0.05) higher platelet factor 3 availability than angionegative patients. Unexpectedly, female patients compared to male patients had significantly (p less than 0.05) larger ADP-release after stimulation with collagen in vitro. Furthermore, when female patients were compared to female controls a significantly (p less than 0.05) increased platelet factor 3 availability was found. The results indicate that platelets in female patients may have an increased tendency to aggregate in vivo. Patients had significantly (p less than 0.01) shortened platelet cyclooxygenase regeneration half times (PRT). This was correlated to high levels of factor VIII related antigen (r=0.59) and high levels of factor VIII biological activity (r=0.67), indicating that platelets may be consumed by platelet adhesion and mural thrombi formation in abnormal vessel walls. PRT appears to be a reliable method to assess platelet function in vivo and to optimize aspirin dose and dose intervals in the individual.

A study of hemostasis in ischemic cerebrovascular disease. III. Abnormalities in vascular plasminogen activators, antiactivators and alpha 2-antiplasmin

A sample of in all 119 patients with ischemic cerebrovascular disease (TIA and minor stroke) below the age of 55 years, were submitted for testing of fibrinolysis in the late recovery phase after acute disease. Defective fibrinolysis, as tested after venous stasis, was found in patients (p less than 0.01) as compared to controls using a conventional fibrin plate method. A new chromogenic peptide substrate method showed a similar tendency. Antiactivator activity, measured as antiurokinase, using a peptide substrate, was significantly higher (p less than 0.01) in young female patients than in female controls. Alpha 2-antiplasmin (peptide substrate method) was significantly (p less than 0.001) higher in female than male patients. However, no correlation was found between inhibitors of fibrinolysis and defective fibrinolysis after venous occlusion. Furthermore, in a pilot study of vein biopsies, normal content of vascular plasminogen activators was found in the majority of cases. Thus, it is suggested that defective fibrinolysis in most cases reflects a disturbed release function.

Effects of adenosine diphosphate administration on the vestibulo-oculomotor reflex of rabbits

Adenosine diphosphate (ADP) injection into a unilateral vertebral artery in rabbits decreased per-rotational nystagmus; the nystagmus beating toward the injected side was predominantly suppressed. This influence became obvious when the animal's blood pressure was low. Field potentials were recorded in the vestibular nuclei upon electric stimulation of the vestibular labyrinth. ADP administration into the vertebral artery also suppressed the amplitude of vestibular evoked potentials (VEPs). Effects of ADP upon the vestibulo-ocular reflex and VEP were considered to be due to regional ischemia induced by the microemboli, which consisted of platelet aggregation.