Failure of platelet function analyser 200 to demonstrate clinical clopidogrel resistance in a patient undergoing intracranial vascular stenting

Abstract

A patient undergoes intracranial stent insertion for stent-assisted coiling of a basilar tip aneurysm and left middle cerebral artery aneurysm. A flow diverting stent is also placed across an anterior communicating artery aneurysm. Prior to the procedure, the patient takes dual antiplatelet medications, being aspirin and clopidogrel. Because of the concern regarding in-stent thrombus and thromboembolic complications related to intracranial stenting and the high rate of clopidogrel resistance, preoperative platelet function testing (PFT) was undertaken to ensure platelet inhibition. In this case, PFT was performed on a platelet function analyser which demonstrated platelet inhibition. Ten days following the procedure, the patient represented with thromboembolic stroke. Repeat PFT performed with whole blood impedance aggregometry and despite full medication compliance demonstrated clopidogrel resistance. Clopidogrel was then ceased and prasugrel commenced. This case demonstrates the importance of appropriate platelet inhibition in patients with intracranial stents and the controversy surrounding PFT.

Background

Complications of intracranial stent placement include in-stent thrombus and embolic stroke. Our current practice is to commence dual antiplatelet therapy (DAPT) 5 days prior to the procedure and continue DAPT for at least 6 weeks in the case of low metal stents for aneurysm neck remodelling or 3 months for flow diverting stents.

Aspirin and prasugrel are our agents of choice; however, there are many other centres and scenarios where clopidogrel will routinely be used as the second agent rather than prasugrel. This is important as there is a well documented population of patients who demonstrate clopidogrel resistance.

To decrease the risk of embolic complication for a patient taking clopidogrel, it is essential to know that the patient responds to clopidogrel, and platelet function testing (PFT) has been proposed as a method to assess this platelet inhibition.

We present a case of PFT using a platelet function analyser (PFA-200) demonstrating platelet inhibition prior to intracranial stent insertion. Following the endovascular procedure, the patient had an embolic stroke. Repeat PFT, employing the technique of whole blood aggregomerty did not demonstrate platelet inhibition with clopidogrel despite full medication compliance by the patient.

Case presentation

A 69-year-old man undergoes MRI of the brain due to a recent history of transient ischaemic attacks (TIA). The MRI demonstrates multiple intracranial aneurysms; however, there is no evidence of aneurysm rupture. He is referred to interventional neuroradiology (INR) for diagnostic cerebral digital subtraction angiography (DSA). This confirms the presence of a 6 mm basilar tip aneurysm, 6 mm wide-necked left middle cerebral artery (MCA) aneurysm and a 3 mm anterior communicating artery (ACOM) aneurysm arising from the junction of the ACOM and right anterior cerebral artery A2 segment. The patient had previously been commenced on 100 mg/day aspirin and 75 mg/day clopidogrel for the recent TIA.

Despite the INR team’s preference for aspirin and prasugrel in cases requiring intracranial stents, as the patient had already commenced clopidogrel, the decision was made to continue with his current medication regime and perform platelet function testing prior to the procedure.

Platelet function tests performed on PFA-200 demonstrated significant closure time prolongation, consistent with platelet inhibition. Subsequently, the patient underwent basilar Y stent coiling (Neuroform Atlas stents and coils, Stryker), left MCA aneurysm Y stent coiling (Neuroform Atlas stents and coils, Stryker) and flow diverting stent of the ACOM aneurysm (Silk Vista Baby stent, Balt). The flow diverting stent was deployed within the ipsilateral anterior cerebral artery A2 segment to the ipsilateral A1 segment of the anterior cerebral artery. All procedures were performed during the same anaesthetic.

There was no intraoperative complication. A low-dose heparin infusion (500 units/hour) was commenced in recovery and ceased the following morning. DAPT was continued.

Postoperative recovery was uncomplicated, and the patient was discharged home 3 days later.

Ten days later, he represented with intractable hiccups. The patient was adamant that there had been full compliance with DAPT.

Investigations

Following readmission to hospital, an MRI brain was performed. This demonstrated acute infarction of the head of the right caudate nucleus, the anterior limb of the right internal capsule as well as the anterior aspect of the right lentiform nucleus. Appearance was consistent with an Artery of Heubner infarct pattern (figure 1). There was also a tiny focus of diffusion restriction within the left centrum semiovale (figure 2).

Figure 1.

MRI brain showing restricted diffusion within the right basal ganglia consistent with acute infarction.

Figure 2.

MRI brain showing small focus of restricted diffusion in left centrum semiovale.

A cerebral DSA was also performed which did not demonstrate in-stent thrombus or stenosis.

PFT was performed at the preoperative clinic appointment prior to the patient’s operation.

This showed a prolonged collagen/epinephrine closure time (301 s) and prolonged collagen/ADP closure time (301 s). This was interpreted as showing satisfactory inhibition of platelets. In our institution, the normal reference ranges for collagen/epinephrine closure time is 94–162 s and 64–127 s for collagen/ADP closure time.

The repeat PFT performed following the embolic stroke with PFA-200 showed borderline high/normal ADP closure time (128 s). A second PFT was then performed via whole blood aggregometry which showed normal platelet aggregation with ADP test (61 units) suggesting no inhibition of platelet function by clopidogrel.

Differential diagnosis

The diagnosis was embolic stroke, secondary to thrombus formation within an intracranial stent due to poor platelet inhibition from clopidogrel resistance.

Treatment

Following test results demonstrating lack of platelet inhibition, the patient was commenced on the antiplatelet agent prasugrel (loading dose of 60 mg, then a dose of 10 mg/day). One hundred milligrams of aspirin a day was also continued.

Outcome and follow-up

Following prasugrel loading, the patient was discharged home. There was no significant disability on follow-up (mRS 0) and no further stroke has occurred.

Discussion

Antiplatelet medications are used in neurovascular procedures to reduce thromboembolic complications associated with insertion of intracranial vascular stents. Due to a lack of randomised controlled studies, there is currently no universally accepted antiplatelet routine for neurovascular procedures, which has led to heterogeneous practices among neurointerventional specialists.¹ In our practice, prior to intracranial vascular stenting we premedicate with 5–10 mg prasugrel (depending on age, weight, renal function) and 100 mg aspirin 5 days before and on the morning of surgery. The patient then continues prasugrel for at least 3 months for flow diverting stents and at least 6 weeks for low metal stents. Aspirin is often continued indefinitely. A recent survey conducted among neurovascular interventionist in the USA performed by Faught et al showed however the most commonly employed antiplatelet regime in preparation for intracranial stenting was premedication with aspirin 325 mg/day and 75 mg/day clopidogrel for 7 days prior to procedure.¹

Clopidogrel is an inhibitor of the platelet ADP receptor P2Y12, thus working to inhibit platelet aggregation. It is a prodrug, which undergoes hepatic biotransformation by cytochrome 450 enzymes.² The use of clopidogrel is widespread among many medical specialties, and resistance has been quoted in up to 40% of patients.³ The predominant mechanism of resistance seems to arise from altered activity and expression of cytochrome p450 or P2Y12 receptors, altered clopidogrel reabsorption, body mass index or potential interactions/competition with other medications which rely on cytochrome 450 enzymes such a statins and proton pump inhibitors.^{2 4}

Prasugrel is a newer antiplatelet agent which inhibits platelet aggregation through irreversible binding of its active metabolite to P2Y12 ADP receptors. Prasugrel demonstrates more potent platelet inhibition, faster onset of action, lower rates of interpatient variability of platelet inhibition and is not affected by cytochrome P450 polymorphs.⁴ There are a number of studies in the cardiology literature such as the TRITON-TIMI 38 and PRINCIPLE TIMI 44 studies which showed significantly decreased ischaemic cardiac events or ischaemic stroke and significantly higher levels of platelet inhibition with prasugrel over clopidogrel.^{4 5} Some studies, however suggest that prasugrel may have increased haemorrhage rates.⁵

Unlike clopidogrel, prasugrel resistance appears exceedingly rare.⁶ Prasugrel is therefore a recommended alternative in patients who demonstrate no or poor response to clopidogrel.⁴

Given the importance of platelet inhibition in neurovascular procedures, platelet function evaluation is becoming an important part of the preoperative workup. There are a number of methods currently available in Australian hospital’s including light transmission aggregometry (LTA), whole blood impedance aggregometry, viscoelastic haemostatic assays and platelet function assays.

A recent study found that 16% of patients who were found to be resistant to clopidogrel developed thromboembolic complications versus 1.6% of clopidogrel responders.⁷ There are also a number of further studies suggesting more frequent thromboembolic complications in clopidogrel-resistant patients such as a study by Cheung et al which showed significantly lower numbers of thromboembolic complications in patients who had undergone PFT and converted to ticagrelor if found to be clopidogrel resistant.³

In LTA, blood is centrifuged to obtain a platelet rich sample. The sample is continuously stirred as a platelet agonist such as ADP is added. A light source shines through the sample, and as the agonist is added, there is increased transmission of light as turbidity decreases as platelets aggregate. This mode of PFT is generally accepted as the gold standard; however, it is more expensive and labour intensive than other available methods.⁴ In whole blood impedance aggregometry (such as the multiplate analyser) whole blood is stirred between two electrodes, an agonist is added causing platelet to adhere to the electrodes which alters the impedance of the circuit.⁸

Platelet function assays such as PFA-200 is in common use. The original PFT undertaken in this case was performed on a PFA-200. This test involves a sample of whole blood passed through an aperture in a membrane coated in collagen and a platelet agonist such as ADP or epinephrine. Platelets are activated by the agonist and a platelet plug forms at the aperture. The time taken for full occlusion of the aperture is measured and expressed as ‘closure time.’⁹ This particular case has been reviewed by our institution’s haematology team, and no definite cause for the change in PFT result was identified. A number of studies have shown that this form of PFT is unable to monitor clopidogrel therapy and lacks correlation with other PFT methods.⁸ In a large Korean study, PFA-100 was found to be insufficiently effective for laboratory screening of drug-induced platelet dysfunction.¹⁰ PFA-100 however has been shown to have a high negative predictive value for platelet-related bleeding disorders, thus when there are normal closure times, further PFT may not need to be performed.⁷

Another assay includes VerifyNow. This method assesses whole blood platelet aggregation via turbidimetric optical detection with cartridges containing ADP and fibrinogen coated beads. Activated platelets bind to fibrinogen attached to the beads. The optical signal increases as more platelets aggregate on the spheres. Some studies demonstrate a strong correlation with LTA and is reliable in detection of P2Y12 platelet inhibition.⁸

Standard laboratory platelet assays and antiplatelet therapies measure and target, respectively, aggregation and platelet activation rather than their procoagulant function important for thrombus initiation and propagation. Using a novel flow cytometry assay, Pasalic et al observed for the first time a significant although partial inhibitory effect of DAPT (clopidogrel or ticagrelor plus aspirin) but not aspirin alone on the procoagulant platelet subset. Furthermore, 40% of clopidogrel-treated patients showing marked suppression of ADP-induced platelet aggregation by Multiplate, remained hyper-responsive to thrombin and collagen on procoagulant platelet testing. The authors speculate that this group of patients would remain at a heightened risk of thrombotic complications despite an adequately inhibited aggregation response.¹¹

It is clear however that there is lack of consensus regarding the most appropriate method of performing PFT in neurovascular patients to assess for platelet inhibition by clopidogrel. LTA, although generally considered the gold standard, is expensive, slow and labour intensive. Point of care devices such as the PFA-200 and VerifyNow clearly have advantages of being fast and simple tests to perform. There is however at present no PFT that that can be declared the optimal test for quantifying platelet inhibition with clopidogrel, and therefore more work is required to standardise and understand short comings of each test.

Learning points.

• We would suggest that for patients who require intracranial vascular stenting and for which clopidogrel has been prescribed as the second antiplatelet agent undergo platelet function testing (PFT) to ensure that the patient is not resistant to clopidogrel. We would suggest avoiding the use of PFA-200 to assess clopidogrel response. If at all possible, we would also suggest the use of prasugrel rather than clopidogrel as the second antiplatelet agent.

• Doctors requesting PFT need to be aware of the various forms of PFT available at their institution and the limitations of each test. We would suggest open discussion with haematologists at their institution to discuss the indications of the test, available PFT and assistance with interpretation.

• Discussion with our institution’s haematology team has been useful in creating a standard protocols when PFT is requested.