Traumatic brain injury causes platelet adenosine diphosphate and arachidonic acid receptor inhibition independent of hemorrhagic shock in humans and rats

Abstract

Background: Coagulopathy in traumatic brain injury (CTBI) is a well-established phenomenon, but its mechanism is poorly understood. Various studies implicate protein C activation related to the global insult of hemorrhagic shock or brain tissue factor release with resultant platelet dysfunction and depletion of coagulation factors. We hypothesized that the platelet dysfunction of CTBI is a distinct phenomenon from the coagulopathy following hemorrhagic shock.

Methods: We used thrombelastography with platelet mapping as a measure of platelet function, assessing the degree of inhibition of the adenosine diphosphate (ADP) and arachidonic acid (AA) receptor pathways. First, we studied the early effect of TBI on platelet inhibition by performing thrombelastography with platelet mapping on rats. We then conducted an analysis of admission blood samples from trauma patients with isolated head injury (n = 70). Patients in shock or on clopidogrel or aspirin were excluded.

Results: In rats, ADP receptor inhibition at 15 minutes after injury was 77.6% ± 6.7% versus 39.0% ± 5.3% for controls (p < 0.0001). Humans with severe TBI (Glasgow Coma Scale [GCS] score ≤ 8) showed an increase in ADP receptor inhibition at 93.1% (interquartile range [IQR], 44.8-98.3%; n = 29) compared with 56.5% (IQR, 35-79.1%; n = 41) in milder TBI and 15.5% (IQR, 13.2-29.1%) in controls (p = 0.0014 and p < 0.0001, respectively). No patient had significant hypotension or acidosis. Parallel trends were noted in AA receptor inhibition.

Conclusion: Platelet ADP and AA receptor inhibition is a prominent early feature of CTBI in humans and rats and is linked to the severity of brain injury in patients with isolated head trauma. This phenomenon is observed in the absence of hemorrhagic shock or multisystem injury. Thus, TBI alone is shown to be sufficient to induce a profound platelet dysfunction.

Figure 1

Overview of TEG and TEG/PM. A, Standard TEG parameters: enzymatic time to clot formation (R), clotting time until 20-mm amplitude is achieved (K), measurement of clot kinetics (α angle), maximum clot strength reported as MA in millimeters, and clot lysis 30 minutes after MA (LY30). B, TEG/PM. The amplitude (millimeters), depicted on the y-axis, in response to platelet agonists MA_ADP or MA_AA is compared with the whole-blood clot strength when clotting is initiated via the usual thrombin pathway (MA_Thrombin) minus the fibrin contribution (MA_Fibrin). This normal control tracing shows ADP- and AA-stimulated thrombus formation similar to the whole-blood clot strength (MA_ADP or MA_AA near MA_Thrombin) with an ADP inhibition of 8% and AA inhibition of 1%. Figure 1 is modified (with permission) from Neurocrit Care 2013;18:201–208.

Figure 2

Platelet inhibition in rats following severe isolated blunt TBI. ADP receptor inhibition at 15 minutes after injury was 77.6% ± 6.7% (n = 25) versus 39.0% ± 5.3% for uninjured controls (n = 20) (p < 0.0001). A parallel trend of lesser magnitude was noted for AA receptor inhibition in the TBI group at 48.6% ± 5.7% versus 28.9 ± 2.8 for uninjured controls (p = 0.0005).

Figure 3

Box and whisker plots of ADP and AA receptor inhibition in control versus TBI cohorts. A, ADP receptor inhibtion of all TBI is 64.5% (IQR, 39.3–95.1%, n = 70) compared with 15.5% (IQR, 13.2–29.1%, n = 10) (Mann-Whitney U-test p < 0.0001) in the healthy controls. ADP inhibition of severe TBI cohort, GCS score of 8 or lower, was 93.1% (IQR, 44.8–98.3%, n = 29) and 56.5% (IQR, 35–79.1%, n = 41) (p = 0.0006) in the cohort with GCS score greater than 8. Mean values denoted by “+.” B, AA receptor inhibition of all TBI is 25.6% (IQR, 3.1–76.7%, n = 70) compared with 2.2% (IQR, 0.0–5.8%, n = 10) (p = 0.0027) in the healthy controls. AA inhibition of severe TBI, GCS score of 8 or lower, was 14.4% (IQR, 0–62.2%, n = 29) and 40.4% (IQR, 12.9–78.9%, n = 41) (p = 0.3460) in the cohort with a GCS score greater than 8. Mean values denoted by “+.”

Figure 4

Representative TEG/PM tracings of a severe TBI patient versus an uninjured control. A, Severe TBI. In this patient with a GCS score of 3, ADP receptor inhibition is 100%, as evidenced by the lack of increase in clot strength when platelets are stimulated by ADP, compared with a fibrin-only clot. B, Uninjured control. ADP receptor inhibition of a healthy volunteer with a GCS score of 15. The ADP receptor inhibition is very low at 13.9%, denoting a normal platelet contribution to clot strength when stimulated by exogenous ADP.

Figure 5

Representative histopathologic sections of post-TBI rat brains (hematoxylin and eosin, original magnification 40×). In the slides taken 30 minutes after injury, subarachnoid hemorrhage was a consistent finding (arrows), confirming disruption of the BBB in our TBI model.