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Patients undergoing surgery for adolescent idiopathic scoliosis (AIS) frequently develop significant bleeding [1]. However, the mechanism of bleeding in these patients remains incompletely understood. Although generally attributed to surgical factors [1], coagulation abnormalities may also contribute to the high incidence of bleeding complications [2–4]. This has important implications for clinical practice because the identification of hemostatic defects prior to surgery may allow affected patients to be targeted with specific interventions to reduce the risk of bleeding.

We investigated the prevalence of coagulation abnormalities among consecutive patients undergoing elective surgery for AIS at our institution over a 2-year period.

Blood was collected by antecubital venepuncture into evacuated tubes containing sodium citrate or ethylenediaminetetraacetic acid (EDTA). Screening coagulation testing [activated partial thromboplastin time (APTT), International Normalized Ratio (INR), fibrinogen] was performed by a STA-R analyzer (Diagnostica Stago, Asnières, France) using Platetin LS (Biomerieux, Durham, USA) for the APTT reagent and Neoplastine Cl plus (Diagnostica Stago) for the INR. In those with abnormal screening studies, individual coagulation factors were measured by a one-stage coagulation assay using plasma deficient in the clotting factor being assayed. The dilute Russell's viper venom test (Gradipore, Sydney, Australia) was used to screen for lupus anticoagulant. Von Willebrand factor antigen was measured by latex immunoassay (Diagnostica Stago) and ristocetin cofactor was measured by the STA-R analyzer using the Dade Behring von Willebrand reagent (Marburg, Germany). Platelet function testing was performed on a Chronolog 680 Aggregometer (Chronolog Corporation, Havertown, MA, USA). Patients on whom platelet function testing was performed had not been on any antiplatelet medications that could potentially interfere with the test.

Between 1 January 2001 and 31 December 2002, 34 patients (median age 16 years; range 13–23 years) underwent surgery for AIS. A preoperative full blood count was performed in all 34 patients and 32 of the 34 patients underwent preoperative screening coagulation tests.

The preoperative platelet count was within the reference range (150–400 × 109 L−1) in all except one patient in whom it was 510 × 109 L−1. Among the 32 patients who underwent screening coagulation tests, eight [25%; 95% confidence interval (CI) 12, 43] had a prolonged APTT. The INR and fibrinogen were normal in all patients tested.

Two patients with a prolonged APTT at baseline did not undergo further laboratory evaluation. In each of the six patients who underwent further laboratory testing, the APTT corrected into the normal range after 50 : 50 mixing with pooled normal plasma and screening tests for lupus anticoagulant were negative.

Among the 32 patients who underwent screening coagulation tests, five (16%; 95% CI 5, 33) were diagnosed with a specific coagulation abnormality potentially associated with an increased risk of bleeding (Table 1). By contrast, inherited hemostatic disorders in the general population are uncommon, in the order of 0.1–0.2%[5].

Table 1.  Results of coagulation and platelet function testing in patients with abnormal preoperative coagulation screening tests
PatientAgeSexHemostatic defectComments
  1. Reference ranges: activated partial thromboplastin time (APTT): 29.5–40.5 s; factor VIII: 50–200%; factor XI: 70–140%; factor XII: 70–140%; von Willebrand factor (VWF) antigen: 50–170%; VWF ristocetin cofactor (RiCoF): 50–200%.

115FAPTT 43 s; factor XI 62%History of menorrhagia and iron deficiency
 Absent secondary wave aggregation with ADP 4 µmol L−1Brother: factor XI 53%; sister: factor XI 68%
217FAPTT 44 sNo bleeding history
 Factor XI 54%Father: factor XI 75%; sister: factor XI 48%
313FAPTT 44 sHistory of menorrhagia
 VWF antigen 36%; RiCoF 32%; factor VIII 50%Blood group O
419FAPTT 42 sNo bleeding history
 VWF antigen 44%; RiCoF 58%; factor VIII 67%  (Several months after surgery: VWF antigen 31%; RiCoF 29%;  factor VIII 51%)Blood group A
513FAPTT 42 sHistory of nose bleeds
 Factor XII 50% 
620FAPTT 41 s; normal on repeat testingHistory of easy bruising
 Primary aggregation only with ADP 10 µmol L−1, slow response to  epinephrine and arachidonic acid 

To our knowledge, an increased prevalence of coagulation factor deficiencies or von Willebrand disease has not been previously reported in patients with AIS. Indeed, two previous studies have reported normal preoperative coagulation tests in patients with AIS [1,6]. However, different APPT reagents have varying sensitivities for coagulation factor deficiencies which could readily explain differences in the rate of detection of mild coagulation disorders between laboratories.

The mechanism of the association between coagulation abnormalities and AIS is unclear. AIS is believed to be caused by a defect in the neuromuscular control system that is exacerbated by biomechanical factors [7]. Because platelets and skeletal muscles contain similar contractile proteins (platelets contain actin and myosin which are similar in structure and function to actomyosin in muscles), abnormalities of platelet function may reflect the underlying neuromuscular defect that contributes to AIS [4]. Supporting this hypothesis, an impaired platelet aggregation response to epinephrine and ADP has been reported in patients undergoing elective surgery for AIS [2–4], and correlated with the degree of curvature and rate of progression of scoliosis [2]. However, other investigators have not found an increased prevalence of platelet dysfunction among patients with AIS [8,9].

It has also been reported that platelets from healthy donors do not aggregate normally in response to collagen from patients with AIS, which suggests an underlying collagen abnormality [10]. Although a defect of collagen may contribute to the increased risk of bleeding in patients with AIS, it is unclear how this could account for the coagulation factor deficiencies or platelet function abnormalities observed in our study.

Our study has several potential limitations. We studied only a small number of consecutive patients at a single tertiary care institution. However, almost all surgery for AIS in Western Australia is performed at tertiary referral centers because the surgery is complex and highly specialized. Therefore, it is unlikely that patient selection or referral bias accounts for the high prevalence of hemostatic defects observed in our patient cohort. We also did not perform detailed hemostatic testing in two of the eight patients with abnormal screening tests and did not perform platelet function studies in all patients. However, this can only result in an underestimate of the prevalence of clinically important hemostatic defects.

In conclusion, our study demonstrates a high prevalence of coagulation abnormalities among patients with AIS which may contribute to an increased risk of bleeding complications. If confirmed, our results would appear to suggest a role for routine preoperative coagulation screening tests in patients undergoing surgery for AIS.

References

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