Reduction of the Six-Minute Walk Distance in Children with Sickle Cell Disease Is Correlated with Silent Infarct: Results From a Cross-Sectional Evaluation in a Single Center in Belgium.
Authors: Chapusette Rudy , Dedeken Laurence , Le Phu-Quoc , Heijmans Catherine , Devalck Christine , Huybrechts Sophie , Ziereisen France , Hanssens Laurence , Ngalula Malou , Ferster Alina 
Published: October 2019

Abstract
Abstract 2107
The 6-minute walk test (6MWT) evaluates the sub-maximal functional exercise capacity and can be used together with the tricuspid regurgitant jet velocity (TRV) and pro-BNP to screen pulmonary hypertension in adults with sickle cell disease (SCD). A reduced 6-minute walk distance (6MWD) is observed in adults with SCD with chronic pain, hip avascular necrosis and osteopenia. In children with SCD, baseline elevated TRV is associated with a decline in age-standardized 6MWD.
The aim of our study is to explore the submaximal exercise capacity of children with SCD followed at the Hôpital Universitaire des Enfants Reine Fabiola, Brussels, Belgium and to analyze the factors affecting the 6MWT and the 6MWD.
Since September 2011, all patients with SCD above 6 years of age were screened with the 6MWT as part of their follow-up in order to test if their functional capacity was altered. The age-standardized predicted value of the 6MWD was established as reported by Geiger. The 6MWT was considered as normal if the 6MWD was more than 80% of the age-standardized predicted value, moderately decreased between 60–80%, and severely altered less than 60%. Baseline hematological values, clinical events, cerebro-vascular disease, cardio-pulmonary parameters and disease-modifying treatment (DMT) were compared between those with normal and abnormal 6MWT and according to the 6MWD.
Forty-six patients (20 boys and 26 girls) with a median age of 12 yrs were investigated. Forty-three were HbSS or HbSβ°, 2 HbSC and 1 HbSβ+. Thirty-two patients had a normal 6MWT and 14 an abnormal 6MWT. Only one patient had a severely altered test. These 2 groups were similar for age, sex, genotype and history of vaso-occlusive crisis or acute chest syndrome (ACS) as well as for the number of patients receiving DMT (either hydroxyurea (HU) or chronic transfusion). The proportion of patients with normal, conditional or abnormal transcranial doppler was also similar in both groups. Silent infarct (SI) on routine cerebral magnetic resonance imaging was found in 42.9% in the group with abnormal 6MWT versus only 19.4% in the group with normal 6MWT (p= 0.087). Pulmonary functional test, blood pressure, heart rate, systolic function and TRV were identical in both groups and only one patient had TRV >2.5m/sec. Baseline pulse oxymetry was slightly but significantly decreased in patients with abnormal 6MWT (98 versus 100%; p=0.022). Biological parameters were not statistically different between both groups. The 6MWD was not modified according to Hb, MCV, HbF, LDH and reticulocytes count or previous history of clinical event, except for the presence of SI (Table 1). Patients with or without SI were similar for age, sex, previous ACS or painful crisis as well as for hemolytic parameters (LDH: 945 versus 825 UI/l, p=0.832; reticulocytes: 273 versus 329 × 103/μl, p=0.548) and basal Hb (9.7 versus 8.8 g/dl, p=0.06). However patients without SI had significantly higher HbF and MCV values, and lower PMN count reflecting that most of them were treated with HU.
In this cross-sectional study, the majority of children with SCD have a normal 6MWT. Abnormal 6MWT was not predicted by any clinical or biological features despite a trend to more SI in the group of children with abnormal test. In this series with only one high TRV patient, the sole factor which influences the 6MWD is the presence of SI. The lower exercise capacity of children with SCD with silent stroke may reflect some subclinical motor or sensitive impairment. Our data suggest also that HU might prevent SI which needs to be confirmed by larger prospective studies.
Table 1. 6-minute walk distance (6MWD) in 46 SCD children according to their biological values and clinical complications Mean 6MWD in meters (SD) p value Mean Age in years (SD) p value Hemoglobin (g/dl) · ≥ 9 (N = 24) 531.5 (95.4) 0.173 11.2 (2.8) <0.001 · < 9 (N = 22) 569.8 (92.2) 14.5 (2.7) MCV (fL) · ≥ 90 (N = 24) 536.1 (100.7) 0.251 12.6 (3.5) 0.518 · < 90 (N = 22) 568.3 (86.8) 13.2 (2.8) HbF (%)* · ≥ 10% (N = 30) 544.0 (101.7) 0.360 13.2 (3.5) 0.352 · <10% (N = 15) 570.1 (81.8) 12.6 (2.4) LDH (UI/l) · ≥ 1000 (N = 14) 526.8 (86.5) 0.229 11.8 (3.0) 0.105 · < 1000 (N = 32) 562.3 (97.4) 13.4 (3.1) Previous ACS* · Yes (N = 38) 548.6 (98.3) 0.625 13.5 (3.1) 0.453 · No (N = 7) 566.9 (85.5) 12.5 (4.2) Silent Infarct · Yes (N = 12) 502.5 (113.9) 0.035 12.1 (2.2) 0.374 · No (N = 34) 568.9 (82.0) 13.2 (3.4) * Missed information for 1 patient.

Disclosures:
No relevant conflicts of interest to declare.