CINXE.COM

<!DOCTYPE html><html lang="en-US"><head><title>Just a moment...</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=Edge"><meta name="robots" content="noindex,nofollow"><meta name="viewport" content="width=device-width,initial-scale=1"><style>*{box-sizing:border-box;margin:0;padding:0}html{line-height:1.15;-webkit-text-size-adjust:100%;color:#313131;font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji}body{display:flex;flex-direction:column;height:100vh;min-height:100vh}.main-content{margin:8rem auto;max-width:60rem;padding-left:1.5rem}@media (width <= 720px){.main-content{margin-top:4rem}}.h2{font-size:1.5rem;font-weight:500;line-height:2.25rem}@media (width <= 720px){.h2{font-size:1.25rem;line-height:1.5rem}}#challenge-error-text{background-image:url(data:image/svg+xml;base64,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);background-repeat:no-repeat;background-size:contain;padding-left:34px}@media (prefers-color-scheme:dark){body{background-color:#222;color:#d9d9d9}}</style><meta http-equiv="refresh" content="390"></head><body class="no-js"><div class="main-wrapper" role="main"><div class="main-content"><noscript><div class="h2"><span id="challenge-error-text">Enable JavaScript and cookies to continue</span></div></noscript></div></div><script>(function(){window._cf_chl_opt={cvId: '3',cZone: "journals.bmj.com",cType: 'managed',cRay: '91377f9f1bfefe00',cH: 'ey5nqiKmpN9l_6ggGu24LiLaGgekQr246mrT5THe0pY-1739813355-1.2.1.1-GRFwvAtEbAFLxCPcH1iQDpYBz.lPYdnDeItz14B.OnqpGSzhtc5iY.ptlnejrUuF',cUPMDTk: "\/home?__cf_chl_tk=SJoVUP7HmRN6wS4Rinxeb7gI.n0qK2MPT71k1DhNr0k-1739813355-1.0.1.1-dERE3tl0VI66V3HxFs7VwdtwLZcHhg7wanJIzcFYRUk",cFPWv: 'g',cITimeS: '1739813355',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/home?__cf_chl_f_tk=SJoVUP7HmRN6wS4Rinxeb7gI.n0qK2MPT71k1DhNr0k-1739813355-1.0.1.1-dERE3tl0VI66V3HxFs7VwdtwLZcHhg7wanJIzcFYRUk",md: "JtbbrNzXIErPYUnFvUclK1DKDXWTsqbFbOX2r955bJU-1739813355-1.2.1.1-LZggGlQoyfPp2mjNzinIwXvkduGy6Cv.mG_rRRDnV2iHUs1RB85_AlFR3zag0raqVCpxve.8OtB1nLYmWR1qDh76aibRsmC4QRfUqrT9wijosTqvQCmGm54MaH7RqtiFhEyy_6CR2SP2DykbX1F5tZKStp3CJe4d5.1T3pR9zEn_3iI.m5bgxn9hrBipDDZ23dxUgfHzVKlqm7oDH.UkRfSLDG8eWBStGZofDUomr3SXiPLyQr7h_CSNg3tOKfqI4kyFSwZPHu9YGWU.o4OeSUwbNAKo1sk53frhg_s01uWma2vAYJL6YBSI6LO1cP6.WpAy921YXBW2HB6fgGDye0C7qxLcJyZnJZc.VgNWZvOqepPqCDMW10AVtz0VQjqE4AeFwZfHt.Kc08CkCo4eZt1te57SgBk6I9izNd6VEyOBryHQNHEpicsK6iyPqaqE4iEejjVY0XGBf2vpazITz4p4EcEwSvSZBD3NzNl5iY7.Cy_1m9gMKSAznM4FvsQKtBvzZmDRoOg43mzf7Ifnd8CKNl7O4W1U8fTepCWpoXBTBIMXCe.njoS94UrWJvAlI9sDjVZthmisqqRqbZj7wSnf5VBRH4.FBwXh9vG5TUKsKkpnDK8huUZRdJX.P6OIoPvIL5NvK_1NUnsOtfRZSiSOZYbyYw2ncuXfIKS8pwhOzGeatKL0cCWUuMowhC__F3O0nSufxtObbpyzjzgL_xXEm2lfgpXP0KEL.n1uX5yQk26Pv1XOaolMnS2w_FY92qLaPp1ndQLk2HOT5MP9rjfcG.DAooAHlh6963sut6Fl0ivWV6WxKgrtG.3NKiN6kknrnkqA4pou2GKD_8vNuRaZUR3osxnx2tBwdlYEnc.AWG9LDUYm9XZkmqigNOfBhIKO6IVtqapgMZZ683S1KOl8CU1mVsks8GgrxNlhrcMBaw3f6Q_VRu_LnUISkJk10_7yIhpbtjvf6_qsY049fM7m7fkzczFdP7F0s6UBvunQGnREZEKhZ6Vpk0sq0F5EISZwgjWxIb6ZgHclvyQE2oE6ojj99s7KqpjrpaBakX2r2FfNb2fWAG4ChFGOtfDoWSlDe_Qcjx1VUpomGb3EyQG_Sy372nsmAqgm47CMd2WJ9CH5S24DLnBcn0x_Z66cRYeomWPqc5k8N9xWXRtzwbREDZbCLHD_zEv.jaNuPSR9Kuo7i2z2MogVw74Y3GpPSqSt1p6gMezv7WLmO.dsK66.MrkKOStsXtujnzz3St6D5x97205tbwNShl0GwKADHroGRGtS8KV.oltt.i4Ii_hEiO9M2td7y_gwbx65mOngx7LQ._I4vI403uYTWl4BLMfd5Lb.r.P2qb1X6ZB6myWReYxE1x2jFR5rvXoY9tcE0T.Lgi8v.vqRyahvNvFLTv6GMTlobVghWVGiW7pdd6m5bxVJ3WLVIbIeAuyPq7lzehvX5OFj6ZUIsvBr_0EiQdSGFfj_MRkmpYytMGJiR2S3igBG58.bB9LJ.2mc_HPv1wjIBQO34psR8P8B1Y2Uz.6paMZbBBG5ScsdWkz6ScdV2z_NO.LiSK2chbzFbvA3oWu_CwGRWiBOLMlPhBntrxe0VkzuGYF4bxcD13EeRSMIkwQyM34qhx2YmIoHU.FXePDELDtZmCZ_htZrba7luUO4p4VdKNotuMtB..5UdV6FrDe7t7AKmQWSLVToDMmhTV7lzprYRpJwJY4A6Chq0Y5damvXcLBejZul0eB3_Xru.Hbk2VC1aJSIGkXhHarIUcLZIlLRa8x0CkLjtt2jlLkyR4JKNQGCeDSw9wXHC.mDJtksFTbczYtycSggtEZN.E_P8Q2RHMfiC.Bw4bMNYvUJKOLPqY8CLW4Fl0mhT4G6xd1UkRIKBSzDxzuYaFpCsGgzfSNkDpiyznoDdqgcAhNrQOcdkoo6spZb41QCk1C7wHkAEqijZW9cO8WPUSOpxAcfS7Kb1ssAU90zqTHsG.TDGX62jzFpz0iJQhk_ds4deKcmiWhGzRxDmp0w5q0rTcq7IJqV644w1O0q3JpR2_G7UdHpcx9DRj8xhdU5QuS30m.0ho0ZBrIQ6u8s8bCzpW8teIjjZr9aYwAR1sXYYq8FSoRJ4tFYUz.MD8z5_Boo7zWCgRsXt3NSBHz85H5pfd4vvBzPqvX64xtPFzqFLbt32B_h.SRrbjZI6NDrqPE.Bf0lznqlZS9lH4FCL9CxhNkjoQ7RTSagahpFNlGaAto7bO0525E.imSUVqkKX9x4oLo.RN6JN.MFey11oL2KV3XnrwlT16sC5iBbURF4JwVsghle55vCYDD5LCHtLrThouAvhNfHrMsLELkPq4VnhCI5lc3n_QShT_m43aRZJgZgb97WFAiv0hjDDxBOJMargfmB3bRvyMwIxlRrPb9NVsZGwqs4x5HjyNE8tybn_3jJmbN7EU63Qr53igkqgM_Bq5HA_lDb.SkVo60pybs",mdrd: "sFc_RZUe7nzBVD1lLfC1KVnCHQQIKWp3sdHvob0Fd2o-1739813355-1.2.1.1-78ECjRQowdXyYY328WLz6JdCE0qG4HEaOASHZ3RLoO9P_EESAQ.Zko2dQgA5ptDVtu0p0D8L8R0GC45HdtoZhfSr06Evgrcn7UgTnrCsuAnEaRe3MRNOzu_ZctbysTrHqvGdsTfywVxzW5gZaZhjVasz2EGw5fY8TC2ir7slPPqvLecaSPBRbb5QVLZE7C6EBNyRKsQFL1JbPErb9ikORdaPhLiPHJ4WJf.kkeUm_Wph0RZGORRFjjmmoEexVUshVJGm74gxX04UKgWgcVrx6Q7g6IQGmWlB3ULYfvyj3k2kVN4WnkEUbR2nPW2VEG5mzHmCMJtP2vwhVKOSj1EzYj318u.9ng5.Ws4UuME.ft9cnYrCau439GH9o37Z07ia6ul7OEm12Ke5Y5CrRcEh8flB.myfng9zGJgXMBHdKVKlNsfsyDjXae7OwHfSKKhsX5bENM5QOE.2XbPOIxb313RiZ2.NdRDzFGFtDqO7pX6Y3jFyKZfKOgpRv6BfxPT2xQRv8tGlaNckHtKqRE7CBbu11ZCmzkNjPe8PT3m7CfvPLbbldUHvI7MckFC_ihRYJWYxVQHeTw1zP_LUYEv6tv9visLtPuofYTTqUqW1UNhSBHKF5VKyhcv9kqL35hpioPpim4n4LDVZhJnVjqwRUv7LNuP5tzuOotiCO89uy58YaQY.2E0xDsEfI9kRhGkVvFwczzqrWiEOww9kgf1E7.8jsqtCrWO_4CbOVg3lQm8jsA4MI7rhjcEIw1Eyl9UPE3TLng2eCD6Knc8zrpVr3fpmICVhKRRtr0oboHRqcc_DwleGJg2Ng0rwK5TE6Wn_AqAu5cDoMjFcgaSk1GxopASWsFEm3BDUUVsXmx5R6Ci646Q5C_FKaBXK98oVgIjjETNLvuFTMEFqJCgXEkFiOr_9pe9XKi6tM_yYddkH841exyPLdhCDn3huGjPdJ0jqv_Mw88o72vZZ7tw9pETnHyqLob5nF.FNjenqtOLmdWXpJx0uyZyg3yEIswJC56C3Nfak6VhtLrufsbuLerp2gFyPjkIBvq7QSZ9GMouUQxinma9Pxq8VMXDtpc0py5QIWkdfQivxGa6Wd2uMP1_8j3Fbow2tUGU.uIgsPUW2V6lGt3Qe3LHN.Hxn7.ui.G_tBh9ufBlKiVWbhQf1SDwiJL.47HTmlKtbX4.VkY0sGWR19fxpLrkrba56IYtAyhrJndhiAnO26dwb18zC.LDwmL1zbN5qacMJejQ7tNAOb7g18TCRNhrjGbhGSNmyVDuQvKtbRWJTHrCTH3.DCv1PAwL1DrTWO.TLlhSKhqoPSd_svwyDnGAciAmkQaZrZ7MgRHkgZAZOjn7hodd2lWlGV7WndEKV3V1aXvhx_vaDDAsWnelPSea15DsyUEl1RTq3jSxXCNT57rMrpBeUCAIMSZUIoRfrsXbacwwlhbos8sAegB5xdqvIfgeYC47BYP92gMoeBiG9XEF443Uw7j7n2GDekxbXz1FIRqMn6JpeRA62WiY58ZmNzttGlXebNT9CVtdU7voZTFw52I2HyIYhxE3R7wXn6EAvfYptsydHFjBonxOwxyGci44QMGtihOu0sk6bCMoNgSCSRenWHl3uQ6AXmmyVHwBkYPMktkIaJwVNfq4KyXdoRWz.ee.GCmykn2l38TsL9jDLVPfHNSuw1H8Cg0_cL5LzVWI9iaB0LTjnWa6Yp_XTtndiZ5goQ51jbaLXgP3NHlLiABh9IQZ1HPA_hskAFLfLyhs14MprnQPumywVjCW5.qjOzaoLa0N_SUYN7JlF9TtBcHi9m9Niel3HfEzv5eJ1dyW6FttNsS3OTS6Hv9qSmKICGGopt2jT5iuRCaxNIKi5NcweM7T5Nee9XJuwTRolbzWQH1aq303i3KXmF7dNSzSeb_JQxCVGk5UpvdbnAd.bsuHikSIWbbfeHQEwX6n.vxrAfrv_Ctsxkf02dM0A4wdpp1_7maBC"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/g/orchestrate/chl_page/v1?ray=91377f9f1bfefe00';window._cf_chl_opt.cOgUHash = location.hash === '' && location.href.indexOf('#') !== -1 ? '#' : location.hash;window._cf_chl_opt.cOgUQuery = location.search === '' && location.href.slice(0, location.href.length - window._cf_chl_opt.cOgUHash.length).indexOf('?') !== -1 ? '?' : location.search;if (window.history && window.history.replaceState) {var ogU = location.pathname + window._cf_chl_opt.cOgUQuery + window._cf_chl_opt.cOgUHash;history.replaceState(null, null, "\/home?__cf_chl_rt_tk=SJoVUP7HmRN6wS4Rinxeb7gI.n0qK2MPT71k1DhNr0k-1739813355-1.0.1.1-dERE3tl0VI66V3HxFs7VwdtwLZcHhg7wanJIzcFYRUk" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>