CINXE.COM

Just a moment...

<!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: "www.bmj.com",cType: 'managed',cRay: '913583d6eca7ce5b',cH: 'naY4AarXWwzpm_YyDE40jmMdzRI_5wEyaKrZ05YwBnc-1739792556-1.2.1.1-2VbFxFBnQ3RRRNsNadC5uI3Y5HMroisClj3bdSbXsoVNG33qAW9Q4g.N5UI_3TxG',cUPMDTk: "\/content\/347\/bmj.f5980.abstract?__cf_chl_tk=Qw1AAkGUiwxOBkWtz9VV2S4ZjUXC5Aw7H4xYbL9qUv8-1739792556-1.0.1.1-DPWxt.bY8EpIX7CZP0eb6R3XMlv9EHjYNB7Ega.CrAM",cFPWv: 'b',cITimeS: '1739792556',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/content\/347\/bmj.f5980.abstract?__cf_chl_f_tk=Qw1AAkGUiwxOBkWtz9VV2S4ZjUXC5Aw7H4xYbL9qUv8-1739792556-1.0.1.1-DPWxt.bY8EpIX7CZP0eb6R3XMlv9EHjYNB7Ega.CrAM",md: "Py9XGVs6QJPj2S.dptVorAzYKb2aeXiwETuJMbQA0Hc-1739792556-1.2.1.1-RQPAOrNlLh5i9ld8mWsP3BFj1TSm.F1_uHXpj85NKlI9HGL79brpVipTx_tEywBsguv0jp12gMWk4KggLnCP4pe.zfxOGbhznQ5Il72NA2.l22VfhsCFOQjsiXom9jhlxKdynC7unBaPIkWIkn86RCQosIfr8N5g7e.hwuMB_uS2IZbsZEapuVoqbhKtW2uwL1DTsYFV0kYUJaYRFyw.pgICV5XUb6g_7EJPafvK0A5ZgOAyMp7zduSOpliO8xHqR33SZNAadmtZH3LJsotHBJdpSXJwzVFPrnOZSVCI3bM9BPYp3bJGkfqNDTLPWBnngekDfIqSTWdVYvtDDV.KEHiEw0z1iUZvOTd29Ity6LrVpAS4qYmxI7sqorN2H7XYayA.z9eGQ1SZWTX7v9BDa8q1F90pGOe021DtQK82zDoHGaJJ8J82Okdj6E.k72LkMeUL.dCWzAg48nKU6baMPq9nje0biSdDjKeZPFySWBvDN_6KgdzB5_6lM3nb.hf2N6satWgi1ZKPnQuhhYBYPnul7ht4sIBpJKH_JELTokYmvZ_jyHH_TScGfQLnTFSdRYLpjJKyfxbBSDZ3hSDHEEILi4gHgw103TuD_k8zSFN1Ii8xWCWfd4U5isR.EmKhLdCRZjHTq3hcAOZnSC1qiddWxpaHMnSz3oA_BHlXDFwzL_lNQIMgAAno5QK2SvuamUXvaXK9eiAL6YslCyIuM7wniUWSOMJoW_LXBCDftXh0bRP.JE5RArhIUXCIZYPKhtd8KoFKRL1FacpVELAGx9uYW0_MxFxFXnV4llRNCRybFzcDZ_6DIpTo4dUTM6TeYKzA.Cqpr.gIhrfO9YyPTNjYelIfkQHbfBzXrtnvopmXRKre4zVxg3u5dHwupxFRU9rfyaEkOtlyy0mLFc5KyBe2h.kTa3lWMVXo57C_cjbM.keQJH1Ucjsg2i5lrTMLOMdGhkXXFd7xlpkZGal_QkJfHwgR6gxDpBxssl2zfve2nRrQ6FFI3_ZZlpUJQGj.A_Qu4aRGDkIpKCydKvx2lWNAWOUtNlSReFXn7IukVh0glwlJyW2roLFAsdHi0x1jsg7Q0DGDR5gFf3lG3qZ7JiTTl_qGsl.pAl1Hx.I.1VU3zP0qHHpJOZE4YdkI4QkiaStZPD99Eti1O.3YGzQSGBeKjaiIirnA2eXaN47I7lPRIRse9fl4SA7AMyf8vnGFASq1Jc6ar_1c2N1E2Fq_b.D33q6FA1M2WsqRV0uCx0dCNim8_c80vCIHcNtyWj4dzVJD6RUHe1Wh7USu.1rzidz1BpOUhNJ0UchEQDeWtniaoFuuzk.7kjzjqNW6IRIfFyxRtuSKAC1UvOamp79Ue1NcUxehcIBs3cehPPXBuNxN1Ml.W1Et2dbFJ_0UuRV0PmflON0HJ37CVZ.WKGw8mcf7NzP.x.XewIntDKmWx2zQ7H70.WhS7qVrmfHRfdH8YbxNzHizl9QGSvzSZ1LkbyDE7TUcsN4IeHC77kdDtgaPqELKQJd8wIzKEJX0Uj4Hil2x.QJmB1lDMJ_ACXvmIZKznPJFLgzhisdVcHNGiODPjspadRPVlnYkdsg8QS16mvXHncurDl388yG4MtlvkVHRFsncFfIKb2f2HuSkws0j7.3TdVu5oG6UZ0GuWrdroYbnkMhqeC3UGdrvSV9Q7K5Ixmg0Ycl1tlFyTezjEZkdoPvtMU0LZODKdqFSDe_7hrSGjxMIA2gRVLZYyr82gTc0fLbc8kvshjEeMvyQ4WPBM3XORSivBhAuC3CgxdeF_AABhLHaHwB.Qnvznpcq7KSoyPHrhbNJG5ZIZ.AOMrQklRVTiXv.4Uf.B8FBMQf6qhEm9_f7LxTXBBwVGPMneAgZH_82x9yGTjwBAhXWoy31OCFtPUjndKTzHwFyYFVbVd1QJkpn4Dv8PKborX9n264kxKywUuQhnaDF4hRU6iHypQyZfMk7pBOoOTDKajeTA5Ai8zVm.jHJaLWHRad2XIRqKv9CUzID5rpELffPfR4Ij7D4tzNTSjJ5hwdpkfyUe4MhrKGByQKm5NNINm3VOw15awh2pR4hb6NxKXOfnpDN2JIHkdamjarPkBGZkERTlrJcZcPJjNhpHv6F3zlFsJwAAmSyaK2QASqayDcSkb9xFpq779rIJQMYUMwUp96ursJxx56f5HoQ3omoP7rKVoJHz0cR3CY7EiY9NNpMu5q8CAXcynkCgY_q1z9hRbRfUrQHNt47TtlXRkBJI08jUn8EGGESrafvTk878A7NuOlLtEyzaJVmB1RVh06z9r7mh6nxHJbZ5MCg7QcvQ3L5qIybV__.HtVJetFKxa7GD4618gtygQVT1snsa0W3qAOSrLZnVigZSdGHlQ4afbWMXFFmptHNwAHe4GZIZbjU8DWxc5oDafxRQ1mE9Iq03i6K_OFRup4X.Wfc9_54QWbOFdunzS6_4cOwmzfc6pHELaw_S7KkB0vX4fG_C5Gj86pi0mxCM.jNcUTHd8MfqXo1EZ2Pvg4ivxjAf2XDvqd8ipI",mdrd: "nX.CfvDo6VRP_C7RxQEKOGpEDqBkqP4flsdyzRBaF5g-1739792556-1.2.1.1-vF42HZEyY8qO8HJ_xerceoEk2eIcXGF5ssCKaLk5HN1ARWs3qV7SKktSz37jdaAm_pRQLcwU4jLvXMTcXKRCRDbFY0c89T.OE3w6fk6zyEbkbDQI2QvxOE7wnoDNiUGWebT.kJMxGgkT.tUoo6OajHurOwY6CCHSbkRc2jyq2jiHkHhlaBImsxWb0q2EPzXrlbT84rktK18AQTkC8dCrb3yZDReWYZmbqlWfXTPoCKsmVRyxXAR0ZYmGt4TpYoUK6oEuGKRPc6g9bNSVUCEQRieZ7zGNVZaMK24rEB6R6lqiclRn10dWE7CIBG5IxybrWuNRVK7MsGq6dX5QXxm.Wk0V2mV22Uv.0K9Jcukfce1t6QPwdtJDjhceQo_TWN42ZA0WTkAkP.DadcqwhZrDnDkbooGt3oZEtVR2o0GF7TGpH5kXXQtKHZWUVf5hz8KkVIYR1Wf8ZpFFR8Mwj64w.MD4ET7ioQfFOvFPCDSqI5cjX0FSpyYOyLJm9nJU9sWwwyRqeYw5ZMxcxmQcatLeKmifCV4L3PJAEj9j6WqjcTiaHVhh7SH8kRwlii27p8pdT96YzVZOAzKdpuvsbRnimHaX2w17Wb0yrm7Ls2Bh70tp27jdfiX_.r5hqwGB0s06JEaLMDOWOneeHCw1gcUTHzhVRxnJvMku4D9SHPnJkY_.RNoq8sgImjA68uQGcT4lUY41BFVLbsjersE1xGyqBGjM2tb0B4Hxi7YgVKyHvNtiwr0.1n86N8PQxcVCzyIhVCc33UW5wsSuhB0SibsR2JwagqzD31tflFyjwsUYym7VnL_0tNA_YKlgxE.8ZshchJ207h4qCXQEEUjJa2JSolfABjbCPpO2hu21ZMN7c9z9YbMSHqjwLzYMjsixaBybnLSIyMC4TV.DRnVRNO6oFGg2AZPhreLqmVwGGXr_ZffMooyvOB6AOCcUW0kgvZy.6cv0K1vMAYYyISuq0WOPJSXKNNn49DsS7rmn_HBUXAnde602gjU.SnbGiwTLuyV4jWFG8yZO9Bt57sBjgGViyQq1kBI.BAU0S4frl.QdiGs2oLrNd44AgKC1Cc7Afxl.Z95pcnhrvurc683mWGpsJlZM7elPuLWYR3mC3q3l0NmDZjDxMfgLMt5SI5_AteAsTDZRtzaVn3gvFe7kvPQEfDjkLImabVjGIZEIDG9.v4sRlX.EBMW2P9TRUUESDjNS8C2zyMEYpk66wm.APc66zNxUsTeFBDz0bnhAE4H3tKS4bKNrPq8ql76t2AScbD0tf.JKygLsR8uEuIuxIqUxFXZbvUqzuCNTG7EUy1Hr_O31wmvAEIKkP3LX1pJr8w1T3gppKBE1RgqAT_PIX05XqLgJ6wo.q73Dg1p.xB_zdxh1sGvr9Mpv8BeTKOkhY7Z8vUVfwNP8CVczsgF6kys8SaKr3OQVFca2C94Q_C4SmATegu7lBOm5iJDGKiRl2OXaIBvhSDyQ6xqnTL_6dxBtmgZRi9QdOfiSIiXkaFK5DTlUJvTaxwKI1Ol2I97x6AEL_xRuZYBlFqxRW_wmIIh_HzCSSvr4Gui_paaQj1yUz2.a0UgyIznlwu3vepJVi8ecSpC1iXw.9OO4PrOeBuN9n4KUA2br1T9.sKonqNXN10jCklXY0mnCwrztHXo4V.3rZBAfnXYF26ULzOCCiqCXLI4iv4FdzzAHxaA7AXLahzJ.PS1ZtsWr2rU7DvumIZTJFIQ.GVs.zveqfgfKU6AaQ7Y6_Nf8fXlGv5_bWS22Tq_5m.Aow7R8QaqSZytSzsisE.ll1eUgD40xXJkvygp7d1B_r.k3gopxu2LCFudBmVsRaYWQ7dTc_aqUg_9IeAgBiI7d2wbHqmakQHMdGaqEVB0zG5yDebj0BUBYaSDuyMdQ.27ZdmUZ4FdBaKcRtE1ro7OmKeQmJfuVVa0f2B0GwRIcrgRSlXqagwYU.ZWPLJA"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=913583d6eca7ce5b';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, "\/content\/347\/bmj.f5980.abstract?__cf_chl_rt_tk=Qw1AAkGUiwxOBkWtz9VV2S4ZjUXC5Aw7H4xYbL9qUv8-1739792556-1.0.1.1-DPWxt.bY8EpIX7CZP0eb6R3XMlv9EHjYNB7Ega.CrAM" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>