1 Alder Hey Children's Hospital NHS Foundation Trust, Jackson Rees Department of Anaesthesia, Liverpool, UK; Pain Relief Institute, University of Liverpool, Liverpool, UK; The Walton Centre for Neurology and Neurosurgery, Pain Department, Liverpool, UK. Electronic address: richard.berwick@liverpool.ac.uk.
2 Alder Hey Children's Hospital NHS Foundation Trust, Jackson Rees Department of Anaesthesia, Liverpool, UK.