Secondary outcome
  1. Repeat ESS in any subsequent admission within 180 days of discharge from the index procedure. The same OPCS-4 codes listed above for ESS were used to define repeat surgery. This follow-up period was chosen to allow sufficient time for complications of index surgery to arise, but to avoid cases of planned repeat surgery.
  2. Mortality at one year post-discharge. Mortality data were taken from the UK Office for National Statistics (ONS).
  3. Likely complications of surgery identified during emergency admission within 30 days post-discharge. These complications were defined within five broad categories: haemorrhage, orbital injury, cerebrospinal fluid leak, meningitis, other post-procedural complication. The ICD-10 codes used to define these complications are given inSupplementary material Table S1 .
  4. Haemorrhage was the most common recorded complication and was also considered separately.