Diurnal brain temperature rhythms and mortality after brain injury: a prospective and retrospective cohort study

Objective: To determine the clinical relevance of brain temperature (TBr) variation in patients after traumatic brain injury (TBI). Design: Cohort study with prospective (healthy participant) and retrospective (TBI patient) arms. Setting: Single neuroimaging site in the UK (prospective arm); intensive care sites contributing to the Collaborative European NeuroTrauma Effectiveness Research in TBI (CENTER-TBI) High Resolution ICU (HR ICU) Sub-Study (retrospective arm). Participants: 40 healthy adults aged 20-40 years recruited for non-invasive brain thermometry and all patients up to May 2020 that had TBr measured directly and were not subjected to Targeted Temperature Management (TTM). Main outcome measures: A diurnal change in TBr (healthy participants); death in intensive care (patients). Results: In healthy participants, mean TBr (38.5 SD 0.4{degrees}C) was higher than oral temperature (36.0 SD 0.5{degrees}C), and 0.36{degrees}C higher in luteal females relative to follicular females and males (95% confidence interval 0.17 to 0.55, P=0.0006 and 0.23 to 0.49, P<0.0001, respectively). TBr increased with age, most notably in deep brain regions (0.6{degrees}C over 20 years; 0.11 to 1.07, P=0.0002). The mean maximal spatial TBr range was 2.41 (SD 0.46){degrees}C, with highest temperatures in the thalamus. TBr varied significantly by time of day, especially in deep brain regions (0.86{degrees}C ; 0.37 to 1.26, P=0.0001), and was lowest in the late evening. Diurnal TBr in cortical white matter across participants ranged from 37.0 to 40.3{degrees}C. In TBI patients (n=114), mean TBr (38.5 SD 0.8{degrees}C) was significantly higher than body temperature (TBo 37.5 SD 0.5{degrees}C ; P<0.0001) and ranged from 32.6 to 42.3{degrees}C. Only 25/110 patients displayed a diurnal temperature rhythm; TBr amplitude was reduced in older patients (P=0.018), and 25/113 patients died in intensive care. Lack of a daily TBr rhythm, or an age increase of 10 years, increased the odds of death 12-fold and 11-fold, respectively (OR for death with rhythm 0.09; 0.01 to 0.84, P=0.035 and for death with ageing by 1 year 1.10; 1.05 to 1.16, P=0.0002). Mean TBr was positively associated with survival (OR for death 0.45 for 1{degrees}C increase; 0.21 to 0.96, P=0.040). Conclusions: Healthy TBr exceeds TBo and varies by sex, age, menstrual cycle, brain region, and time of day. Our 4-dimensional reference resource for healthy TBr can guide interpretation of TBr data in multiple clinical settings. Daily temperature variation is frequently disrupted or absent in TBI patients, in which TBr variation is of greater prognostic use than absolute TBr. Older TBI patients lacking a daily TBr rhythm are at greatest risk of death in intensive care. Appropriately controlled trials are needed to confirm the predictive power of TBr rhythmicity in relation to patient outcome, as well as the clinical utility of TTM protocols in brain-injured patients. Registration: UK CRN NIHR CPMS 42644; ClinicalTrials.gov number, NCT02210221.