Comparison of two automated pupillometry indices in patients with acute brain injury: the prospective multicentre LYNX cohort study

Automated pupillometry quantifies the pupillary light reflex in acute brain injury. The neurological pupil index (NPi) is the established composite index and carries prognostic value. A second index from a different device, the quantitative pupil index (QPi), has recently become available, but whether it conveys equivalent information in non-anoxic acute brain injury is unknown. LYNX was a prospective cohort study in three European intensive care units (September 2022–July 2024). Consecutive adults with traumatic brain injury, subarachnoid or intracerebral haemorrhage, or ischaemic stroke ventilated for more than 12 h were enrolled and followed for 6 months. Paired NPi (NPi-300) and QPi (NeuroLight) measurements were obtained at least every 6 h to day 7. Ancillary cohorts of healthy volunteers (dark vs. bright light) and surgical patients (before and after sufentanil and propofol) were also studied. The primary outcome was the correlation between the two indices; secondary outcomes were their association with intracranial hypertension and unfavourable 6-month outcome (Glasgow Outcome Scale–Extended ≤ 4). Values below 3 were considered abnormal, with QPi thresholds of 2 and 4 as sensitivity analyses; multivariable models were adjusted for age, sex, admission Glasgow Coma Scale score and focal deficit. Among 108 patients (median age 57 years [IQR 44–67], 60.2% male), 7089 paired measurements were obtained. The two indices were moderately to strongly correlated ( r = 0.717, 95% CI 0.705–0.730). Each 10-percentage-point increase in abnormal values independently predicted unfavourable outcome (NPi: OR 1.49, 95% CI 1.09–2.04, p = 0.013; QPi: OR 1.76, 95% CI 1.31–2.37, p < 0.001). QPi was associated with intracranial hypertension (OR 1.38, 95% CI 1.04–1.82, p = 0.025), whereas NPi was not. Ambient lighting and contralateral eye opening had no meaningful effect in 75 volunteers. In 20 sedated non–brain-injured patients, propofol reduced QPi to a median of 1 (IQR 1–3) and NPi to 4.0 (IQR 3.6–4.3). NPi and QPi were correlated and both predicted 6-month outcome, but QPi was far more sensitive to anaesthetics, which probably confounds its association with intracranial hypertension. The indices are not interchangeable in sedated critically ill patients. ClinicalTrials.gov, NCT05567978. Registered 5 October 2022.