Rethinking the Postmortem Pupil: Biological Insights Through the Standardization of Miosis and Mydriasis

Background/Objectives: Postmortem pupillary assessment remains qualitative and lacks standardized quantitative criteria. The Pupil-to-Iris Ratio (PIR), a described photographic morphometric measure, enables objective postmortem pupil characterization independent of image magnification. This study aimed to establish a standardized PIR framework, characterize baseline postmortem pupillary morphology, and investigate its biological determinants. Methods: Retrospective study of 83 forensic cases (166 eyes) with postmortem intervals of 3–288 h. Digital photographs were analyzed using a semi-automated workflow. A predefined proposed operational PIR classification was applied. Associations with age, sex, postmortem interval, cause of death, and toxicology were evaluated using mixed-effects models and generalized estimating equations. Results: Quantitative PIR assessment was successful in all eyes. Neutral morphology predominated (119/166 eyes, 71.7%), while miotic and mydriatic phenotypes, including their extreme categories, accounted for 17/166 (10.2%) and 30/166 eyes (18.1%), respectively. Age showed a significant inverse association (ρ = −0.352, p = 0.001). Toxicological analyses showed poor discriminatory performance (direction-normalized AUCs, defined as max [AUC, 1 − AUC], of 0.553–0.606). Intra-observer repeatability across a randomly selected subset of 62 eyes was excellent (ICC(2, 1) = 0.919), with no statistically significant systematic difference between the two measurement sessions (p = 0.052). Conclusions: Standardized PIR assessment provides an objective framework for postmortem pupillary characterization. This study establishes a reproducible methodological foundation for future postmortem pupillometry research.