Abstract
Vitreous humor (VH) is a gelatinous substance contained in the posterior chamber of the eye, playing a mechanical role in the eyeball. It has been the subject of numerous studies in various forensic applications, primarily for the assessment of postmortem interval and for postmortem chemical analysis. Since most of the xenobiotics present in the bloodstream are detected in VH after crossing the selective blood-retinal barrier, VH is an alternative matrix useful for forensic toxicology. VH analysis offers particular advantages over other biological matrices: it is less prone to postmortem redistribution, is easy to collect, has relatively few interfering compounds for the analytical process, and shows sample stability over time after death. The present study is an overview of VH physiology, drug transport and elimination. Collection, storage, analytical techniques and interpretation of results from qualitative and quantitative points of view are dealt with. The distribution of xenobiotics in VH samples is thus discussed and illustrated by a table reporting the concentrations of 106 drugs from more than 300 case reports. For this purpose, a survey was conducted of publications found in the MEDLINE database from 1969 through April 30, 2015.
Highlights
Vitreous humor (VH), known as the vitreous body, is a gelatinous substance contained in the posterior chamber of the eye, between the crystalline lens and the retina
The expressions of concentrations reported in the table are identical to those published in the original articles 25-NBDMe 2-(4-chloro-2,5-dimethoxyphenyl)-N-[(2-methoxyphenyl)methyl]ethanamine; 25I-NBOMe 2-(4-iodo-2,5-dimethoxyphenyl)-N-[(2-methoxyphenyl)methyl]ethanamine; 4-MTA 4-methylthioamphetamine; 6-MAM 6-monoacetylmorphine; BZE benzoylecgonine; EME ecgonine methyl ester; GHB Gamma-hydroxybutyric acid; LSD Lysergic acid diethylamide; mCPP meta-chlorophenylpiperazine; MDMA 3,4-methylenedioxymethamphetamine; EDDP 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine; THC-COOH 11-nor-9-carboxy-delta-9-tetrahydrocannabinol; ND not detectable; NQ not quantified properties of esterases in VH are different from those in blood, as demonstrated for brain synapse acetylcholinesterase, which was unable to hydrolyze heroin, unlike erythrocyte acetylcholinesterase [262]
The authors explained the increase in morphine and M3G concentrations in VH by diffusion from ‘‘tissue’’ without, specifying the tissue in question. These studies suggest that VH is a matrix protected against the main sources of postmortem redistribution from the abdominal cavity, but that ocular tissue may be a region of accumulation of xenobiotics liable to diffuse into VH postmortem
Summary
Vitreous humor (VH), known as the vitreous body, is a gelatinous substance contained in the posterior chamber of the eye, between the crystalline lens and the retina. Drug penetration into the retina depends on various factors, including plasma concentration, compound physicochemical and pharmacological properties, distribution volume, plasma protein binding and relative BRB permeability [70]. In a study of numerous compounds of forensic interest, Holmgren et al [73] found significant correlation between blood/VH concentration ratios and percentage of plasma protein binding. Findings were similar for 3,4-methylenedioxymethamphetamine (MDMA) [89], phenytoin [90], barbiturates [90] and cocaine [91] For compounds such as digitalis-glycoside, which accumulates dramatically in the retina [92], sampling problems such as choroid and retinal cell aspiration may affect observed concentrations, and we recommend separate sampling of the two VH specimens, without pooling. One sample, dedicated to toxicology analysis, should be performed with a stabilizer (1.5 % NaF or KF) to prevent ethanol neoformation and degradation of xenobiotics such as benzodiazepines, 6-MAM or cocaine.
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