By Stuart Kurtz, M.S. D-ABFT-FT
The purpose of these next two blog posts is to discuss the utility that vitreous humor provides in the context of a case. This first post will focus on what vitreous humor is and the caveats associated with its interpretation. The second post will discuss some scenarios in which it can be very useful.
Vitreous humor is the fluid found in the eyes between the lens and retina. There is approximately 2 mL of fluid in each eye meaning the volume is limited for testing. Because of this, we don’t recommend the use of vitreous humor as the primary specimen, and we do recommend careful consideration before attempting testing on it. The context of the case is always important and sometimes that is the most useful sample to test. Therefore, from a whole case investigation approach, we recommend always collecting vitreous humor, alongside blood and/or urine, at autopsy – if possible. Collection should be done in a tube without sodium fluoride or potassium oxalate as a preservative. The clear top tubes that are provided in our toxicology kits are well suited for vitreous collection, having no preservatives.
The primary components of vitreous humor are a connective tissue gel and transparent liquid that is primarily water. Its composition and location mean that it is quite stable and resistant to putrefactive changes postmortem. Movement of substances in and out of the vitreous happens by diffusion, osmotic pressure, convection, or active transport. Low molecular weight substances, such as ethanol, acetone, methanol and isopropanol, will travel in and out via diffusion. Most drugs that are important in postmortem testing are a much higher molecular weight and will travel in and out via convection.
Ultimately, the mechanisms for most substances entering the vitreous are mostly unknown and/or not well studied. Drugs such as fentanyl, methamphetamine, and cocaine might be found in vitreous humor but the ability to correlate to a blood concentration is extremely limited and not recommended. Most drugs are very fat soluble and the vitreous humor is mostly water. So just like with oil and water, drugs will not be easily dissolved in the small volume present.
Whereas with urine we expect that the majority of drugs will be found there in one form at some point post-exposure, that may not be the case with vitreous humor. There may be some drugs that are so poor in their ability to enter the vitreous humor that they would be undetectable. It can have some ability to show recent exposure, similar to urine, of a drug, but the absence of a drug should be interpreted with caution.
Our next post will discuss vitreous humor’s important use in forensic toxicology. As always, we are here to assist you. Please reach out to us if you have any questions regarding this topic or want help with deciding what testing may be necessary. Our email is [email protected] and our phone number is 317-759-4869 option 3.
Guidelines for the Interpretation of Analytical Toxicology Results. Disposition of Toxic Drugs and Chemicals in Man. Twelfth Edition. Randall C. Baselt. Biomedical Publications. Pages xxx-xlii. (2020).
Introduction to Forensic Toxicology. Clarke’s Analytical Forensic Toxicology. Sue Jickells and Adam Negrusz. Pharmaceutical Press. Pages 1-12. (2008).
Postmortem Toxicology. Clarke’s Analytical Forensic Toxicology. Sue Jickells and Adam Negrusz. Pharmaceutical Press. Pages 191-218. (2008).
Postmortem Forensic Toxicology. Principles of Forensic Toxicology. Fourth Edition. Barry Levine. AACC, Inc. Pages 3-14. (2017).