Significant correlations
Results showed that 55 of the 77 participants (71.4%) had at least 20 pg/ml of NfL protein present in the vitreous humor. The median level was 68.65 pg/ml. Statistically significant associations were found between NfL levels in the vitreous humor and Abeta40, Abeta42, and total tau; higher NfL levels were associated with higher levels of all three biomarkers. On the other hand, NfL levels were not positively associated with increased vitreous levels of p-tau181.
Vitreous NfL concentration was significantly associated with inflammatory cytokines, including interleukin-15, interleukin-16, and monocyte chemoattractant protein-1, as well as vascular proteins such as vascular endothelial growth factor receptor-1, VEGF-C, vascular cell adhesion molecule-1, Tie-2, and intracellular adhesion molecular-1.
Despite these findings, NfL in the vitreous humor was not associated with patients’ clinical ophthalmic conditions or systemic diseases such as hypertension, diabetes, and hyperlipidemia. Similarly, NfL was not significantly associated with APOE genotype E2 and E4, the alleles most commonly associated with Alzheimer’s disease.
Finally, no statistically significant associations were found between NfL and Mini-Mental State Examination (MMSE) scores.
A “first step”
Most research currently examining the role of the eye in neurodegenerative disease is focused on retinal biomarkers imaged by optical coherence tomography, the investigators noted. Although promising, data obtained this way have yielded conflicting results.
Similarly, while the diagnostic potential of the core CSF biomarkers for AD (Abeta40, Abeta42, p-tau, and total tau) is well established, the practical utility of testing CSF for neurodegenerative diseases is limited, wrote the researchers.
As such, an additional biomarker source such as NfL–which is quantifiable and protein-based within eye fluid – has the potential to play an important role in predicting neurodegenerative disease in the clinical setting, they added.
“The holy grail of neurodegenerative-disease diagnosis is early diagnosis. Because if you can implement treatment early, you can slow down and potentially halt the progression of these diseases,” Dr. Subramanian said.
“This study is the first step toward determining if the eye could play a potential role in early diagnosis of conditions such as Alzheimer’s disease,” she added.
That said, Dr. Subramanian was quick to recognize the findings’ preliminary nature and that they do not offer reliable evidence that vitreous NfL levels definitively represent neurodegeneration. As such, the investigators called for more research to validate the association between this type of biomarker with other established biomarkers of neurodegeneration, such as those found in CSF fluid or on MRI and PET scans.
“At this point, we can’t look at eye fluid and say that people have neurodegenerative diseases,” she noted. “The other thing to consider is that vitreous humor is at the back of the eye, so it’s actually a fairly invasive procedure.
“I think the next step is to look at other types of eye fluids such as the aqueous fluid in the front of the eye, or even tear secretions, potentially,” Dr. Subramanian said.
Other study limitations include the lack of an association between NfL levels and MMSE scores and that none of the study participants were actually diagnosed with Alzheimer’s disease. Validation studies are needed to compare vitreous levels of NfL in patients with mild cognitive impairment/AD to normal controls, the investigators noted.
Fascinating but impractical?
Commenting on the findings, Sharon Fekrat, MD, professor of ophthalmology, Duke University, Durham, N.C., agreed that there’s potential importance of the eye in diagnosing neurodegeneration. However, she suggested that vitreous humor may not be the most expedient medium to use.
“I commend the authors for this fascinating work. But practically speaking, if we ultimately want to use intraocular fluid to diagnose Alzheimer’s and perhaps other neurodegeneration, I think aqueous humor might be more practical than the vitreous humor,” said Dr. Fekrat, who was not involved with the research. “What might be even better is to have a device that can be held against the eyeball that measures the levels of various substances inside the eyeball without having to enter the eye,” added Justin Ma, a Duke University medical student working under Dr. Fekrat’s guidance. “It could be similar technology to what’s currently used to measure blood glucose levels,” Mr. Ma added.
The study was supported in part by the National Institute of Aging. Dr. Subramanian, Dr. Fekrat, and Mr. Ma have disclosed no relevant financial relationships. Disclosures for other study authors are listed in the original article.
A version of this article originally appeared on Medscape.com.