Review: Emerging Eye-Based Diagnostic Technologies for Traumatic Brain Injury

Abstract

The study of ocular manifestations of neurodegenerative disorders, Oculomics, is a growing field of investigation for early diagnostics, enabling structural and chemical biomarkers to be monitored overtime to predict prognosis. Traumatic brain injury (TBI) triggers a cascade of events harmful to the brain, which can lead to neurodegeneration. TBI, termed the "silent epidemic" is becoming a leading cause of death and disability worldwide. There is currently no effective diagnostic tool for TBI, and yet, early-intervention is known to considerably shorten hospital stays, improve outcomes, fasten neurological recovery and lower mortality rates, highlighting the unmet need for techniques capable of rapid and accurate point-of-care diagnostics, implemented in the earliest stages. This review focuses on the latest advances in the main neuropathophysiological responses and the achievements and shortfalls of TBI diagnostic methods. Validated and emerging TBI-indicative biomarkers are outlined and linked to ocular neuro-disorders. Methods detecting structural and chemical ocular responses to TBI are categorised along with prospective chemical and physical sensing techniques. Particular attention is drawn to the potential of Raman spectroscopy as a non-invasive sensing of neurological molecular signatures in the ocular projections of the brain, laying the platform for the first tangible path towards alternative point-of-care diagnostic technologies for TBI.

Fig. 1.

(a) Inter-relationship between the primary and secondary injuries of TBI. Secondary injuries can contribute to the initial primary ones, creating a cycle causing further damage . (bi) Flowchart of the pathophysiological responses to TBI at a cellular level, reproduced with permission from (ii) Timescale of neurochemical and metabolic changes that take place following moderate to severe TBI .

Fig. 2.

The neuroanatomy of the human visual tract. (a) Photoreceptors in retina transform incident light into changes in membrane potential. This signal is received by bipolar cells and transmitted to retinal ganglion cells whose axons travel to the lateral geniculate body (LGN) in the optic nerve (surrounded with CSF), the optic chiasma and the optic tract. In the lateral geniculate body, part of the midbrain, RGC axons synapse with cells of the optic radiation that travel to the striate cortex (primary visual cortex), reproduced with permission from . (b) Cranial nerves II-VII located within the skull base, responsible for vision and eye and facial movement Created with BioRender.com. (c) The posterior segment of the human eye including the choroid with a dense vascular network to supply the outer retina, and the fovea in the centre of the macula containing only cone receptors to allow for sharp images in photopic conditions Created with BioRender.com.

Fig. 3.

OCT measurements of control and indirect, blast mediated TBI, murine eye samples, illustrating significant loss in the peripapillary RNFL thickness, reproduced with permission from .

Fig. 4.

Illustration of the concentrations of validated TBI biomarkers in the acute, subacute, and chronic phases following the primary injury. Early, on-site diagnostics would be completed within minutes to hours following injury and thus, necessitates the need to be sensitive to acute-phase indicative biomarkers, reproduced with permission from .

Fig. 5.

(a) Mean UCH-L1 levels in uninjured and in TBI patient groups, illustrating increased levels after trauma, with measurements taken from serum 4h post trauma. Analysed using an enzyme-linked immunosorbent assay (ELISA) kits, reproduced with permission from . (b) Time-course of eNAA using microdialysis in severe TBI survivors and non-survivors, demonstrating irregular levels in non-survivors and low levels followed by a large spike in survivors . (c) Plasma concentrations of S100-B, GFAP and NSE of moderate-severe TBI patients within the first 24 hours of hospital admission .

Fig. 6.

Examples of chemical sensing techniques used to measure TBI biomarkers. (a) NAA levels measured as metabolite ratios in mild TBI patients and healthy volunteers using proton MRS. Measurements were taken 1-20 days following trauma and indicate higher NAA concentrations in mild TBI patients than controls . (b) Surface Enhanced Raman Spectroscopy (SERS) used to measure ex-vivo samples, determined that SERS can detect a clear change in spectra when measuring multiplex immunosensors after incubation with varying concentrations of S100B and NSE . (c) In-vivo measurements of S100B, from brain extracellular fluid, using MD. S100-B levels peak in alignment with periods of raised ICP following TBI, reproduced with permission from . (d) SERS used to measure NAA concentrations in finger-prick blood plasma samples at t = 0 (black) and t = 8 hrs (red) following TBI, compared to healthy volunteers (navy) .

Fig. 7.

(a) Diagram of the energy transitions involved in Raman (inelastic) scattering compared to Rayleigh (elastic) scattering. In Stokes scattering, the incident photon has greater energy than the scattered photon, whereas the incident photon in anti-Stokes scattering has lower energy . (b) Schematic diagram of a generic Raman Spectroscopy system, reproduced with permission from . (c) Representative Raman spectrum of Ethanol with a prominent characteristic peak at 882cm⁻¹ of the C-C-O bond symmetric stretching vibration . (d) Schematic of a Raman fibre optic probe, an example of a popular RS development for clinical applications, used here for an optical core needle biopsy for in-vivo detection of brain cancer tissue .

Fig. 8.

Raman spectra of eye tissue undergoing chemical changes indicative of neurodegeneration. (a) Raman spectra of en-face murine retina using 785 nm laser, grouped into wild mice and AD model mice. Chemometric analysis revealed biochemical changes indicative of structural and pathological manifestations of AD, reproduced with permission from . (b) Spectra of murine, retinal cultures modelling MS using LPS, measured using a 785 nm Raman system. Increasing incubation with LPS lead to changes in the heights of characteristic peaks in the spectra, indicative of neuroinflammation .

Fig. 9.

Raman spectra of in-vivo samples and models which simulate an in-vivo environment. (a) Raman spectrum of healthy, human retina, measured in-vivo with dilated pupil (∼8 mm diameter), using a 488 nm laser. The top spectrum is from 3 summed measurements and the bottom spectrum is the same measurements with baseline subtracted. Characteristic carotenoid peaks are present at 1008, 1159 and 1525 cm⁻¹, reproduced with permission from . (b) Comparative study of Raman spectra taken in a human eye in-vivo and macular carotenoid zeaxanthin in liquid form within an eye model, reproduced with permission from . (c) Comparative spectra of flat-mounted, murine retina using 785 nm lasers within a commercial Raman system (bottom) and an in-house built Raman set-up (top) which simulates in-vivo parameters in the eye. Characteristic carotenoid peaks are present in both . (d) Spectra taken of ex-vivo macular pigment tissue samples which were fixed using formalin fixative, using a 488 nm excitation wavelength . (a)–(d) demonstrate that the same characteristic carotenoid peaks are present in in-vivo, fresh ex-vivo and fixed ex-vivo eye samples. (e) Raman spectra of fresh, ex-vivo, porcine eyes that were dissected into 5 main features. Measurements were taken using a commercial system, 785 nm laser and settings were chosen based on the maximum permissible exposure defined by eye-safe limits .