Functional imaging of the retinal layers by laser scattering: an approach for the study of Leão's spreading depression in intact tissue

The use of time-lapse optical coherence tomography to image the effects of microapplied toxins on the retina

PURPOSE

We developed a novel technique for accelerated drug screening and retinotoxin characterization using time-lapse optical coherence tomography (OCT) and a drug microapplication device.

METHODS

Using an ex vivo rabbit eyecup preparation, we studied retinotoxin effects in real-time by microperfusing small retinal areas under a transparent fluoropolymer tube. Known retinotoxic agents were applied to the retina for 5-minute periods, while changes in retinal structure, thickness, and reflectance were monitored with OCT. The OCT images of two agents with dissimilar mechanisms, cyanide and kainic acid, were compared to their structural changes seen histologically.

RESULTS

We found the actions of retinotoxic agents tested could be classified broadly into two distinct types: (1) agents that induce neuronal depolarization, such as kainic acid, causing increases in OCT reflectivity or thickness of the inner plexiform and nuclear layers, and decreased reflectivity of the outer retina; and (2) agents that disrupt mitochondrial function, such as cyanide, causing outer retinal structural changes as evidenced by a reduction in the OCT reflectivity of the photoreceptor outer segment and pigment epithelium layers.

CONCLUSIONS

Retinotoxin-induced changes in retinal layer reflectivity and thickness under the microperfusion tube in OCT images closely matched the histological evidence of retinal injury. Time-lapse OCT imaging of the microperfused local retina has the potential to accelerate drug retinotoxicological screening and expand the use of OCT as an evaluation tool for preclinical animal testing.

Patterns of non-embolic transient monocular visual field loss

The aim of this study was to systematically describe the semiology of non-embolic transient monocular visual field loss (neTMVL). We conducted a retrospective case note analysis of patients from Moorfields Eye Hospital (1995-2007). The variables analysed were age, age of onset, gender, past medical history or family history of migraine, eye affected, onset, duration and offset, perception (pattern, positive and negative symptoms), associated headache and autonomic symptoms, attack frequency, and treatment response to nifedipine. We identified 77 patients (28 male and 49 female). Mean age of onset was 37 years (range 14-77 years). The neTMVL was limited to the right eye in 36 % to the left in 47 % and occurred independently in either eye in 5 % of cases. A past medical history of migraine was present in 12 % and a family history in 8 %. Headache followed neTMVL in 14 % and was associated with autonomic features in 3 %. The neTMB was perceived as grey in 35 %, white in 21 %, black in 16 % and as phosphenes in 9 %. Most frequently neTMVL was patchy 20 %. Recovery of vision frequently resembled attack onset in reverse. In 3 patients without associated headache the loss of vision was permanent. Treatment with nifedipine was initiated in 13 patients with an attack frequency of more than one per week and reduced the attack frequency in all. In conclusion, this large series of patients with neTMVL permits classification into five types of reversible visual field loss (grey, white, black, phosphenes, patchy). Treatment response to nifidipine suggests some attacks to be caused by vasospasm.

Correlation of visually evoked intrinsic optical signals and electroretinograms recorded from chicken retina with a combined functional optical coherence tomography and electroretinography system

Visually evoked fast intrinsic optical signals (IOSs) were recorded for the first time in vivo from all layers of healthy chicken retina by using a combined functional optical coherence tomography (fOCT) and electroretinography (ERG) system. The fast IOSs were observed to develop within ∼5 ms from the on-set of the visual stimulus, whereas slow IOSs were measured up to 1 s later. The visually evoked IOSs and ERG traces were recorded simultaneously, and a clear correlation was observed between them. The ability to measure visually evoked fast IOSs non-invasively and in vivo from individual retinal layers could significantly improve the understanding of the complex communication between different retinal cell types in healthy and diseased retinas.

Near-infrared imaging of fast intrinsic optical responses in visible light-activated amphibian retina

High performance functional imaging is needed for dynamic measurements of neural processing in retina. Emerging techniques for visual prosthesis also require advanced methodology for reliable validation of electromagnetic stimulation of the retina. Imaging of fast intrinsic optical responses associated with neural activation promises a variety of technical advantages over traditional single and multichannel electrophysiological techniques for these purposes, but the application of fast optical signals for neural imaging has been limited by low signal-to-noise ratio and high background light intensity. However, by using an optimized near-infrared probe light and improved optical system, we improve the optical signals substantially, allowing single pass measurements with approximately micron resolution. We image fast intrinsic optical responses with different optical modalities, i.e., bright field, dark field, and cross-polarization, from isolated retina activated by visible light stimulation. At single cell resolution, bright-field imaging discloses the maxima of optical responses approximately 5% dI/I, where dI is the dynamic optical change and I is the baseline light intensity. Dark-field imaging techniques further enhance the sensitivity of optical measurements, and show the maxima of optical responses exceeding 10% dI/I. Cross-polarized imaging provides optical sensitivity similar to dark-field imaging, but different patterns of neural activation are observed.

Dynamic neuroimaging of retinal light responses using fast intrinsic optical signals

Transient intrinsic optical responses associated with neural activation offer an attractive strategy for dynamic imaging of neural activity, and may provide a noninvasive methodology for imaging of retinal function. Here we demonstrate the feasibility of near infrared imaging of fast intrinsic optical changes in isolated frog retina activated by visible light. Using a photodiode detector in a transmitted light geometry, we routinely measured dynamic transmitted optical responses in single passes, at the level of one part in 10(4) of background light. Rapid CCD image sequences acquired with transmitted light (bright field) illumination disclosed larger fractional responses and showed evidence of multiple response components with both negative- and positive-going signals with different timecourses. Dark field imaging further enhanced the contrast and sensitivity of optical measures of neural activation. High-resolution imaging disclosed optical responses in single pixels often exceeding 5%, of background light, allowing dynamic imaging at the resolution of single cells, in single passes. Fast optical signals are closely related to identified response components of the electroretinogram. Optical responses showed complex but consistent spatial organization from frame to frame. Our experimental results and theoretical analysis suggest that the optical responses may result from dynamic volume changes corresponding to ion and water flow across the cell membrane, directly associated with the electrophysiological response.

Visual stimulus-induced changes in human near-infrared fundus reflectance

PURPOSE

Imaging studies from anesthetized feline, primate, and human retinas have revealed near-infrared fundus reflectance changes induced by visible light stimulation. In the present study, the spatial and temporal properties of similar changes were characterized in normal, awake humans.

METHODS

Five normal human subjects were studied. A modified fundus camera was used to image changes in retinal reflectance of 780-nm near-infrared light imaged onto a 12-bit charge-coupled device (CCD) camera in response to a green (540 nm) visual stimulus. During 60 seconds of recording (frame rate, 3 Hz) 10 cycles were recorded, during each of which 3 seconds of blank and then 3 seconds of either vertical bar or blank stimulus was projected. The change in the average near-infrared reflectance of the stimulated retinal region relative to an equal-sized nonstimulated region (r is the ratio of reflectance between the two retinal areas) was analyzed with a mixed model for repeated measures.

RESULTS

The mixed model showed a significant average decrease in r of 0.14% (95% CI, -0.25 to -0.03) over all subjects induced by bar stimulus cycles, with a gradual return to baseline after stimulus offset, compared with only a 0.04% (95% CI, -0.11-+0.20) decrease in r induced by blank, nonstimulated cycles. The mixed model for individuals showed a decreasing linear trend in r over time during bar stimulation, but no decrease for blank cycles in three of five subjects.

CONCLUSIONS

There was a localized decrease in reflectance in response to 780-nm near-infrared light in the retinal region exposed to a visual stimulus, which was significant in three of five subjects. It is presumed that the reflectance change represents the functional activity of the retina in response to a visual stimulus.

Migrainous scintillating scotoma and headache is ocular in origin: A new hypothesis

Brain neuronal dysfunction has been implicated in pathogenesis of migraine but direct evidence is lacking. Scintillating scotoma of migraine is generally believed to originate at the visual cortex. While cortical spreading depression is a relatively late physiological alteration in migraine, its protective role in neuronal ischaemia is increasingly being recognized. Atenolol, nadolol, or verapamil prevent migraine but do not readily cross the blood-brain barrier or critically influence any brain or peripheral neuronal function. Typical migraine headache, aura, or scintillating scotoma has not been reported following enucleation or evisceration of the eye. In humans, pain and temperature fibres from only the ophthalmic division of the trigeminal nerve reach the upper cervical spinal segments. Pain in migraine attacks including occipital and nuchal discomfort reflects selective involvement of the ophthalmic nerve. Photophobia is largely a retinal reflex involving the ophthalmic division of the trigeminal nerve. Key clinical features of the migrainous scintillating scotoma are consistent with retinal origin. Spreading depression in the retina is well-established. A subtle regional ocular sympathetic deficit prevails in migraine patients and possibly impairs regulation of intraocular choroidal blood volume and intraocular pressure. Several first-line migraine prophylactic agents lower the intraocular pressure. The neuro-ophthalmological basis for a monocular origin of migrainous scintillating scotomata due to mechanical deformation of the posterior segment of the corneo-scleral envelope consequent to choroidal venous congestion and rise in intraocular pressure is presented. Study of distribution and displaceability of the migrainous scintillating scotoma can settle its site of origin. Headache of migraine possibly arises from a similar mechanical deformation of the anterior eye segment followed by antidromic discharge in the trigeminovascular system. Lateralizing negative deficits such as homonymous hemianopia probably reflect vasospastic complications of migraine. A rational explanation for the most characteristic clinical features of migraine and a new template to elucidate the pharmacological basis of anti-migraine drugs is offered.

Simulation of the effect of Na+ and Cl- on the velocity of a spreading depression wave using a simplified electrochemical model of synaptic terminals

In the study of the spreading depression (SD) wave phenomenon and its dynamics, it is necessary to describe the ionic movements along the extracellular space, as well as between this and the intracellular space. In both cases, the ionic movement includes a double coupling involving the concentration and the potential gradients and hence must be described by electrodiffusion mechanisms. Based on this, the effects of the ionic composition on the characteristics of the wave propagation can be predicted. The influence of varying extracellular sodium and chloride concentrations on the velocity of propagation of the SD wave was investigated by simulation. The results achieved are close to the experimental measurement from the literature. These findings suggest the potentiality of the model proposed in supporting the interpretation of experimental data in neuronal tissues, particularly the SD.

Modeling Extracellular Space Electrodiffusion During LeÃo's Spreading Depression

Computational modeling of spreading depression (SD) has been used increasingly to study the different mechanisms that are involved in this phenomenon. One of them that is still under discussion involves the mechanisms that originate the extracellular electrical field responsible for the dc potential shift. The main goal of this paper is to present a mathematical derivation for the extracellular electric field that is incorporated in a SD model that has the basic structure of Tuckwell and Miura's model, but with the ionic variations calculated electrochemically. Electrodiffusion equations were used to describe the ionic movement of the four ions Na+, K+, Cl-, and Ca2+. These are mutually coupled by the electric field within the extracellular space (ECS). The results from the simulations show that the model is able to calculate the effect of the ionic changes along the ECS on the electric field, and to reproduce the SD in respect to the most important features that characterize the phenomenon experimentally in the retina or hippocampus. It is suggested that the extracellular negative field-potential shift during SD is due to an electrical field generated by a Goldman-Hodgkin-Katz equation acting within the ECS.