As an example, many glaucoma models involve induced elevation of IOP but do not model the increased susceptibility to glaucomatous damage that may be a contributing factor in many glaucoma patients.
Second, the way glaucoma is conventionally phenotyped is relatively unsophisticated and it is likely that different mechanisms are relevant to varying extents in different patients in ways that are not modelled effectively in animals at present. Third, differences in outcome measures used when assessing laboratory studies and human patients are another potential issue.
In addition, neuroprotective agents are often administered prior to the onset of damage in animal studies which limits their relevance when comparing any effect to therapeutic intervention in patients who already have the disease. Slow progression of glaucoma in patients recruited to clinical trials of neuroprotection would suggest lengthy trials would be required and the individual variability that occurs when performing functional tests would be expected to necessitate large group sizes to determine whether there is evidence of any therapeutic effect.
It is therefore of considerable interest that recently, the UK Glaucoma Treatment Study UKGTS , 38 which evaluated vision preservation in patients taking the prostaglandin analogue Latanoprost for open-angle glaucoma in a randomised, multicentre, placebo-controlled trial demonstrated statistically significant differences between treatment groups in only one year with approximately patients per group.
It is conceivable that recruitment of patients progressing at a pre-defined rate prior to trial entry and enhancement of functional endpoints by clustering of measurements at the beginning and the end of studies could help to shorten neuroprotection trials and reduce the group sizes required still further.
Over clinical trials using MSCs have been registered in the NIH database with 8 trials in multiple sclerosis alone. Advantages of MSCs compared to other types of stem cells include the fact that they are easy to obtain from a variety of sources including adult bone marrow, avoid ethical concerns and can be used without immune suppression.
MSCs have also been shown to be neuroprotective in a rat glaucoma model. However, despite these encouraging results, it should be noted that PDGF and MSCs can induce reactive gliosis 44 in retinal Muller cells and astrocytes with upregulation of intermediate filament proteins and retinal folding. This is consistent with increasing evidence that Lcn2 is involved in astrogliosis, neuroinflammation and reactive glia-mediated neurotoxicity in animal models of neurodegeneration, 45 perhaps through chemokine upregulation 46 and the promotion of glial migration.
There have been similar recent reports of proinflammatory vitreous clumping of MSCs injected intravitreally 47 as well as thick epiretinal membrane formation following MSC administration in humans. An attempt to standardise the identification of these cells led to the development of the International Society for Cellular Therapy criteria 49 but problems continue to be reported despite this and may limit the degree to which MSCs can successfully be used to confer neuroprotection to RGCs.
Although there are significant concerns with regard to the unregulated use of MSCs in various centres around the world, there are currently at least three registered clinical trials evaluating the use of stem cells in glaucoma:. The Intravitreal Mesenchymal Stem Cell Transplantation in Advanced Glaucoma study: 50 a Phase 1 safety study in 10 patients who meet the legal definition of bilateral blindness, with intravitreal injection of autologous MSCs administered to the worst-affected eye.
Outcome measures include visual acuity, visual fields, optical coherence tomography and electroretinography. The study is patient funded and claims to be associated with a risk of potential complications of 0. The study website declares that it hopes for visual improvement in the vast majority of individuals enrolled, but very little information is provided with regard to outcomes of those already treated.
Effectiveness and Safety of Adipose-Derived Regenerative Cells for Treatment of Glaucomatous Neurodegeneration study: 52 an open-label safety and efficacy study in Russia.
It remains conceivable that stem cells do have the potential to provide a useful treatment strategy for glaucoma, either by a neuroprotective or neuroregenerative mechanism. However, in order to assess such strategies we need randomised, masked, controlled clinical trials and further work on the optimal mode of delivery.
Care needs to be taken to prevent the exploitation of vulnerable patients and false promise of success with unproven treatments. Additional gene-based strategies being investigated include the delivery of genes encoding therapeutic proteins such as neurotrophic factors. Channelrhodopsin-2 depolarises when expressed and exposed to light, thereby generating a signal for transmission along RGC axons to the brain.
The delivery of intravitreal agents has revolutionised the treatment of retinal conditions such as age related macular degeneration, diabetic retinopathy and retinal vascular pathologies and their success and widespread therapeutic use demonstrates that patients are able to tolerate intravitreal injections repeatedly. There remains a strong clinical need for RGC neuroprotection with gene therapy and cell-based therapies showing considerable promise. As we continue to develop these strategies, the need to ensure that vulnerable patients are not taken advantage of with the false promise of success from unregulated trials that rely on patient funding is paramount.
Refinement of clinical trial design to ensure that endpoints are feasible and clear also remains a priority. The number of people with glaucoma worldwide in and Br J Ophthalmol ; 90 : — Predictors of long-term progression in the early manifest glaucoma trial.
Ophthalmology ; 11 : — Article Google Scholar. An evidence-based assessment of risk factors for the progression of ocular hypertension and glaucoma. Am J Ophthalmol ; 3 Suppl. Lifetime risk of blindness in open-angle glaucoma. Am J Ophthalmol ; 4 : — Long-term trends in glaucoma-related blindness in Olmsted County, Minnesota.
Ophthalmology ; 1 : — Nature ; : — Neural activity promotes long-distance, target-specific regeneration of adult retinal axons. Nat Neurosci ; 19 8 : — Goldberg JL, Guido W. Invest Ophthalmol Vis Sci ; 57 3 : — Retinal regeneration mechanisms linked to multiple cancer molecules: a therapeutic conundrum. Prog Retin Eye Res ; e-pub ahead of print 30 August ; doi From mice to mind: Strategies and progress in translating neuroregeneration. Eur J Pharmacol ; : 90— Chronic low-dose glutamate is toxic to retinal ganglion cells: toxicity blocked by memantine.
Invest Ophthalmol Vis Sci ; 37 8 : — The density of RGC bodies was 10 times higher in the perifoveal retina than for retinal locations corresponding to points at the outer measurement zone of the field test 20—30 o from fixation. If we assume that some of the cells we counted in the ganglion cell layer were actually amacrine cells and not RGCs, our estimates of percentage of loss would actually be understated.
In addition, if amacrines are included in our counts and if they atrophy in proportion to the loss of RGCs, then our estimates would be unaffected. Some measures of field test results could not be closely correlated with the RGC numbers. There is substantial variation in the total number of RGCs from one eye to another, placing relatively broad confidence limits on any estimate of histologic damage. Furthermore, there is very significant variability in field test results both among persons and for the same subject within and between tests.
The reproducibility of our histologic counting methods is excellent and adds only minimally to the variability in correlations. Finally, some field measures would not be expected to correlate with glaucoma damage very closely, because they are measures of general sensitivity loss that can be influenced by other disorders, age, and test conditions.
The validity of our RGC counting was supported by the correlation of the retinal data with the optic nerve fiber counting in the same eyes.
The number of fibers in the normal optic nerve has varied from , to 1. Age-related loss estimates have varied from as few as 2, to as many as 12, RGCs dying per year.
The normal number of RGCs in this report was lower, and our estimated loss with age was higher than our own previous estimates. This probably resulted from the substantially older age of our control subjects compared with past reports , who were selected to match the age of the glaucoma patients. Inspection of data from other investigators reveals that the loss of RGCs with age may accelerate after middle age.
If we assume that the loss of RGCs increases with advancing age, subjects with glaucoma who have loss of the majority of RGCs could undergo progressive impairment with an age-related loss of fibers per year, despite any effort to treat the disease.
We have shown that larger RGCs are preferentially susceptible to death from glaucoma in human eyes, 3 4 6 8 9 11 12 and this was corroborated in studies of the retina, 5 the optic nerve, 7 or the lateral geniculate body 10 of persons with glaucoma and in experimental monkeys.
If RGC axons were to decrease their diameter before death, an apparently selective loss of larger axons might be simulated.
We have previously demonstrated that our data are not compatible with this hypothesis. Axon diameter and cell body size are correlated with functional RGC behavior, and psychophysical tests that exploit the loss of the functions subserved by larger RGCs including scotopic, 28 motion, 29 and frequency-doubling paradigms 30 show promise in glaucoma diagnosis.
The translation of anatomic selectivity into useful psychophysical tests depends on the sensitivity with which loss of particular RGCs can be detected by functional testing. Submitted for publication February 23, ; revised July 26, ; accepted October 26, Commercial relationships policy: N. Corresponding author: Harry A. Quigley, Wilmer , N. T able 1. View Table. T able 2. Visual Field Data from Subjects with Glaucoma. Type of field test: 12, program ; 5, program Algorithm: 16, standard Humphrey Field Analyzer 1; 1, full from prior strategy.
Reliability: All 17 tests within reliability range: fixation loss and false-positive results. T able 3. Glaucoma Historical Data. F igure 1. View Original Download Slide. F, fovea. F igure 2.
F igure 3. F igure 4. F igure 5. The average loss of sensitivity in decibels of all points in the lower visual field was correlated with the percentage of normal RGCs in the corresponding upper retina. F igure 6. Comparison of data for two points in the upper visual field that fall within cluster 1 of the Glaucoma Hemifield Test Humphrey, San Leandro, CA and their two mirror-image locations in the inferior visual field.
Cluster 1 consists of points close to central fixation. The average percentage of normal RGCs in the two upper retinal locations was subtracted from that of the paired lower locations horizontal axis.
The loss from age-normal values in visual field threshold in decibels for the lower visual field pair of points was subtracted from that of the upper field pair vertical axis. F igure 7. Optic nerve axon diameter data from 16 eyes of 16 normal and 11 eyes of 11 glaucoma subjects. The axon distribution of the glaucomatous eyes circles , with error bars falls generally within 1 SD of the normal mean for small fiber diameters and at or below this level for larger diameters.
T able 4. F igure 8. The glaucoma-affected eyes triangles , regression line are represented as a percentage of normal axons surviving at each diameter. Optic nerve damage in human glaucoma, III: quantitative correlation of nerve fiber loss and visual field defect in glaucoma, ischemic neuropathy, disc edema, and toxic neuropathy.
Arch Ophthalmol. Morphologic changes in the lamina cribrosa correlated with neural loss in open-angle glaucoma. Am J Ophthalmol. Example of a standard automated perimetry printout from the left eye of a patient with a superior arcuate scotoma due to open-angle glaucoma.
Text bubbles describe the main parameters evaluated. Standard automated perimetry measures achromatic differential light sensitivity with the purpose of quantifying visual function and RGC loss. A correlation between perimetric sensitivity and histological RGC counts has been demonstrated in experimental glaucoma in non-human primates, 3 , 4 and in human glaucoma patients. The most commonly used parameter is the peripapillary retinal nerve fibre layer RNFL thickness. Similar to RNFL thickness, width, area, or volume measurements of the NRR are influenced by its non-axonal components, including glia and blood vessels.
Cases of documented complete loss of RGC axons can provide an estimate of the magnitude of the non-axonal components in the NRR. Drance and King 13 reported 4 patients with complete loss of RGCs due to trauma or tumor compression of the optic nerve, who had their NRR area measured with fundus photography. Additionally, NRR measurements are influenced by remodelling and biomechanical changes in the connective tissues of the optic nerve head. For example, as neural tissue is lost in the optic nerve head, remodelling changes of the connective tissues and gliosis can lead to a higher non-neural component of the NRR.
Intraocular pressure changes can also impact the axial position of the lamina cribrosa and scleral canal opening area. However, limitations in current OCT technology do not always permit differentiation of the ganglion cell layer from the adjacent ones, namely the RNFL and inner plexiform layer, and therefore are often measured together, 21 , 22 increasing the non-RGC components of these measurements.
All parameters currently used in clinical practice are indirect measurements of RGC counts, with significant interference of non-RGC factors, reducing their accuracy to assess RGC damage. Cross-sectional studies may suggest strong correlation of these parameters with RGC count, but this may be related to the wide range of damage typically included in these studies, from normal to advanced glaucoma.
When detecting smaller amounts of RGC damage, such as diagnosing early disease or detecting disease progression over few years, the noise included in these parameters from non-RGC factors may overcome the real signal of RGC damage. Furthermore, the estimated inter-subject variation in normal RGC counts is impressively high, 20 making any estimates of actual RGCs highly tenuous.
The low signal-to-noise ratio in current parameters is evidenced by the frequent scenario of patients showing changes in some parameters but not others, for example, a normal visual field examination with abnormal OCT, or vice-versa. If all clinical parameters were reliable indicators of RGC damage, their outputs would be in high agreement, unlike what is observed in most clinical studies. Longitudinal imaging of RGCs would be a most appropriate method to monitor the progression of glaucoma, yet to date, the ability to directly image RGCs is possible only in laboratory animals.
For more than a decade, in vivo longitudinal imaging has been reliably performed in experimental rodent models of optic nerve damage causing RGC loss. Commonly used methods include fluorescence imaging of retrogradely labelled RGCs, 26 , 27 structural imaging of the optic nerve head 28 , 29 , 30 , 31 or axons, 32 and imaging of RGC apoptosis.
This can be accomplished with the use of transgenic animals or administering an exogenous label in a minimally invasive manner. Regardless of the method, it is important that 1 the RGCs remain labelled for longitudinal imaging and 2 RGC labelling decreases with the damage or death of the cell, for reliable measurements.
The ability to introduce markers into living tissue for the monitoring of cellular health in specific cells is important when studying disease detection and progression. The best-established method of RGC labelling in experimental animals, specifically rodents, is the administration of a tracer, commonly Fluorogold, to the optic nerve or region of the brain that RGCs project.
Optic nerve labelling is assumed to label the entire RGC population and can be completed via the intact optic nerve or by stump labelling after the optic nerve is transected.
Furthermore, the tracer is taken up by microglia and macrophages when RGCs die after inducing injury, reducing the specificity of the label to RGCs.
Recently, there has been interest in introducing markers via intravitreal or subretinal injection to fluorescently label RGCs. These methods do not require the use of transgenic mice and can be used in a variety of species. Cholera toxin subunit B has also been used as an anterograde tracer to label RGC axons by administering it via intravitreal injection.
Despite the poor specificity, it is expected that RGCs previously labelled by an intravitreal injection of CTB would exhibit a loss of fluorescence when damage is induced. A variety of transgenic mouse strains have been developed to express fluorescence in RGCs and are especially useful for longitudinal imaging. These fluorescence signals are easily detectable by non-invasive in vivo imaging of the retina.
These mice have been used for studies of acute injury to RGCs, including optic nerve transection Figure 3 , 45 optic nerve crush, 46 N-methyl-D-aspartate NMDA receptor induced excitotoxicity, 47 and retinal ischemia induced by increased intraocular pressure.
In vivo fluorescence imaging of the retina in a transgenic mouse expressing cyan fluorescent protein CFP under the Thy-1 promoter. Images were acquired longitudinally beginning at baseline BL and then 3, 7, 10, 14, and 21 days after optic nerve transection. From Chauhan et al With this strain characterization of at least six different RGC types has been accomplished ex vivo 51 and in vivo.
Visualizing and measuring dendritic atrophy of RGCs can be useful for studying the effects of neuroprotective interventions in the early stages of damage. However, if it is desirable to study RGC damage in an animal that does not have a fluorescent reporter endogenously expressed in RGCs, alternate labelling methods must be utilized.
In vivo fluorescence imaging of the retina in a transgenic mouse expressing yellow fluorescent protein YFP under the Thy-1 promoter. Images were acquired longitudinally, beginning at baseline BL and then 3, 5, 7, 14, and 28 days after optic nerve transection.
AAV vectors are available in a variety of serotypes, which show preferential uptake to cell types, and have the ability to incorporate differing promoters, allowing for improved specificity to cells.
These two factors make AAV vectors desirable for labelling an exclusive cell population, such as RGCs, in tissue with many cell types, like the retina. The greatest challenge when utilizing AAV vectors for in vivo labelling of RGCs is to overcome the inner limiting membrane, which acts as a physical barrier to achieving a high transduction rate. In vivo fluorescence images of GFP-labelled retinal neurons in mouse following intravitreal injection of an adeno-associated viral vector with a ubiquitous promoter AAV2-CAG-GFP at a 1 week post-injection and b 5 weeks post-injection.
Genetically encoded calcium indicators, such as GCaMPs, are a useful tool for measuring intracellular calcium concentration of neurons 58 and have the potential to be used for studying RGC calcium transients after optic nerve transection.
Alternatively, genetically encoded calcium indicators can be administered with viral vectors, eliminating the need for transgenic animals. In vivo fluorescence imaging of the retina a transgenic mouse expressing GCaMP under the Thy-1 promoter a with baseline fluorescence in darkened room and b during exposure to UV light stimulus. Arrows indicate cells with decreased intracellular calcium and circles indicate cells with increased intracellular calcium during UV light exposure.
Currently, there is a large gap, both in the techniques and measures, between how we assess RGC damage in humans and in animals. Regardless of the technology or technique, it is important that RGC loss is measured reliably.
Furthermore, it is important to develop techniques that are a true measure of RGC changes that can one day be used in the clinic.
BMJ Open ; 5 : e Nomenclature by Masri et al. These cells are located mainly in the central retina and project to the parvocellular pathway 66 , Midget RGCs have a one-to-one connectivity with midget bipolar cells, which draw their input from a single cone There are two types of midget RGCs: the outer stratified OFF-midget cells show smaller dendritic fields and higher cell densities than the inner ON-midget cells.
The parvocellular pathway is dominated by midget RGCs. Functional assessments of these cells demonstrate that their luminance contrast sensitivity is lower than that of parasol RGCs and most show clear chromatic opponency However, recent studies suggest that some OFF-midget cells receive signals from short wavelength blue sensitive cones 14 , Electron microscopy reconstructions of retinal circuits suggest the possibility that a small proportion of midget ganglion cells might have blue—OFF, yellow—ON receptive fields.
In addition to colour discrimination, midget RGCs also subserve pattern, texture and stereoscopic depth perception As with midget cells, there are two types of parasol cells in primates: ON-parasol cells respond with an increase in firing rate at the onset of light in the centre of their receptive field whereas OFF-parasol cells react to off stimuli Parasol RGCs have larger receptive fields and cell bodies, have higher sensitivity to luminance contrast, and present little or no chromatic antagonism, in contrast to midget RGCs Parasol RGCs play a role in motion perception, flicker perception and depth processing based on motion parallax They largely comprise the magnocellular pathway.
This arrangement is thought to give good colour vision with low spatial resolution. The inputs of large bistratified RGCs have not been elucidated.
However, the precise role of this cell type is not yet clear Smooth monostratified RGCs smRGCs have irregular receptive fields with multiple distinct hotspots of light sensitivity. They might contribute to signalling spatial information via a non-linear mechanism, whereby output is not linearly related to input The recursive RGCs have moderately densely branched dendritic trees in which many secondary branches tend to curve back towards the soma or close loops of apposing and recursive dendrites.
In addition, many dendrites overlap those of neighbouring cells To date, only one population of a bistratified ON-OFF type has been described in the macaque retina 78 — Broad thorny RGCs are given various names such as thorny diffuse, T-group cells, S3 narrow thorny, and hedge cells 30 , 49 , The dendrites of broad thorny RGCs span a whole layer of the inner plexiform layer.
Additionally, two narrowly stratified cells, including outer and inner, are found in primates and their connectivity has not been clarified yet.
These cell types are monostratified cells that receive input from amacrine and bipolar cells. The transcription factor Satb2 is expressed in large sparse RGCs in macaque and human retina These cells have large, sparse dendritic fields. They are intrinsically photosensitive because of the expression of the melanopsin photopigment and capable of phototransduction independently of rods and cones The main function of ipRGCs, in particular of the Brn3b-M1subtype, is to contribute to circadian photoentrainment through the projections to the suprachiasmatic nucleus SCN of the hypothalamus 86 , but they are also relevant for other non-image forming functions of the eye, including the regulation of the pupillary light reflex through the projections to the OPN.
M4 ipRGCs have a larger cell body compared with M5 ipRGCs that have small, highly branched dendrites arrayed uniformly around the soma. Melanopsin-containing intrinsically photosensitive RGCs in primates. M1 ipRGCs are reported to receive an inhibitory input from short-wave cones via an S-cone amacrine cell 90 , whereas M2 ipRGCs receive input from S-On bipolar cells and contribute to the blue cone pathway However, ipRGCs are affected in other optic neuropathies, such as glaucoma, and late-onset neurodegenerative disorders, such as Alzheimer disease and Parkinson disease 92 — There are a small number of unclassified RGCs in primates that do not fit with any of the previously described RGCs 8.
Further work is needed to elucidate the characteristics of this miscellaneous group of RGCs. Although RGCs have been extensively studied in primates, the clinical assessment of RGCs has proven more challenging as they cannot be evaluated directly.
Optical coherence tomography OCT is a non-invasive imaging technique that uses low-coherence light waves to capture a cross-section of various tissues. Major advances have led to the development of spectral domain OCT, which can produce a segmentation of ten layers of retina, including the retinal nerve fibre layer RNFL and ganglion cell layer.
OCT has become a standard tool to investigate changes with RGCs as it is non-invasive, rapid, highly reproducible 96 — The RNFL can be measured in both the peripapillary and the macular areas.
Several studies suggest that changes can be detected earlier by assessing the thickness of the RNFL in the macula compared with the peripapillary RNFL, owing to the latter's thickness 99 , There is a good correlation between RNFL thickness and both visual acuity and visual field changes, offering an objective structural parameter for assessing glaucoma and other optic neuropathies — However, to avoid misinterpretation of OCT, several factors need to be considered: segmentation errors can occur particularly in the presence of a tilted optic disc ; and RNFL thickness is also affected by age as well as by refractive error and axial length.
In addition, there is lag time before any changes in the thickness of the RNFL can be detected after disease onset , and the thickness can be overestimated in the presence of optic disc or RNFL swelling.
In addition, RNFL thickness exhibits a floor effect that must be considered in advanced optic neuropathies. RNFL thinning reaches a trough at a certain level owing to residual tissues such as vessels and glial cells , Furthermore, RNFL loss usually signifies irreversible damage and functional tests as described below might be needed to identify ganglion cell dysfunction at a potentially reversible stage.
It is well-established that visual acuity and visual fields can recover despite extensive RGC layer thinning , Microcysts in the inner nuclear layer have been reported on macular OCT imaging in some patients with advanced loss of macular RGCs.
They do not seem to be specific to a particular aetiology, having been reported in patients with inherited optic neuropathies, demyelinating optic neuritis, compressive and nutritional optic neuropathies, endemic optic neuropathy and advanced glaucoma — It is not clear why these microcysts develop in only a subgroup of patients.
They are seen more often in younger patients who may have a more adherent vitreous surface and ILM tension has been implicated as part of the pathophysiology However, microcysts have also been reported as a long-term consequence associated with RGC loss in patients with silicon oil-related visual loss These patients have undergone prior removal of the vitreous suggesting that simple vitreous traction may not be sufficient to explain the development of these microcysts.
The detection of apoptosing retinal cells DARC is a new technique that enables visualisation of real-time RGC apoptosis using fluorescently-labelled annexin A5. This 36 kDa protein is expressed in humans and it is a well-established indicator of apoptosis DARC has the advantage of early detection of RGC loss before visual deterioration has occurred, and it being considered for the evaluation of optic neuropathies, including glaucoma disease progression A number of psychophysical measurements can be used to investigate changes in RGC function.
However, as visual acuity tests central foveal function, patients can have widespread ganglion cell loss with preserved central visual acuity. Achromatic stimuli of low and high spatial frequencies can be used to differentiate responses from the magnocellular and parvocellular systems. The magnocellular pathway has lower spatial resolution and responds to higher temporal frequencies than the parvocellular pathway However, this difference is relatively small and the two pathways have a degree of overlap.
Colour vision impairment is a frequent feature of ganglion cell pathology, but outer retinal dysfunction can also affect colour vision, such as anomalies of the cone photoreceptors. Congenital stationary red-green colour deficiencies commonly affect men, owing to loss or alteration of the long or medium wavelength opsin genes on the X-chromosome Rarely, abnormalities in the same genetic region can give rise to S-cone monochromacy.
Congenital tritan anomalies, arising from abnormalities in S-cones are also rare. Progressive or later onset cone or macular dystrophies, or congenital achromatopsia, will also affect colour vision, but in these conditions visual acuity is also usually impaired In acquired ganglion cell pathology, however, visual acuity can be preserved with colour vision being preferentially affected.
Many optic neuropathies affect red-green discrimination, although glaucoma commonly affects the blue-yellow axis Colour vision tests are widely used to screen patients with congenital colour vision defects and to investigate acquired pathology. There are three broad types of colour vision tests in practice In arrangement tests, such as the Farnsworth-Munsell FM Dichotomous D tests and hue test, the patient is required to arrange a set of colours in order.
The FM hue test is highly sensitive, but time-consuming. Lastly, anomaloscopes are based on colour-matching where the observer adjusts a mixture of red and green lights to match a monochromatic orange light. As congenital anomalies of colour perception more commonly affect red-green discrimination, many standard tests such as the Ishihara plates and the Nagel anomaloscope do not probe for tritan disorders, which are common in acquired pathologies.
In addition, more specialised psychophysical methods, including measurement of the three primary colour vision mechanisms, colour adaptometry, and colour perimetry can be applied Among them, SWAP, a specialised type of perimetry, can also be considered a colour vision test, as the targets are short-wave and the field is of long wavelength and high intensity in order to adapt the long- and middle-wave cones In addition to conventional visual field testing, short wavelength automated perimetry SWAP probes the small bistratified ganglion cells and the konioceullar pathway, and high-pass resolution ring perimetry tests the parvocellular pathway, whereas flicker perimetry, motion perimetry, and frequency doubling technology FDT target the magnocellular pathway Modern FDT uses targets of low spatial frequency that flicker at a high temporal frequency and that predominantly stimulate the magnocellular pathway, which corresponds to motion detection and flicker detection FDT has been put forward for the early detection of glaucoma on the basis that the magnocellular pathway is more vulnerable in glaucoma , However, there is evidence that both the parvocellular and magnocellular pathways are affected early in glaucoma with no significant differences between these two pathways in terms of their vulnerability Furthermore, a recent study indicated that FDT is neither sensitive nor specific as a screening tool for glaucoma Further studies are, therefore, needed to evaluate the role of FDT in the early detection of glaucoma.
Unlike standard visual field testing, which uses a white stimulus on a white background, SWAP employs a blue stimulus on a yellow background. Several studies suggested that SWAP is more sensitive for the early detection of glaucomatous changes compared with standard visual field testing — There is, however, no definitive evidence that the small bistratified ganglion cells short-wave response are more vulnerable in glaucoma.
However, SWAP has some limitations as it is time-consuming, it needs a higher level of cooperation, and it has lower reproducibility compared with standard perimetry The primate pupil responds to signals from ipRGCs, which additionally receive input derived from cone responses. Chromatic pupillometry uses selective wavelengths to quantify pupil size before, during, and after a light stimulus has been applied.
Comparison of pupillary responses to short-wavelength and long-wavelength light can selectively probe the function of outer retinal photoreceptors or the intrinsic response of ipRGCs.
The ipRGCs are blue light sensitive and maximally sensitive to wavelengths that lie between the peak sensitivities of the rods and S-cones.
Several studies using chromatic pupillometry in experimental animal models have shown that the light sensitive ipRGCs were spared in retinitis pigmentosa characterised by marked photoreceptor loss Generally, the ipRGCs are relatively preserved in mitochondrial optic neuropathies, such as LHON and ADOA , , but affected in other optic neuropathies such as glaucoma, non-arteritic anterior ischemic optic neuropathy and demyelinating optic neuritis Bichromatic pupillometry has been used to differentiate between mitochondrial and non-mitochondrial optic neuropathies 94 , Electrophysiology allows direct objective assessment of electrical responses in vivo.
The visual evoked potential VEP , recorded over the visual cortex, has long been used as a means of assessing the function of the visual pathway, as well as demonstrating developmental abnormalities, such as the misrouting of ganglion cell axons in albinism In addition, the electroretinogram ERG , which represents the summed electrical response of the retina to light stimuli, can be recorded non-invasively.
In contrast, the full-field ERG, which is generated from the stimulation of the whole retina, is usually used to evaluate responses from photoreceptors and bipolar cells. However, a late component, the photopic negative response PhNR has been shown to arise in ganglion cells. The PERG is recorded in response to a patterned stimulus typically a checkerboard pattern reversing 4 times per second , which stimulates the central 15 degrees of the retina The test is performed in photopic conditions with undilated pupils and it requires optimal refraction.
The response is driven by the macular cone photoreceptors, but it appears to arise largely from the macular RGCs, whose signals appear to give rise to the N95 component and the majority of the P50 component , Various optic neuropathies that affect the ganglion cells within the retina either as the primary site of impairment or from retrograde degeneration from an optic nerve lesion , for example demyelinating optic neuritis, ischemic optic neuropathy, compressive optic neuropathy, toxic optic neuropathy, and inherited optic neuropathies can result in a reduction of the N95 and P50 amplitudes, with N95 being reduced more than P50, and a shortening of the P50 peak time — Whilst it can be detected in standard white-on-white flash responses, specific chromatic protocols can be used to optimise the PhNR signal Unlike in PERG recordings, optimal refraction is not needed, but the pupils need to be dilated.
In addition, a hand-held mini-Ganzfeld stimulator is available to test PhNR The PhNR can be used to examine the parvocellular pathway whereas the steady-state PERG is focused on the magnocellular pathway in glaucoma Inherited and acquired optic neuropathies are important causes of registrable blindness. Treatment options remain limited, and when available, they mostly slow down or prevent further loss of RGCs. Visual loss is usually irreversible although in some cases, spontaneous visual recovery can occur owing to the functional recovery of RGCs that have not undergone apoptosis.
To better inform future treatment strategies, it is essential to gain a better understanding of the pattern of RGC loss and whether different aetiological triggers result in global or more selective loss of RGCs, and how these relate to the visual deficits and eventual outcome.
It remains a challenging task as patients are not always examined in the acute stage of the disease and serial measurements are needed to document progression over time. Nevertheless, we are gaining a better understanding of the structure-function relationship in different optic neuropathies aided by the availability of high-resolution retinal imaging with OCT and more sophisticated visual electrophysiological and psychophysical tools Figure 5.
Pattern of RGC involvement in optic neuropathies. The types of RGCs affected in inherited optic neuropathies and acquired optic neuropathies are indicated by black and red lines, respectively. The dotted green line indicates the preservation of ipRGCs in inherited optic neuropathies. The minimum prevalence of inherited optic neuropathies has been estimated at 1 in 10, This group of disorders is genetically heterogeneous with disease-causing mutations occurring in both mitochondrial and nuclear DNA Remarkably, all genes identified to date encode proteins that are either directly or indirectly involved in regulating mitochondrial function.
The generation of ATP by the mitochondrial respiratory chain is central to cell survival and mitochondria also regulate other key pathways, including the level of reactive oxygen species and the tight control of apoptosis. An intriguing aspect of inherited optic neuropathies is the preferential vulnerability of RGCs compared with other neuronal populations despite the ubiquitous expression of the genes involved.
There have been limited post mortem studies on the pattern of RGC loss in inherited optic neuropathies owing to the lack of access to diseased human tissues. Nevertheless, useful insight has been obtained with the application of high-resolution OCT imaging and psychophysical evaluation of patients at different stages of the disease process. Although bilateral simultaneous onset can occur in some patients, sequential involvement of the second eye within a few months is more typical.
LHON is characterised by severe visual loss with dyschromatopsia and a dense central or cecocentral scotoma on visual field testing. Childhood-onset LHON and the m. In LHON, RGCs with the smallest calibre axons, which have smaller mitochondrial reserve per energy requirement, are preferentially affected and these are predominantly located within the papillomacular bundle , Measurement of ganglion cell and inner plexiform layer GC-IPL thickness in the macular area indicate that pathological thinning is already evident in the pre-symptomatic stage about 6 weeks before the onset of visual loss in the fellow eye These findings suggest that midget RGCs, which are a major component of the papillomacular bundle, could be more vulnerable to the underlying mtDNA mutation.
Selective attenuation of four of the six layers in the LGN that are connected to the parvocellular pathway have been reported, but this feature is controversial as the magnocellular pathway is known to be also affected in LHON , Further investigations are needed to determine the primary defect.
The ipRGC subtype is relatively preserved in LHON, explaining why the pupillary light reflex is maintained even in severely affected patients The mechanisms that account for this enhanced resilience of ipRGCs remain unclear, although several hypotheses have been proposed From an anatomical perspective, ipRGCs are predominantly located in the parafoveal area and at the far end of the nasal hemiretina, rather than feeding into the papillomacular bundle In a post mortem study of a patient carrying the m.
It is possible that ipRGCs are protected because of their higher concentration of mitochondrial cytochrome c oxidase and a greater abundance of mitochondria ADOA is the most common inherited optic neuropathy with an estimated prevalence of 1 in 25, in the general population The classical clinical features of ADOA are progressive bilateral visual loss starting in early childhood, dyschromatopsia, a central or cecocentral scotoma, and optic disc pallor that is more prominent temporally due to the preferential involvement of the papillomacular bundle The disease process is thought to start in utero with OPA1 carriers having a reduced number of RGCs at birth compared with normal healthy individuals In ADOA, midget RGCs, parasol RGCs and small bistratified RGCs are all affected, impairing sensitivity to high spatial frequencies, long- and middle-wave colour discrimination, sensitivity to high temporal frequencies, and short-wave sensitivity.
The S-cone—related losses showed a significant deterioration with increasing patient age and could therefore prove useful biomarkers of disease progression in ADOA The S-cone chromatic response and koniocellular pathway are impaired in the early stage of the disease, which suggest a vulnerability of small bistratified RGCs Although tritanopia has been reported as the characteristic colour vision defect in ADOA, only 7.
Studies using chromatic pupillometry also reported preservation of ipRGCs in ADOA patients with severe visual loss and optic atrophy , There is a long list of aetiological factors that can result in RGC injury and optic nerve degeneration. Compared with inherited optic neuropathies, fewer studies have focused specifically on RGC pathophysiology in acquired optic neuropathies.
More work is, therefore, needed to elucidate subtype selectivity, if any, of RGC loss in ischemic, compressive, inflammatory, autoimmune and paraneoplastic optic neuropathies. However, we do know that most toxic optic neuropathies have an underlying mitochondrial aetiology There is a growing body of evidence that mitochondrial dysfunction plays a prominent pathophysiological role in glaucoma, demyelinating optic neuritis and toxic optic neuropathies , This aetiological link is relevant and comparing the pattern of RGC loss between these acquired optic neuropathies and classical monogenic optic neuropathies could reveal common pathways amenable to therapeutic intervention.
Extrafoveal RGCs usually deteriorate in the early stages resulting in arcuate scotomas in the visual field. Traditional anatomical studies reported greater loss of axons of large diameter, corresponding to the magnocellular pathway parasol cells , and the magnocellular LGN layers were more affected compared with the parvocellular LGN layers However, there are rarer types of retinal ganglion cells with large axons and further investigations are needed to evaluate the changes of these cells in glaucoma.
The relative vulnerability of large axons in glaucoma may simply reflect the anatomical location of the affected ganglion cells. Glaucoma patients have poor response to high temporal frequency light stimuli that correspond to the magnocellular pathway. This specific vulnerability was ascribed to calcium-permeable receptors, the relative proximity of RGCs and their dendrites to blood supply in the IPL layer, and the differing metabolic requirements of these particular large cell types However, other studies suggested no predilection for a specific pathway , Compared with inherited optic neuropathies, the ipRGCs are vulnerable in both patients with confirmed glaucoma and glaucoma suspects , In contrast, ocular hypertension does not seem to result in significant loss of ipRGCs Inflammatory demyelination resulting in optic neuritis is a major manifestation of multiple sclerosis.
Inflammation of the retinal vascular endothelium can precede demyelination and perivascular cuffing and oedema of the optic nerve sheath leads to breakdown of myelin Idiopathic demyelinating optic neuritis leads to visual loss with minimal axonal loss.
Optic neuritis is associated with alteration of both the parvocellular and magnocellular pathways Viret et al.
Despite the recovery of visual acuity, the magnocellular pathway did not fully normalise Fallowfield and Krauskopf suggested that chromatic discrimination is more severely impaired than luminance discrimination in the demyelinating diseases
0コメント