Lecture 4: Pathogenesis of Glaucomatous Optic Neuropathy Flashcards
The optic nerve head usu vertically elongated with horizontal diameter of __ mm and about __ mm (2DD) nasal to the fovea
This position projects to a point in visual space that is approx __-__ degrees temporal to and slightly below horizontal meridian passing through the fovea
1.5mm, 3mm
10-15 degrees
___ and __ had significantly smaller optic disc sizes than the other races studied
- Caucasian - american
- Hispanic - american
With both in similar ranges and with opposing directional vectors, differences between them can pinch the edge of the optic nerve, either bow the ___ ___. back of forward depending on which pressure is higher at any give time. Either of these could lead to what?
- lamina cribrosa
- these could lead to axonal compression and/or vascular perfusion presure, leading to ganglion cell apoptosis
Heidelberg retinal tomograph HRT 2 showed that __ had smaller optic discs than all other races,
caucasian-americans
Women tend toward __ optic nerve heads and __ CD ratios
smaller, smaller
Notice that the retinal vessels are rarely “dead-center”. They are usually…?
countouring the nasal edge of the cup
As the axons are lost in glaucomatous atrophy, the vessels move __ to the nasal rim of the disc
Closer
When looking at the optic nerve head, what do you assess?
- size (S,M,L) of the disc
- color
- neuro-retinal rim
- cup
- Estimate ratio of area of the cup to the area of the disc
The normal/Cup/disc ratio varies, and if measured along a dimater, rather than as an area, CD ratios are often __ horizontally than vertically
Larger
C/D ratio should theoretically be the ….?
estimate of the AREA of the nerve head occupied by the cup , rather than the % of horizontal or vertical diameter
An inter-eye asymmetry in the c/d ratio of __ suggest a glaucomatous optic nerve damage unless…?
>0.2, unless the asymmetry in the c/d ratio is accompanied by an inter-eye asymmetry in the disc size
According to the OHTS study, the higher incidence of POAG in african americans was related to baseline __ and __ (CCT is __ in african americans)
C/D and CCT, thinner
In less than __% of normal eyes the horiztonal C/D ratio is smaller than the vertical one
7%, ex. the quotient of horizontal-to-vertical C/D ratios is usu higher than 1.0. It is important for the diagnosis of glc, in which, the early to medium-advanced stages, the vertical C/D diameter ratio increaes faster thant he horizontal one, leading to an increae of the quotient of horizontal to vertical C/D ratios to values lower than 1.0
Describe the ISNT rule
- IN a normal eye, regardless of C/D ratio, the thickness of neuro-retinal rim generally varies in a predictable pattern that is described by the ISNT rule
- Beginning from thickest to thinnest (as seen this photo), the descending order of neuro-retinal rim thickness are
- Inferior
- Superior
- Nasal
- Temporal
Retinal nerve fiber layers will measure what?
- measure these realtive thickness and give you an automated risk assessment of each quadrant after comparing with a normative data base
What does this image show?
- Notice fuzziness and loss of striation detail in the sector between the yellow arrows (compare 7:00 position to *). This indicates loss of NFL and should correlate witha. decreasein NFL thickness in that sector and corresponding loss of VF
Describe the image
- There are 2 distinc, somewhat concentric zones of atrophy adjacent to, and seen most often along temporal margin of the disc. But these may surround the entire optic nerve head
- These 2 zones termed zone beta and zone alpha are visible ophthalmoscopically
- Zone beta (inner of the two -black arrows) is at the peripheral edge of scleral rim of elschnig in which RPE and choriocapillaris are absence but underlying choroidal vasculature (sattlers and hallers) are still present. Areas of zone beta atrophy are more indicative of glc
- Zone alpha (white arrows) located peripheral to, but contiguous with beta (if present). It can have areas within it that are either hyper or hypopigmented adn is associated with thinning, but not absnce of the choriocapillaris
Label the structures
a. Internal limiting membrane of elschnig (thin layer of astrocytes)
b. Internal limiting membrane of the retina (thick layer of foot plats of mueller cells)
c. central meniscus of kuhnt (astrocytic)
d. Intermediary tissue of kuhnt, formed mostly by astrocytes
e. border tissue of jacoby, formed mostly by astrocytes
g. lamina cribrosa
h. meningeal sheaths
* attachment of posterior vitreous face to edge of disc. If this comes off, it’s a weiss ring
tent of martegiani leading to cloquet’s canal
Note no permeabilit barrier between the choroidal stroma and the optic nerve
Describe the lamina cribrosa
- It is important to appreciate the lamina cribrosa is not as it appears, a simple interweaving of scleral collagen and optic nerve axons
- the lamina cribrosa matrix is formed of a core of elastic fibers interwoven with a matrix of collagen types I and III, which is associated with collagen type IV and laminin
- by contrast, the scleral is composed mostly of type I collagen
Describe the blood supply of the optic nerve head
- In the laminar region, with no central blood source, axons rely upon astrocytes to deliver them their meals, just as photoreceptors rely upon mueller cells in the reitna
Describe pial septae
- The fibrovascular pia mater tightly surrounds the outside of the optic nerve and then penetrates the substance of nerve, providing connective conduits that surround bundles of axons and convey the microsvaculature into the core of the nerve
Describe optic nerve head glia
- oligodendroglia: these cells make myelin in the CNS (recall that this is the job of schwann cells in the periperal nervous system). Oligos begin abruptly at the lamina cribrosa
- Astrocytes: the main regulatory and supportive glial cells, also provide tissue support, including formation of the internal limiting membrane over the optic nerve head
- Microglia: the reticuloendothelial cells of the CNS. Roles similar to macrophages, including inflammatory modulation, phagocytosis, antigen recognition and processing
Embryologically, myelination begins from where?
- the brain and travels to the eye, normally stopping abruptly at the lamin cribrosa
Myelination __ the diameter of the optic nerve during its passage through the __ mm long scleral canal
Oligodendrocytes from myelin in the CNS
Describe astrocytes and their subtypes
- Protoplasmic (akak, type 1, with a and b sub-types) found mostly in grey matter, show a more spherical cell body, with tortuous varicose and thorny processes radiating in all directions. (picture a - notice the overall spherical profile). These all exhibit glial fibrially acid protein (GFAP). Type 1A astrocytes express GFAP but not neural cell adhesion molecule (NCAM). type 1B astrocytes, however, express BOTH GFAP and NCAM and are the dominant astrocyte in the optic nerve head
- Fibrous (aka, type 2) found primarily in white matter. These cells usu exhibit an oblong cell body, with relatively long and unramified varicose and thorny processes extended mainly in 2 opposite directions. These cells often give rise to “vascular feet” that make connections to the outside of vascular walls (picture b, notice the overall ellipitcal or axial profile)
What is the difference between resting and reactive astrocytes?
- Astrocytes communicate between neurons, blood vessels, and other types of glial cells. They form a coordinated syncytium by communicating through numerous gap junctions. They form the internal limiting membrane of the optic nerve head and line the border tissues (Kuhnt, jacoby) of the scleral. They represent a significant fraction of the tissue mass of the pre-laminar nerve, but they also continue throughout the length of the nerve
- Rest astrocytes are called “nonreactive,” function to maintain homeostasis, regulate blood flow, recycle neurotransmitters, maintain synapses, and participate in neurogenesis. Astrocytes also help maintain the BBB and interact closely with vessel capillaries, axon bundles, and neuronal somas
- Reactive astrocytes fomr a barrier to isolate an injury from the surrounding CNS tissue by forming a glial scar, which may prolong survival, but offers little benefit for tissue repair. There is evidence that activated astrocytes can become antigen-presenting cells, and generate cytokines such as tumor necrosis factor alpha (TNF a)
Describe NVD vs NVE in diabetic retinopathy
- NVD breaks out into the vitreous earlier than NVE in most cases.
- Part of the reason is that the internal limiting membrane of elschnig, over the optic nerve head, is formed by a thin covering of astrocytes.
- This membrane is much less robust a barrier than the footplates of the mueller cells that form the internal limiting membrane of the retina
__ is the most commonly used immunocytochemical marker of type 1 astrocytes
- Glial fibrillary acidic protein (GFAP)
- In the photo, GFAP is stained red
- In the pre-laminar (non-myelinated) region of the optic nerve head, axons are surrounded by astrocytes. IN this region, astrocytes form glial tubes (the red lines). These glial tubes are larger and less numerous, the closer one gets to the NFL.
what is the relationship between ganglion cells, astrocytes and blood vessels?
- Note the anatomic relationships between ganglion cells (green), thin astrocyte process (red) and blood vessels. The traditional notion is that these processes surround blood vessels, pick up nutreients and then delive them to RGC and their axons. This is especially important in the laminar region where ther eis not a central blood source, only peripheral vessels from the pia
- Astrocytes and RGCs have an unusual relationship. When RGC mitochondria wear out they are not degraded by the RGC, they are handed off to astrocytes to complete mitochondrial degradaiton
Where are astrocytes present?
- Astrocyte is present throughout the optic nerve. THis astrocyte is localized to intraconal (myelinated) portion of the optic nerve
- Notice that its cell body and overall general shape is elliptical (axial), characteristic of fibrous (type 2) type astrocytes commonly found in myelinated nerve tissues (i.e. white matter)
With both in similar ranges and with opposing directional vectors, differences between them can pinch the edge of the optic nerve, either bowing the lamina cribrosa back or forward depending on which pressure is higher at any given time. Either of these could lead to what?
- Either of these could lead to axonal compression and/or vascular perfusion pressure, leading to ganglion cell apoptosis
Describe the mechanical change that occurs with glc
- much as tissue strain from IOP elevation leads to JCT cell activation that leads to extracellular matrix remodeling of the TM
- Mechanical strain created by the changing translaminar pressure gradient at the lamina cribrosa leads to activation of both astrocytes and microglia and probably reduction in axoplasmic flow and optic nerve head perfusion pressure
__ cell activation is a hallmark of CNS injury, characterized by an increase size and number of __ cells and upregulation of __, with additional celluarl changes that may cause or relieve neuronal impariment
- Glial, glial, GFAP (glial fibrillary acidic protein)
- Glial cells in the optic nerve an peripapillary area of a normal (A) and glaucomatous (B) retina stained with GFAP. The astrocytes in the laminar region (LC). of normal and glaucomatous eyes were FGFAP-IR positive. In the prelaminar (PL) and NFL in the glaucomatous eye, the GFAP-IR was much stronger than in normal eye. Note that the Muller cells in the peripapillary area of glaucomatous eye were also stained with (GFAP) (arrowhead)
astrogliosis is characterized by what?
- characterized by alterations in astrocytic morphology, like thickened processes and hypertrophy of the cell body, and in protein expression patterns int he prelaminar region of the ONH. IN the laminar region the astrocytes show round shaped cell bodies and a loss of cell processes
- In, addition to these morphological changes, ONH astrocytes of the prelaminar and laminar region show increase GFAP immunoreactivity compared with normals following chronic elevation of IOP and a moderate or advanced glaucomatous axonal damage (Normal GFAP expression, top photo and increased expression in glc, bottom photo)
Describe microglia activation in glc
- Inactive, quiescent astrocytes and microglia
- Resting microglia cannot mount an immune response
- But microglial activation is one of the first events in glaucomatous neurodegeneration
- When activated, microglia change their phenotype markedyly and become migratory, antigen presenting cells that induce the expression of new proteins, cytokines, receptors, and mitogens
- Activated microglia have altered morphology, may undergo mitosis and become scavenging phagocyting cells with cytotoxic and degradative enzymes
Describe activated microglia in the optic nerve head in glc
- cellular activities of microglia in the lamina cribrosa and in the parapapillary chorioretinal region in glaucomatous optic nerve heads are likely to make these areas dynamic region of change
- As lamina becomes compressed, fewer and fewer microglia are found here, except near remaining blood vessels
- The distribution and activation of microglia in teh parapapillary chorioretinal region of glaucomatous eyes (the intermediary tissue of kuhnt and the border tissue of jacoby)
- In glaucomatous tissue, activated microglia appear to be strategically positioned in relation to blood vessels
- Activated microglia, as clusters of cells in the parapapillary chorioreitnal region, appear to form a discontinuous, linear barrier in the parenchyma near the vessels of the choriocapillaris bordering the neural tissue (Border tissue of jacoby).
- In the medium sized vessles in the compress prelaminar region, activated microglia form concentric rings in the parenchyma around the vasculature.
- As the glaucomatous optic nerve becomes compressed, disorganized, and remodeled, blood vessels may become leaky and comprise the blood retinal barrier. Strategically positioned, activated microglia may serve a neuroprotective function with respect to a damage blood retinal barrier
- This likely leads to inappropropriate entry of T cells that may initiate an autoimmune response that sustains neurodegeneration even if IOP is controlled
Compare microglia in a normal optic nerve vs in a glaucomatous optic nerve
We know that with progression tehre is increased posterior bowing of the lamina cribrosa and widening of the optic nerve within the scleral canal. But how does that happen?
- There are 2 components of acute intraocular pressure (IOP) - induced optic nerve head (ONH) deformation in normal and early-glaucoma eyes
- The lamina dipslaces posteriorly due to the direct action of IOP (B)
- Much of this posterior laminar displacement is counteracted as the lamina is pulled taut by simultaneous scleral expansion
- This tissue strain is known to give rise to release of TFG beta-2 from ECM stores
- TFG-beta is pro-fibrotic and will increase tissue stiffness through its effects on ECM turnover
- TGF-beta is also know to be a potent signaling molecule that induce an astrogliotic activity
- It is important to note that even though the net result of these IOP-related deformations is a small amt of posterior displacement of the lamina, substantial levels of IOP related strain are induced in both the peripapillary sclera and lamina in this scenario and these activate astrocytes and microglia
- Glial activation and the further ECM remodeling that is triggered, increase CT volume in and around the ONH by 50-100% in early glc
Describe the restructuring and remodeling of ONH in early glc
Describe restructuring and remodeling of ONH: Alterations in CT architecture, cellular activity, axoplasmic transport and blood flow in later stages of glaucomatous damage
- Classic descriptions of
- Profound laminar deformation
- Excavation of scleral canal beneath the optic disc margin
- Compression
- Increase rigidity of the lamina
- But its most important to appreciate that the important part is the increase in stiffness and deformation of the matrix, not the fact that its in the shape of a lamina cribrosa
- Increased IOP induces ONH axonal degeneration both in rodents and primates despite the fact that rodents do not have a clear lamina cribrosa
Glutamate is stored mostly in __ __
- subcellular compartments
- In astrocytes its uptake is coupled with its conversion to glutamine. When glutamate is in excess, it can become toxic to retinal neurons by overstimulation of the glutamate receptors. Therefore, efficient transport of glutamate from the extracellular space (by glial cells) is critical for maintenance of retinal function
- It is hypothesized that glutamate transport is compromised under conditions of high IOP
- Accumulation of glutamate leads to reitnal cell death through overstimulation (excitotoxicity). But when glutamate is exposed to molecular oxygen and hydoxyl radicals in extracellular spaces, it also generates hydrogen peroxide
- Since glutamate transport is the only mechanis for removing glutamate from extracellular fluid, it is hypothesized that functional impairment of glutamate transporters (as those in astrocytes) may play a major role in excitotoxicity and contributes to the pathogenesis of glaucoma
- Similar toxicity may injure RGCs if glutamate level rise after elevation of IOP (N.B. years ago some data suggested that glutamate was dramatically increased in the vitreous in glc. When other groups could not verify this finding, the Harvard MD/PhD author of the paper was accused of falsifying his data (incentivized by his control of a patent on an NDMA receptor antagonist for glc) lost his grant and his grant and his Harvard position and his right to bill medicare and Medicaid)
Describe optic nerve head perfusion in glc
- Optic nerve head vascular perfusion pressure is normally autoregulated but this can become deranged int eh face of sustained elevations as of IOP, once innate defensive mechanisms such as eNOS-induced vasodilation have not been sufficient
- Using hi-speed OCT, researches at OHSU were able to visually image diminished vascular perfusion of glaucomatous vs normal optic nerve heads and calculate an optic disc flow index (below) showing sig change between these groups. These findings may support a role for focal ischemica of the optic disc as a causative factor for glc at leas in some patients, either by itself or in conjunction with elevated IOP. Hence the quest for -nod drugs (nitric oxide donors) to generate vasodilators within the optic nerve head
Describe the systemic vascular findings in glc
- Epidemiologic links have been reported between ocular perfusion pressure (OPP) and glaucoma
- Baltimore eye survey
- 6 fold risk of glc with those with lowest OPP
- Collaborative normal tension glaucoma study
- Highly significant association between the rate of progression and presence of migrain in NTG patients
- Other studies have also shown
- Exaggerated nocturnal blood pressure dips in POAG and NTG patients with progressive VF loss
- Thought that such drops compromise ONH perfusion
- Baltimore eye survey
Describe ocular perfusion pressure
- NTG patients were reported to have a clearly increased prevalence of systemic hypotension
- Patients with progressive glc were found to have lwoer systemic BP, particularly at night
- Pt with stable glc did not have this association
- Little doubt that BP is a clear risk factor
- Addition supportive evidence
- Systemic HTN is protective against glc in younger pt
- Presumably though improved OPP
- Systemic HTN is deleterious in older glc patients
- Presumably through atherosclerosis and loss of autoregulation
- Loss of autoregulation can also lead to uncontrolled vasospasm
- Systemic HTN is protective against glc in younger pt
Describe the improvements in ocular blood flow following intraocular pressure reduction
- Measurements of optic disc rim blood flow in POAG and OHTN groups before and after IOP reduction
- Clearly, blood-flow increased as IOP decreased. Remember that ischemica is a pro-inflammatory event once intrinsic survival strategies such as production and release of eNOS-induced vasodilation have not succeded
- Remember, autoregulation maintains a relatively constant blood flow in spite of changes in OPP
- In the absence of autoregulation, there is an inverse relationship between OPP and IOP
- The higher the IOP, the lower the OPP
Describe the blood-retinal barrier compromise in glc
- The prelaminar part of the ONH has only a marginal BRB. ONly the astrocyte laden, permeable border tissue of Jacoby separates the edge of the nerve from the choroid at this location (upper picture)
- At the end of a normal retinal angiogram it is normal to see fluorescein leakage from teh choroid into the edge of the optic nerve head (green arrow in picture uppr right and in angiogram, lower right)
- The direct access of certain molecules diffusing into the nerves influences the barrier function. If, simultaneously, ET-1 reduces endothelial tight junctions and matrix-metalloproteinase (MMP)-9 degrades the basement membrane, not only macromolecules but even RBC may cross the blood brain barrier and lead to what is clinically observed as optic disc hemorrhages (aka Drance hemorrhages)
- Compromise of BRB may also allow immunocompetent cells normally prevented from patrolling in the ON and retina (aka immune privilege) to gain access, risking mis-identification of “self” or molecular mimicry to set off autoimmune rxn because these antigens have never previously encountered immune surveillance and they are presume foreign
How to think about glc
What is the susceptibility quotient?
- Notice that if we could find the right weighted formula to calculate a susceptibility quotient then a high IOP is no longer intrinsically BAD
- It is BAD only if the susceptibility quotient demosntrates that a given ONH cannot wtihtand whatever pressure we have measured
