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«Glaucoma and ocular hypertension - assessment of the disc and RNFL Dr Simon Barnard PhD FCOptom FAAO DipCLP DipClinOptom DipTh(IP) Contents Glaucoma ...»

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31st March 2013

© Dr Simon Barnard 2013 Synsam

Glaucoma and ocular hypertension -

assessment of the disc and RNFL

Dr Simon Barnard

PhD FCOptom FAAO DipCLP DipClinOptom DipTh(IP)

Contents

Glaucoma and ocular hypertension -assessment of the disc and RNFL

What’s important for Norwegian optometrists

What are the glaucomas?

Epidemiology

Diagnosis of glaucoma and OHT

Anterior segment assessment

Optic nerve head and retinal nerve fibre layer assessment

Quantitative Evaluations of the Optic Nerve Head

Summary points

Example referral letters

Acknowledgments

References

31st March 2013 © Dr Simon Barnard 2013 Synsam What’s important for Norwegian optometrists

• Primary Open Angle Glaucoma

• Primary Closed Angled Glauoma

• Pseudoexfoliation

• Ocular hypertension (OHT) This lecture w will concentrate on these conditions.

What are the glaucomas?

Glaucoma is the name applied to a group of potentially blinding diseases that share the common feature of a characteristic optic neuropathy and corresponding loss of visual function including progressive visual field loss (Salmon & Kanski, 2004) There are a number of disease processes that lead to the many, varied sub-types of glaucoma, which explains the variety of clinical features that exist between the subgroups such as differences in symptoms and signs.

Epidemiology A leading cause of blindness throughout the world. Prevalence of glaucoma throughout the world was 66.8 million cases in the year 2000. About 6.7 million bilaterally blind as a result of the disease. Chronic open angle glaucoma is the 3rd leading cause of blindness in the developed world Has recently found to be the second most common singledisease cause of UK blindness and partial sight registrations, accounting for 8% of all certifications.

Diagnosis of glaucoma and OHT There is no one test.

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Although assessment of the optic nerve is important, it is imperative to understand that glaucoma and glaucoma risk can only be properly evaluated by carrying out a number of procedures Anterior segment assessment Corneal endothelium Examine for – deposited pigment (PDS) – pseudoexfoliative (PXF) material – and keratic precipitates (KPs) Aqueous convention currents, driven by the temperature differential between the iris and cornea, typically cause pigment granules to be deposited in a spindle pattern (Krukenberg’s spindle), orientated with its long-axis vertical.

KP are accumulations of inflammatory cells that settle on the corneal endothelium during episodes of uveitis. Ocular inflammation is generally associated with a decrease in IOP, because ciliary body involvement reduces aqueous humour production.

However, it can elevate pressure, directly by involving the trabecular-meshwork (trabeculitis), obstructing trabecular outflow with leaked viscous protein and leukocytes, and encouraging the formation of peripheral anterior synechiae (PAS); and indirectly, by its management with corticosteroids.

Glaucoma is more commonly associated with uveitis when the inflammation is recurrent or chronic, and in the special case of Fuch’s heterochromic iridocyclitis Crystalline lens Pigment granules, liberated by pigment dispersion syndrome (PDS) or pseudoexfoliation syndrome (PXF), or other causes may settle on any structure in the anterior chamber, including anterior lens capsule. In PXF syndrome, epithelium of lens, pigmented epithelium of iris, non-pigmented epithelium of ciliary body, and possibly zonules of lens, produce PXF material.

PSX is important for Norwegian optometrists as there is evidence that 50% of open angle glaucoma in Scandanavian countries is caused by PSX.

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Deposited PXF material on anterior lens surface classically has bull’s eye configuration, because the mid-peripheral region is rubbed clear by the movement of the touching inner border of the iris.

Bright slit-lamp → pupil constriction = visibility of scalloped edge of these zones.

Therefore, often requires mydriasis.

PXF syndrome is an ocular manifestation of a systemic disorder. In affected individuals, PXF material can be detected in visceral organs and the skin, but it is only in the eye that it is known to be of consequence.

Optic nerve head and retinal nerve fibre layer assessment Introduction Open angle glaucoma is defined as a slowly progressive optic atrophy, characterized by midperipheral visual field loss and excavated appearance of the optic disc.” (HA Quigley 1996).

Clinical ability to identify individuals with optic nerve head (ONH) and retinal features consistent with glaucomatous optic neuropathy (GON) by ophthalmoscopic examination remains a fundamental skill and cornerstone of clinical practice Qualitative Evaluation of the Fundus and Optic Nerve Head in Glaucoma Retinal Nerve Fibre Layer (RNFL) RNFL best viewed with bright, red-free light (green filter), although may also be reasonably visualised with bright achromatic illumination. Best viewed through a dilated pupil, easier to see in eyes with heavily pigmented retinal pigment epithelium (RPE) and choroid and may be indistinguishable in eyes with blonde fundi.

The RNFL appearance consists of bright ‘silver‘ striations emanating from the ONH towards the retinal periphery Rather than being actual retinal nerve fibres, the striations consist of groups of nerve-fibres bundles (grouped ganglion cell axons) held within supportive Müller cell processes. Physiologically, RNFL is most easily seen within 2 disc diameters (DD) of the ONH and is brightest infero- and supero-temporally, where the fibre bundles are most dense, being least visible superior, inferior, temporal and nasal meridians Visibility decreases with both age and presence of media opacities

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shown to precede development of glaucomatous ONH signs and achromatic visual field defects In primary open angle glaucoma (POAG), the loss of ganglion cell axons follows a characteristic pattern. Initially, there is loss of fibres, most frequently at the inferior and superior poles of the ONH. This results in the development of wedge-like defects in the RNFL. Localized RNFL defects consist of slit, or wedge shaped defects (not spindle-shaped) wider than retinal vessels that are darker than the adjacent areas, within 2DD of the disc margin The visibility of retinal arterioles provides a clue to the depth of an RNFL defect In healthy eyes, the vessel walls appear blurred. In contrast, in eyes with RNFL defects the vessel wall appears to stand out in relief due to lack of overlying nerve fibres.

The amount of fundus pigmentation affects the visibility of the RNFL. A dark pigment epithelium enhances nerve fibre layer visibility.The visibility of the RNFL is less good in lightly pigmented eyes, eyes of older subjects and in those with a tessellated fundus.

Loss of striations results in a matt appearance and retinal vasculature within areas of RNFL loss appear more sharply demarcated. Defects smaller than retinal vessels within 2DD of the disc margin, or broader than vessels but 2DD from the disc margin are usually physiological Diffuse thinning of the RNFL, unless segmentally affecting a single horizontal hemiretina is difficult to identify.

Because RNFL is not always visible and the difficulties associated with determining the presence of diffuse loss, RNFL defects have been shown to occur in only around 20% of eyes with GON and so have limited sensitivity.

It is also very important to remember that they are not pathognomonic of GON and are markers of optic nerve atrophy of any cause Parapapillary Atrophy (PPA) PPA consists of irregularities of the retinal tissues surrounding some or all of the disc margin. PPA represents disruptions of the retina, with the underlying choroid remaining intact. It differs histologically from myopic scleral crescents and scleral crescents found in tilted discs in which both retina and choroid are affected. It has been divided into two distinct sub-types;

It has been divided into two distinct sub-types:

31st March 2013 © Dr Simon Barnard 2013 Synsam Βeta zone (β PPA). This is a pale zone bordering the peripapillary scleral ring and is characterized by visible sclera and large choroidal blood vessels Alpha zone (α PPA). This is characterized by variable irregular pigmentation of the RPE. On its outer edge it is adjacent to normal retinal tissue, with the inner edge either next to beta PPA, or the peripapillary scleral ring if no β PPA is present Degrees of α PPA are present in almost all normal eyes, with β PPA being found in about 15-20% or normal individuals. When present in normal eyes, both zones tend to occur in the temporal hozontal, supero- and infero-temporal regions of the disc margin.

PPA is a valuable sign in identification of glaucomatous structural damage.

GON has been shown to be associated with larger size of both PPA zones, and frequency of β PPA occurrence. PPA location has also been shown to be spatially correlated to angular region of greatest glaucomatous disc damage. Change in PPA does not appear to occur in non-glaucomatous optic neuropathies.

Neuroretinal Rim When studying ONH features, the first step in any decision-making process should be identification of the relevant landmark structures of the disc margin, (peripapillary ring) and the edge of the optic cup.

NRR configuration The normal NRR has a characteristic configuration whereby it is usually widest inferiorly, followed by superiorly, nasally and thinnest temporally (80% of normal individuals). This has been termed the ISNT rule.

This configuration is produced by a number of predisposing anatomical factors including thicker disc arteriole vessels inferiorly, position of the fovea below above the disc centre and least lamina tissue (and greater numbers of nerve fibres) at the disc poles.

The ISNT rule is valuable in detection of glaucomatous changes in the NRR, however it important to remember that: not all normal discs obey the ISNT rule.

Sometimes the differences in NRR width between vertical and horizontal meridians is so slight as to be hard to be sure about whether the ISNT rule is obeyed and in some eyes with glaucoma diffuse NRR loss may be not disturb the ISNT configuration.

–  –  –

The healthy NRR has an orange or pink colour provided by its internal capillary plexus. The NRR typically takes on a white or even greyish hue with GON which may occur throughout the rim in localised areas. In GON, areas of affected NRR are often associated with pallor within the base of the adjacent cup Glaucomatous rim pallor is hard to identify in early disease, and so is often one of the later signs to be identified. As with RNFL defects, it should be remembered that pallor is associated with all causes of optic neuropathy and is not pathognomonic of GON.

If the NRR is initially pale, and other features of glaucoma are not present or mild, care should be taken to exclude other optic neuropathies as the cause

–  –  –

The optic cup is the 3-dimensional depression in the ONH centre devoid of neural tissue. Cups are present in most ONHs, although size is dependent upon disc size, whereby larger discs are physiologically associated with larger cups.

Physiological cups are usually horizontally oval with depth dependent upon size, larger cups being deeper. Normal cups can appear can be ‘dimple’ shaped punched out or have a sloping temporal wall When looking at the cup, care should be taken to distinguish the true cup edge, as identified by a contour change on the disc surface (‘contour cupping’) from the deepest part of the cup.

True cup edge is best evaluated from looking at small blood vessels as they bend away from the plane of the retina into the cup. It is easy to mistake the pale cup centre closest to the pale underlying lamina cribrosa for the entire cup.

Use of this pale area, which represents the deepest part of the cup (‘pallor cupping’), will result in underestimation of cup size – a problem with direct ophthalmoscopy.

As NRR is progressively lost in GON, cup shapes will alter regardless of their initial appearance. Erosion of NRR is such that the cup shape will become increasing distorted, eventually undercutting the disc margin with most pronounced changes usually occurring at the superior and inferior poles of the disc.

–  –  –

Lamina Cribrosa The lamina cribrosa underlies the NRR. It is a connective tissue structure composed of a series of aligned sieve-like plates through which nerve fibre bundles pass on exiting the eye. Although the lamina may not be seen directly, it is often faintly visible through neural tissue at the base of deep optic cups. Visualisation of the lamina (‘lamina sign’) is therefore useful in discrimination of deep from shallower cups Physiologically, the lamina is usually relatively flat and normal lamina pores tend to be round In GON, the lamina has been shown to be progressively pushed backwards away from the scleral plane and the array of plates compacted, disrupting their alignment.

These changes tend to be most pronounced where least lamina connective tissue is present at the disc poles, and occur least along the better supported horizontal plane. These glaucomatous pathological changes results in elongation of the lamina pores, making them oval or slit-like in appearance when viewed ophthalmoscopically Haemorrhages Splinter haemorrhages at or directly adjacent to the ONH the disc occur rarely in normal eyes and in between 4-7% of eyes with chronic open angle glaucoma.

They are occur more frequently in cases of normal pressure type primary open angle glaucoma (POAG).

Observation of such haemorrhages in both normal and high pressure types of POAG suggests that they are caused by a common pathological mechanism.

Although they are therefore a highly specific sign (≈99%) and their presence implies that the eye is unlikely to be normal, their low prevalence in glaucoma means they are an insensitive sign for glaucoma detection, although studies are clearly limited by the minimum clinically observable haemorrhage size and review frequency.



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