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Gonioscopy and optic nerve head evaluation
BY AHMED FAIK
Gonioscopy
Gonioscopy is a method of evaluating the AC angle, and can be used therapeutically for procedures such as laser trabeculoplasty and goniotomy.
Other methods of evaluation are
As OCT U BM
The optical principle
The angle of the AC cannot be visualized directly through the intact cornea because light from angle structures undergoes ‘total internal reflection’ at the anterior surface of the precorneal tear film. When light travels from a medium of higher to one of lower refractive index (such as cornea to air) it will be reflected at the interface between the two unless the angle of incidence is less than a certain ‘critical angle’ dependent on their refractive index difference (46° for the tear film–air interface). Because the refractive index of a goniolens is similar to that of the cornea, it eliminates total internal reflection by replacing the tear film–air interface with a tear film–goniolens interface ). Light rays can then be viewed as they exit the contact lens, directly or indirectly
Types of gonioscopies
Indirect non indentation gonioscopy
o A viscous coupling substance is needed to fill the gap
o Excessive pressure may close the angle and cause folds in cornea which impair the view
Indirect indentation gonioscopyo No need for coupling substanceo It can easily open angle giving
misleading pictureo Not suitable for laser procedures
Direct gonioscopyo exiting light rays strike the
contact lens/air interface at a steeper than critical angle so that they will pass through to the observer
o They do not require a slit lamp
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Identification of the angle structures
ACCURATE IDENTIFICATION OF ANGLE STRUCTURES IS NOT ALWAYS STRAIGHTFORWARD, EVEN FOR HIGHLY EXPERIENCED GONIOSCOPISTS.
The Schwalbe lineThis is the most anterior structure, appearing whitish to variably pigmented. Anatomically it demarcates the peripheral termination of Descemet membrane and the anterior limit of the trabeculum. It may be barely discernible in younger patients. In contrast, there may be pigment deposits on or anterior to the Schwalbeline – a Sampaolesi line – especially in heavily pigmented angles (e.g. pseudoexfoliation syndrome). It may have a double-line configuration, when the posterior component may be mistaken for the pigmented meshwork
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The corneal wedge
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useful in locating an inconspicuous Schwalbe line. Using a narrow slit beam, two distinct linear corneal reflections can be identified, one on the inner and one on the outer corneal surface; the outer reflection will arc round across the corneoscleral interface – due to the sclera being opaque – to meet the inner reflection at the apex of the corneal wedge that coincides with the Schwalbe line
The trabeculum
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extends from the Schwalbe line to the scleral spur, with an average width of 600 µm. In younger people it has a ground-glass translucent appearance. The anterior non-functional part lies adjacent to the Schwalbe line and has a whitish colour. The posterior, pigmented functional part lies adjacent to the scleral spur and has a greyish-blue translucent appearance in the young. Trabecular pigmentation is rare prior to puberty, but in older eyes involves the posterior trabeculum to a variable extent, most marked inferiorly. Patchy trabecular pigmentation in a suspiciously narrow angle raises the possibility of intermittent iris contact.
The schlemm canal
may be identified in the angle, especially if non-pigmented, as a slightly darker line deep to the posterior trabeculum. Blood can sometimes be seen in the canal (Fig. 10.13), either physiologically (sometimes due to excessive pressure on the episcleral veins with a goniolens), or in the presence of low intraocular or raised episcleral venous pressure.
The scleral spur Click icon to add picture
is the most anterior projection of the sclera and the site of attachment of the longitudinal muscle of the ciliary body. Gonioscopically it is situated immediately posterior to the trabeculum and appears as a narrow whitish band that yellows with age
The ciliary body
Click icon to add picturestands out just behind the scleral spur as a pink, dull brown or slate grey band. Its width depends on the position of iris insertion and it tends to be narrower in hypermetropic eyes and wider in myopic eyes. The angle recess represents the posterior dipping of the iris as it inserts into the ciliary body. It may not be visible in some eyes dueto a physiological anterior iris insertion, though fixed pathological angle narrowing due to peripheral anterior synechiae (PAS) – adhesions between the iris and angle structures – should be excluded.
Iris processes small, usually tenuous extensions of the anterior surface of the iris that insert at the level of the scleral spur and cover the ciliary body to a varying extent (see Fig. 10.13). They are present in about one-third of normal eyes and are most prominent during childhood and in brown eyes. The processes should not be confused with PAS, which typically extend more anteriorly and are more substantial.
Radial vessels at the base of the angle recess are often seen in normal eyes. Pathological blood vessels run randomly in various directions. As a general principle, any blood vessel that crosses the scleral spur onto the trabecular meshwork is abnormal. Larger circumferential vessels may also be seen
The grading of angle width
many angles are narrowest superiorly, though this difference may be reduced by decreasing the ambient illumination.
Shaffer System
The Shaffer system records the angle in degrees between two imaginary lines tangential to the inner surface of the trabeculum and the anterior surface of the iris about one-third of the distance from its periphery. The system assigns a numerical grade to each quadrant of the angle.
Grade 4 (35–45°) is the widest angle, characteristic of myopia and pseudophakia; the ciliary body can be visualized without tilting the lens.
Grade 3 (25–35°) is an open angle in which the scleral spur is visible.
Grade 2 (20°) is an angle in which the trabeculum but not the scleral spur can be seen.
Grade 1 (10°) is a very narrow angle in which only the Schwalbe line and perhaps the top of the trabeculum can be identified.Slit angle is one in which there is no obvious iridocorneal contact but no angle structures can be identified.Grade 0 (0°) is closed due to iridocorneal contact. • Indentation will distinguish appositional from synechial angle closure.
Van Herick method
uses the slit lamp alone to estimate the AC angle width
A thin but bright slit beam is set approximately perpendicularly to the corneal surface (offset from the optics by about 60°) to the patient’s temporal side for each eye.
○ The beam is used to estimate the ratio of the corneal thickness to the most peripheral part of the AC.
It is useful as a screening tool, but overestimates angle width in a proportion of patients, particularly those with a plateau iris conformation.
Changes in optic Nerve head in glaucoma
Normal optic disc head
The neuroretinal rimThe neuroretinal rim (NRR) is the orange-pink tissue between the outer edge of the cup and the optic disc margin. The inferior rim is the broadest followed by the superior, nasal and temporal (the ‘ISNT’ rule – Fig. 10.15); this has high sensitivity for glaucoma but is not very specific, i.e. eyes without glaucoma often do not respect the rule.
C/D ratio
The C/D ratio indicates the diameter of the cup expressed as a fraction of the diameter of the disc; the vertical rather than the horizontal ratio is generally taken. Small diameter optic discs have small cups (Fig. 10.16A) and vice versa (Fig. 10.16B); only 2% of the population have a C/D ratio greater than 0.7. In any individual, asymmetry of 0.2 or more between the eyes should also be regarded with suspicion, though it is critical to exclude a corresponding difference in overall disc diameter
Optic disc size
normal median vertical diameter (for non-glaucomatous discs) is 1.5–1.7 mm in a white population.
Large discs are believed to be more likely to sustain damage, particularly in NTG. This may be the result of the larger diameter conferring relative mechanical weakness and hence greater vulnerability to IOP-induced displacement of the lamina cribrosa; the lamina cribrosa has been found to be thinner in eyes with NTG
“
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Glaucomatous damage results in characteristic signs involving (a) the optic nerve head, (b) the peripapillary area and (c) the retinal nerve fibre layer.
Optic disc changes
Pathological cupping is caused by an irreversible decrease in the number of nerve fibres, glial cells and blood vessels.
If an eye with a small optic disc and correspondingly small cup develops glaucoma, the cup will increase in size, but even in the presence of substantial damage may still be smaller than that of a large physiological cup.
Subtypes of glucomatic damage
Focal ischaemic discs (Fig. 10.17A) are characterized by localized superior and/or inferior notching and may be associated with localized field defects with early threat to fixation.
Sclerotic discsare characterized by a shallow, saucerized cup and a gently sloping NRR, variable peripapillary atrophy and peripheral visual field loss. The peripapillary choroid is thinner than in other disc types. Patients are older, of either gender, and there is an association with systemic vascular disease.
Myopic disc with glaucoma (Fig. 10.17B) refers to a tilted (obliquely inserted), shallow disc with a temporal crescent of parapapillary atrophy, together with features of glaucomatous damage. Dense superior or inferior scotomas threatening fixation are common. This morphology is most common in younger male patients.
Concentrically enlarging discs (verified by serial monitoring) are characterized by fairly uniform NRR thinning (Fig. 10.17D) and are frequently associated with diffuse visual field loss. IOP is often significantly elevated at presentation.
Non-specific signs of glaucomatous damage
Disc hemorrhages (Figs 10.18A and B, and see Fig. 10.17A) often extend from the NRR onto the retina, most commonly inferotemporally. Their presence is a risk factor for the development and progression of glaucoma. are more common in NTG, but can also occur in healthy individuals as well as patients with systemic vascular disease
Baring of circumlinear blood vessels is a sign of early thinning of the NRR. It is characterized by a space between the neuroretinal rim and a superficial blood vessel
Bayoneting is characterized by double angulation of a blood vessel. With NRR loss, a vessel entering the disk from the retina may angle sharply backwards into the disk and then turn towards its original direction to run across the lamina cribrosa
Collaterals between two veins at the disc (Fig. 10.18E), similar to those following central retinal vein occlusion (CRVO), are relatively uncommon. They are probably caused by chronic low-grade circulatory obstruction. Retinal vascular tortuosity may also occur
Loss of nasal NRR (Fig. 10.18F) is a sign of moderately advanced damage; a space may develop between the NRR and the central retinal vasculature.
laminar dot sign occurs in advancing glaucoma. Grey dot-like fenestrations in the lamina cribrosa (see Fig. 10.18F) become exposed as the NRR recede
The peripapillary changes
Peripapillary atrophy (PPA) surrounding the optic nerve head may be of significance in glaucoma (Fig. 10.19), and may be a sign of early damage in patients with ocular hypertension.
• Alpha (outer) zone is characterized by superficial retinal pigment epithelial changes. It tends to be larger and possibly more common in glaucomatous eyes.
• Beta (inner) zone is characterized by chorioretinal atrophy; it is distinct from the scleral rim, the white band of exposed sclera central to the beta zone. The beta zone is larger and more common in glaucoma, and is a risk factor for progression; the location of beta-zone PPA seems to indicate the orientation of likely visual field loss.
RNFL changes
In glaucoma subtle retinal nerve fibre layer (RNFL) defects precede the development of detectable optic disc and visual field changes; their onset often follows disc haemorrhages.
Two patterns occur: (a) localized wedge-shaped defects and (b) diffuse defects that are larger and have indistinct borders.
OCT and scanning laser polarimetry are highly effective means of quantifying the RNFL.
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Its also not suitable for defining structures that are difficult to assess such as double Schwalbe line from pigmented meshwork
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