A small vertical diameter is equal to or less than 1. The thickness of the central retinal vein CRV can also be used to help estimate optic disc size. Optic disc surface area is between 2. Optic disc size varies from small to large according to racial heritage as follows: Caucasians, Hispanics, Asians, African-Americans. This may cause a large optic disc to seem glaucomatous. Contrarily, a small optic disc might hide the NRR thinning.
Thus, a large cup in a large optic disc can be normal, while a small optic cup in small optic disc suggests glaucomatous optic neuropathy GON. Non-glaucomatous cupping can occur in AION, optic nerve compression optic nerve tumour or hemorrhage , contusion, inflammation, and trauma of optic nerve, and hereditary optic neuropathies. In contrast to GON, in which the optic cup deepens and enlarges as associated with the glaucoma type and the level of intraocular pressure IOP , the optic cup does not show significant enlargement in eyes with non-GON.
It has been recommended that patients who have one or more of the Greenfield criteria need to have neuroimaging performed to identify the causes of non-glaucomatous cupping. However the absence of these criteria does not rule out the presence of nonglaucomatous cupping. Optic cup and cup-to-disc ratio The optic cup is a central pale excavation in the ONH.
The optic cup includes glial tissue but not retinal nerve fibre or RGC axons.
The pale color of the cup is due to the exposure of the collagenous LC and the loss of glial tissue. The area of pallor in the center of the disc frequently corresponds well with the area of the cup. The evaluation of CDR in normal subjects, subjects with physiological cup, and glaucoma patients is difficult. Additionally, the application of the evaluation rules of CDR is not appropriate for subjects with an optic nerve or disc anomaly tilted, hypoplasic, dysplasic nerves.
The size of the optic cup always seems smaller in monoscopic examination than in stereoscopic examination. The direction or point of deviation of small blood vessels on the surface of the ONH is used to determine the size of the optic cup contour method but not the area of pallor in the center of the optic disc colorcontrast method.
Physiologically large cups can occur more commonly in patients with normally large chorioscleral canals and large discs.
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Optic cup size has high inter-individual variability. The area of the optic disc and cup are correlated in normal eyes. This means that large discs have larger cups and smaller discs have smaller cups.
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A large cup in a large optic disc is most likely normal, while an average CDR in a small disc may be a sign of glaucomatous optic cupping. Optic cup depth depends on the cup area in normal eyes.
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This means that a larger cup has a deeper cup. In normal eyes, CDRs are larger horizontally than vertically. The average CDR is 0. However, CDR ratio in normal patients can range from 0.
Normal CDRs range from less than 0. A CDR less than 0. A CDR between 0. A CDR equal or over 0. More than CDR asymmetry, a difference of 0. Vertical elongation of the optic cup is due to the loss of RGC axons and the thinning of NRR in inferior and superior poles. Saucerization is a shallow diffuse excavation in the cup and it is very hard to detect in some cases because of the need for careful detection of the contour.
Estimation of CDR has only limited value in the identification of GON because of the wide variation in the size of the optic disc and cup in the normal population. In addition, there is the significant individual measurement variability among glaucoma specialists in the evaluation of CDR.
Neuroretinal rim NRR is the tissue in between the outer boundary of the cup and the optic disc margin. Normally, circumlinear blood vessels rest on the NRR. Therefore, in most cases, the boundaries of the optic cup are best identified by following the track and kink of the blood vessels within the ONH. PPA can cause the wrong determination of the boundary of the optic disc and the misinterpretation as NRR. The ISNT rule gives to the optic cup a round or horizontally oval shape in healthy subjects.
NRR color in normal healthy subjects should be pink or orange due to there being about 1—1. The width of NRR is the distance between the border of the optic disc and the position of blood vessel bending. In glaucoma, NRR always seems pink except in terminal glaucoma. This means that NRR is pallor, it is not glaucoma.
Pallor of the NRR increases the likelihood that a non-GON is present, especially when pallor is greater than cup size. Increasing pallor of the optic disc and especially of the NRR is a typical sign of optic nerve damage.
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A mis-match between pallor and NRR margin can be a sign of early glaucoma. Additionally, it has been demonstrated that NRR notching corresponds well with VF loss in most cases, although not all. NRR notching usually does not occur in the temporal or nasal region. Temporal NRR notching without other glaucomatous optic disc changes is most likely associated with an optic disc insertion anomaly. In early glaucoma, NRR thinning predominantly occurs in the inferotemporal and supero-temporal disc regions while in moderate glaucoma, it is observed in the temporal horizontal disc region. In advanced glaucoma, the remnants of NRR are present mainly in the nasal optic disc region.
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