Symptoms of glaucomatous optic neuropathy

The optic nerve contains more than 1 million axons of retinal ganglion cells, the bodies of which are located in the superficial layers of the retina. Despite some variability in the size and shape of the optic nerve disc, most often the disc is a vertically oriented oval. In the center of the disc there is an area of the excavation, which usually has the shape of a horizontally located oval. The central part of the disc is usually paler, since there are no axons there, the lamina cribrosa (cribriform plate), located deeper, shines through. The tissue between the physiological excavation and the edges of the disc is the neuroretinal belt (NRP), on which the location of the bulk of the retinal ganglion cell axons is projected. This tissue is usually orange-red in color due to the abundance of capillaries in it; in diseases it acquires a pale color.

Determining the size of the optic disc is essential in assessing glaucomatous optic neuropathy. Its size correlates with the size of the physiological cup and the neuroretinal rim: the larger the disc, the larger the cup and the ring. A large cup in a large disc may be normal, while a small cup in a smaller disc may indicate pathology. In addition, the depth of the cup is related to its area and indirectly to the size of the normal cup.

The area of the neuroretinal zonule is positively correlated with the area of the optic disc: larger discs have larger neuroretinal zonules and vice versa. The determination of the zonule width generally follows the ISNT rule: the widest part is the inferior part of the ring (inferior), then the superior part (superior), the nasal part (nasalis), and the narrowest part is the temporal part (temporalis). A predominant reduction in the width of the neuroretinal zonule, especially in the inferior and superior parts of the disc, occurs in the early or intermediate stages of glaucoma. Non-glaucomatous damage to the optic disc is rarely associated with loss of the neuroretinal zonule.

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Evaluation of the glaucomatous optic disc

Loss of the neuroretinal zonule

Degeneration of retinal ganglion cell axons in glaucoma leads to enlarged cup and loss of neuroretinal zonule tissue. Its mean area is typically reduced in glaucomatous discs compared with normal discs, a better indicator than the cup/disc ratio in differentiating early glaucoma from the normal variant. Neuroretinal zonule loss may be focal or concentric.

Focal loss of the neuroretinal rim often begins with a small, localized defect in the contour of the inner edge of the excision, resulting in narrowing of the neuroretinal rim.

This condition is called focal notch or pitting changes.

This defect can increase and lead to the development of an excavation breakthrough. When the optic disc narrows to the edge and there is no neuroretinal rim tissue, a marginal excavation appears. The vessels crossing the thinned ring bend sharply. This phenomenon is called forced bending, it is important when assessing the width of the rim.

Concentric glaucomatous atrophy with an increase in the excavation in the form of concentric circles is sometimes more difficult to distinguish from physiological excavation. In this situation, it is important to remember the ISNT rule and that the excavation normally has the shape of a horizontally oriented oval, not a vertically oriented oval.

Lattice dots sign

On the surface of the optic nerve papilla, the axons are strongly curved, leaving the eye through fenestrated sheets of connective tissue or the cribriform plate.

Deepening of the optic disc cup in glaucoma may result in the openings of the cribriform plate showing through, a sign of cribriform dots. It is unclear whether the deepening of the cup itself has any clinical significance.

Disc hemorrhages

Splintered or flame-shaped hemorrhages at the optic disc margin are called Drance hemorrhages. These hemorrhages are considered an unfavorable prognostic sign for the development of glaucomatous optic neuropathy. Drance hemorrhages are often observed in low-pressure glaucoma. They are associated with defects in the nerve fiber layer, notches in the pseudoretinal rim, and ring scotomas of the visual field.

Nerve fiber layer defects

Normally, retinal nerve fiber layer striations are visible ophthalmoscopy as light reflections from nerve fiber bundles. Loss of retinal ganglion cell axons in glaucoma leads to loss of neuroretinal zonule tissue and visible nerve fiber layer (VNFL) defects. Retinal nerve fiber layer depletion is visible ophthalmoscopy as dark wedge-shaped defects directed toward or affecting the optic disc margin. Nerve fiber layer defects are best seen in green light or without red light. Their detection is used for early diagnosis of glaucomatous damage. However, it is not pathognomonic of glaucomatous damage, since the defects also occur in eyes with optic neuropathy of other origins.

Parapapillary chorioretinal atrophy

Parapapillary atrophy, especially in the beta zone, is more often of a larger size in eyes with glaucomatous damage. It correlates with the loss of the neuroretinal zonule. The sector of greatest loss has the largest area of atrophy. Because parapapillary atrophy is less common in eyes with non-glaucomatous optic disc damage, its detection helps to differentiate glaucomatous optic neuropathy from non-glaucomatous optic neuropathy.

Type of vessels

The appearance of vessels on the optic disc can help in assessing glaucomatous damage to the nerve. In addition to forced bending, some researchers consider the trestle phenomenon to be a sign of glaucomatous damage. The trestle sign consists of vessels crossing over the deepened excavation in the form of a bridge. With progressive loss of underlying tissue, the vessels lose support and appear to hang over the empty space of the excavation.

Many other changes are non-specific. Focal narrowing of the retinal arterioles and diffuse narrowing of its vessels, more pronounced in the area of greatest loss of the neuroretinal rim, can be observed in optic neuropathies of various origins.

Non-glaucomatous optic neuropathy

Distinguishing glaucomatous from nonglaucomatous optic neuropathy can be difficult. Paleness out of proportion to the cup or pallor with an intact neuroretinal rim are features of nonglaucomatous optic neuropathy. Examples of nonglaucomatous optic neuropathy include giant cell arteritis and optic nerve compression lesions. Nonglaucomatous optic disc lesions are not always associated with loss of the neuroretinal rim, so its shape is little changed. In contrast, glaucomatous optic neuropathy involves loss of neuroretinal rim tissue with increasing pallor due to enlarged cup size.

Stereophotographs

Color stereophotographs can be used to evaluate changes in the optic nerve over time. Stereophotographs can be obtained by taking two photographs in succession, with the camera either moved manually or with a sliding adapter (Allen separator). Another way to obtain stereophotographs is to take two photographs synchronously with two cameras, using the principle of indirect ophthalmoscopy (Donaldson stereoscopic fundus camera) or a two-prism separator. In general, simultaneous images of the disc are more reproducible.

Other techniques that can image and measure the optic disc for comparison over time include HRT, GDx laser polarimetry, and optical coherence tomography (OCT).

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