Corpus: Optic disc

The optic nerve papilla is the place visible in the fundus of the eye where the optic nerve leaves the eye. It is the anatomical correlate of the blind spot and lies 15° nasal to the fovea centralis on the retina. The blind spot (physiological scotoma) is located 15° temporally in the visual field according to the switching system of the visual pathway.

The papilla nervi optici has a diameter of approx. 1.6 mm and is clearly visible during ophthalmoscopy or fundoscopy. The axons of the ganglion cells, which leave the bulb as the myelinated optic nerve (2nd cranial nerve), gather in the area of the papilla. Therefore, there are no photoreceptors at this point and no information can be perceived from this area of the visual field. This results in a "blind spot", a physiological scotoma, which is compensated for in binocular vision by the contralateral eye.

Two vessels, the central retinal artery and the central retinal vein, enter and exit in the area of the optic disc. The optic disc is supplied with blood via the Zinn-Haller vascular ring.

As a remnant of the fetal hyaloid artery, a small patch of connective tissue sometimes appears in the centre of the papilla, the so-called Bergmeister papilla.

The optic disc is assessed as part of ophthalmoscopy during every ophthalmological examination, whether in miosis or mydriasis. However, an assessment in mydriasis is more accurate and in some cases can differ considerably from the assessment in miosis.

The assessment criteria are the colouring (vitality), edge sharpness, hollowing (excavation) and size of the optic disc. Difficulties in assessing the optic disc can be caused by narrow pupils, lack of patient cooperation, corneal opacities, but also advanced lens opacities. Myopic optic discs pose particular challenges for the ophthalmologist because they can appear very oblique (oblique optic nerve insertion, SSNE), but also with a large cone, so that it is not always possible to assess the cup, for example.

A healthy optic disc is usually described as vital and sharp-edged with central vascularisation.

Colouration[Bearbeiten]

The colour of the optic disc reflects the vitality of the optic nerve. A normal optic disc is light pink in colour. An increasing fading to white colouration (atrophy) of the optic nerve is found, for example, in advanced glaucoma but also in cases of central retinal artery occlusion or optic atrophy of a different origin (e.g. due to compression). A bilaterally blunted optic disc should also make the examiner think of a compression syndrome caused by a meningioma in the area of the pituitary gland with pressure on the optic chiasm.

Marginal acuity[Bearbeiten]

When assessing the rim, look for demarcation, notches, papillary rim haemorrhages and bayonet-shaped vascular kinks. A healthy disc has a sharp margin. A blurred edge may be present, for example, in a congestive papilla due to increased intracranial pressure. The finding is then usually present in both eyes. However, the absence of a congestive papilla does not rule out increased intracranial pressure. A hypertensive crisis or a central retinal vein occlusion can also lead to a peripherally blurred papilla. Papillae with blurred edges always require further internal investigations.

Excavation[Bearbeiten]

An important criterion is the hollowing out (excavation) of the papilla. It is described by the cup-disc ratio (CDR). The CDR is the ratio of the cup to the total size of the papilla head or its diameter. A cup-disc ratio of 0.0 - 0.3 is considered physiological. Findings with a CDR of 0.4 - 0.5 are to be regarded as borderline, from a CDR of 0.6 one generally speaks of a pathological papilla excavation. For example, a CDR of 1.0 is typically found in final-grade glaucoma. Final-grade, but also increasing papillary excavations can be associated with a nasal shift of the vascular tree, but also with a view of the lamina cribrosa.

In the case of borderline findings, the neuroretinal rim should also be described using the so-called ISNT rule. An asymmetry of the optic disc excavation of 0.2 or more between both eyes is considered suspicious and should be investigated further (glaucoma diagnostics, duplex sonography of the common carotid artery, cranial MRI if necessary).

Diameter[Bearbeiten]

Measuring the papilla diameter helps to avoid misjudgements of the papilla excavation. With a micropapilla, the cup is often underestimated, with a macropapilla it is overestimated. The physiological median of the vertical papilla diameter is approximately 1.5 mm.

Optical coherence tomography (OCT) of the papilla head enables precise measurement of the papilla size. Alternatively, the size of the papilla can also be estimated using a slit lamp and a magnifying glass. For this purpose, a narrow beam of light from the slit lamp is projected vertically onto the papilla head and then adjusted until it corresponds to the distance from the superior to the inferior rim. The length of the light beam is then read from the slit lamp and multiplied by a factor that varies depending on the magnifying glass used. The papilla diameter is then calculated from this.

Correction factors for estimating the papilla diameter are:

• 60D loupe (x 0.88 - 1.0)
• 90D loupe (x 1.3)
• 78D loupe (x 1.1)
• Goldmann three-mirror contact glass (x 1.27)

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