Basic Optic Disc Grading

an intorduction to Planimetri

Author: Mr. Rupert R A Bourne BSc FRCOphth
Clinical and Research Fellow in Glaucoma, Hamilton Glaucoma Center, San Diego, USA
Specialist Registrar Ophthalmology (Moorfields Eye Hospital, City Road, London)
Clinical Research Registrar (Institute of Ophthalmology, 11-43 Bath St, London)
Honorary Senior Lecturer (International Centre for Eye Health, London School of Hygiene & Tropical Medicine, London)

  1. Introduction to planimetry
  2. The Normal Optic Nerve Head
  3. The Optic Nerve Head in:
  • Myopia
  • Hypermetropia
  • Tilted disc syndrome
  • Optic nerve drusen

The Normal Optic Nerve Head

The optic nerve head can be imagined as a 'plug-hole' down which over 1 million nerve fibres descend through a sieve-like sheet known as the lamina cribrosa. These fibres are then bundled together behind the eye as the optic nerve which continues towards the brain.

The retinal nerve fibres are spread unevenly across the surface of the retina in a thin layer. As the nerve fibres converge on the edge of the disc they pour over the the scleral ring and then down its inner surface. This dense packing of nerve fibres just inside the scleral ring is visualized as the neuroretinal rim.

The inner (with respect to the centre of the optic nerve head) edge of this neuroretinal rim marks the most central of the nerve fibres. This edge is usually sloped, yet may be range from an overhang to vertical to a gentle slope towards the centre of the disc. This inner edge marks the cup edge.

The Normal Optic Nerve Head

Figure 1:

The Scleral Ring

This ring is usually pale allowing it to be distinguished from the neuroretinal rim tissue which is pink. The ring may not be visible in a given disc image, or the visibility may vary in different areas of the circumference of the disc. It is often easier to see on the temporal side of the disc than on the nasal side.

The Scleral Ring

Figure 2:

The Cup Edge

This is undoubtedly the most difficult contour to identify, and is subject to more variability than the disc edge. Elschnig described a series of shapes of the cup, however there are countless variations.

The inner edge of the neuroretinal rim (=cup edge) may be sloped (especially on the temporal side of the disc) or vertical. In some cases the edge may be an overhang. The most effective way of drawing this ring is to identify certain points on the cup edge where you are sure of its location, and then using a dot-to-dot procedure link up these points into a ring. First look for a blood vessel, preferably a small to medium-sized one (large vessels do not hug the surface and may not be tethered to the surface and therefore are unhelpful). Trace its path across the scleral ring and then over the rim tissue- at some point it will change direction as it bends inward towards the centre of the disc. If the slope is shallow this will be a gradual change in direction, however if vertical it will be an obvious bend, and in the case of an overhang it will suddenly disappear from view. It is the point of maximum change of direction of the vessel that marks the cup edge. When viewing the disc in stereo, the edge of the cup can often be clearly seen in areas where there are no vessels as a guide.

There can be a temptation to mark the cup edge where the colour changes from the pink of the rim to the pallor of the cup. In many situations this would be correct, yet in some situations the edge of pallor is not necessarily the edge of the cup, and hence it is better to rely on vessels and stereo cues as described above. The following images illustrate these points.

Cup Edge 01

Figure 3:

Cup Edge 02

Figure 4:

Cup Edge 03

Figure 5:

Cup Edge 04

Figure 6:

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