‘Optic Disc’ is frequently used to describe the portion of the optic nerve clinically visible on examination. This, however, may be slightly inaccurate as ‘disc’ implies a flat, 2 dimensional structure without depth, when in fact the ‘optic nerve head’ is very much a 3 dimensional structure which should ideally be viewed stereoscopically.
The optic nerve itself is a cylindrical structure of approximately 50mm in length, between the retina and the optic chiasm. This can be divided into 4 main parts:
- Intraocular (the optic nerve head)
- Intraorbital (between globe and optic canal)
- Intracanalicular (within the optic canal)
- Intracranial (between optic canal and chiasm)
The optic nerve head, or disc, is defined as the distal portion of the optic nerve extending from the myelinated portion of nerve that begins just behind the sclera, to the retinal surface. Typically, it is slightly oval with the vertical diameter being about 9% greater than the horizontal. On average the vertical disc diameter is approximately 1500 micrometers, although this may be greater in a myopic eye and less in a hypermetropic eye.
This can also be divided into 4 component parts:
- Superficial nerve fibre layer - contiguous with the nerve fibre layer of the retina
- Prelaminar area - consists of nerve fibre bundles and astroglia, which form sheaths around each bundle
- Laminar (Scleral) portion - contains a modification of sclera called the Lamina Cribrosa. This is made up of sheets of connective and elastic tissue, and contains fenestrations which give passage to the nerve fibre bundles and retinal blood vessels. It also serves to maintain intra-ocular pressure (IOP) against a gradient between the intra-ocular and extra-ocular spaces.
- Retrolaminar portion - myelinated nerve fibres, circumscribed by leptomeninges of the CNS
There are various ways to examine the optic disc, the techniques and merits of some are described below.
When assessing a disc for glaucoma there are many subtle characteristics which should be examined, and are covered in detail in the Glaucoma Evaluation Tutorial
At the very least, all one should remember for an assessment of the optic disc is “the 3 Cs” - the cup, colour and contour.
The borders of the optic disc should be clear and well defined. If not one becomes concerned that the disc may be swollen such as in the case of papilloedema - disc swelling secondary to raised intracranial pressure. Alternatively, the disc margins may appear blurred due to presence of optic disc drusen.
Typically the optic disc looks like an orange-pink donut with a pale centre. The orange-pink appearance represents healthy, well perfused neuro-retinal tissue. There are many pathological reasons why a disc may lose this orange-pink colour and appear pale ie optic atrophy. These include advanced glaucoma, optic neuritis, arteritic or non-arteritic ischaemic optic neuropathy or a compressive lesion. The causes of an optic neuropathy can be remembered by NIGHT TICS
As mentioned above the disc has an orange-pink rim with a pale centre. This pale centre is devoid of neuroretinal tissue and is called the cup. The vertical size of this cup can be estimated in relation to the disc as a whole and presented as a “cup to disc ratio”. A cup to disc ratio of 0.3 ( i.e. the cup occupies 1/3 of the height of the entire disc ) is generally considered normal, and an increased cup to disc ratio may indicate a decrease in the quantity of healthy neuro-retinal tissue and hence, glaucomatous change.
Whilst this is quick and straightforward to do it is not often an accurate reflection of this damage. A more comprehensive disc assessment tool is the DDLS.
There are many methods by which one can exam the optic nerve.
Outside of an ophthalmic environment the most common tool employed to examine the optic nerve is the direct ophthalmoscope. This hand held device is cheap and readily available in most clinical settings. It offers a uni-ocular view with x15 magnification, and allows the “3 Cs” above to be evaluated.
Full details of how to use a direct ophthalmoscope can be found in a separate tutorial.
Where more detailed examination is required, indirect slit-lamp biomicroscopy should be carried out.
Indirect fundus lenses provide an excellent view of the retina and optic disc. These come in various strengths - traditionally +90, +78 and +60 dioptres, although other lenses have been manufactured with specific desirable properties. A 90D lens provides a wide field of view but less magnification in comparison to a +60D, which provides a narrower field of view but x1.15 magnification. It is this magnification and depth of field that makes the +60D lens a excellent choice for examining the optic disc ( and macular examination). A +78D lens lies between these lenses with properties of both.
Unlike +20 to +30D lenses which are used with a binocular indirect ophthalmoscope (BIO), the indirect slit lamp lenses do not have to be held in any particular direction.
It should be remembered, however, that all indirect lenses provide an inverted image of the object being viewed. When learning this skill, therefore, you might find it useful to turn the piece of paper upside down before drawing the findings of your fundus examination ‘as you saw it’, which then normalises the image when you turn the paper back round.
Another useful aspect of indirect slit-lamp examination is the ability to measure objects such as the optic nerve head. This is done by getting the disc in sharp focus then adjusting the height of the light beam on the lamp so that it corresponds to the height of the optic disc. If this is done using a +66D lens (Volk) then the height of the beam corresponds exactly to the height of the disc, as it has a x1 magnification factor. This magnification factor varies between lenses and must therefore be taken into account when measuring the disc.
The BIO can also be used in assessment of the optic nerve. Unfortunately, the lenses employed in this technique (usually 20D - 30D) provide an excellent field of view but limited magnification, thereby making it difficult to examine the disc in fine detail which would make a glaucoma assessment difficult. However, as a gross examination tool this is excellent and is therefore commonly used in assessing the posterior pole in paediatric eyes which is otherwise difficult by other techniques.
All methods described above rely on a degree of patient cooperation, but it is essential that pupils are dilated prior to the examination. Most commonly tropicamide 1% drops 15-20 minutes beforehand will suffice. This may be more difficult, or longer, in those with dark irides, diabetics or concurrent ocular inflammation. Phenylephrine 2.5% or cyclopentolate 1% may also be added in such situations to aid mydriasis.