Glaucoma is a group of conditions characterised by a progressive optic neuropathy. As glaucoma is a multifactorial condition, detection and monitoring requires consideration of multiple factors such as patient risk factors, intraocular pressure, central corneal thickness and visual field changes. Through the Ocular Hypertension Treatment Study (OHTS) study, it was demonstrated that the most common first primary end point for progression from ocular hypertension (OHT) to primary open angle glaucoma (POAG) was optic nerve head changes. As glaucoma is often detected by these characteristic optic disc changes, having an understanding of how to assess the optic disc for changes seen in glaucoma is essential. Part 1 in this series (Optician 22.01.16) looked at the SIGN guidance introduced in Scotland. This article describes the interpretation of disc changes that all optometrists need to consider when assessing possible glaucomatous changes.

The Normal Optic Nerve Head

The optic nerve head or optic disc describes the area of the optic nerve that is visible clinically on examination. Over a million nerve fibres pass through the lamina cribrosa and are bundled together through the optic nerve towards the brain. The optic nerve head is typically vertically oval although there can be significant variation in the size and shape. An oblique exit of the optic nerve from the eye can give a tilted optic disc. Tilted optic discs are more common in myopic eyes and can be more difficult to interpret, with a broader sloping rim in one sector and a narrower well defined rim in the opposite sector. The average optic disc has a vertical disc diameter of approximately 1.88mm and a horizontal disc diameter of 1.77mm. The optic disc diameter, although largely independent of refractive errors in low to moderate refractive errors (-5.00DS to +5.00DS), is generally larger in myopes (greater than -8.00DS) and smaller in a hypermetropic eye (greater than +5.00DS). The scleral ring represents the edge of the optic nerve head. The densely packed retinal nerves within this ring are known collectively as the neuroretinal rim. Neuroretinal rim tissue is pinkish in colour and is a good sign of vascular perfusion. The optic cup represents the central area of the optic nerve head where axons are absent. This area is generally fairly round or horizontally oval in healthy eyes, where the horizontal diameter is about 8% bigger than the vertical diameter. The depth of the optic cup tends to increase with the size of the optic cup. It is common for inferior neuroretinal rim to be thicker than the superior rim, which is thicker than the nasal rim, which in turn is thicker than the temporal rim. This typical configuration is known as the ISNT rule (Figure 1).

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Clinical assessment of the optic disc

This assessment is best carried out with binocular indirect ophthalmoscopy using a condensing lens. These lenses allow for a magnified, stereoscopic image essential for detecting subtle changes in optic disc morphology. While dilation is not always indicated or practical in routine practice, dilation does provide optimal viewing conditions for optic nerve assessment. It is common to underestimate disc cupping while assessing undilated, and dilation makes it easier to assess the RNFL.

Size matters

The degree of optic disc cupping is related to the size of the optic disc in normal eyes. A big optic cup in a big optic disc can sometimes be mistaken as glaucomatous and a small or moderate cup in a very small glaucomatous optic disc can be assumed normal. The measurement of optic disc size is therefore an essential part of glaucoma assessment. This measure can be performed clinically using the slit lamp or imaging techniques such as optical coherence tomography (OCT). The estimation of the optic disc diameter may vary dependent on the technique used.

The most commonly used technique is with the slit lamp and a condensing lens, performed with the slit beam coaxial to the observation system. Once the slit beam is focused over the centre of the optic disc, the beam can be narrowed, illumination increased and the light beam adjusted vertically until it falls to the white margin of Elschnig’s ring. The value can be read off the graticule in millimetres. The orientation of the beam can be rotated to adjust for tilted optic discs or to measure horizontal width. This value can then be adjusted dependent on the condensing lens used to calculate the actual disc height using the following magnification factors (Figure 2).

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It should be noted that these values are estimates and will vary dependent on the distance between the condensing lens and the eye.

The European Glaucoma Society guidelines suggest the following classification for disc size:

  • Small: <1.45mm

  • Medium: 1.45mm-1.9mm

  • Large: >1.9mm

The cup to disc ratio

The cup to disc (CD) ratio was first introduced by Armaly in the 1960s as a way to standardise optic disc assessment. His studies showed that the CD ratio is genetically determined. The CD ratio is still commonly used both in community and hospital practice, as a way of describing the optic disc appearance. It is quick and easily recognisable to healthcare professionals. However, the measure doesn’t take into consideration optic disc size and focal rim loss. Focusing on the width of the central cup in just the vertical axis can lead the examiner to overlook focal thinning elsewhere. The CD ratio alone has been shown to have a poor sensitivity and specificity in glaucoma detection, with significant inter/intra-observer variability.

Spaeth’s Disc Damage Likelihood Scale (DDLS)

Devised by Spaeth et al in 2002 (Figure 3), this grading scale for assessing glaucomatous disc damage incorporates the effect of disc size and focal rim width.

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Method:

  • Measure disc size and correct for BIO lens used. The disc size is categorised as:

    Small < 1.5mm

    Medium 1.5-2.0mm

    Large > 2.0mm

  • Measure width of thinnest part of the rim

  • Calculate the Rim:Disc ratio

    If no rim present at thinnest part value = 0

    If rim as thick as possible (no cup) value = 0.5

  • The extent of rim absence is measured in degrees

  • The DDLS grading scale can then be applied

The DDLS is highly reproducible and correlates very strongly with the degree of visual field loss.

Key optic disc changes in glaucoma

Enlargement of the optic cup

Glaucoma is characterised by progressive thinning of the neuroretinal rim. As retinal nerve fibres are lost due to glaucoma, the optic cup enlarges. The optic disc is normally vertically oval in dimension, but the cup itself is normally horizontally oval. Vertical elongation of the optic cup can be a common feature of the glaucomatous disc, as narrowing is greater in the superior and inferior poles. Structural variability of the lamina cribrosa at the vertical poles has been proposed as the explanation for this change. Changes to the neuroretinal rim can be either focal or diffuse.

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Diffuse loss can give a generalised enlargement of the cup (Figure 4), while localised loss can give rise to small notches in the neuroretinal rim (Figure 5), most commonly inferotemporally.

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Disc haemorrhage

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Disc haemorrhage is an important ‘warning’ sign of glaucomatous optic nerve damage and is often associated with progressive change of the optic nerve head and visual field. The prevalence of disc haemorrhages in the normal population is estimated at less than 0.2%. The prevalence is significantly higher at between 2.2% and 4.1% in glaucomatous eyes. Although disc haemorrhages are seen in primary open angle glaucoma, they are more commonly seen in normal tension glaucoma. Disc haemorrhages are commonly splinter- or flame-shaped and located in the pre-laminar area of the optic disc in the superficial retinal nerve fibre layer. They can also be found in deeper parts of the disc margin and in the disc cup.

Optic disc haemorrhages must be considered in the context of other clinical findings as they may also be due to other ocular and systemic causes. Disc haemorrhages can be associated with posterior vitreous detachment, optic disc drusen, non-glaucomatous optic neuropathies, and vascular occlusive diseases of the retina. Systemic disorders associated with optic disc haemorrhage include diabetes, systemic hypertension, systemic lupus erythomatosis and leukaemia. They can also be associated with use of anti-coagulants. Caution should be given when considering disc haemorrhage as a risk for progression in glaucoma in the presence of any of these other ocular or systemic conditions.

Haemorrhages associated with glaucoma are often observed in the inferotemporal sector of the optic disc. This location correlates with the distribution of early perimetric defects, which occur most commonly in the superior field, although haemorrhages can occur anywhere at the disc. Although high intraocular pressure could play a part in the pathogenesis of disc haemorrhage, given the prevalence in normal tension cases, these local vascular crises are also likely to be related to ischaemic events.

Budenz et al’s study on the prognostic significance of optic disc haemorrhages during the OHTS showed that the majority of ocular hypertensive patients with optic disc haemorrhage did not develop glaucoma during the study period; however, it did suggests that optic disc haemorrhage in patients with ocular hypertension places them at a higher risk of developing POAG, as does older age, a thinner cornea, higher pattern standard deviation on automated perimetry, and larger vertical cup to disc ratio. Optic disc haemorrhage should therefore be considered in the decision process when deciding whether to initiate treatment in a patient with ocular hypertension.

As optic disc haemorrhages can be a strong predictive factor for the development of POAG, glaucoma suspects with optic disc haemorrhages should be monitored carefully. Optic disc photography can be extremely useful in these circumstances and has been shown to be even more effective at detecting optic disc haemorrhages than clinical examination.

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Blood vessels

A number of changes to the vessels within or around the optic disc may occur due to glaucoma and can act as signs to alert clinicians to possible glaucoma.

Baring of the circumlinear vessel occurs when areas of pallor develop between these small branches of the central retinal vessels and the cup margin (Figure 7). Although this sign can be present in non-glaucomatous eyes, particularly in large optic discs with larger cups, it is more commonly seen in glaucoma.

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Fly-over vessels are seen when rim tissue underlying the retinal vessels that cross the disc margin is lost, which can give the appearance of vessels floating above areas of deeper cupping.

In areas where the rim has been lost, retinal vessels can sharply change direction as they pass under the overhanging edge of the cup. This is known as bayonetting (Figure 8).

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Although not pathognomonic for glaucoma, nasalisation of the central retinal vessels can commonly be seen in glaucoma.

Changes to vessel calibre (Figure 9) can be seen in glaucoma, particularly in areas where there has been most significant loss of the neural retinal rim.

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Retinal nerve fibre layer

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The retinal nerve fibre layer (RNFL) describes the radial expansion of the optic nerve fibres from their point of entrance at the optic disc. These opaque striations form arcuate bundles of nerve fibres where changes can occur in glaucoma. Examination of the RNFL can be difficult and is best performed with red-free (green) light to enhance contrast, examining the peripapillary region for slit shaped, wedge shaped or diffuse defects. Imaging modalities such as OCT can be incredibly useful in quantifying changes in the RNFL and has become an integral part of glaucoma assessment in many glaucoma services. Clinically detectable nerve fibre layer changes have been shown to precede the onset of glaucomatous field loss and can be useful in predicting progression.

Peripapillary atrophy

Peripapillary atrophy (PPA) is another morphological change that can be used to help detect glaucoma. This atrophy surrounding the disc can be divided into an inner crescent or a-zone and an outer crescent or ß-zone.

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The a-zone atrophy is more common in normal eyes and is located on the outer surface of zone beta if present. The inner ß-zone beta represents loss of retinal pigment epithelium and is uncommon in non-glaucomatous eyes.-zone atrophy is more common in normal eyes and is located on the outer surface of zone beta if present. The inner ß-zone beta represents loss of retinal pigment epithelium and is uncommon in non-glaucomatous eyes. Its size significantly correlates with other factors used to determine the severity of glaucoma such as visual field defects. These areas of atrophy are often located temporally. Although PPA is often considered to less crucial in the detection of glaucoma, as other disc of field changes are often noted first, it can be useful in identifying progression and to differentiate between glaucomatous and non-glaucomatous optic disc damage.

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The Manchester model for disc assessment

All patients referred to the MREH with a suspected diagnosis of glaucoma are triaged into the Optometrist-Led Glaucoma Assessment (OLGA) clinic. Patients seen in the OLGA clinic will undergo a comprehensive glaucoma assessment, working to the 2009 NICE guidance for glaucoma: diagnosis and management. This assessment will include:

  • IOP measurement using Goldmann applanation tonometry (slit lamp mounted)

  • Central corneal thickness (CCT) measurement using ultrasound pachymetry

  • Peripheral anterior chamber configuration and depth assessments using gonioscopy

  • Visual field measurement using a Humphrey visual field analyser

  • Dilated optic nerve assessment using stereoscopic slit lamp biomicroscopy

  • Optic disc imaging with fundus photography or OCT imaging.

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Clear documentation of optic disc assessment is critical to provide evidence to support a diagnosis and to help identify progression. This documentation will include a small sketch of the optic disc that should include the height of the optic disc (including the lens used) and any important features. This information may include location of rim loss, cup to disc ratio, neuroretinal rim configuration, vessel changes, disc haemorrhage and peripapillary atrophy.

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Imaging will be used at the initial visit and thereafter at regular intervals as deemed appropriate. Imaging should be performed if any changes to the appearance of the optic disc are noted.

Summary

Optic disc assessment is the most important aspect of glaucoma diagnosis. It doesn’t fluctuate like IOP and isn’t dependent on performance like in visual field assessment. It is a difficult skill that requires practice. Use of indirect BIO, dilated assessment and making the most of imaging techniques will optimise the clinician’s ability in making appropriate diagnoses and management decisions when assessing patients with glaucoma.

Special thanks

A special thanks to all the OLGA optometry team at the Manchester Royal Eye Hospital, particularly Dr Robert Harper, Mrs Amanda Harding and Miss Joanne Marks in helping prepare this article. 

Patrick Gunn is principal optometrist (Training and Education) at Manchester Royal Eye Hospital and optometrist at Muldoon and Tonge Opticians, Ashton-under-Lyne

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