Glaucoma Evaluation Tutorial
to establish to role of optic disc evaluation in the detection and long-term monitoring of glaucoma
to describe the features of a normal optic disc
to recognise features of a glaucomatous disc
Glaucoma is a group of conditions defined by a progressive optic neuropathy with accompanying visual field changes. Raised intra-ocular pressure (IOP) is classified as a risk factor but is not part of the definition.
When evaluating patients we suspect of having glaucoma or to monitor patients with the disease we are monitoring three things. The easiest parameter to monitor is intra-ocular pressure and this is done using a form of tonometry, Goldman applanation tonometry, Tonopen or non-contact tonometry (the puff of air). We measure and record an actual figure for IOP and therefore it is easy to chart alterations. In reality this is the least useful parameter and although its variation with time in each patient can be useful, the actual figure on its own is no indication of the severity of the disease or the impact on the patient.
The next parameter we monitor is visual fields. This is carried out in the majority of cases using the Humphrey field analyser which produces a computerised print out marking areas within the central part of the patients visual filed that they can see and those areas that they cannot. Glaucoma causes permanent loss of the peripheral field which can be demonstrated by a visual field test. The visual field is very dependant of patient co-operation and concentration. Although the test can take 3-4 minutes it is a very difficult test to do and there can be wide variations in the field from one day to the next. It is a very valid way of detecting progression but must be interpreted with care and acknowledgement that the result is dependant on the individual patient.
The final parameter we routinely monitor is the optic disc appearance. This parameter is not dependant on the whims of the patients but on the whims of the clinician!! Unlike IOP, the appearance does not fluctuate from day to day and unlike fields is not dependant on patient co-operation. It has the potential therefore to be the most useful indicator of disease and disease progression. However, disc evaluation to be of use in glaucomatous patients has to be done with great care and attention to detail. Direct examination with a stereoscopic view as obtained with a slitlamp and 90/78/66D lens preferably with a dilated pupil is the best method. Stereoscopic disc photographs can be equally good and have the added advantage that they provide a permanent record of the disc configuration that is not dependant on the artistic skills (or lack of them) of the clinician. Disc evaluation takes not only an understanding of the normal disc appearance and the pathological process of glaucoma but also a lot of clinical experience.
At present the exact cause of the disease is unknown. We theorise that mechanical pressure in the form of IOP compress the nerve fibres as they exit from the eye through the optic nerve en-route to the visual cortex within the brain. This compressive force is enough to cause the nerves to die off. The peripheral part of the retina sends in fewer nerve fibres than the macular so as the overall number declines the loss is first noticed within the peripheries. The loss of nerve fibres manifests itself as field loss.
The optic disc is the site of exit for retinal ganglion cell axons (collectively referred to as the optic nerve) as they head off towards the brain. The nerve fibres exit the eye through the scleral canal The size of the scleral canal, which governs the size of the optic disc, varies from 1-2.5mm in diameter. Generally it is smaller in hypermetropic (long sighted) eyes than in emmetropic (no refractive error) and is largest in myopic (short sighted) eyes. Although the scleral opening for the optic nerve varies in size between person to person, the number of fibres, in the normal eye, is fairly consistent. We are born with about 1.2 million nerve fibres, this number gradually declines as we age but in glaucoma the rate of decline is accelerated.
When assessing the optic nerve we are looking not only at the area of the scleral outlet that is occupied by nerve fibres but also at the area that is not. The optic disc refers to the entire scleral outlet. The optic cup refers to the central portion of the disc that is not occupied by fibres. The size of this cup is dependent not only on the size of the scleral opening but also on the number of fibres passing through.
Cup to disc ratio
Armaly back in the ‘70’s introduced the idea of cup disc ratios (CDR). That is, expressing the proportion of the disc that is occupied by the cup. This notation is commonly used in glaucoma clinics to classify discs and you will see in the casenotes CDR 0.4 or CDR 0.6. But opinions vary as to what CDR constitutes a glaucomatous disc some say 0.3, some 0.5 and some say 0.8.
The enormous physiological variation in optic disc size means that a myopic disc with a normal compliment of fibres may have a CDR of 0.6 and be normal but a hypermetropic disc with severely diminished number of fibres as a result of extensive glaucomatous damage may also have a CDR of 0.6. One disc normal, one disc grossly diseased but both having the same cup disc ratio.
The CDR notation is also used to chart progression, an increase from 0.3 to 0.7 for example, being a sign of progression. However this technique of evaluating the optic disc has been shown to be inaccurate and highly variable in the way that it is recorded. Studies have shown that clinicians vary widely in the value they give a CDR when compared to each other and when retested themselves.
Optic disc appearance, even in a normal population, is hugely diverse. Early glaucomatous changes are subtle and may appear similar to normal optic discs. What we rely on when evaluating a disc for evidence of glaucoma or progression of glaucoma are specific patterns of change.
Four morphological pathological catagories
Attempts have been made to draw up a glaucomatous disc classification system however because of the wide variation of disc appearance it is of limited use. Some glaucomatous discs will carry features of more than one type and so when evaluating a disc it is best to have a system for looking at various features and assessing each of these. If the disc that is being assessed falls neatly into one of the categories well and good, if not, it is important to just document what you see.
focal glaucomatous optic discs
association with migraine and vasospasm.
Disc size is smaller than other glaucomaout disc types.
myopic glaucomatous discs
Older patient group.
Hypertension and cardiovascular disease is more common
generalised cup enlargement
The area of the disc occupied by the retinal nerve fibre axons is referred to as the neuroretinal rim and it is changes in the appearance of this area that is the key to quantifying glaucomatous disc damage. When examining the neuroretinal rim the pattern of thickness and areas of focal thinning are the most important features.
The ISNT rule is useful in evaluating whether thinning is physiological or pathological. A healthy disc tends to have it’s thickest portion of neuroretinal rim inferiorly, then superiorly, then nasally, with the thinnest portion being temporally.
In fact the temporal rim being the thinnest is probably the most important.
However, it is also important to remember that some healthy discs will not obey the ISNT rule, and similarly some glaucomatous discs may obey the ISNT rule.
Focal notching is very characteristic of glaucoma and tends to be inferio- or superio- temporal. It can become a full thickness notch with complete abscence of the rim. It may be very deep and it may be associated with peri-papillary atrophy. Progression will cause this area to enlarge. Notches usually correspond to specific visual field defects.
Senile sclerotic discs are more difficult to assess. Essentially the disc has a shallow saucerised cup with gently sloping sides and peripapillary atrophy. The disc also has a moth eaten neuroretinal rim.
Pallor can precede cupping and can occur in place and can be in place of cupping in a small disc.
overpass-blood vessel crossing over an area of neuroretinal rim loss prior to sinking in and taking on the bayonet appearance.
laminar dot appearance-exposure of the lamina cribrosa
Optic disc haemorrhage
splinter haemorrhage-can be seen in normals but are associated with progression of disease particularly in NPG.
particular seen in infero-temporal region.
may precede optic disc change and field loss.
Optic cup enlargement can progressive in advance of pallor.
Retinal nerve fibre layer (RNFL)defects
The retinal nerve fibre layer is the innermost layer of the eye and corresponds to the nerve fibres passing from the photoreceptors over the retina as they course towards the optic disc. In the same way the loss of these fibres causes the optic cup to expand, the loss of fibres can be evaluated by looking at the presence or absence of this layer of nerve fibres particularly in young people.
wedge defects-their presence is significant, their absence is of no significance.
RNFL is seen with the green light on the slit lamp.
RNFL can be photographed.
RNFL analysers looking for loss of the retinal nerve fibre layer are available but are of limited use at present. They are very poor at screening for glaucoma but may be useful in monitoring ocular hypertension/early glaucoma for progression. Once there is moderate disease they are very poor at demonstrating subtle changes.
this is a feature of the normal ageing process and of high myopia. It may however also be a feature of glaucoma.
In glaucoma patients the extent of the atrophy has been shown to correlate with the severity of the disease and progression of disease particularly in NTG.
In non-glaucoma PPA is most common temporally. In glaucoma the area of PPA is usually in the sector with the rim loss.
In a senile sclerotic disc it has been reported to be present for the full 360dgrees.