There’s more to glaucoma than visual fields
PhD.BSc.FCOptom.FAAO.DipCLP.DipClinOptom.
Primary open angle glaucoma (POAG)
A multifactorial optic neuropathy in which there is a characteristic acquired loss of optic nerve fibres. (American Academy of Ophthalmology, 1996).
POAG is a chronic, generally bilateral disease which is often asymmetrical and
Characterised, at least in one eye, by all of the following (AAO 1996):
What happened to the IOP? Ocular tensions are far from being the most important diagnostic sign. What is important is evidence of damage to optic nerve. Therefore it is fundus examination and visual field analyses that are diagnostically paramount.
An interesting problem exists when reviewing epidemiological studies of prevalence. The problem with most studies that have taken place in the past was the very definition of POAG which often combined two out of three of fields / discs / IOP.
Reanalysis of results from previous studies, using AAO criteria, shows a prevalence that is higher by 30 to 50% than originally construed.
The Baltimore Eye Survey (Tielsch et al. 1991) used AAO criteria. From this study it is apparent that prevalence rates are significantly affected by risk factors such as race, age and family history. The prevalence of glaucoma depends upon the characteristics of the population being examined.
The effect of age on the prevalence of POAG (The Baltimore Eye Survey, 1991)
|
Ethnicity |
Age (years) |
Prevalence (%) |
|
White |
40 - 49 |
0.9 |
|
White |
> 80 |
2.2 |
|
Black |
40 - 49 |
1.2 |
|
Black |
> 80 |
11.3 |
Effect of positive family history on the prevalence of POAG (The Baltimore Eye Survey, 1991)
|
Adjustments |
||
|
for age and |
POAG in parent |
1.3 |
|
race |
POAG in sibling |
2.9* |
|
POAG in children |
1.1 |
|
|
POAG in 1st degree relative |
1.8* |
* Statistically significant (p <0.05)
Less well settled is the effect of gender. However, both the Barbados and Rotterdam studies showed a higher prevalence of POAG in males compared to females (Leske et al, 1995; Dielmans et al, 1994).
The higher the IOP, the higher the rate of POAG.
Whilst high myopia may be a risk factor appropriate studies need to be made to develop the theory of this relationship.
Diabetes mellitus and other medical conditions
Again, further studies are needed. An intriguing epidemiological question will require addressing. Namely, does POAG appear to be more common in the diabetic population simply because these patients are examined more often and (?) more thoroughly than the non-diabetic population?
Smoking, alcohol and migraine do not appear to be risk factors.
Systemic vascular hypertension has been found to be a risk factor in some studies but rejected in others. It is important to appreciate that the pathophysiology of POAG is affected by the relationship between disc perfusion pressure and intraocular pressure.
Additional care should be taken with certain patients such as ocular hypertensives who are coincidentally placed on therapy for vascular hypertension.
The case finding eye examination
This is important because of risk factors.
Automated quantitative analysis (e.g., Humphrey; Henson; Dicon) is preferable. These tests are:
Humphrey visual field analyser
The Humphrey Central 30-2 Sita may be considered as a ‘gold-standard’ with regard to POAG field detection analysis. It examines 76 points centred around the fovea giving retinal thresholds (dB) for each point. High numbers = good sensitivity (greater attenuation = dimmer stimulus).
Total, Pattern & Mean deviations
Total deviation
This provides an array of differences of measured thresholds from age-corrected normals. The probability plot shows the probability of obtaining the value exhibited by the patient in the reference population. MD = mean deviation of all points from that of the reference population
Pattern deviation
This is a software correction applied to the field for factors that affect all points and allows focal defects to be readily identified. A probability is also displayed.
PSD = pattern standard deviation of points for that eye
GHT (Glaucoma Hemi Threshold)
This provides a useful statistical indicator of any difference in sensitivities between the two vertical hemifields thus showing asymmetry across the horizontal meridian
Both multiple stimuli and single stimulus available for suprathreshold test
Ability to repeat individual points within a field rather than have to repeat the whole plot
Ability to increase number of stimuli and specificity
The proportion of true positives that are correctly identified by the test
The proportion of true negatives that are correctly identified. Specificity can be increased by extending the test and setting new pass/fail criteria
The Henson gives a graded approach: 26-point test; 68 point test; 136-point test
Henson argues that extendibility is important as one cannot have a quick, highly sensitive and highly specific test. So the Henson allows for a quick screening test with reasonably high sensitivity with the ability to extend to obtain higher specificity. Further, the Repeat and Add facility improves sensitivity and improves specificity without needing to repeat the whole test. The practitioner can choose selective repeats and/or add neighbouring stimuli.
Anterior chamber angle & depth
The anterior chamber characteristics of every adult should be examined at least on the first visit to the practitioner. There are a number of techniques available and these are covered fully in a separate Optometric Educators lecture.
These are:
Viewing the optic nerve in glaucoma
The method of choice for assessing the optic nerve head is with the slit lamp microscope providing a magnified, stereoscopic view.
Dilate to view disc using Tropicamide 0.5% or 1%. If the angle is narrow (Van Herrick grade 1) and has not been examined by gonioscopy, check the IOP again twenty minutes after dilation. Never instil pilocarpine to routinely produce reversal as this in itself can produce an attack of acute glaucoma. If there is a significant rise in IOP following dilation then the use of pilocarpine is indicated to bring the pressure down in the short term. However, this case scenario will occur very rarely indeed.
The following signs or diagnostic points are of importance:
The presence of localised nerve rim thinning ("notching") is significant and corresponds to visual field change and NFL loss.
Neural rim haemorrhages occur more common temporally and appear to be more common in normal-tension glaucoma.
Nerve fibre layer defects, which may be visible, represent axonal loss at the optic nerve head.
The blood supply to the ONH comes from the short posterior ciliary arteries (not CRA) and these vessels also supply the peripapillary area. Poor perfusion of these vessels in POAG may produce visible peripapillary atrophy of the choroid and RPE.
The diurnal variation of IOP in glaucomatous and ocular hypertensives > "normals", and a diurnal variation > 4 mmHg suspicious. Checking IOPs at different times of the day may provide additional diagnostic information.
The mean IOP = 16 mmHg with a SD = ± 2.5 mmHg. Thus the normal distribution =16 ± (2 x SD) and = 11 to 21 mmHg. Thus > 21 mmHg = "ocular hypertensive."
Whether positive or negative, you are making a tentative diagnosis.
However, do not come across that you are definitive in your discussion with your patient or in the referral letter to GP or ophthalmologist. You will have your own way but one style example might be to lay out your findings in your referral letter and finish:
" in view of my findings I would be grateful if you would consider referring Mr. Rock Hardeye for an ophthalmological opinion to rule out the possibility of primary open angle glaucoma" .
Dr Barnard wishes to thank Dr David Henson for the use of selected slides and for information relating to the Henson Pro.
American Academy of Ophthalmology (1996) Preferred practice pattern: primary open-angle glaucoma, San Francisco, CA
Choplin N & Lundy D (1998) Atlas of Glaucoma, Martin Dunitz, London
Dielmans I, Vingerling JR, Wolfs RCW et al (1994) The prevalence of primary open-angle glaucoma in a population-based study in the Netherlands, Ophthalmology, 101, 1851-1855
Leske MC, Connell AMS, Wu S-Y, Hyman LG, Schachat AP (1995) The Barbados Eye Study Group: Risk factors for open-angle glaucoma Arch Ophthalmol, 113, 918-924
Tielsch JM, Katz J, Toyall RM, Quigley HA, Javitt J (1991) Racial variations in the prevalence of primary open-angle glaucoma: the Baltimore Eye Survey, JAMA, 266, 369-374
Dr Simon Barnard
January 2002
sb@eye-spy.co.uk