Visual Field Test

[Nocton]

I have carried out this test at the opticians and hospital several times over the last few years as my mother had mild glaucoma, although it has not been confirmed for me. The test is usually inconclusive, i.e. too many false positives/negatives/noise. The problem I have is to decide what is a 'spot'. Neither the technician nor ophthalmologist seems to be able to say whether all the spots should be sharp and bright, equal brightness or fuzzy. I find that most of the spots are not sharp and bright and I wonder if it is to do with the fact that I wear bifocals, so that the focus/lens selected is never correct. I did mention this and the settings were then selected from my prescription rather than from my glasses, but it did not improve matters. I do not know if they base the lens settings on my short or long focus lenses, in which case neither will be perfect, so next week when I go again I was going to ask this question and perhaps suggest that the lens be set halfway between the two - or perhaps they already do? Are there any experts who know what I should see and how to get the optimum results? any advice would be appreciated.

[supremetwo]

The spot brightness and size does vary as part of the test.

International Glaucoma Association video:- https://www.youtube.com/watch?v=zTmIsUD ... e=youtu.be

Technical article:- http://eyewiki.aao.org/Standard_Automat ... _luminance

[Nocton]

Many thanks, supremetwo, - very useful and clear info.

[Snowbadger]

Hi Nocton. The prescription used in a field test is your reading i.e. bifocal segment portion. When You do your field test relax and do your best. If you think you have seen a spot, hit the button. In the early part of the test ( presuming you were tested with the Humphrey VFA ) the machine locates and measures your blind spot. Thus as the test progresses, occasionally a dot will be flashed into your blind spot. If you press the button, this is recorded as a false positive. Unfortunately people often move during the test thus changing the position of the blind spot and large number of false positives occur. Field tests are just plain difficult, but the clinician is not looking at the fields in isolation. Damage to the optic nerve precedes visual field loss so often there is an expectation of where the field loss should be. Also, you never rely on a single field test for diagnosis repetition should show a consistent pattern even with the background noise.

Regards,

SB

[Lootman]

My eye doctor (who is an ophthalmologist and not just an optometrist) actually uses two different machines to perform visual field tests.

The results are then combined to ensure that any outlier results from one test are eliminated. He claims this is the most accurate way to be confident in the results.

He tests the interocular pressure two different ways as well. Glaucoma is such a difficult disease to diagnose that such redundancy makes sense, if you're willing to pay for it anyway.

[Nocton]

Thank you, Lootman and Snowbadger, for the extra info and fits in the a discussion I had with the eye consultant at the hospital when I visited a couple of weeks ago. He showed me the Field Test results and explained them clearly.
"if you're willing to pay for it anyway." doesn't apply as the eye test is free on the NHS (im over 65) where they use the "puff" method to test pressure and at the hospital (NHS) they use the "probe" method. They seem to give similar results well within the 'healthy range.

[Lootman]

Thank you, Lootman and Snowbadger, for the extra info and fits in the a discussion I had with the eye consultant at the hospital when I visited a couple of weeks ago. He showed me the Field Test results and explained them clearly.

"if you're willing to pay for it anyway." doesn't apply as the eye test is free on the NHS (im over 65) where they use the "puff" method to test pressure and at the hospital (NHS) they use the "probe" method. They seem to give similar results well within the 'healthy range.

What I was referring to about having to pay for it is the case where you want extra tests or second opinions. As it happens I see a private doctor.

There are actually three tests for pressure that I know of:

1) The puff test - the least accurate and most annoying
2) The pressure pad AKA "the blue light", for which you get an eye drop as an anaesthetic
3) There's a new test from Finland that "stabs" your eye. Sounds painful but it isn't.

The two visual field tests are different in that the one just has one light that varies in position and intensity. The other one has up to four lights at the same time, where you indicate how many lights you see at once. Sorry but I don't know the technical name for those pieces of equipment.

Other diagnostic tools used (for glaucoma) are a test of corneal thickness, and imaging of the retina and optic nerve, plus a visual examination of course.

I exhibit some of the symptoms of glaucoma but do not officially have it, and don't take drops.

[Snowbadger]

Hi Guys. As I retired after 40yrs as an optometrist five years ago, I have no doubt the debate has moved on from when I left but over the years the definition and diagnosis of glaucoma has changed on a frequent basis. Don't be too harsh on optoms, they are on the front line. Glaucoma is very easy to diagnose as it is a progressive disease and all you have to do is wait to be certain. Optoms are between a rock and a hard place constantly facing complaints from eye depts for referring too soon or too late while giving little or no guidance or criteria to work to. Eye depts have an easier task as they receive most referrals from optometry and so can refine a subsection of the population. There is (was ) also much debate as to the usefulness of lowering eye pressure. Many people with progressive VF loss have low IOP, many with high IOP show no signs of VF loss. It has been suggested that thicker corneas result in higher IOP measurements but thinner corneas are thought to be a greater risk of glaucoma.

So!

You need to look at three things to diagnose and treat,

Pressures : Puff Puff. Very much used by high street opticians as a screening test. Ping thing ( Tonopen ) from Finland is much the same.
Normally a high result would be then refined using Goldmann ( the gold standard ) You need to use an anaesthetic fluorescein dye and poke them in the eye while shining blue light at them.

21mmHg is taken as the upper limit of normality as it is two std dev from the mean of 16mmHg.

Visual Fields : They are just difficult. On the front line at high st opticians they often use more simple screening tests, which are easier to do but will miss early defects. The Humphrey VFA is the gold standard and most research has used this tool as do most Eye Depts. The Full Threshold Test measures the VF by varying the size and intensity of the light until it is not seen and then increasing until it is seen again. This is what makes it so difficult as it is so tantalising. However if you have a field loss you would see a progression by regular testing through the noise.

Optic Nerve : Glaucoma is essentially loss of nerve fibres so clinicians look for signs of thinning of the fibres. Because of this you can often predict where the field loss should be before you measure the visual field. More progressive high street optician have Optical Coherence Tomographers that can measure nerve fibre very accurately. However the normal variation in human beings is 750,000 to 1,750,000 ie massive variation.

So in a nutshell. To diagnose and treat glaucoma takes a lot of time and repetition of testing to find a pattern of optic nerve change and VF change.

Hope that helps.

SB

[Nocton]

Thanks again for the very useful information. Actually the eye consultant I see at the hospital does all three things that Snowbadger mentions every year. In fact I have only incipient signs of glaucoma and I was initially referred because my mother had glaucoma, but not that it ever caused serious loss of vision. But the consultant is keen that I should take drops as he says the condition is irreversible and best to be on the safe side.