In the first part of this blog I had told you what the so called ‘computerized eye testing’ was all about. As I said in that post, it is essentialy just taking the measurement of your eye, prescribing a spectacle and earning good money.
While I’m not against the use of the autorefractor (AR), as the machine really is called, or the money that comes from dispensing a specatcle (some people earn a living from that), I’m against the way it is misused.
The AR is a machine. It’s job is to take a measurement and that is it. It is the duty of the optometrist or ophthalmologist to verify your real refractive status (the number of your eye) before prescribing the final spectacle.
How does the autorefractor work?
If you’ve had the opportunity to look into the opening on the AR you’d have seen a picture that keeps going in and out of focus. The machine bounces off infra-red rays from the retina (innermost light sensitive layer of the eye) and takes a series of measurements to determine your eye’s refractive power. The unfortunate part is that this machine can be fooled easily!
The human eye is so designed that it focuses almost instantaneously on anything that is aligned to its visual axis. In children, this property that is called accommodation, is extremely strong. Even if you are instructed to de-focus or ‘leave your eye relaxed’ instinct induces you to focus on the picture within the AR. That is why it is suggested that the AR be used after cycloplegia (using dilating drops to temporarily knock out this auto-focus mechanism). It is very rarely opticians and optometrists do this because it would be a waste of time and consquent reduction in business. After all, who would want to go to a ‘computerised eye test’ center to get things done ‘manualy’!
The only problem is that without knocking out the accommodation the AR will throw up numbers that are to the ‘minus’ side. It will show that a patient or customer has a higher ‘minus’ number (pseudomyopia) than there really is. Sometimes even people with ‘plus’ powers end up having a ‘minus’ numbered spectacle! If you look at the pictures below you will see that the print out on the left (done before cycloplegia) always shows a higher ‘minus’ number than the one on the right (after cycloplegia).
You will notice that the figures on the left side can either come down by a significant amount after cyloplegia or shift completely to the plus side. Sometime even the nature of your astigmatism (cylidrical power) can change to exact opposite axis and power, as shown within the red rings on the last picture.
Here is a question asked by an intelligent person who noticed that she is overcorrected.
So the next time you are told that you need spectacles for ‘short sight’ ask for a dilated examination especially if you have no problem with your vision! Otherwise you will be overcorrected towards the minus side and that usually worsens symptoms of eye strain for near (reading, computer, etc) though your far vision will seem very sharp; sometimes, uncomfortably sharp.
If you really have no problem with your vision get a detailed eye test done along with a general health check. Sometimes eye symptoms can be caused by general systemic disease like fever, anaemia, diabetes, sinusitis and even medicines that you maybe taking.
UPDATE – 29/03/2013
I’m adding another picture here of a similar patient, a child of 13, who was discovered to have poor vision during a school screening program. The number circled in red (handwritten) is the power of the spectacle lens the child is wearing in her right eye. The left eye has 6/6 vision and has a plano lens (with zero power). The auto-refractor threw up a number lesser than her prescription (number circled in blue) before instillation of dilating drops. That number became even lesser after dilating (number circled in green). Even the left eye shows a shift to the plus side.
The child was brought because she was extremely uncomfortable with her spectacles. Naturally; if the difference between the two eyes exceeds 2 to 2.5 diopters the eyes find it difficult to fuse the images properly and there is a tendency for double vision. In such cases it is better to under correct the larger number so that there is no spectacle induced discomfort, a common cause for children to avoid wearing spectacles. If the child is old enough and capable of handling it, contact lenses are a good option, as in this case.
Always ensure that you or your child are not over corrected. Don’t rely completely on ‘computerized eye testing’ especially in children.
A Peek into your Eyes 
pj
October 5, 2013 at 1:36 AM
Your data is neither measured or recorded in a consistent format ! The astigmatic cylinder and axis did not shift from – to + as a result of dilation. ( Large print with red circle ) The operator of the auto refractor changed from minus cylinder form to plus cylinder form, had nothing to do with dilation.
Dilation is certainly not needed for better refraction in 90% patients, however, some patients may benefit from a cycloplegic refraction. Let me say this another way, a skilled refractionist can control and compensate for accommodation in 90% of the patients. Cycloplegia and repeat refraction are of value when you cannot and hopefully the person performing said refraction has enough clinical skill to make that distinction.
adithyakiran
October 5, 2013 at 1:07 PM
Hi pj, I’m not sure if you looked beyond those red ovals in that particular picture. It very clearly says ‘MIX’ for the settings for the cylinder. My AR is usually set in ‘MIX’ by default. Since I am the operator, I can vouch for the fact this change appeared from before to after cycloplegia.
What I’m trying to show here is just how different the numbers can be ‘before’ versus ‘after’, because in the part of the world I live in the gullible patient think that they have had comprehensive eye test if they visit an optician with a board saying ‘Computerized Eye Testing’ outside the workplace. I still trust my faithful streak retinoscope and I use the AR to make my job faster!
ajay
April 15, 2015 at 2:28 PM
hello there i just came across your blog .i had lasik done 3 yrs back .no issues now since last 2 yrs i’ve been regularly doing eye exercises .my vision has remained the same it was just after lasik .now i find no change .just out of curiosity i has “computerised eye testing ” done in a shop .it showed the following detains on 14/2/2015 (rt eye sph: nil cyl:-0.75d axis 180) (lt eye sph:-0.25d cyl:nil axis : nil) of course no dialtion test was done .after 2 months on 15/4/2015 (rt eye sph: ± cyl: -1d axis 180) (lt eye sph: ± cyl: -0.5d axis : 110) . in just 2 months there seems to be a wide fluctuation .now after reading your blog i’m sure though i might have a no. it wn’t be so high .even with the latest no.s i can see everything clearly with sharp borders ..it happens it’s a little difficult to distinguish 8 or E but everything else is clear .i’m continuing with eye exercises and use relaxation techinues during comp use .i just wanted your opinion on this
adithyakiran
February 19, 2016 at 12:30 PM
I am sorry for this delayed response. I seemed to have missed the notification altogether. A 0.25 diopter change in refraction cannot be called a ‘wide fluctuation’. As a matter of fact the cylindrical number and axis will fluctuate a bit depending on many factors. Commonest factor is the position of your head and alignment of the eyes when the measurement is taken. A slight tilt will make the machine measure in a completely different axis and so will give you a different number. In any case the numbers are not important as long as you are able to see what you want to see. 6/6 is an average value. Not everyone has 6/6 vision and some see more than that, upto 6/5 or 6/4. If you are comfortable with your present vision, I think you should stop getting your eyes checked at such centers. If you were a high myope the serious changes are inside the eye, on the retina. LASIK will not change that. It is therefore mandatory that you should have an annual dilated examination with your ophthalmologist.
jhansi
February 18, 2016 at 7:26 PM
thanks for enlightening us
JH
March 5, 2017 at 8:21 PM
I just came across this interesting piece as I have just discovered that my myopia has been over-corrected for going on 15 years, by qualified ophthalmologists, giving prescriptions between -4.5 to -5.5 (on this last occasion I refused to change my lenses, I could see perfectly at -5.0), and have now had confirmation from an ophthalmologist in France and an optometrist in the UK of -3.75.
All these years my correction was calculated without using machines but with the old fashioned specs with lenses changed by hand.
So maybe if a machine gives only a slight variation on the truth, as opposed to the significant human errors I have suffered (and their consequences of years of headaches) it’s maybe not such a big deal?