Let’s get back to basics…what is the fundamental requirement of a refraction eye-based exam, which is historically known in the UK as a sight test?
Well, according to the Rougier definition of a sight test as enunciated by the General Optical Council in the 1960s, a sight test is a three-fold process. First, it is the determination of whether or not there is a defect of sight. Secondly, it is measuring it by a process of ‘determination.’ And thirdly, it is prescribing an appliance to correct said defect.
By and large this is all fine. However, the salutary question remains, when do you know when making a prescribing decision that it is the correct one? And when is there no ‘significant’ change?
Enter the Freeman Near Vision Unit – or more accurately the near duochrome, which is also available on a Mallet unit. I use this tool every day in my practice.
A trusting tool
My routine for determining a near refractive change and a near add is based around this test. Of course, the first step is the distance refraction. I always start with a check of corrected visual acuity (if a correction is worn), right, left and both eyes open (logMAR of course).
I then check right and left duochrome dominance at distance. I also measure unaided vision using a Sheridan-Gardiner, if vision is < 6/60.
Anecdotally, and having worked with many optometrists trained at different institutions and with experience in varying types of practice, I find that there is a predilection not to measure unaided vision. This is, in my opinion, poor practice. Firstly, for medicolegal reasons, I think it is important to know the unaided vision of a patient, even if, in a high prescription, it is relatively low.
Secondly, it is vital to know whether or not a patient might achieve the MOT number-plate standard without prescription. And this may well change with time, even in a patient familiar to you and when you have prior records.
Thirdly, in taking a history, patients frequently mention that their vision is worse. To us as optometrists, this means their vision is worse with their prescription. However, it doesn’t necessarily mean the same to the patient, who may think you are asking about unaided vision, especially if they do not habitually wear spectacles.
Following that, I check near reading acuity at what seems to be their habitual working distance. So far, so good...
But near acuity, especially in the clinical lighting of a consulting room, and in a patient with small pupils, is a capricious measurement because of the depth of focus.
This is where the Freeman Near Vision Unit comes in. I would find the R=G sweet spot for the centre of the depth of focus vision and measure it. If it’s greater than 40cm, I probably need to up a patient’s near add.
However, don’t forget to measure a patient’s PC screen or music distance as well because that is the first clue as to whether the near prescription needs changing. I also find this helpful later on when I switch to the automated-phoropter and get to the point where I want see if the new add is appreciated.
"I absolutely use this tool every day in my practice"
There are two pairs of R/G targets that are polarised, allowing any difference in optimum focus to be visualised monocularly with both eyes open. I set the near focusing rod at the previously measured habitual working distance (often closer than the R=G point). Then I can jump between the dialled in habitual prescription and the new prescription, and the effect is instantaneous. Patients can immediately appreciate the benefit of a prescription change.
The test takes seconds and enhances the flow of the subsequent refraction. There are some people who simply cannot discriminate the nature of the task or appreciate a discreet end point of R=G equality, but, in my experience, that’s rare. Of course, I could just motor on and use acuity only, or use a mallet unit (which also has near duochrome), but I find the former less precise and the latter a bit more clunky to hold.
So what else does the Freeman Near Vision Unit offer? There is an illuminated vocational near chart with text N5-N6-N8-N10 in short paragraphs. However, this is less useful as the illumination biases the result, but you can turn the light out.
A particularly useful feature of this tool is the near tangent screen that is internally illuminated in red and links with a green Maddox rod. It’s a fantastically quick measure of quantifying a completely dissociated near muscle balance. It is also quite useful with latent hypermetropic esophores, and for young myopes within near esophoria who might progress faster or benefit from a near add or multifocal.
The instrument also includes a test of clinical, as opposed to vocational, acuity, which could be used for near cross-cyl and to test for near prism. I never use either of these.
As optometrists, despite having lots of fantastic new technology at our fingertips, we should remember what the public needs from us and how we can achieve that in the most efficient way. Sometimes we can achieve that with something really basic.