Prescribing near magnification & acuity reserve

As a low vision practitioner I am amazed at how many different ways practitioners calculate the amount of magnification patients require prior to demonstrating low vision aids. With that in mind I have covered one of two of these here with some comments based upon practical experience in the clinic.

1. Using the charts

There is a direct mathematical relationship between letter size notations on the Times New Roman Faculty of Ophthalmologists reading chart:

N12 is half the size of N24
N6 is half the size of N12
N9 is twice the size of N4.5

By using this method: if a patient sees N10 with their current spectacles then one would assume that 2x magnification is required to see N5.

This method is quick and useful for patients with relatively good distance and near acuities. However, this is not an accurate method to use when the VA is poor as there will be several other factors affecting the result.

2. Using the ‘divide by three’ rule.

This ‘rule’ apparently gives an estimation of near acuity based on the distance acuity. For example, if the distance acuity is 6/24, then the expected near acuity should be 24/3 = N8 at a working distance of 25cm (and assuming any relevant near correction in place).

It must be stressed however, that this is just estimation, as in practice, patients with low vision rarely exhibit this relationship. Reading acuity does not correlate well with distance visual acuity and significantly less so with visually impaired patients. Patients with ARMD for example, will often demonstrate a significantly reduced near acuity than what would be expected from their distance acuity, due to the presence of a central scotoma. Posterior subcapsular cataracts can also give anomalous results. The cataract is close to the nodal point of the eye and despite relatively good distance acuity, the near VA can be quite poor. Furthermore, pupillary constriction will enhance the influence of scotomata and opacities. Reading is also a complex visual task; if words are used as a target then it is easy for a patient to guess subsequent words on a page from the meaning of the text.

3. Using a low vision routine.

The technique below is a well-established methodical routine, which is an extremely useful method to assess magnification. It gives a good foundation upon which the rest of the consultation is based. Most low vision practitioners will use this method in some format or another.

Begin by establishing the patient’s baseline near acuity with a +4.00D addition at 25cms. (25 cm being the focal length of the +4.00D lens)
If the level of VA is less than what the patient needs to achieve the desired task, then start by increasing the addition in the trial frame until the desired acuity is met, usually in +4.00 D steps.

When the desired near visual acuity is met, the magnification of the high reading addition thus demonstrated will give the practitioner a starting point magnification with which to demonstrate the device of choice, such as a hand magnifier.
Explain the reduced working distance, and demonstrate the effect of illumination.
If the patient finds the closer working distance unacceptable, then they may need encouragement to try a spectacle magnifier. Instead, try demonstrating alternative aids around the calculated magnification.
Record patient handling, motivation and achieved VA with each aid shown.
With higher magnification, it is important to demonstrate that an increase in magnification means a smaller field of view not to mention a smaller magnifier.

Why +4.00D?

Magnification, in the above example, is merely the comparison of the old object distance to the new object distance. For example, if we cannot see the number plate of a car, we move up closer to it, the retinal image size increasing all of the time. If a patient moves in towards the television by half the total distance, then the image of the television, or the retinal image size, will twice the size. This is termed relative distance magnification.

Magnification = Old object distance / New object distance

When establishing near magnification, as in the above method, by convention, we call the old object distance 25 cm.

For example, if a patient needed to bring the print in to 12.5 cm in order for it to be seen, then the retinal image size is 2x larger than it was when it had been held at 25cm, our conventional starting point:

M = 25/12.5 = x2

What the lenses do is to merely focus the retinal image of the object at the given distance, in this case a +4.00D addition for a 25 cm working distance, and a +8.00D addition for a 12.5 cm working distance. Alternatively, the patient may wish to use their accommodative effort, if they have it. Thus it is typical for a visually impaired child to hold text very close in order to achieve ‘magnification’ of the print or object.

From the above equation, and on the assumption that 25cm is the conventional ‘old’ object distance, we derive the magnification formula,

M = F/4

+4.00D @ 25cm 1x (by convention)
+8.00D @ 12cm 2x
+10.00D@ 10cm 2.5x
+20.00D @ 5cm 5x etc.

From this, we call +4.00D add ‘unit magnification.’ Each successive addition of another 4 dioptres adds another unit of magnification.

From the practical viewpoint, this method quickly establishes the estimated magnification a patient will require to see an object of a given size, most typically, the printed text.

Patient Mr AT

+3.00/-1.00×165 = 6/24+2 +4.00/-1.25×180 = 6/60
LogMAR 0.54 LogMAR 1.00

Add +2.75 = N10@ 30cm Add +2.75 = N24@32cm

Add+4.00 = N6@ 25cm Add +4.00 =N24@ 25cm
Add+6.00 = N5 fair @ 17cm
Add+8.00 = N5 well @ 12cm

• Although he appreciated the significant improvement to his near acuity, Mr AT complained that the short working distance was unacceptable to him. This is not at all unusual. It is typically the younger patients, who have been accommodating to hold work close, who are more tolerant of the shorter working distances.
• The power required to improve his vision to N5 (approx a +6.00D or +8.00D lens) is the power of his own magnifier. As would be expected, it is the poor quality image caused by the scratches on the lens, which is resulting in an under-achievement in his near VA.
• By using the formula M+F/4 by convention, one would expect Mr AT to require a 1.5x – 2x) magnifier to improve his near acuity to N5.

One advantage of using this step-by-step method is that for each additional unit of magnification added in the trial frame, the patient should see an improvement, which does help in restoring confidence. However, there are many patients who resist the close working distance that each additional lens brings, and this could indicate a challenge if a spectacle magnifier is to be prescribed

It is also worth mentioning here that patients with a recent central scotoma often do not show an improvement with each addition. This may indicate the need for eccentric viewing training.

The above method describes the minimum magnification required to view a high contrast near acuity test card, of pre-determined size. However, the actual amount of magnification that is actually prescribed will depend several upon other factors:

Reading speed required for the task: i.e. whether the task involves a degree of fluency or sustained reading, or whether the task is merely a spot checks.
The contrast of the task
The patient’s visual field.

Reading rates & Acuity Reserve

During the previous ‘work-up’, the minimum magnification required for a patient to reach the target acuity has been estimated.

Whilst this minimum amount of magnification may be all that is necessary to look at a washing machine dial or a can label, it is not possible to work at or on this threshold for sustained or fluent reading tasks, such as reading the newspaper or a novel.

For sustained reading, more magnification will be required than what has previously been estimated. In other words, the patients will keep a degree of acuity in reserve.

This acuity reserve is calculated as the ratio of the target acuity to the patient’s threshold acuity (with a device). For sustained reading tasks, this ratio has been estimated to be between 2:1 and 3:1

For example, if a patient needs to be able to read newsprint successfully, (N8 approx.), then we need to consider enough magnification for him or her to be able to read at least N4 or better. If the patient is given an aid with which he can only achieve N8, then it is less likely that reading newsprint will be sustained for any length of time and the outcome will be deemed less successful.

However, for survival or spot reading, an acuity reserve of 1:1 is all that is considered necessary, and a patient can have a magnifier that allows him or her to work close to threshold. Usually in such cases, by giving an unnecessarily strong magnifier would compromise the patient’s field of view and further reduce the working distance of the task.

Rumney, N. J. (1995).  Using Visual thresholds to establish low vision performance. Ophthalmic and Physiological Optics. 15 (Suppl 1), 18-24.
Whittaker SG, Lovie-Kitchin JE.(1993) Visual requirements for reading. Optometry and Vision Science. 70:54-65.
 

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