Bill Harvey describes a new lens from Essilor aimed at the pre-presbyope suffering from visual fatigue
As a +2D hypermetrope well into his 41st year, I am very conscious of the insidious onset of presbyopia. I was, therefore, intrigued by the new Essilor Anti-Fatigue lens which was launched in the UK on the June 5.
The lens is marketed as suitable for the pre-presbyope (defined by the company as 20 to 45 year-olds) who is experiencing visual fatigue or who has a 'very heavy near vision load', say three or more hours of intermediate or near work per day. One might argue this was the majority of the UK workforce today.
In effect, the lens is a very weak progressive lens. There is a stable clear distance portion which changes progressively down the lens by +0.60D from a mounting reference point which is fitted over the patient's pupil centre. The lens should not be offered to the under-16s (though the large accommodative reserve in the under-20s would make a +0.60D addition an irrelevance) and the company also realises the potential problems of reducing accommodative effort in those patients with a poorly compensated exophoria and does not recommend it for these people also.
So why not give all pre-presbyopes with symptoms a varifocal? Well, the obvious initial answer is that it should not be needed as that is what the eye's accommodative ability is for.
As long as the accommododative demand represents a small portion of accommodative facility then any asthenopic symptoms are more likely to be influenced by changing postural and ergonomic circumstance, as well as viewing behaviour (taking breaks).
Also, progressives with the usual addition range necessarily have some impact (such as unwanted astigmatism) upon peripheral vision and range of vision for various viewing levels, usually requiring the wearer to adjust their viewing position accordingly.
The Anti-Fatigue lens, with such a low addition, has negligible visual distortion.
What makes this particular lens interesting, in my view, is that it is being promoted against a background of scientific study into microfluctuations of accommodation.
Microfluctuations
It has been found that when an eye focuses on a near target, accommodation is not constant but rather fluctuates around an average position.
The microfluctuations do not exceed 0.25D in amplitude and seem to increase with age.1,2 Furthermore, the fluctuations appear to have two components; a low frequency fluctuation (LFC) and a high frequency one (HFC). It is thought by some researchers3,4 that microfluctuations play some role in accommodation regulation based on visual feedback.
It has further been proposed that microfluctuations are related to visual fatigue (and corresponding symptoms of tiredness, poor focusing and so on) and it has been suggested5 that there is an increase in LFCs after sustained near vision tasks. Others have proposed, however, that the HFC is artefactual and linked to ocular pulse influence while the LFCs might be linked to intraocular changes associated with the respiration cycle.6
The Essilor claim is that the Anti-Fatigue lens is able to both reduce LFCs which may be a sign of visual fatigue and yet elevate HFCs which may help in the regulatory feedback for maintenance of optimum accommodation. What is clear is that there still appears to be some debate about the exact nature of the microfluctuations, whether regulatory or artefactual, and further research is needed to qualify some of the claims categorically.
Experience
The lens has been available in France and Spain since autumn 2005 and consumer feedback suggests it has proved particularly popular with the French. The lens also won the Silmo d'Or 2005 and the Janus 2006 de la Sante. It is now available in the UK in Orma 1.5 and Stylis 1.67 materials and with optional coatings. A trial of the lens undertaken recently at Anglia Ruskin University will be reported in a future issue of Optician.
References
1 Heron C, Schor C. The fluctuations of accommodation and ageing. Ophth Physiol Opt, 1995; 15, 445-449.
2 Daras C. Elements de réflexions d'optique physiologique 2006.
3 Miege C, Denieul P. Mean response and osscillations for various stimuli vergences in relation to accommodation feedback control. Ophth Physiol Opt, 8, 1988; 165-171.
4 Denieul P. Etude desfluctuations d'accommodation de l'oeil par optométrie infra rouge. Phd Thesis 1980.
5 Iwasaki T, Kurimoto S. Objective evaluation of eye strain using measurements of accommodative oscillation. Ergonomics, 1987; 30, 581-587.
6 Gray L S, Gilmartin B, Winn B. Accommodative microfluctuations and pupil size during sustained viewing of visual display terminals. Ophth Physiol Opt, 2000, 20, 5-10.