By Dr Frank W Howes, MBChB MMed FCS FRCS FRCOphth FRANZCO
Presbyopia is a frustrating phenomenon reaching everyone eventually, some people more than others, some earlier in life and some later in life, but eventually everyone. Solutions from past to present have revolved around scleral expansion techniques to retain natural accommodation, to corneal inlay techniques, producing pinholing, around monovision induction using corneal steepening techniques, with holmium laser and conductive heating but mostly with the excimer laser ablation. There have been lens based techniques, usually lensectomy or cataract surgery, with low myopia induction, and in appropriately selected people, using multifocal intraocular lenses (piggy back or bag – multifocal phakic IOLs have been available in the past and may be so in the future). This myriad of options suggests that none are perfect, and some simply don’t work! This also confirms the necessity for the surgeon or optometrist to perform some sort of simulation trialing in most patients (contact lenses, loose lenses, or whatever is deemed closest to the effects of the intervention) to demonstrate both the positive and negative attributes of the technology and the all important permanency of the intervention.
While all ophthalmologists have their preferred surgical approach to presbyopia (as is outlined by the different views of my colleagues within this newsletter), my choice is usually a refractive lens exchange. As IOL technology improves and our understanding of optics and optical aberration improves newer options for presbyopia will potentially become available:
The cause of presbyopia relates, in the majority, to progressive inelasticity of the crystalline lens with ciliary accommodative effort no longer achieving the quantum of lenticular power change required to bring a near focal point into sharp focus. The crystalline lens shows an almost linear loss of elasticity, and hence accommodative power, from birth to approximately age 65 when the lens becomes motionless, ie a decline in accommodative power from approximately 18 diopters at the age of 5years to zero at the age of 65 years, with approximately 3 diopters left at age 45years; 3 diopters of focal power provides a focal point at 33cms, a comfortable ‘arms length’ for reading in an average sized person. So when arms become too short – Presbyopia has arrived.
Now, not only do we lose elasticity in our crystalline lenses with time, so does the optics of the lens change, with a shift of the youthful negative asphericity (which balances the usual positive corneal asphericity, thus providing the excellent ‘High Definition’ vision of youth), to the more positive asphericity which accompanies the ageing lenticular nuclear sclerosis. The loss of this balance degrades the quality of vision as we get older.
The discussion above refers to the average. As usual there are outliers to the general rule on both sides, with some people showing presbyopic symptoms earlier than others and some showing symptoms later. There is information emerging that seems to demonstrate that the shift in the asphericity balance spoken of above to more net positive asphericity in an eye with time (or high base corneal positive spherical aberration) may lead to early presbyopia and vice versa. More retained negative spherical aberration (SA) may promote the retention of a form of accommodation dubbed pseudo-accommodation (probably in this case an increase in depth of field, supplementing pupil size). This status quo is aside from the base myopic and hyperopic prescriptions and pupil size which, as is well known, retards or advances presbyopic symptoms.
If we can harness this status with more understanding we would be able to provide people with greater near function without excessive distance function loss by this phenomenon of psuedoaccommodation. This would allow multifocal vision (or shall we use the catch phrase blended vision), without the penalty of multiple retinal imagery of multifocal lenses, or the excessive monovision induction which increases anisometropia and asthenopia and avoids the corneal risks of laser and inlays.
Are we there yet? Yes!! With the new range of aspheric monofocal IOLs which are available with asphericity values from +0.4um RMS SA to -0.3um RMS SA in 4 steps, we have the capability of restoring youthful total eye asphericity. The obvious target for Hi Def vision is Oum SA (perfect asphere) to +0.1um RMS of spherical aberration (this is the spherical aberration measured in the sharp shooting US fighter pilots), a great place for our dominant eyes to be in for distance vision. However, by measuring the total corneal asphericity and selecting an IOL with the appropriate asphericity value, we can now target greater degrees of negative asphericity thus enhancing depth of field and psuedoaccommodatino. By this mechanism, small degrees of monovision (eg -0.75DS) can provide greater near vision in the nondominant eye without significant loss of distance acuity!
This type of correction still provides excellent contrast sensitivity and MTF (modulation transfer function) – good, sharp, ‘nonhaloed’ vision, as opposed to some other forms of presbyopic correction. The question remains whether targeting a similar ‘defocus’ in the positive spherical aberration range will provide a similar effect. With these new aspheric monofocal lenses discussed above now available we should be able to find this out. Obviously the usual risks of surgery still apply, but the safety, speed, techniques and technology of microincisional surgery in this day of modern surgery are exceptional.
Roll on Refractive Lens Exchange!