Pinholes and presbyopia: solution or sideshow?

The gradual decline with age in the amplitude of accommodation, leading to presbyopia at the age of about 40 years and the complete absence of active accommodation after the age of about 55 years, has always presented problems for those lucky enough to survive into middle age and beyond. They resent the fact that they no longer possess their youthful ability to see clearly over a wide range of distances. The introduction of spectacles in the 13 C provided, for the first time, a practical solution to these difficulties. Later spectacle lens developments, starting with bifocals and proliferating into many different types of multifocal and progressive lens, have allowed the differing visual needs of many presbyopes to be adequately satisfied. Nevertheless, spectacles do have optical, cosmetic and other limitations which cause some presbyopes to wish for spectacle-independence. The search has therefore continued for improved alternative methods for overcoming the visual difficulties associated with presbyopia. The world population of presbyopes exceeds 1 billion, with more than 30 million of these in the UK, so that the potential market for any method which successfully meets the needs of even of small fraction of this population is huge. Contact-lens (CL) methods have figured prominently among the various newer approaches that have been explored. Many of the optical concepts pioneered by the CL workers, such asmonovision, simultaneous-image lenses and modified monovision, have been transferred to presbyopic refractive surgery techniques and intraocular lens designs. It is, however, surprising to find that, in recent years, variants of the very old technique of using a circular stenopaic aperture (‘pinhole’) to increase ocular depth-of-focus (DOF) and hence allow reasonable imagery over a range of object distances in the absence of any active accommodation have re-emerged as serious contenders as aids for presbyopes who have low levels of distance ametropia. This pinhole effect, by which a reduction in the effective pupil diameter reduces the diameter of the retinal blur circle formed for any error of focus and hence improves the clarity of the associated retinal image, is, of course, widely used in the clinical pinhole test to determine whether reduced acuity is due to refractive error or amblyopia. Theoretically, for an aberration-free eye the DOF would be expected to vary with the inverse square of the pupil diameter but the DOF found in practice is increased by the monochromatic and chromatic aberrations of real eyes, particularly at larger pupil diameters. Figure 1 shows recent data for through-focus acuity at two pupil diameters when aberrations are corrected. Evidently acuity declines more slowly with defocus when the pupil diameter is smaller, although optimal acuity is better with larger pupils due to the greater diffractive blur associated with smaller diameters. The value of the DOF derived from plots like Figure 1 depends upon the criterion used to assess it, e.g. the dioptric range over which acuity is 0.1 logMAR or better. Figure 2 shows some typical experimental measurements of the pupil-dependence of DOF for normal aberrated eyes. Values differ between studies due to the differences in the blur criteria and methodology used but it is clear that DOF in dioptric terms increases rapidly as the pupil diameter is reduced below 2 mm. However, although ‘pinhole’ corrections can undoubtedly give many near-emmetropic presbyopes adequate visual acuity at both distance and near under some conditions, can they really be recommended as the ideal solution for the visual needs of the older patient? Might patients equipped with such devices sometimes suffer from vision which is degraded to an extent which could cause them to be put at risk? To answer such questions, it is helpful to consider the history of devices in which some form of pinhole is placed in the optical path between the object and the retina.