Refractive Errors and Glasses Prescriptions
Eyesight Vs. Vision – A Behavioural Perspective
To finish this introduction to vision, we will revisit the difference between eyesight and vision, and review basics of refractive error. This time, we’ll look at these conditions in a little more depth, and from primarily a behavioural perspective. Refractive error creates a load on vision that the child must overcome in order to even begin to deal with teacher demands. Let’s begin by reconsidering the distinction between eyesight and vision – this is critical for a few reasons.
Firstly, the concept of eyesight is the most common public conception of what vision is. It is of course a limited view, but an important perspective to understand, especially when trying to explain why vision has a key role in reading and learning. Secondly, vision can be ‘difficult’ for many reasons, but in all cases, there are two likely consequences: Discomfort and difficulty finding and targeting objects and symbols of interest. Finally, while the terms ‘nearsighted’, ‘farsighted’, and ‘astigmatism’ are fairly commonly used, not many laypeople will understand the distinction between them. Again, it is not simply a matter of images being ‘blurry’. These terms will be revisited later in the context of how they impact reading and learning. For now, we will simply review their basic meanings.
Accommodation, or focusing, is triggered by blur occurring due to refractive errors in one or both eyes, or due to a difference in refractive status between the eyes (anisometropia). The focusing reflex is a very low level neural process, but can be modulated through conscious effort, optical correction, and visual therapy. The purpose of accommodation is to bring images into sharp clarity onto the retina. If the eyeball were a camera, the retina would be the film onto which light is focused to make the image clear. The image we are viewing can be focused too far in front of the retina in which case the eye must ‘relax’ focus to let the image fall back, or the image might be too far behind the retina and the eye must work to focus to bring it forward. As these terms suggest, ‘relaxing’ vision feels good, and forceful focusing is felt as muscular strain.
Neutral Eyesight (Emmetropia): The eye’s focusing system is tuned for the distance, that is, distant objects are clear with no effort from the focusing muscles inside the eye. As objects are brought closer to the eye, the image inside the eye is pushed back behind the retina, so the focusing system must engage to bring the image closer in, or forward towards the retina.
Nearsightedness (myopia): Images in the distance come to focus in front of the retina, and so they appear blurred on the retina. The focusing system must relax to let the image fall back onto the retina, but since the eyes are already fully relaxed, there is no room to move. ‘Minus power’ lenses help to push the image focus back onto the retina so distance objects can be seen clearly. Nearer objects will appear clear without glasses. Nearsighted people find near work is easy on the eyes, if they are mildly or moderately nearsighted; with no glasses on, they will see perfectly well and in good comfort at near distances. People who are highly nearsighted must bring objects in so close to see them clearly that the eyes must cross inwards, and the arms, head and neck must be manipulated to hold that near posture. Nearsighted children, because they will always struggle with distant eye charts, will almost always be detected by simple sight tests in school.
Farsightedness (hyperopia): Images always come into focus behind the retina, and so the image on the retina is blurred. The eyes must engage the focusing system to bring the objects into clear view, even if the objects are in the distance. As nearer objects are viewed, the focus is pushed further behind the eyes, and so more effort is required to focus to bring the clear image forward and closer to the retina. When farsighted eyes exert too much effort, or for too long a time, the focusing mechanism relaxes, or releases, making images blurry whether they are near or far. In severe cases, focusing requires so much effort that the eyes simply give up trying to focus, and the child is left in a permanent state of blur which leads to amblyopia (sometimes called ‘lazy eye’). Because farsighted children can often make enough of an effort to focus, even for a short period, they can often see the small letters on distant and even near eye charts. Farsightedness is a leading obstacle to learning and makes near work especially uncomfortable, but because children can manage to see distant eye charts clearly, they are almost never detected using this technique.
Astigmatism: In nearsightedness and farsightedness, the problem is that the ‘plane’ of focus is too far forward of, or too far behind the retina, respectively. An adjustment of focus either with glasses or with the focusing system will move that plane of focus to the retina so we can see the object clearly. In astigmatism, there are two planes of focus, one in front of the other. These planes might both be behind the retina, in front of the retina, or one in front and one behind. In any these configurations of astigmatism, the eye is faced with a dilemma: Which of the two planes of focus should it adjust to see the object clearly? Since the object image is represented on two planes of focus, there is no answer to the question, the eye must try to clear both planes simultaneously. Practically, this means that if one plane is in focus, the other will not be. The end result is that the eye, with the brain’s input, must continuously analyse the images and try to make them clear. The constant strain can be both painful and discourage a child from looking at detailed objects, such as letters and words. Distance or near viewing is not the concern with astigmatism, both will be blurred, and both will require effort – all the time, just like with farsightedness. Astigmatism will also cause objects to appear distorted, and this leads to difficult reading and viewing of finely detailed objects. Astigmatism, then, can also lead to amblyopia if not corrected early enough.
Anisometropia is another condition that is often uncomfortable. Anisometropic refractive errors, such as the name suggests, describes a condition where there is a significant difference in optical properties of one eye compared to the other. One eye might be more nearsighted or farsighted than the other, or perhaps one eye is farsighted while the other is either emmetropic or nearsighted. There are many combinations of anisometropic errors, but significant anisometropia is relatively uncommon compared to simple astigmatism, hyperopia, or myopia. Moderate amounts of hyperopia and astigmatism can be especially uncomfortable and noxious to reading and learning, while relatively lower amounts of anisometropia can have a similar effect, depending on the magnitude and type of differences between the eyes.
Eyesight, then, can be described in terms of blur, but more correctly in terms of when objects appear blurry – distant, near, or both. Approximately 80% of this focusing power of the eye is from the cornea, which is invariant. The remaining 20% of accommodation is from the effort of the ciliary body pulling and releasing tension on the lens, which lies immediately behind the iris. The refractive error, otherwise known as the glasses prescription is the optical description of the eye’s natural image resolving power, that is, its ability to make an object focus clearly onto the retina with no assistance from the focusing system. Correcting for the distance refractive error will provide maximal resolving power at infinity, provided there is no pathology to prevent this.
In optometry, the refractive error is expressed in the following format:
OD: +1.50 – 1.00 x 090
OS: -2.00 – 3.00 x 060
These are the powers of the lenses required to correct these ‘errors’. OD is from the Latin ‘oculus dexter’, meaning ‘right eye’, and OS is ‘oculus sinister’, for the left eye. The numbers “+1.50” and “-2.00” refer to the amount of farsightedness and nearsightedness, respectively. ‘Plus’ is used to denote farsightedness, and ‘minus’ to denote nearsightedness: This reflects the type of lens required to correct for the error. Convex, or outwardly curving lenses are ‘plus’ lenses and will correct for a deficit in accommodation caused by hyperopia – that is, the eye is already stressed for need of constant accommodation, so this is considered a deficit position as accommodative strain is generally unwanted. In the case of myopia, the eye’s static accommodative status is already over-focused with no effort at all, with near object appearing clearly while distant objects are blurred. Myopes start from a natural focusing posture that is nearer to the page than that of their emmetropic and hyperopic peers, and therefore less effort is required to focus on the page in front of them. Since myopes are naturally over-focused for the distance, we must ‘unfocus’ the light by using ‘minus’ power lenses, that is to say concave or inwardly curving lenses.
The second sets of numbers, ‘– 1.00 x 090’ and ‘– 3.00 x 060’ describe the power and ‘axis’, or orientation, of the astigmatism. The axis is measured across 180 degrees, with zero degrees marked as 180o as opposed to 0o for clarity, given they both denote a perfectly horizontal axis. Horizontally and vertically oriented astigmatism is easier to deal with in the classroom than diagonal, or ‘oblique’ astigmatism. The horizontal and vertical varieties compresses letters vertically or laterally, respectively, while oblique astigmatism twists or distorts letters making them more unrecognizable. It has been shown that as little as one diopter of oblique astigmatism will measurably slow reading.
Finally, while blur is important, what is more critical is the effort the eyes must provide in order to create clear retinal imaging. Astigmatism, like farsightedness, not only leads to blur, but causes the focusing system to engage much more frequently and for greater durations than nearsightedness in which the focusing system is most often in a relaxed state. It is this relative difference in effort of focusing that gives nearsighted children an advantage in the classroom.
The Neurophysiology of Near Vision
The focusing system consists of the natural lens and a few small muscles inside the front of the eye that tug on it to change its shape. The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion and controls the ciliary muscle causing accommodation, and also to pupillary sphincter muscle leading to pupil constriction. Sympathetic post-ganglionic fibers also join the nerve from the internal carotid artery plexus in the wall of the cavernous sinus, and these serve to counterbalance the parasympathetic input.
The Edinger–Westphal nucleus also supplies collateral connections to four of six extra-ocular muscles, linking pupillary constriction to accommodation and convergence of the eyes. This is known as the near triad. The synergy in the near triad provides the necessary optical response for viewing near objects: The eyes converge to target the near object simultaneously with the greatest degree of visual overlap possible (binocular fusion), the lenses accommodate to bring the object into clear focus, and the narrowed aperture of the constricted pupil acts as a camera’s aperture on a higher f-stop providing greater depth of field.
The synergy in the near triad can also work against a child when astigmatism and hyperopia create a constant accommodative strain leading to discomfort, pain, and amblyopia. In some cases, the accommodative impulse is strong enough and persistent enough to lead to esotropia, an inward-turning strabismus, which will further deepen the amblyopia in the turned eye. These cases are often successfully managed non-surgically through visual neuro-rehabilitation.
The accommodative response occurs from the blurring of the retinal image created by effortful viewing of an object of interest. Certain rules apply to gauging accommodative effort.
- Astigmatism induces a constant accommodative strain.
- This can be significant at values above 0.75 diopters, and especially uncomfortable between 1.5 and 3.5 diopters. Beyond 3.5 diopters, human physiology tends to prefer to not attempt accommodation, choosing rather to simply relax focus and leave the image blurred, thus leading to amblyopia.
- Hyperopia requires a constant accommodative response, even while looking into the distance.
- Distance viewing provides the most comfortable vision for emmetropes (neutrally-sighted people). Myopes are comfortable viewing the distance, but the view is blurred.
- Nearer objects require an accommodative response as well as convergence of the eyes.
- The required visual response increases geometrically with increasing proximity of the object viewed.
It becomes clear that difficulties with accommodative and convergence responses, or significant astigmatic or hyperopic refractive errors provide a notable disadvantage in a classroom where most work is effected within the Harmon distance.