Almost all patients who enter an ophthalmologist’s office require a determination of the refractive status of their eyes, for either diagnosis or treatment. Only by correcting the patient’s refractive error can the ophthalmologist distinguish between visual loss caused by organic disease and that caused by a refractive error. Any visual loss not amenable to correction by lenses is regarded as a pathologic condition. Unexplained visual loss not corrected by glasses must always be investigated.
Many patients, regardless of their problem, be it fatigue with driving or headache, expect to receive a pair of glasses to remedy their complaints. Some patients are methodic about having their glasses changed every year or every 2 years; they believe that glasses, like tires, will wear out in time. Others think that glasses have a therapeutic effect on the eyes, maintaining them in good performance and preserving their integrity. Vision, especially in younger individuals and the presbyopic patient, may deteriorate with time, necessitating a change in correction but the health of the eye is not affected, for better or worse, regardless of changes made in the glasses. There is some evidence, however, that in the young, plus correction may prevent the normal loss of hyperopia and minus correction may exacerbate the development of myopia.
Glasses function to improve visual performance, to relieve the symptoms of refractive errors and muscular imbalance of the eyes, and to prevent suppression of one eye in children younger than 5 years, when the refractive difference between the two eyes is great. This chapter deals with the signs and symptoms of refractive errors and the therapy available for their treatment.
The emmetropic eye is a normal eye in which all the rays of light from a distantly fixated object are imaged sharply on the retina without the necessity of any accommodative effort. This is a relatively uncommon condition ( Fig. 12.1 ).
There are three basic abnormalities in the refractive state of the eye: hyperopia or hypermetropia, myopia, and astigmatism.
The hyperopic or far-sighted eye is one that is deficient in refractive power so that rays of light from a distant object come to a focus at a point behind the retina with respect to the unaccommodated eye ( Fig. 12.2 ). Consequently, the image that falls on the retina is blurred and can be brought into focus only by accommodation or by placing a plus, or convex, lens in front of the eye. The convex lens supplies the converging power that the eye is lacking.
In most cases of hyperopia, the chief cause is a shortening of the anteroposterior axis of the eye. Such an eye is smaller than the normal, or emmetropic, eye. At birth almost all human eyes are hyperopic or shorter than normal, to the extent of 2.00 or 3.00 diopters. With growth, the eye lengthens and approaches the normal length of an adult eye. Each millimeter of shortening of the eye is represented by 3.00 diopters of refractive change. This shortening of the globe results in axial hyperopia .
Another cause of hyperopia is found when the front surface of the eye (the cornea or lens) has less curvature than normal so that the image formed is focused at a point behind the normally placed retina. This is called curvature hyperopia .
From a practical standpoint, the cause of the hyperopia is not of great importance. What is significant is whether the accommodative system of the eye can supply an additional plus power to correct the hyperopic error. Young people are usually not handicapped by hyperopia because of their excellent range of accommodation.
Hyperopia may be latent, manifest, or absolute. Latent hyperopia is the portion of the hyperopic error that is completely corrected by the eye’s own accommodation. The compensation is so complete that any attempt to place a plus lens in front of such an eye will merely blur the vision. Manifest hyperopia (facultative hyperopia) is the element of the refractive error that can be corrected either by convex lenses or by the patient’s own accommodation. In both latent and manifest hyperopia, the patient has normal visual acuity ( Fig. 12.3 ). Absolute hyperopia is the portion of the refractive error that is not compensated for by accommodation ( Fig. 12.4 ).
To understand the three types, consider the following case. A 32-year-old man is found to have a visual acuity of 20/50. A +1.00 lens is given, which improves his vision to 20/20. This means that the patient has 1.00 diopter of absolute hyperopia. It is found, however, that the patient can still see 20/20 if an additional 1.50 diopters are placed before the absolute correction. Thus the patient is found to have 1.50 diopters of manifest hyperopia. A cycloplegic examination is performed and it is found that the patient requires 3.50 diopters of plus lenses to enable him to see 20/20. Of the 3.50 diopters, 1.00 diopter we know has been accounted for in the form of absolute hyperopia , 1.50 diopters were present as manifest hyperopia , and 1.00 diopter remains in the form of latent hyperopia . Such a patient requires 1.00 diopter, will accept up to 2.50 diopters, but cannot be given the full hyperopic correction of 3.50 diopters.
Role of cycloplegia
Cycloplegic drops paralyze accommodation and thereby prevent the accommodative effort required to compensate for hyperopia. Therefore under cycloplegic examination all of the hyperopia is uncovered. Full correction of hyperopic errors, however, can never be based on cycloplegic findings because correction of the latent factor will only blur distance vision. The findings may be unreliable, however, in the management of accommodating esotropia. Cycloplegic examination indicates the magnitude of the refractive error. Noncycloplegic examination reveals the acceptability of a particular correction.
In the young, the condition may cause no symptoms, because a healthy youngster has an ample reserve of accommodation and, if hyperopic, accommodates for distant and near objects without being conscious of the act. Thus a 5-year-old may have 4.00 diopters of hyperopia and not require any spectacle correction whatsoever. It is usually in older adults that the symptoms of hyperopia become apparent, as educational demands and the time allotted for close work increase and accommodative reserves decrease ( Fig. 12.5 ).
The symptoms of eyestrain are many and varied. They include headaches, burning of the eyes, and a pulling sensation of the eyes. These symptoms are generally related to the constant excessive accommodation that is required for close work. In older patients, no symptoms may appear until the power of accommodation has diminished to the extent that the near point is beyond the range of comfortable reading distance, so that close work has to be held farther away than usual to be seen clearly. The greater the degree of hyperopia, the sooner this symptom arises; therefore presbyopia commences at an earlier age than usual in the uncorrected hyperopic eye.
The treatment of hyperopia is based on the patient’s symptoms, occupation, and ability to compensate for close work. In the very young, the treatment of hyperopia is usually unnecessary. The only exception to this rule occurs with accommodative strabismus . In this condition, part or all of the strabismus may be corrected by the use of convex lenses, which decrease the need for accommodation and thus for the associated excessive convergence.
In older adults, hyperopia is always corrected to improve near vision. Some believe the facultative component is never fully corrected unless the patient complains of fatigue and headaches. Whereas a 5-year-old may be oblivious to 4.00 to 5.00 diopters of hyperopia, a young college student may be very distressed by the presence of even 1.00 diopter of hyperopia. Such a patient needs to wear glasses only when the demands on accommodation are the greatest, that is, for performing close work. Some doctors, however, believe that the facultative component should be fully corrected.
In a middle-aged person, reading glasses become a necessity. The decline in accommodative power becomes so great that the patient is totally unable to see at a comfortable reading distance without convex lenses. Moreover, the power of the lenses exceeds the absolute and facultative demands of the hyperopia so that the patient can see comfortably with reading glasses for close work, but the vision is totally blurred when these lenses are used for distance vision.
Older adults, particularly those between 55 and 65 years, find it difficult to accommodate even 1.00 diopter. This type of hyperopic patient usually needs convex lenses for both distance vision and close work.
Myopia, or near-sightedness, is that condition in which parallel rays of light come to focus at a point just in front of the retina with respect to the unaccommodated eye ( Fig. 12.6 ). The myopic eye has basically too much plus power for its size. The myope has a fixed far point in space. For example, a person with 1.00-diopter myopia can see an object clearly if it is 3 feet (1 m) from the eye ( Fig. 12.7 ).
In axial myopia , the eyeball is too long for the normal refractive power of the lens and the cornea. Parallel rays of light are brought to a converging point usually somewhere in the vitreous in front of the retina. This type of eye is larger than normal.
In curvature myopia , the eye is of normal size but the curvatures of the cornea and lens are increased.
Index myopia is a result of a change in the index of refraction of the lens. This is witnessed in two pathologic states: diabetes and cataract. In diabetes, the lens loses water because of the high level of blood sugar in the anterior chamber, and therefore its index of refraction increases. In the cataract patient, the lens becomes increasingly hard because of the constant lamination of lens fibers being pushed to the center of the lens. The hard inner core increases the index of refraction of the entire lens structure, thereby increasing the converging power.
Almost everything has been blamed as a cause of myopia: diet, obesity, allergy, lighting conditions, vitamin deficiencies, and even wearing glasses too much or too little. Controversy and heated debate have raged about whether excessive close work or reading is a primary cause of myopia.
Much research and clinical investigation have been carried out in trying to understand the development of myopia. The results, however, have been inconclusive.
In the great majority of cases, the near-sighted eye is longer than the normal eye. Just as some people are tall and some are short, so some people are far-sighted and some are near-sighted. The near-sighted eye has grown longer than normal.
Most authorities agree that some myopia is familial, passed from one generation to another as a dominant trait. In fact, it is uncommon to find a myope who does not have one parent or siblings with a similar condition.
In the past, the causes of myopia were a subject of heated debate and its treatment was often based on speculative theories. One school of thought held that myopia was a result of an excessive accommodative effort. In this regard, children often were given bifocals to prevent them from using their own accommodation to see objects at near. Often limitations were placed on them at school and the child was allowed to use the eyes for homework only for a period not exceeding 1 hour. A variation of this line of thinking led to the undercorrection of myopia. Because these children could never see adequately at a distance, they would not use their accommodation. It was believed that not making an effort at accommodation would prevent the progress of the myopia.
Another group believed that myopia was caused not by excessive accommodation but by a lack of it. The contention was that a myope has to make less of an accommodative effort at near than an emmetrope or a hyperope. Therefore to increase the circulation to the ciliary muscle and improve the health of the eye, this group advocated overcorrecting the myopia so that the individual would have to accommodate more than necessary.
Another prevalent speculation was that myopia was a result of a vitamin deficiency, especially during the growing years. To cope with this deficiency, calcium and vitamin D were prescribed during the active growth period, especially during adolescence, when myopia was thought to increase the most.
One group believed that myopia was related to deficient lighting conditions while children were reading. With this in mind, many parents became alarmed if they discovered their children reading in bed during the twilight hours and using only the available natural light.
Refractive surgery has become popular as a way of treating myopia (see Chapter 36 ). With the higher degrees of myopia, removal of the lens (lensectomy) has been advocated as a way of countering myopic effects. Some of these patients were treated by replacing the lenses of their eyes with artificial lenses. Other operations for the relief of high myopia included shortening the eyeball or flattening a central portion of the cornea.
The overwork theory of accommodation excess in myopia has been resurrected. It has been shown that some groups of students seem to have a higher incidence of progression of their myopia than do individuals who leave school at an early age and who do not do any close work. Furthermore, some doctors treat myopia by placing atropine or homatropine drops into myopic eyes to relax the ciliary muscle.
Myopia is rare at birth. It usually manifests after the fourth year of life. Its progression is relatively constant until the time of puberty. At that time, the myopia may change alarmingly and progress rapidly, requiring changes of glasses every 6 months. Normally, the myopia becomes arrested when full maturity is reached. Therefore the ages between 20 and 40 years are relatively quiet and the myope’s correction may remain virtually unchanged during this period except for a condition called progressive myopia.
The most outstanding symptom of myopia is inability to see objects clearly at a distance. Near vision is always good ( Fig. 12.8 ). Myopic children often regard this as the natural order of life: objects in the distance are fuzzy, whereas those at close range are clear. In many cases, the myope is not detected until the school runs a visual screening program. Older children often learn of their condition when they discover that classmates sitting beside them can see the blackboard with ease, whereas they see it only with difficulty.
The antics of the near-sighted movie cartoon character Mr. Magoo are well known. The humorous episodes in which Magoo mistakes a gorilla for his wife make us laugh, but they also make us aware of the danger and menace that a near-sighted person can be to society. Just imagine what a hazard Magoo would be on today’s highways!
Many myopic children have a tendency to squeeze their lids around their eyes to create the effect of a pinhole camera. This is one expedient the myope uses to obtain better vision in the distance. The constant squeezing of the lids, however, may lead to headaches and general eyestrain. Squinting is not a substitute for spectacles.
Occasionally, myopia develops in early childhood and reaches alarming degrees. Children younger than the age of 10 years who have myopia of −6.00 diopters or more often develop secondary visual complications because of the elongation of the eyeball and a thinning of the sclera ( Fig. 12.9 ). These complications, such as seeing spots before the eyes because of vitreous degeneration, may be relatively harmless. Others, such as retinal degeneration and detachment and macular hemorrhage, may be more serious. Invariably, all myopia more than 10.00 diopters in magnitude is axial, that is, associated with an elongated eyeball. Because of these complications, the myope in particular should be examined yearly, not only for alteration of the spectacle correction but also for a thorough retinal examination.
Most doctors feel myopia should be fully corrected at all times so that the person can enjoy comfortable and clear distance vision ( Fig. 12.10 ). Some doctors, however, believe that myopia should be undercorrected and that the myope should read without glasses. They believe this prevents a further increase in myopia. The full correction, however, enables the myope to establish a normal relationship between accommodation and convergence. Myopic children require no special inducements to wear their glasses. These children, on receiving their glasses, soon learn to enjoy the sharp, clean edges of clear vision and will reach for their glasses the first thing on arising in the morning.
Moreover, myopes are among the most conscientious regarding reappointments. Although they do not initially recognize their own visual defect, once they receive glasses they become acutely aware of the progress of their myopia and changes in the clarity of things.
People with myopia of high magnitude, such as 4.00 diopters or greater, are slightly handicapped by the fact that their image size is smaller. A high myopic lens placed at a distance from the eye has a minifying effect. This effect may be offset by the use of contact lenses. Not only is the image size more normal with contact lenses, but also the aberrations of the thick lens are eliminated and the field is enlarged. The use of contact lenses in high myopia, that is, − 10.00 diopters or greater, is especially recommended because in these ranges secondary disadvantages of spectacle correction are great.
Astigmatism is the condition in which rays of light are not refracted equally in all directions, so that a point focus on the retina is not attained.
Regular astigmatism is a refractive condition that is amenable to correction by cylinders. The axes of the principal meridians of the astigmatism are at right angles to each other. If the axis of the astigmatism deviates from either horizontal or vertical meridians, generally the deviation is symmetric in the two eyes.
Regular astigmatism may be subdivided into the following groups:
In simple astigmatism , one of the focal lines always falls on the retina; that is, one meridian is emmetropic. The other meridian may have its focus behind the retina or in front of it. The condition then is referred to either as simple hyperopic astigmatism or as simple myopic astigmatism , respectively ( Fig. 12.11A and B ).
In compound astigmatism , the rays of light are refracted so that both focal points lie either in front of the retina or behind it. The former is referred to as compound myopic astigmatism and the latter as compound hyperopic astigmatism ( Fig. 12.11C and D ).
In mixed astigmatism , one focal point lies behind the retina, whereas the other focal point lies in front of it ( Fig. 12.11E ).
If the cornea has been damaged by trauma, inflammation, scar tissue, or developmental anomalies so that a geometric form is not adhered to, the resultant condition is called irregular astigmatism . In view of the irregularity of the corneal surface and the lack of any geometric form, this condition cannot usually be completely corrected by cylinders.
In most instances, astigmatism results because the radius of curvature of the cornea is not equal in all directions. Although at birth the cornea is usually a perfect sphere, by the age of 4 years, it loses its spherical qualities. The horizontal axis (vertical radius) of the cornea becomes more steeply inclined so that rays of light are refracted more acutely than those rays being refracted along the vertical axis of the cornea. This type of astigmatism is commonly referred to as with-the-rule astigmatism . The astigmatism in which the vertical axis (horizontal radius) of the cornea is stronger than the horizontal one is referred to as against-the-rule astigmatism . In the formative years, astigmatism may alter in small increments, but its axis usually remains relatively unchanged.
Although astigmatism most commonly results from a cornea that is not spherical, in some instances the astigmatism may be the result of an unequal bending of light by the crystalline lens, the so-called lenticular astigmatism .
Problems of astigmatic individuals
Because neither the horizontal nor the vertical axis forms a point focus, the person with an astigmatic condition usually chooses the more normal, or emmetropic, axis for seeing. If the two axes are equally in focus, then the vertical focal line is, as a rule, preferentially chosen. The object being viewed will appear somewhat indistinct.
Consider an individual with an astigmatic problem in which the vertical axis is focused on the retina and the object of regard is a cross. Each point of the cross that is imaged on the retina is elongated in a vertical direction. The horizontal line therefore appears as a series of short vertical lines that elongate into a broad, blurred band; in the vertical line, the vertical strokes are superimposed and cover each other so that the whole line appears sharply defined and black, with only the uppermost and lowermost of the vertical lines having a vertical, brush-like appearance ( Fig. 12.12 ).
Obviously, the most common complaint of the patient with astigmatism is inability to see at both distance and near, whereas the hyperopic person normally can see efficiently at a distance and the myope sees quite adequately at near.
As with other refractive errors, the astigmatic patient uses many compensatory movements to improve vision. There may be a tendency to half close the lids to make a horizontal slit between the lids and cut off the rays in one meridian. Reading matter may be held very close to the eyes to obtain a large, even though blurred, retinal image.
Refractometry and refraction
Refractometry is defined as the measurement of refractive error and it should not be confused with the term refraction . Refraction is defined as the sum of steps performed in arriving at a decision as to what lens or lenses (if any) will most benefit the patient. These steps include, in addition to refractometry, measurement of visual acuity, measurement of accommodative ability, and the exercise of clinical judgment. Refraction, often referred to as an art, is generally considered to require a license for its practice and constitutes a major activity of ophthalmology and optometry.
Refractometry, however, is strictly limited to clinical application of optical principles. This measurement function can be performed at the highest level of precision by technicians and, in some cases, even by sophisticated instruments and computers.
The exercise of clinical judgment included in the foregoing definition of refraction refers to a consideration of such factors as the patient’s occupational requirements, muscle balance, impairment of vision by other than refractive error (such as cataract, macular degeneration, or suppression amblyopia), the extent and type of refractive error present, and even the emotional “set” of the patient with respect to wearing glasses. (For some patients, even though there may be a significant error, the maximum benefit is achieved by prescribing no lenses at all.) Refractometry may be classified in several different ways: preliminary versus refining, objective versus subjective, and cylinder versus sphere. These classifications are integrated in Table 12.1 .
|Objective (retinoscope and objective separators)||Approximate||Approximate|
|Subjective (dials and cylinders)||—||—|
|Subjective (cross cylinder)||—||Precise|
Methods of refractometry
For convenience, refractometric methods are considered here under two headings: objective and subjective. Objective methods provide the advantage of permitting measurements to be made without requiring the patient to give answers; adequate measurements can be made in patients who are unable or unwilling to answer questions but who cooperate to the extent of fixating a distant target. The disadvantages of objective methods are that they require a moderate-sized or dilated pupil and they cannot be relied on to provide data that are sufficiently accurate to provide the basis of a prescription. The advantages of subjective methods are that they can be performed even when the pupil is very small and they provide data that are much more precise and reliable; their use, however, is limited by the amount of patient judgment and participation required.
Steps for refractometry
At the beginning of any refraction, a decision must be made about whether to use drops and, if so, what drops to use ( Table 12.2 ).
|Drug||Onset of maximum cycloplegia||Duration of activity||Comment|
|Atropine sulfate 0.5%, 1%||45–120 min||7–14 days, especially in a blue-eyed child||Not used routinely except for the assessment of accommodative strabismus in children|
|Scopolamine hydrobromide 0.25%||30–60 min||4–7 days||Used in atropine-allergic patients|
|Homatropine hydrobromide 2%, 5%||30–60 min||3 days||Requires half an hour to an hour to take effect and lasts 3 days; not used routinely|
|Cyclopentolate hydrochloride 0.5%, 1%, 2% (Cyclogyl)||30–60 min||6–24 h||Active in 30–60 min; two sets of drops given 5 min apart; a good rapid-acting cycloplegic drop for office use|
|Tropicamide 0.5%, 1% (Mydriacyl)||20–40 min||4–6 h||A good drug for office use with an effect similar to Cyclogyl|
|Phenylephrine hydrochloride 2.5%||30–60 min (no cycloplegia)||30 min–2 h||Little effect on accommodation; 2.5% most commonly used because of potential systemic effects. Avoid 10%|
Most refractionists use cycloplegic drops on anyone up to the age of 20 years. The drops impair the power of accommodation by inhibiting the ciliary muscle. They also dilate the pupil. Thus the drops have two basic functions:
They arrest accommodation or focusing, which in a young person with a powerful accommodative ability may not be achieved any other way.
They dilate the pupil to make a retinal examination more complete by exposing a greater part of the peripheral retina.
The drawback to drops is that adult patients who drive to the examination have to return home with blurred vision, photophobia, and fear.
Systemic absorption of the drugs in Table 12.2 can cause a toxic reaction. For instance, atropine can cause a fast pulse, a fever, and a skin rash. To minimize the systemic absorption of these drops, pressure should be exerted with a cotton ball or tissue held over the tear sac for a minute after the drop is instilled in the eye.
The next step in the process of refraction is to check the old glasses for power and optical centration. Frequently, the refractionist uses this information as a basis for refining the prescription or for the overrefraction.
A lensmeter records the optical center of the lens, its power, and the axis of the correcting cylinder, including its power. The newer lensmeters are fully automated and digitalized. These basic mechanical lensmeters are accurate but require some technical expertise. Details are presented in Chapter 8 .
Notation of the axis of the cylinder must be precise inasmuch as a patient will not tolerate a large deviation from his or her old prescription, especially in high degrees of astigmatism.
The retinoscope is the most useful instrument in the refractionist’s armamentarium. Retinoscopy is the chief objective method of determining the refractive error of an eye (see Fig. 10.6 ). It is the only way to assess the refractive error in children, people who are illiterate, people who speak a different language from the examiner’s, and people who are too confused, suffer from dementia, or are too ill to add a precise subjective component to the total refraction.
The retinoscope has a viewing system and an illuminating system. The viewing system consists of a small aperture at the head of the retinoscope that enables the examiner to see. The illuminating system shines diverging rays of light into the patient’s eye. This light enters the patient’s eye and is reflected back again as a reflex in the patient’s pupil. This reflex appears as a red-orange glow with a slight shadow around it. The vergence of the rays of light that leave the patient’s eye depends on the refractive error of that patient. In a myope the rays leave converging, in a hyperope the rays leave diverging, and in an emmetrope the rays leave parallel.
The degree of divergence of the rays that leave the illuminating system of the retinoscope depends on the distance of the retinoscope from the patient’s eye. Most ophthalmologists use a working lens in the phoropter or trial frame to account for this distance. The working lens conventionally is held at 66 cm, being a +1.50 diopter lens, or at 50 cm, being a +2.00 diopter lens. If the patient is far-sighted, the examiner will see, in the patient’s eye, a reflex that moves with the movement of the retinoscope. In this instance, plus lenses are added until there is no movement at all; in other words, until the refractive error has been neutralized. The working lens is then removed or subtracted and what is left in the trial frame is a measure of the hyperopia. Usually +1.50 or +2.00 diopters are subtracted to allow for the working distance of 66 or 50 cm, depending on the arm length.
If the patient is a myope, the movement of the examiner’s retinoscope will create a movement of the reflex that is opposite to the movement of the retinoscope. In this instance, the examiner places concave lenses or minus lenses in front of the patient’s eye until the “against” motion is converted to a nonmoving reflex.
The reflex seen in the patient’s pupil becomes much brighter and moves much faster as the refractive error becomes reduced. In other words, small refractive errors have a bright and fast reflex, whereas large ones have a dull and slow reflex. The reflex fills the entire pupillary space when the refractive error has been eliminated.
The following list provides aids to retinoscopy:
The patient should be looking at a distant object of regard at least 20 feet (6 m) away.
The patient’s left eye should be examined with the examiner’s left eye and vice versa.
Plus cylinders should be used in astigmatic cases because it is easier to see “with” motion.
Small differences in astigmatism are difficult to see, yet most astigmatism is 1.00 diopter or less. The power of the correcting cylinder should be moved forward and backward as a check for accuracy.
The eye not being measured should be fogged or occluded. The patient should not be told to close one eye during retinoscopy.
Irregular reflexes will appear in the following cases:
Dirty soft contact lenses
Warped corneas from poorly fitting hard contact lenses
There are two main types of retinoscopes: the spot retinoscope and the streak retinoscope. The Copeland streak retinoscope has been the most popular. Other streak retinoscopes are the Nikon, the Welsh-Allyn, the Keeler, and the Reichert. The last three retinoscopes work by holding the bar on the handle down, whereas the Copeland and the Nikon work by holding the bar up.
Many ophthalmologists use the streak retinoscope ( Fig. 12.13 ). The streak reflex illuminated in the pupil can be aligned easily with an astigmatic error. Moreover, it can be rotated to any desired meridian. With the streak retinoscope, the point of neutrality sometimes is evidenced by a cleavage in the streak (scissors reflex) so that half the streak moves in one direction and the other half moves in the opposite direction.