An easily detectable and correctable condition like refractive errors still remains a significant cause of avoidable visual disability in our world. A child, whose refractive error is corrected by a simple pair of spectacles, stands to benefit much more than an operated patient of senile cataract in terms of years of good vision enjoyed and in terms of overall personality development. In developing countries, like India, it is estimated to be the second largest cause of treatable blindness, next only to cataract. Measurement of the refractive error is just one part of the whole issue. The most important issue however would be to see whether a remedial measure is being made available to the patient in an affordable and accessible manner, so that the disability is corrected. Because of the increasing realization of the enormous need for helping patients with refractive error worldwide, this condition has been considered one of the priorities of the recently launched global initiative for the elimination of avoidable blindness: VISION 2020 – The Right to Sight.
Ministry of Health and Family Welfare, Government of India has issued the Standard Treatment Guidelines for Refractive Errors. Following are the major recommendations :
i. Myopia or Short sightedness or near sightedness
ii. Hypermetropia or Long sightedness or Far sightedness
These errors happen because of the following factors:
a. Abnormality in the size of the eyeball – The length of the eyeball is too long in myopia and too short in hypermetropia.
b. Abnormality in the curvature of the cornea – The curvature of the cornea is too steep in myopia and too flat in hypermetropia. In addition, irregularity of the corneal surface may give rise to astigmatism.
c. Abnormality in the refractive index – Light passes through the major refracting surfaces like the cornea and lens, before impinging on the retina. A high refractive index may cause myopia, while a low refractive index may cause hypermetropia.
Of all these factors, the abnormality of the size of the eyeball is most common in producing refractive errors.
Myopia is a form of refractive error, wherein parallel rays of light come to a focus in front of the retina, when the eye is at rest.
It is the commonest cause of refractive error and usually detected in the younger age group.
It consists of two main types:
Simple myopia : is just a variant of the normal and is not a major problem. Apart from the refractive error, no anatomical or functional complications of the ocular structures occur. The progression of myopia, more commonly happens during the growing phase of one’s life. The maximum growth happens between the ages of 12 and 20 years. Frequent change of glasses maybe required during these years and the myopic error usually stabilizes after the age of 20. Hence permanent surgical procedures like laser surgery have to be performed only after documented stabilization of the myopia.
Pathological (or progressive) myopia : also called as degenerative myopia, as the name suggests is of a more serious nature a type of severe, progressive nearsightedness characterized by changes in the fundus of the eye, posterior staphyloma, and deficient corrected acuity. Refractive error is greater than -8.00 diopters, and axial length is greater than 32.5 mm. In pathological myopia the retina becomes very thin and is stretched at the periphery. The peripheral retina is at risk of developing tears or holes.
Hypermetropia is a form of refractive error, wherein parallel rays of light come to a focus behind the retina, when the eye is at rest.
In this condition, the person is unable to see near objects well. The eye tries to focus the images by increasing the refractive index of the lens (by making it thicker) – the process is called Accommodation.
In this condition the eye is unable to form a point focus of light upon the retina. Instead of a single focal point of the image being formed, there are two foci that are separated by a distance. Hence the person is unable to see an image clearly in one meridian when he is focusing on another meridian.
- In regular astigmatism, the refractive power varies successively from one meridian to the next, and each meridian has a uniform curvature at every point across the entrance pupil. The meridians of greatest and least power, the so-called principal meridians, are always located at meridians 90 degrees apart.
- In irregular astigmatism, which can be clinically significant in conditions such as keratoconus and other corneal ectasias; corneal basement membrane and stromal dystrophies; corneal scarring; and post-surgical corneas (e.g., following penetrating keratoplasty, radial keratotomy (RK), complicated refractive surgery), the magnitude and the axis of astigmatism vary from point to point across the entrance pupil.
In addition to these refractive errors, there is another condition called as presbyopia which occurs in all people, usually above the age of 40 years.
In this condition,the eye is unable to focus near objects. The problem in presbyopia is the aging of the lens and the accommodating system, which fail to focus near objects on the retina. Although not truly a refractive error, presbyopia will be considered in this document because its correction has similarities to the correction of refractive errors.
Incidence of The Condition In Our Country
With blindness defined as presenting distance visual acuity <3/60 in the better eye, the prevalence of blindness due to refractive error has been reported to be as high as 0.2% in India, for all age groups in the population considered together. A recent survey shows an increase in the incidence of refractive errors as a cause of social blindness from 12% to 15%, whereas the burden of cataract has actually decreased from 80% to 62% (Source: National Survey of Blindness 1986-89 and 2001-02).
Patients with refractive errors often complain of defective vision, tiredness of eyes and headache. Even though refractive errors is the most common causative factor for defective vision, other organic causes such as the following should be examined and ruled out by a comprehensive eye examination.
ii. Retinal disorders
iii. Optic nerve pathology
v. Other media opacities
It should be once again emphasized that estimation of refractive errors should be a part of a complete ophthalmic evaluation. In addition, other causes of headache like sinusitis or migraine should be suspected especially when there are complaints of recurrent headache in the setting of a normal refractive status.
Prevention And Counseling
For all practical purposes, there are no strategies which are effective in preventing this condition from occurring. However, counseling on the importance and compliance to treatment optimizes visual performance and prevents further deterioration of the functioning of the eye. Most myopic refractive errors develop and progress during childhood and adolescence, which corresponds to their general physical development. The importance of school teachers in suspecting this condition cannot be over emphasized. They can be trained for screening of this condition and experience has shown that this system is very useful to identify children who need ophthalmic care and support at the school going level. The positive effect of outdoor activity for reducing myopic progression has been documented in numerous studies. Physical activity, sports, and low accommodative demands have been postulated as the cause of this protective effect.
Optimal Diagnostic Criteria, Investigations, Treatment & Referral Criteria
Situation 1: At Secondary Hospital/ Non-Metro situation: Optimal Standards of Treatment in Situations where technology and resources are limited
Clinical Diagnosis :
A proper history taking remains a cornerstone of the diagnosis of this condition. A child with uncorrected refractive error is restricted to the limits of its vision. Those who are not so eloquent with their complaints would tend to read their books very close to their face and sit closer to the black board or the television. The frustrated child becomes an introvert, keeping away from other children of his age and avoids outdoor activities.
The myope’s main complaint is that he /she don’t see distant objects clearly. The defect may also manifest as headache, watering from eyes and constant itching of the eyes all these symptoms are a result of the eyestrain induced by the refractive error. Conditions like pathological myopia, if not diagnosed early and followed up regularly can result in permanent loss of vision due to retinal scarring or retinal detachment. The progression of myopia happens more often in the growing phase of a person’s life. The maximum growth happens between the ages of 12 and 20. Frequent changes of spectacles may be required during these years, and the myopic error usually stabilizes after the age of 20.
The hypermetropes complain that they are unable to do near work very well. Headache and eyestrain following any prolonged near work like desk jobs or sewing can be accompanying symptoms. These symptoms are collectively termed asthenopia. Although hypermetropia can be detected at any age, it generally manifests more with increasing age Presbyopes are usually people in their fourth decade, who find it increasingly difficult to read the newspaper at their usual working distance in dim light and tend to keep it at a distance to make out the letters. Simple tasks like threading a needle, putting one’s signature on a document or making out the fine print on a visiting card become increasingly difficult and sometimes embarrassing. For artists and artisans who depend on their near vision for livelihood, it can even become economically crippling. Headaches and eye – strain may occur after long hours of near work. The uncorrected disability hampers his/her financial productivity and compromises their occupational skills. Left uncorrected, he/ she loses interest in performing near work like reading and writing leading to a compromise in quality of life.
The evaluation of refractive errors requires an assessment of both the refractive status of the eye, the patient’s current mode of correction, symptoms, and visual needs. As emphasized repeatedly, refraction is often performed in conjunction with a comprehensive ophthalmic evaluation. A thorough history with a special focus on the vocational needs of the patient should be asked for and kept in mind before finalizing the prescription. The examination should include both undilated and dilated evaluations. While pupillary reactions, ocular alignment and movements, visual field evaluation (in indicated cases) and dynamic refraction can be performed on a reactive pupil, posterior segment evaluation and cycloplegic retinoscopy and refraction can be done after dilating the pupil.
1. Visual acuity testing
Distance visual acuity is usually measured in a dimly lit room as the patient looks at a chart of high-contrast characters. It should be measured separately for each eye with current correction. For clinical purposes it is desirable that test chart luminance not be less than 80 cd/m2. It is desirable that the luminance level employed be specifiable. visual acuity testing conditions should be standardized in each examination room and at each visit, so that the same viewing distance and lighting conditions are used. Near acuity is usually measured while the patient looks at a well-lit reading card of high-contrast characters held at a specified near working distance, typically 14 inches or 30 cm
Nonverbal child (upto 1 year)
Estimating visual acuity in a non verbal child is a challenge. It should comprise of assessing the following parameters.
1. The assessment for this age involves evaluation of ocular fixation and following to appropriate visual stimuli: for an infant under 4 months of age, the examiner’s face or the parent’s face is used as a target. For older infants, appropriate toys can be used to induce fixation. Attempts should be made to assess the quality of the fixation response (central, eccentric, steady, unsteady, maintained) to the targets used. Even a subtle difference in the ocular fixation response of an infant with an otherwise normal eye examination requires monitoring to evaluate the presence or development of amblyopia.
2. Ability to fixate a light held at 33cm.is assessed
3. Blink reflex in response to sound is observed.
Verbal but preliterate child (1 to 5 year)
Preschool going children can be assessed by either of the following methods.
1. Ability to locate small objects (cake decorations).
2. Marble game test; in which child is asked to place marbles in holes of a card.
3. Illiterate E-card test.
4. Sheridan Gardiner (S-G) test.
Literate child (>5 year) and adults
These patients can be checked using a regular Snellen’s visual acuity chart. In illiterate people, Landolt C chart or a tumbling E chart may be used. Near vision testing can be performed using a Jeager chart.
Each eye should be evaluated independently. A dynamic refraction is done first for all adults followed by a cycloplegic refraction if necessary. Presbyopic correction is determined before applying dialating drops. All children, below the age of 15 years should be refracted only after applying cycloplegics to neutralize the effects of accomadation, to identify accomadative spasm and to diagnose strabismus of accomadative aetiology. An objective retinoscope followed by a subjective refinement of refraction is preferred. Distance refraction should be performed with accommodation relaxed using manifest (noncycloplegic) refraction with fogging or other techniques to minimize accommodation with care not to provide excess minus power correction to the patient. The common dialating agents are tropicamide and cyclopentolate. Tropicamide provides a more rapid onset of action and a shorter duration of effect while cyclopentolate provides greater cycloplegia that can allow a more accurate refraction but a longer duration of effect. A significant difference between manifest and cycloplegic refraction is frequent in children; In adults,a substantial difference between manifest and cycloplegic refraction may require a post-cycloplegic dynamic refraction on a subsequent day where the cycloplegic refraction is used to guide the final manifest prescription. The post-cycloplegic refraction is performed when full accommodation has returned.
3. Measurement of interpupillary distance (IPD) to determine the distance in millimeters between the centre of the pupils of the two eyes for a given viewing distance with a ruler. IPD is to be measured for both distance and near.
4. Determination of vertex distance and precise astigmatic axis is especially important in patients with high refractive errors.
5. Determination of Muscle Balance
This is done for people complaining of asthenopic symptoms.
Spectacle Correction :
1. Myopia : Full correction can be given to patients who are having visually demanding activities so that they can have a good distance vision. Over correcting myopic patients will cause excessive accommodation, which may create symptoms. Some patients may become symptomatic from increasing myopia at low levels of illumination. These patients may require increased correction for clear vision at night.
2. Hypermetropia : Slight under correction may be desirable in young and middle aged hypermetropes because there is some physiological accommodative tone. With the onset of presbyopia as the patient ages, full correction may be necessary to minimize difficulties with near vision.
3. Astigmatism : Full correction may not be needed for individuals with regular astigmatisms. Adults with astigmatism may not accept full cylindrical correction in their first pair of spectacles, or in subsequent spectacles if their astigmatism has been only partially corrected or its oblique axis astigmatism.
4. Presbyopia : In presbyopic patients, accommodative amplitudes are measured to determine if the available accommodation is sufficient for the near task. It is suggested that half of the accommodative amplitude be held in reserve for comfort. Individuals with myopia must exert more accommodative effort when using contact lenses, or after refractive surgery, than when using eyeglasses. Individuals with hyperopia must exert more accommodative effort when using eyeglasses than contact lenses.
A brief note of the basics of spectacle fitting is given below:
The types of frames can be broadly classified into Metal and Plastic Frames.
Alloys of varying constituents and consistencies are used to make frames. The most commonly used Nickel alloys, are cheap, strong and are tensile but may cause allergic reactions in the skin and so are not advisable. Stainless steel is a good alternative as it is inert, less allergenic and lightweight at the same time. Aluminium is lightweight but soft, and is less allergenic. Metals such as Titanium are also becoming popular, as it is very strong but lightweight.. The color and protective coatings given to the frame are just as important as the base material. Electroplating is a sturdy and reliable way to coat the frames. Cheaper frames are usually dipped or spray-painted – these coats usually peel off fast and may not be uniform. Powder coats on frames have a longer life as the coat is fused with the base metal. A final protective lacquer coat on top of the colour coat adds lusture and binds the color coat to the frame.
Most ‘plastic’ frames are actually all natural. The common constituent of this frame is Cellulose acetate, which is derived from treated wood pulp. These are sometimes called “shell” frames since the tortoise shell was initially used to make frames and later bone and animal horns were also used. Though this practice no longer exists, the name has remained.
All these shell frames are less allergenic, flexible and elastic – so they last longer. However, low-grade plastic frames which are more thermosetting (become rigid with heating) tend to become brittle and cannot be reused. Thermoplastic frames on the other hand become pliable on heating and can be reused when one wants a change of lenses.
These frames are actually disjointed three parts of a frame – the two temples of the frame and the nose bridge. These are then attached to the lenses and held in place by screws.
These are available in both metal and shell frames. The advantage here is that the lens shape can be free form. However, these must be handled carefully as the lenses have no protection.
These frames appear to have the frame rim running only along one side of the frame, but if at a closer look one can see a clear nylon wire running along a groove holding the lens in place. These are quite fashionable and are more reliable than rimless frames.
Once the frame is chosen the lenses need to be selected. Lenses are available in three basic materials:
Traditionally lenses were made out of glass. These had very high clarity and were easy to produce and fit into frames. Alternatives for lens materials were sought because glass was heavy and broke easily. This was a cause for concern where wearers had an active lifestyle.
Lenses made of plastic are lightweight and do not break easily. However, as plastic is softer than glass, it tends to scratch easily. But today with the developments of high quality scratch resistant coatings plastic is fast replacing glass in the market. Plastic lenses weight only 40% as much as glass and are especially suitable for an active lifestyle or a high refractive error. It is especially safe for children, as it does not shatter like glass lenses.
These are called “bulletproof” lenses and for good reason too. They are impact resistant, that makes them long lasting and lightweight. They have a very high refractive index, which means that these lenses are very thin even for very high powers. Polycarbonates are usually used for sports eyewear. But their lightweight, longevity combined with a good scratch resistant coating make them one of the best choices.
Hi Index Lenses
For a high myope heavy, thick ringed lenses make their eyes seem small and unsightly. An excellent alternative are hi-index lenses that give the same refractive power at half the thickness of the normal glass lens. These are available in both plastic and glass lenses.
Lenses admit most of the light through them – but some of the incident rays are reflected off the surface of the lens. This causes glare, which is disturbing when driving in the night with glasses on or while using the computer.
Anti-Reflection Coatings (or ARC) that is applied onto the lens ensures that the lens admits up to 99% of the light. This not only gives clearer vision to the wearer, it is a cosmetically better choice as the ugly reflections on the lens surface are avoided and the eyes are visible to the onlooker.
Correction for Presbyopia
Presbyopes require glasses called bifocal lenses (two focal lengths); their main disadvantages being discomfort if line of sight falls at the division in the lens; and being able to see clearly only at two focal lengths.
To help them focus at all distances new lenses have been designed called ‘varifocals’. These are designed by increasing the focal power gradually (so also called Progressive Addition Lenses). They eliminate the discomfort caused by segments in the lenses and improve the quality of vision.
These are special eyewear systems designed for protecting the eye from excessive heat, dust, light, harmful rays, or impact. This eyewear is suitable for people working in extremes of environment like welders, bikers, those living in extremes of weather, those handling harmful chemicals etc.
Polarized lenses have long been known and proven to enhance visibility and reduce glare under all conditions. The lenses provide a sharper vision under any environmental condition. Polarized lenses are the only lenses that provide visibility below the surface of the water in a marine environment. A polarized lens filters and eliminates annoying glare and transmits only efficient light to the eye. Wherever there are horizontal surfaces producing glare, the use of polarizing lenses is recommended because it reduces eye fatigue and maintains the health of your eyes while exposed to the sun.
Ultra Violet Protection
UV Filter capacity of a lens is measured and certified. Lenses marked UV400 mean that they block 100% of UVA & UVB. As a result, your eyes are not subjected to any harmful rays. All sunglasses offer UV protection to some extent but not 100%. Exposure to UV rays accelerates development of cataract, degeneration of cornea and retina. It can also cause tumors of the eyelid.
Standard Operating procedure
a. In patient
b. Out Patient: All patients are treated as outpatients.
c. Day care
The following patients can be referred to a higher centre for the following reasons:
- Pathological Myopia
- Refractive Error with associated Strabismus
- Visual Acuity not improving with glasses
- Oblique Astigmatism
- Sudden refractive change
Those patients who desire contact lenses and refractive surgeries.
*Situation 2: At Super Specialty Facility in Metro location where higher-end technology is available
Apart from the regular history and examinations, measured above, special emphasis on evaluating the visual expectations should be performed on patients seeking refractive surgery.
While regular patients seeking spectacle correction go through the same investigations mentioned above, patients with special needs such as contact lenses and laser refractive surgeries have to undergo special procedures. The refraction may be done objectively by retinoscopy, with an autorefractor, or with a wavefront analyzer; or it may be done subjectively. In cooperative patients, subjective refinement of refraction using a phorometer or trial lens set is preferred
An attractive alternative to spectacles for correcting refractive errors, contact lenses are also used for various therapeutic, pharmacological and cosmetic procedures. Modern contact lenses are polymers which are relatively safe, non-toxic and non-allergic. Since the cornea derives its oxygen supply mainly from the atmosphere, placing a lens on its surface can compromise its intake starving the cornea of oxygen. Newer lenses however, allow oxygen to pass through them to the cornea.
There are basically three types of contact lenses:
Soft contact lens: These are well flexible lenses made of a polymer called HEMA (Hydroxy Ethyl Metha Acrylate). They are very comfortable to wear and permit oxygen to reach the cornea.
Semi soft lenses or Rigid Gas Permeable Lenses: These lenses are made from special materials like Cellulose or Silicon acrylate. They are less flexible than soft contact lenses but allow some oxygen to pass through the cornea. Though less comfortable than soft lenses, they can be used to correct steep corneas and high astigmatism.
Hard contact lenses: Made of a polymer called PMMA (Poly Methyl Metha Acrylate), hard lenses are least flexible and permit almost no oxygen to pass through to the cornea. The advantage of a hard contact lens is its durability.
Advantages of a contact lens:
- A contact lens provides a larger field of vision than spectacles and a better range of mobility. Hence it is ideally suited for sportspersons and people engaged in outdoor physical activities.
- In people with high refractive error in one eye alone, spectacles will produce double vision due to the image size discrepancy. Contact lenses can be used for such people.
- Cosmetically, they offer an advantage over spectacles.
- Apart from correcting refractive errors, contact lenses are also used to deliver drugs to the eyes and to treat corneal diseases.
- Nowadays contact lenses are also available in various colors and can be used by people even with normal vision for the cosmetic appeal.
With all these advantages, there are also some precautions to be taken before opting for contact lenses.
An ophthalmologist should examine the eyes to rule out infections and other disorders of the eye.
- A qualified contact lens specialist alone should do the contact lens fitting. This includes measuring the corneal curvature and diameter and issuing a suitable pair of contact lens. After wearing them, the specialist examines the eyes for a good, comfortable fit on the eyes.
- Hand washing with soap before inserting and removing the contact lens from the eye is mandatory.
- Proper maintenance of the contact lens by cleansing it with the appropriate lens fluid is necessary.
- If the eyes are red or there is a white discharge from the eyes, the contact lens should not be used and the eye care professional should be contacted for further instructions.
- One should not sleep overnight with the regular daily wear contact lens on. The oxygen supply to the cornea is totally cut off during sleep by the contact lens. The oxygen deprived cornea is liable to damage and ulceration.
- The eyes should be periodically examined by an eye care professional to look for any infections or corneal damage, possible due to contact lens wear.
The latest innovation in the field is extended wear contact lenses which can be used over a longer period of time. Wearing of contact lenses during waking and sleeping hours, i.e., continuous 24 hour wear for a specified number of days. NOTE: The FDA recommends no longer than six (6) consecutive nights wear for traditional extended-wear lenses. These lenses allow free passage of oxygen through them to the cornea, but even they must be used with care.
Refractive Surgery for Myopia, Astigmatism and Hyperopia
Refractive surgery may be considered when a patient wishes to be less dependent on eyeglasses or contact lenses, or when there are occupational or cosmetic reasons not to wear eyeglasses.
Patient expectation will be the main pillar around which all the other structures of the preoperative examinations should be built. The motives for desiring refractive surgery should be carefully discussed with the patient by the surgeon. Individuals who expect only 6/6 and nothing less should be discouraged from having refractive surgery, while patients motivated by a desire to reduce their dependence on glasses or contact lenses, will be good candidates.
Keratorefractive surgery can be applied to a broad range of refractive errors, but in some circumstances, the surgeon may consider an intraocular procedure like the placement of an intraocular lens (IOL) implant, either in front of the crystalline lens (phakic IOL) or in place of the crystalline lens (refractive lens exchange).
1. Patients to discontinue contact lenses at least 1 week before surgery because of the possibility of contact lens induced corneal warpage. Toric soft contact lenses and rigid contact lenses should be discontinued for a longer period (atleast 2 weeks) because they are associated with a greater potential for corneal warpage and refractive instability.
2. Visual acuity without correction
3. Manifest, and where appropriate, cycloplegic refraction: The stability of the refraction should be well documented. If the patient is seen for the first time, it may be prudent to ask for his previous refraction powers and compare with the present.
4. Detailed anterior and posterior segment evaluation: A thorough anterior segment evaluation before and after pupillary dilation is crucial. Potential anatomical problems like a prominent brow, narrow palpebral fissure and peripheral vascular pannus should be looked for, since these may pose problems during microkeratome operation. As the cornea is evaluated, close attention should be paid to any anterior membrane dystrophy, stromal scarring or vascularization that may influence the type of surgery or whether it should be done at all. Any variation in stromal thickness, as in pellucid or Terrien’s degeneration, keratoconus or stromal dystrophies should alert the surgeon to reconsider performing a refractive procedure. The crystalline lens should be examined in detail after full dilatation, especially in patients above the age of 40 years. A progressive myopia, during middle age should alert the clinician about the early stage of immature cataract. In case of doubt, an axial length measurement can be estimated and if there is any discrepancy, further testing can then be ordered. A baseline intraocular pressure measurement, like the preoperative keratometry reading, serves
as an useful adjuvant, since post lasik intraocular pressure measurement using Goldman’s applanation tonometry is less than accurate.
A detailed indirect ophthalmoscopic evaluation with and without indentation should be an integral component of the whole screening processes. Lattices with multiple holes and horse shoe tears can be subjected to barrage laser and laser surgery can be performed after a period of 1 month. Round atrophic holes and simple lattice degenerations need not be treated as a routine. Even as there is no proof of increased occurrence of retinal detachment following refractive surgery, it will be prudent to treat any predisposing condition first.
5. Evaluation of tear film: The level of tear meniscus and the quality of the tear film should be assessed before any instrument touches the eye. Although mild dryness due to intolerance of contact lens wear may be an indication for refractive surgery, more significant dryness may pose a threat to successful epithelial healing.
6. Evaluation of ocular motility and alignment
7. Topography: for evidence of irregular astigmatism, corneal warpage, or abnormalities suggestive of keratoconus or other corneal ectasias, because all may be associated with unpredictable refractive outcomes and latter with progressive ectasia following surgery. The detection of the posterior corneal elevation adds an extra safety dimension before Lasik procedure.
8. Pachymetry: To identify unusually thin corneas and estimate residual stromal bed thickness. Corneas with central corneal thickness values less than 500 μm should be considered potentially abnormal.
Excimer Laser Systems
Laser assisted in situ keratomileusis (LASIK) is the most commonly performed refractive surgery worldwide. Good visual results, quick rehabilitation and negligible complication rates have been the main reasons for the widespread acceptance rate of this procedure.
Technological advancements have brought in many evolutions from the initial era of broad beam lasers to the more recent ones using flying spot technology. These refinements have helped the new generation lasers to have more accurate predictability than the earlier ones. A recent addition in this field is the area of customized correction, by lasers using wave front technology. Even though the customized correction procedures are becoming popular by the day, controversies still surround many of its presumed theoretical advantages. Since conventional LASIK surgery by itself is a highly accurate procedure, it becomes all the more difficult to establish the superiority of the customized correction over the conventional one.
By varying the ablation pattern, the excimer laser can alter the anterior corneal curvature to modify a particular refractive error described by sphere and cylinder. The laser delivery methods currently being utilized to achieve the ablation pattern are broad beam, scanning slit, or flying spot systems. Eye-tracking technology is integrated into the current commercially available excimer laser systems and permits the ablation to remain centered on the pupil in the presence of small ocular movements.
Potential advantages of customized treatment
Wave front based aberrometry has certain theoretical advantages, the notable one being the evaluation of whole eye optics. In certain instances, visual symptoms can be correlated with certain higher order aberrations. However the measurements of these aberrations are not reproducible. For example, it has been shown that Postoperative wavefront aberrations do not always correlate with contrast sensitivity measurements. The magnitude of higher order aberrations have been documented to increase with increasing papillary size. Hence there is a theoretical beneficial effect of correcting higher order aberrations in people with large pupils, who have to work mainly in dim light conditions.
Potential disadvantages of customized treatment
1. All aberrations are not treated: The aberrometer generates a lot of data from a human eye. Not all of these are treatable using customized LASIK procedure. In fact, customized LASIK has been demonstrated to induce additional higher order aberrations as well. This again brings us to the notion that the significance of these aberrations is still not well understood at this point of time and further refinements are yet to be done in this field.
2. All aberrations are not reproducible: The aberrations generated from a human eye are not reproducible and this is one of the major draw backs. Daily fluctuations in wave front aberrations have been noted in several studies and are attributed to accommodation, tone of the pupil and tear film distribution.
3. Wave front changes with age: The wave front measurements of the eye alter with age, because of changes that occur in the lens and cornea with age. Hence even if one performs customized corneal ablation, it will be interesting to watch whether these changes would be permanent.
4. Pupillary size: Pupillary alterations happen almost on a continuing basis in our day to day routine. The output of the wave front images changes with minute changes in the papillary size. If this be the case, it is interesting to speculate whether the treated area corrects the aberrations, over the entire spectrum of papillary dilatation and constriction, which happens during our day to day activities.
5. Limitation of the retina: Even if we correct all the aberrations of the eye, there is a biological limitation of the retina to see beyond approximately 6/3 vision. It is not clear whether the neural retina will be able to interpret the improved image.
6. Depth of field: Removing the eye’s higher order aberrations increases optical quality for objects that lie at the best focus. However, it reduces optical quality for objects that are far out of best focus. Exactly how this benefits and cost tradeoff functions in everyday vision has not yet been carefully studied.
7. Change in posture: It has to be understood that the wave front measurements are taken with the patient in a sitting position, while the treatment is performed in a patient in a supine position. Since the wave front changes with these postural alterations, this factor needs to be studied in detail.
- Unstable refraction
- Certain abnormalities of the cornea (e.g., keratoconus or other corneal ectasias, thinning, edema, interstitial or neurotrophic keratitis, extensive vascularization)
- Insufficient corneal thickness for the proposed ablation depth
- Visually significant cataract
- Uncontrolled glaucoma
- Uncontrolled external disease (e.g., blepharitis, dry eye syndrome, atopy/allergy)
- Uncontrolled connective tissue or autoimmune disease
- Unrealistic patient expectations
- Functional monocularity
- Ocular conditions that limit visual function
- Excessively steep or flat corneas
- Abnormal corneal topography suggestive of forme fruste of keratoconus, keratoconus, or other corneal ectasias
- Significant irregular astigmatism
- Corneal stromal or endothelial dystrophies
- History of herpes simplex virus (HSV) or varicella zoster virus (VZV) keratitis
- Significant dry eye syndrome
- Prior incisional or lamellar keratorefractive surgery
- Diabetes mellitus
- Pregnancy or lactation
- Connective tissue or autoimmune diseases
- Certain systemic medications (e.g., isotretinoin, amiodarone, sumatriptan, levonorgestrel implants, colchicine)
- Under 21 years of age (FDA labeling should be consulted for each laser)
Surface Ablation Techniques
In PRK, the central corneal epithelium is removed and the excimer laser is used to ablate Bowman’s membrane and superficial corneal stroma over the entrance pupil.
Laser Epithelial Keratomileusis and Epi-LASIK
Laser epithelial keratomileusis (LASEK) is a modification of PRK that attempts to preserve the epithelium. After alcohol is applied to the corneal epithelium, an epithelial trephine and spatula are used sequentially to score, loosen, and roll up the epithelium, which remains attached at a nasal or superior hinge. Photoablation is then performed, and the epithelium is unrolled back over the central corneal stroma. A bandage contact lens is used for several days until the surface re-epithelializes.
An alternative surface ablation procedure to LASEK is epi-LASIK. Instead of using alcohol to loosen the epithelium, an epikeratome is used to dissect an epithelial sheet from Bowman’s membrane. The epikeratome is similar in design to a mechanical microkeratome used for LASIK. Instead of using an oscillating sharp blade to incise the cornea beneath Bowman’s membrane, the epikeratome uses a blunt oscillating separator that moves across the cornea held under high pressure with a suction ring. This separator lifts a sheet of epithelium from Bowman’s membrane. The laser ablation is then performed and the epithelial sheet is either replaced or discarded. Visual recovery and discomfort with LASEK and epi-LASIK are similar to PRK and are prolonged relative to LASIK.
Alternatives for patients who are Lasik Rejects:
For eyes with THIN CORNEA
- PRK / LASEK / EPI-LASIK
- SBK (Sub Bowman’s Keratomileusis)
- Intra corneal Rings
- Phakic IOL (ICL)
For eyes with ABNORMAL SHAPE
- Intra corneal Rings (INTACS, Kera rings )
- Phakic IOL
Phakic IOLs fall into three broad categories:
- Anterior chamber angle-fixated lenses, originally introduced by Baikoff and Joly
- Anterior chamber iris-fixated lens, introduced by Fechner and Worst
- Posterior chamber sulcus- fixated lens introduced by Fyodorov and subsequently modified by other companies.
Indications for phakic IOLs:
• Correction of myopic errors of -8.00 D or more.
• In eyes contraindicated for LASIK:
- Relatively thin corneas,
- Stable keratoconus;
- Corneas steeper than 48.00 D or with expected post-LASIK keratometry readings flatter than 35.00 D.
- in eyes with residual refractive errors after LASIK, corneal transplant, intrastromal corneal ring segment (ICRS) implantation, corneal collagen crosslinking (CXL), and pseudophakia.
- Unstable refraction
- Visually significant cataract in the case of phakic IOLs
- Corneal endothelial disease, including Fuchs dystrophy
- Uncontrolled glaucoma
- Uncontrolled external disease
- Uncontrolled connective tissue or autoimmune disease
- Unrealistic patient expectations
Standard operating procedure
a. In patient: Not applicable
b. Out Patient: Not applicable
c. Day care: Refractive surgeries are done as day care procedures.
WHO DOES WHAT? And TIMELINES
1. History taking with examination of the patient
2. To carry out the necessary investigations
3. To satisfy the patient as per his/her needs
Role of Ophthalmic Assistants is to
1. Test vision, conduct refraction and prescribe corrective lenses.
2. Assist medical officers in providing primary care including treatment for trachoma, ocular injuries, and conjunctivitis etc.
3. Assist in conducting eye camps at the primary health centre.
4. Help in organizing vision-screening programmes in schools.
5. Carry out eye health education activities.
6. Maintain the dark room and ophthalmic equipment in primary health centre.
|RESOURCES REQUIRED Situation||HUMAN RESOURCES||INVESTIGATIONS||DRUGS & CONSUMABLES||EQUIPMENT|
|1) Secondary level||1)Doctor
|1)Mydriatics and 2)Cycloplegics
3)Spectacle frames and lenses
4)Antibiotic and steroid eye drops
3)Indirect ophthalmic with 20D lens
4)Slit Lamp Biomicroscope with
5)Visual acuity charts
|2) Tertiary level||1)Doctor (with a refractive surgeon)
2)Nurse – 1
Assistant – 1
4)OT Nurse – 1
|1)Mydriatics and 2)Cycloplegics- 3)Spectacle frames and lenses
4)Antibiotic and steroid eye drops
2) Trial set
4)Slit Lamp Biomicroscope
8)Indirect ophthalmic with 20D lens
10)Visual acuity charts
Guidelines by The Ministry of Health and Family Welfare :
Dr. Venkatesh Prajna Chief- Dept of Medical Education, Aravind Eye Hospitals, Madurai