REFRACTIVE ERRORREFRACTIVE ERROR

MRJAYDIP J. NINAMALECTURERSCON/SSON,ZUNDAL.

MYOPIAMYOPIA

DefinitionDefinition Myopia is a condition of the eye Myopia is a condition of the eye

where the light that comes it does where the light that comes it does not directly focus on the retina but in not directly focus on the retina but in front of it.front of it.

EtiologyEtiology1) Axial Myopia:1) Axial Myopia:It results from increase in the antero-It results from increase in the antero-

posterior length of the eyeball.posterior length of the eyeball.2) Curvatural Myopia:2) Curvatural Myopia:It Occurs due to increased curvature of It Occurs due to increased curvature of

the cornea, lens or both.the cornea, lens or both.3) Index Myopia:3) Index Myopia:It results from increase in the refractive It results from increase in the refractive

index of the crystalline lens associated index of the crystalline lens associated with nuclear sclerosis.with nuclear sclerosis.

ClassificationClassification1) Congenital Myopia:1) Congenital Myopia:It is also known as in fantile myopia, is present at birth It is also known as in fantile myopia, is present at birth

and persist through infancy.and persist through infancy.2) Simple Myopia:2) Simple Myopia:It is more common than other types of myopia, is It is more common than other types of myopia, is

characterized by an eye that is too long for its characterized by an eye that is too long for its optical power or typically too powerful for its axial optical power or typically too powerful for its axial length.length.

Both genetic and environmental factors, particularly Both genetic and environmental factors, particularly significant amount of near work, are thought to significant amount of near work, are thought to contribute to the development of the simple myopia.contribute to the development of the simple myopia.

3) Pathological Myopia or Degenerative Myopia:3) Pathological Myopia or Degenerative Myopia:It is also known as malignant, or progressive myopia, is It is also known as malignant, or progressive myopia, is

characterized by marked fundus changes, such as characterized by marked fundus changes, such as posterior staphyloma and associated with a high posterior staphyloma and associated with a high refractive error and subnormal visual acuity after refractive error and subnormal visual acuity after correction.correction.

Sign and SymptomsSign and Symptoms

Nearsightedness is often first noticed in Nearsightedness is often first noticed in school-aged children or teenagers. Children school-aged children or teenagers. Children opten cannot read the blackboard, but they opten cannot read the blackboard, but they can easily read a book. can easily read a book.

Myopia presents with blurry distance vision, Myopia presents with blurry distance vision, but generally gives good near vision.but generally gives good near vision.

Squinting.Squinting. Eye Strain.Eye Strain. Headache.Headache. Feeling Fatigued when Driving or Playing Feeling Fatigued when Driving or Playing

Sports.Sports. Flashing Lights.Flashing Lights.

HYPEROPIAHYPEROPIA

DefinitionDefinition Hyperopia is a refractive error in which Hyperopia is a refractive error in which

parallel rays of light entering the eye parallel rays of light entering the eye reach a focal point behind the plane of the reach a focal point behind the plane of the Retina, while accommodation is Retina, while accommodation is maintained in a state of relaxation. Thus maintained in a state of relaxation. Thus the posterior focal point is behind the the posterior focal point is behind the retina, which therefore receives a blurred retina, which therefore receives a blurred vision.vision.

EtiologyEtiology1) Axial Hypermetropia:1) Axial Hypermetropia:It is by far the commonest form. It occurs due to It is by far the commonest form. It occurs due to

short axial length of the eyeball.short axial length of the eyeball.2) Curvatural Hypermetropia:2) Curvatural Hypermetropia:It occurs due to comparatively flatter curvature of It occurs due to comparatively flatter curvature of

the cornea or lens or both. the cornea or lens or both. 3) Index Hypermetropia:3) Index Hypermetropia:It results due to change in the refractive index of It results due to change in the refractive index of

the lens. the lens. 4) Congenital Hypermetropia:4) Congenital Hypermetropia:Absence of the lens (Aphhakia) either congenital Absence of the lens (Aphhakia) either congenital

or acquired leads to high Hypermetropia. or acquired leads to high Hypermetropia. 5) Pathological Hypermetropia:5) Pathological Hypermetropia:It results from other than normal biologic It results from other than normal biologic

variation of the refracting components of the variation of the refracting components of the eye.eye.

Sign and SymptomsSign and Symptoms

Red or tearing eyes.Red or tearing eyes. Squinting and facial contortions while Squinting and facial contortions while

reading.reading. Ocular fatigue (Astenopia).Ocular fatigue (Astenopia). Frequent blinking.Frequent blinking. Constant or intermittent blurred vision.Constant or intermittent blurred vision. Frontal or fronto-temporal headache.Frontal or fronto-temporal headache. Patient may have difficulty in near Patient may have difficulty in near

vision or both near & far vision.vision or both near & far vision.

ASTIGMATISMASTIGMATISM

DefinitionDefinition Astigmatism is a type of refractive Astigmatism is a type of refractive

error where in the refraction varies in error where in the refraction varies in the different meridia of the eye. the different meridia of the eye. Consequently, the ray of light Consequently, the ray of light entering in the eye cannot converge entering in the eye cannot converge to a point focus but from focal lines.to a point focus but from focal lines.

EtiologyEtiology1) Corneal Astigmatism:1) Corneal Astigmatism:In corneal astigmatism the cornea has an In corneal astigmatism the cornea has an

irregular shape. So that cornea does not curve irregular shape. So that cornea does not curve perfectly-one half is flatter or steeper than the perfectly-one half is flatter or steeper than the other-the light that hits it will not refract other-the light that hits it will not refract properly & the retina at the back of the eye will properly & the retina at the back of the eye will receive an imperfect image. receive an imperfect image.

2) Lenticular Astigmatism:2) Lenticular Astigmatism:In lenticular astigmatism the lens has an irregular In lenticular astigmatism the lens has an irregular

shape. The lens has variations in its curvature, shape. The lens has variations in its curvature, rather than having a perfect curve, causing rather than having a perfect curve, causing images to reach the back of the eye (retina) images to reach the back of the eye (retina) imperfectly. Patients with lenticular imperfectly. Patients with lenticular astigmatism have a normally shaped cornea-astigmatism have a normally shaped cornea-the defect is only in the curvature of the lens.the defect is only in the curvature of the lens.

Sign and SymptomsSign and Symptoms

Headache.Headache. Eyestrain.Eyestrain. Squinting.Squinting. Distorted or blurred vision at all Distorted or blurred vision at all

distances.distances. Difficulty driving at night.Difficulty driving at night.

PRESBYOPIAPRESBYOPIA

DefinitionDefinition Presbyopia is the normal aging Presbyopia is the normal aging

process, where the lens progressively process, where the lens progressively loses its capacity to increase its loses its capacity to increase its power for near vision. The distance power for near vision. The distance vision may be normal, but the near vision may be normal, but the near vision becomes blurred with age vision becomes blurred with age greater than about 45 years. greater than about 45 years.

EtiologyEtiology1)1) Decrease in the accommodative Decrease in the accommodative

power of crystalline lens with power of crystalline lens with increasing age, leading to Presbyopia increasing age, leading to Presbyopia occurs due to occurs due to

Decrease in the elasticity and Decrease in the elasticity and plasticity of the crystalline lens (which plasticity of the crystalline lens (which results from age related sclerosis) results from age related sclerosis)

Age related decrease in the power of Age related decrease in the power of ciliary muscle.ciliary muscle.

Sign and SymptomsSign and Symptoms

Decreased focusing ability for near Decreased focusing ability for near objects.objects.

Eye Strain.Eye Strain. Headache.Headache. Fatigued.Fatigued.

Diagnosis for Refractive ErrorDiagnosis for Refractive Error

1) Slit Lamp Examination:1) Slit Lamp Examination:The slit-lamp examination looks at structures that are at The slit-lamp examination looks at structures that are at the front of the eye.the front of the eye.An instrument used in ophthalmology for examining the An instrument used in ophthalmology for examining the external, surface, and internal segments of the eye, external, surface, and internal segments of the eye, including the eyelid(s), lashes, conjunctiva, cornea, including the eyelid(s), lashes, conjunctiva, cornea, anterior chamber, pupil, iris, vitreous, and retina. A high-anterior chamber, pupil, iris, vitreous, and retina. A high-intensity beam of light is projected through a narrow slit, intensity beam of light is projected through a narrow slit, and a cross section of the illuminated part of the eye is and a cross section of the illuminated part of the eye is examined through a magnifying lens. A second, hand-held examined through a magnifying lens. A second, hand-held lens is used to examine the retina.lens is used to examine the retina.

2) Visual Acuity:2) Visual Acuity:Visual acuity is acuteness or clearness of Visual acuity is acuteness or clearness of vision, which is , which is dependent on the sharpness of the retinal focus within the dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the and the sensitivity of the interpretative faculty of the brain. brain. Visual acuity is a measure of the spatial resolution of the Visual acuity is a measure of the spatial resolution of the visual processing system. VA is tested by requiring the visual processing system. VA is tested by requiring the person whose vision is being tested to identify characters person whose vision is being tested to identify characters (like letters and numbers) on a chart from a set distance. (like letters and numbers) on a chart from a set distance. Chart characters are represented as black symbols against Chart characters are represented as black symbols against a white background. The distance between the person's a white background. The distance between the person's eyes and the testing chart is set at a sufficient distance to eyes and the testing chart is set at a sufficient distance to approximate approximate infinity in the way the in the way the lens attempts to focus. attempts to focus. Twenty feet, or six metres, is essentially infinity from an Twenty feet, or six metres, is essentially infinity from an optical perspective.optical perspective.

3) Retinoscopy:3) Retinoscopy:Retinoscopy is a technique to obtain an objective measurement Retinoscopy is a technique to obtain an objective measurement of the of the refractive error of a patient's of a patient's eyes. The examiner uses a . The examiner uses a retinoscope to shine light into the patient's eye and observes the retinoscope to shine light into the patient's eye and observes the reflection (reflex) off the patient's reflection (reflex) off the patient's retina. While moving the streak . While moving the streak or spot of light across the pupil the examiner observes the or spot of light across the pupil the examiner observes the relative movement of the reflex then uses a relative movement of the reflex then uses a phoropter or or manually places lenses over the eye (using a trial frame and trial manually places lenses over the eye (using a trial frame and trial lenses) to "neutralize" the reflex.lenses) to "neutralize" the reflex.

4) Refraction Test:4) Refraction Test:Refraction is the change in direction of a Refraction is the change in direction of a wave due to a change in its due to a change in its medium. It is essentially a . It is essentially a surface phenomenon. The phenomenon is . The phenomenon is mainly in governance to the law of conservation of mainly in governance to the law of conservation of energy and and momentum. Due to change of medium, the . Due to change of medium, the phase velocity of the wave of the wave is changed but its is changed but its frequency remains constant. This is most commonly remains constant. This is most commonly observed when a wave passes from one observed when a wave passes from one medium to another at any to another at any angle other than 90° or 0°. Refraction of angle other than 90° or 0°. Refraction of light is the most commonly is the most commonly observed phenomenon, but any type of wave can refract when it observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when interacts with a medium, for example when sound waves pass from pass from one medium into another or when water waves move into water of a one medium into another or when water waves move into water of a different depth. Refraction is described by different depth. Refraction is described by Snell's law, which states , which states that for a given pair of media and a wave with a single frequency, the that for a given pair of media and a wave with a single frequency, the ratio of the sines of the ratio of the sines of the angle of incidence θ1θ1 and and angle of refraction θ2θ2 is equivalent to the ratio of phase velocities ( is equivalent to the ratio of phase velocities (v1 / v2v1 / v2) in the two ) in the two media, or equivalently, to the opposite ratio of the indices of refraction media, or equivalently, to the opposite ratio of the indices of refraction ((n2 / n1n2 / n1):):

5) Tonometry:5) Tonometry:Tonometry is the procedure Tonometry is the procedure eye care professionals perform perform to determine the to determine the intraocular pressure (IOP), the fluid (IOP), the fluid pressure inside the inside the eye. It is an important test in the . It is an important test in the evaluation of patients at risk from evaluation of patients at risk from glaucoma. Most . Most tonometers are calibrated to measure pressure in tonometers are calibrated to measure pressure in millimeters of mercury (millimeters of mercury (mmHg).).

TREATMENT FOR TREATMENT FOR REFRECTIVE REFRECTIVE

ERRORERROR

EYE GLASSES:-EYE GLASSES:-GlassesGlasses, also known as , also known as eyeglasseseyeglasses (formal), (formal),

spectaclesspectacles or simply or simply specsspecs (informal), are frames (informal), are frames bearing bearing lenses worn in front of the worn in front of the eyes. They are . They are normally used for normally used for vision correction or or eye protection. . Safety glasses are a kind of Safety glasses are a kind of eye protection against against flying debris or against visible and near visible flying debris or against visible and near visible light or or radiation. . Sunglasses allow better vision in bright allow better vision in bright daylight, and may daylight, and may protect against damage from high from high levels of levels of ultraviolet light. Other types of glasses may . Other types of glasses may be used for viewing visual information (such as be used for viewing visual information (such as stereoscopy) or simply just for aesthetic or fashion ) or simply just for aesthetic or fashion purposes purposes

CONTACT LENCES:-CONTACT LENCES:-People choose to wear contact lenses for many People choose to wear contact lenses for many

reasons. Aesthetics and cosmetics are often reasons. Aesthetics and cosmetics are often motivating factors for people who would like to motivating factors for people who would like to avoid wearing glasses or would like to change avoid wearing glasses or would like to change the appearance of their eyes. Other people wear the appearance of their eyes. Other people wear contacts for more visual reasons. When contacts for more visual reasons. When compared with spectacles, contact lenses compared with spectacles, contact lenses typically provide better peripheral vision, and typically provide better peripheral vision, and do not collect moisture such as rain, snow, do not collect moisture such as rain, snow, condensation, or sweat. This makes them ideal condensation, or sweat. This makes them ideal for sports and other outdoor activities. for sports and other outdoor activities. Additionally, there are conditions such as Additionally, there are conditions such as keratoconus and and aniseikonia that are typically that are typically corrected better by contacts than by glasses.corrected better by contacts than by glasses.

CORRECTIVE LENCES:-CORRECTIVE LENCES:-A corrective lens is a A corrective lens is a lens worn in front of the worn in front of the eye

, mainly used to treat , mainly used to treat myopia, , hyperopia, , astigmatism, and , and presbyopia. Glasses or . Glasses or "spectacles" are worn on the face a short "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are distance in front of the eye. Contact lenses are worn directly on the surface of the eye. worn directly on the surface of the eye. Intraocular lenses are surgically implanted Intraocular lenses are surgically implanted most commonly after cataract removal, but most commonly after cataract removal, but recently for purely refractive purposes. Myopia recently for purely refractive purposes. Myopia (near-sightedness) requires a divergent lens, (near-sightedness) requires a divergent lens, whereas hyperopia (far-sightedness) requires whereas hyperopia (far-sightedness) requires convergent lens.convergent lens.

RADIAL KERATOTOMY:-RADIAL KERATOTOMY:-In RK, incisions are made with a diamond knife. In RK, incisions are made with a diamond knife.

Incisions that penetrate only the superficial Incisions that penetrate only the superficial corneal stroma are less effective than those corneal stroma are less effective than those reaching deep into the cornea, and reaching deep into the cornea, and consequently, incisions are made quite deep. consequently, incisions are made quite deep. One study cites incisions made to a depth One study cites incisions made to a depth equivalent to the thinnest of four corneal-equivalent to the thinnest of four corneal-thickness measurements made near the center thickness measurements made near the center of the cornea Other sources cite surgeries of the cornea Other sources cite surgeries leaving 20 to 50 micrometres of corneal tissue leaving 20 to 50 micrometres of corneal tissue unincised (roughly equivalent to 90% of unincised (roughly equivalent to 90% of corneal depth, based on thickness norms).corneal depth, based on thickness norms).

PHOTO REFRACTIVE PHOTO REFRACTIVE KERATECTOMY:-KERATECTOMY:-

The outer layer of the cornea, or epithelium, is a soft, The outer layer of the cornea, or epithelium, is a soft, rapidly regrowing layer in contact with the tear film that rapidly regrowing layer in contact with the tear film that can completely replace itself from limbal stem cells can completely replace itself from limbal stem cells within a few days with no loss of clarity. The deeper within a few days with no loss of clarity. The deeper layers of the cornea, as opposed to the outer layers of the cornea, as opposed to the outer epithelium, are laid down early in life and have very epithelium, are laid down early in life and have very limited regenerative capacity. The deeper layers, if limited regenerative capacity. The deeper layers, if reshaped by a laser or cut by a microtome, will remain reshaped by a laser or cut by a microtome, will remain that way permanently with only limited healing or that way permanently with only limited healing or remodelling. With PRK, the corneal epithelium is remodelling. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate removed and discarded, allowing the cells to regenerate after the surgery. The procedure is distinct from LASIK after the surgery. The procedure is distinct from LASIK (Laser-Assisted in-Situ Keratomileusis), a form of laser (Laser-Assisted in-Situ Keratomileusis), a form of laser eye surgery where a permanent flap is created in the eye surgery where a permanent flap is created in the deeper layers of the cornea.deeper layers of the cornea.

LASER ASSISTED IN SITU LASER ASSISTED IN SITU KERATO MILEUSIS (LASIK):-KERATO MILEUSIS (LASIK):-

Before LASIK, the most common form of Before LASIK, the most common form of laser eye surgery was a treatment called laser eye surgery was a treatment called PRK which involved scraping away the PRK which involved scraping away the surface cells of the cornea before surface cells of the cornea before reshaping it with the laser. reshaping it with the laser. Unfortunately, the healing process after Unfortunately, the healing process after PRK tends to be very painful and can PRK tends to be very painful and can take weeks. Furthermore, both eyes take weeks. Furthermore, both eyes cannot be treated at the same time and cannot be treated at the same time and the patient must wait several months the patient must wait several months between treatments to allow each eye to between treatments to allow each eye to recover sufficiently.recover sufficiently.