MYOPIA

....... MYOPIA

± Short sightedness

± term myopia was derived by Galen (131-201 AO) from the words myein ("to close") and ops ("eye").

± Parallel rays of light coming from infinity are focused in front of the retina with minimal accommodation.

Ø  Mechanism of production of ametropia

± Ametropia, or the absence of emmetropia, may be produced by variations in :

1. The relative location of the optical elements of the eye with respect to the retina.
2. The relative refractive power of the optical elements with respect to the location of the retina.

Ø  Optics of myopia

± Far point is finite (In front of the eye)

±  Emmetropic eye it is at infinity

± Higher the myopia the shorter the distance

±  Far point is 1mt from the eye ,there is 1D of myopia

± Nodal point is further away from retina

±  CLASSIFICATION

The proposed classifications may be grouped under the following broad headings:

± Rate of myopic progression

± Anatomical features of myopia

± Degree of myopia

± Physiological and pathological myopia

± Hereditary and environmentally induced myopia

± Theory of myopic development

± Age of myopia onset

± Rate of myopic progression

Donders" classified myopia on the basis of its rate of progression, describing three categories of myopia:

± Stationary (low degree: -1.50 to -2.00 D) and arises "in the years of development.“ The degree of myopia remains stationary during adulthood and may occasionally diminish with the approach of old age. However, Donders incorrectly suggested that the apparent reduction in myopia with increasing age was probably due to age-related pupillary miosis with an associated increase in the depth of focus of the eye.)

± temporarily progressive (Temporarily progressive myopia generally arises in the early teens and progresses until the late 20s. After this age, the rate of myopia progression approaches zero. Interestingly, Donders reported that it was rare for myopia to develop after 15 years of age in previously normal eyes and, falsely, that it never developed after the 20th year of life )

± permanently progressive.(Permanently progressive myopia ascends rapidly until around 25 to 35 years of age, and thereafter advances more slowly. Subsequent increases in myopia are said to occur in jumps, rather than in a smooth progression. Donders observed that because of pathological conditions such as retinal detachment and macular degeneration, in these cases it was rare at 60 years of age "to find a tolerably useful eye

± Classification by anatomical features of myopia

Berish" stated that myopia could be:

Axial, whereby the eye is too long for its refractive power.•

Refractive, whereby the refractive system is too powerful for the axial length of the eye. An increase in axial length may occur in the anterior or posterior portions of the globe individually, or may occur throughout the eye. The site of elongation may have implications for determining the etiology. For example, it has been suggested that expansion of the posterior portion of the globe may be related to the actions of the superior and inferior oblique muscles during vergence. 12

Berish" further divided refractive myopia into:

• Index myopia, in which one or more of the refractive indices of the media are anomalous.

• Curvature myopia, in which the reduced radius of curvature of one or more refractive surfaces produces increased dioptric power.

• Anterior chamber myopia, in which a decrease in anterior chamber depth increases the refractive power of the eye.

± Classification by Degree of myopia

± Hirsch divided the population into three groups on the basis of the degree of myopia, which he designated the alpha, beta, and gamma groups, respectively.

± Alpha group followed a normal distribution curve, with a theoretically assumed peak of +0.50 D.

± The beta group was represented by a second normal distribution curve, with its peak around -4 D. Hirsch suggested that the myopia in this group may be hereditary in origin.

± The gamma group ranged from -9 to -15 D, and this degree was described by Hirsch as malignant, pathological, degenerative, or congenital.

± Classification by Physiological and pathological myopia

± Physiological myopia was defined by Curtin as myopia in which each component of refraction lies within the normal distribution for that population. Thus, the myopia arises from a failure of correlation between the refractive components. However, physiological myopia may be defined as normal as opposed to pathologic myopia Therefore, physiological myopia might simply and more accurately be defined as nonpathological myopia.

± Pathological refractive errors ( by Duke-Elder and Abrams) as "those refractive anomalies determined by the presence in the optical system of the eye of an element which lies outside the limits of the normal biological variations." Pathological myopia may also be described as malignant or degenerative myopia. These authors adopted the term degenerative myopia to describe myopia that is accompanied by degenerative changes, particularly in the posterior segment of the globe. This is most frequently found in high (>6 D) degrees of myopia, but Duke-Elder and Abrams suggested that a classification merely by degree of ametropia is inappropriate because degenerative changes may also occur in cases of low myopia. Moreover, in 1913 Harman" described a case of more than 17 D of myopia without any pathological changes.

± Classification into Hereditary and
Environmentally Induced Myopia

± The debate of hereditary versus environmental influences on the development of myopia has persisted for more than 400 years and is still unresolved." Kepler, writing in 1604, was the first to have suggested an association between the development of myopia and the performance of sustained near-vision tasks.' However, Rosenfield' noted that the case for such an association remains unproven. It is frequently impossible to distinguish between environmental and hereditary influences, and hence other means of classification have been adopted (e.g., age of onset or degree of myopia) in an attempt to provide additional information regarding the etiology of refractive error development.

± Classification According to Theory of
Myopic Development

± In a review of the etiology of refractive error, Mc-Brien and Barnes" described three major theories of myopic development:

    • The biological-statistical theory

    • The use-abuse theory

    • The theory of emmetropization

± The biological-statistical theory

± The biological-statistical theory" considered variations in refractive error as forming a biological continuum ranging from high myopia to high hyperopia. Thus, ametropia simply represented the normal biological variation of a physiological component. However, data by both Stenstrom" and Sorsby et al. clearly demonstrated that the distribution of refractive error was not normal.

± The use-abuse theory

± The so-called use-abuse theory proposed by Cohrr“ suggested that myopia onset was an adaptation to use or abuse of the eyes during sustained near vision. Cohn examined the prevalence of myopia in more than 10,000 German schoolchildren. He observed that in the youngest children there was little myopia, but the prevalence increased with age. Cohn concluded that because the increased prevalence of myopia occurred during the educational process, a substantial portion of which entailed reading and other close work, the onset of myopia was related to increased near-vision activities.

± Numerous investigators have reported a higher prevalence of myopia among people whose occupations involve substantial amounts of close work, In addition, Young" demonstrated that when adolescent monkeys are restricted to a near-vision environment they exhibit significant increases in myopia. Other studies indicated an increased prevalence of myopia in an Eskimo population after the introduction of formal education, with its increased near-vision requirement. However, other factors, such as intelligence and changes toward a Western diet, may also have been at least partly responsible for the change in refractive error distribution in this population.

± The theory of emmetropization

± In view of the higher prevalence of emmetropia than might be predicted on purely statistical grounds, it would appear that the components of the eye do not grow independently, but rather undergo a process of coordinated growth. This proposed correlated growth of the ocular biometric components has been referred to as emmetiopizauon. Van Alpherr“ suggested that emmetropization was achieved by a negative- feedback, self-focusing control system. Variations in ciliary muscle tone could produce changes in refractive error by interfering with this self-focusing mechanism.

± Classification Based on Age of Onset

Grosvenor" classified myopia into the following categories:

     • Congenital myopia-Myopia is present at birth and persists through infancy.

     • Youth-onset myopia-The onset of myopia occurs between 6 years of age and the early teens.

     • Early adult-onset myopia-The onset of myopia occurs between 20 and 40 years of age.

     • Late adult-onset myopia-Myopia onset occurs after 40 years of age.

± Late-Onset Myopia

Late onset Myopia can be categorized the change in myopia during adulthood (i.e., beyond 18 years of age) into the following three groups:

± Adult stabilization-Rapid increases in myopia during early adolescence were followed by stabilization during early adulthood. Minor adjustment of the refractive error sometimes occurred after stabilization, but this change was generally small, on the order of ±0.25 D. Sixty-eight percent of male subjects and 87% of female subjects fell into this category.

± Adult continuation-The rapid myopic progression seen during adolescence continued through adulthood. This pattern represented 25% of male subjects and 13% of female subjects.

± Adult acceleration-Myopic progression increased after adolescence. This was the least common pattern, representing 6.3% of male subjects and no female subjects.

± Other Myopias

Night myopia

± The phenomenon of increased myopia under low luminance conditions was first reported in 1789 by the Reverend Nevil Maskelyne, the Astronomer Royal. He found that his astronomical observations at night were facilitated by the use of concave spectacle lenses.

± More recent evidence has demonstrated that night myopia is produced by an increased accommodative response (typically on the order of 0.50 to 1.00 D) under degraded stimulus conditions. However, there is also some suggestion that changes in chromatic aberration may also be involved in this myopic shift. The chromatic aberration of the eye results in blue light being refracted more than red light.“Furthermore, as the eye transfers from photopic to scotopic luminance

levels, its peak sensitivity shifts from approximately 55 nm to around 510 nm. This change in sensitivity is termed the Purkinje shift.78 Thus, at extremely low luminance levels, the eye becomes most sensitive to those wavelengths undergoing a greater degree of refraction, and therefore appears to be more myopic than it is under photopic viewing conditions. To determine the extent to which chromatic aberration could account for the myopic shift under reduced illumination, Wald and Criffin" used a spectral stigmatoscope to measure the refractive state of the eye under monochromatic light. They assessed axial chromatic aberration by measuring the eye's refractive state under nine narrow monochromatic conditions over a range of 365 to 750 nm. They concluded that the Purkinje shift in spectral sensitivity would produce a myopic shift in refractive power of approximately 0.35 to 0.40 D. However, the mean magnitude of the refractive error shift observed under reduced illumination in their study was -0.59 D (range =-1.40 to +3.40 D). Thus, chromatic aberration may account for a significant proportion of the increased myopia observed under degraded stimulus conditions. Direct evidence that night myopia is primarily produced by ocular accommodation comes from researchers who examined variations in the form of the third Purkinje image (reflected from the anterior surface of the crystalline lens) to assess the accommodative response under very low illumination levels.8 ! - 84 These investigators all reported mean changes in accommodation of approximately 0.75 D.

This confirmed that the change in the dioptric power of the eye resulted directly from a shift in accommodation, that is, a change in the refractive power of the crystalline lens. Indirect evidence that changes in accommodation must be the primary source comes from the observation of equivalent levels of tonic accommodation under a number of widely varying test conditions. For example, in the measurement of tonic accommodation, the accommodative loop may be opened by having the subject view a Ganzfeld field, a low spatial frequency difference of Gaussian (DOG) grating, a distant target through a 0.5 mm pinhole, or by placing the subject in total darkness.“’:” Clearly, the magnitude of spherical and/or chromatic aberration will exhibit wide variations under these different conditions, and yet equivalent values of tonic accommodation have been recorded." Therefore, both chromatic aberration and tonic accommodation appear to be the main determinants of the relative myopic shift observed under degraded stimulus conditions.'

± Pseudomyopia

Pseudomyopia has been defined as a reversible form of myopia that results from a spasm of the ciliary muscle? It is apparent that this does not meet the standard definition of a refractive error, that is, one that occurs under conditions of minimal accommodation. The excessive accommodative response produces an apparent myopic shift that will disappear when a cycloplegic agent is administered to produce relaxation of accommodation. These patients are frequently detected by the presence of a significantly greater (more than 1 D) amount of relative plus power (i.e., more hyperopia or less myopia) on retinoscopy compared with the subjective refractive findings, or by the observation of either an eso shift in oculomotor balance or a reduction in distance visual acuity, particularly toward the end of a working day.

Clinical Classification

± Congenital Myopia

± Simple Myopia

± Degenerative Myopia

± Nocturnal Myopia

± Pseudo Myopia

± Induced Myopia

Degree of Myopia

± Low Myopia(<3D)

± Medium Myopia(3-6D)

± High Myopia(>6D)

AGE OF ONSET

± Congenital Myopia

± Youth-Onset Myopia(<20 yrs of age)

± Early Adult-Onset Myopia(20-40 yrs of age)

± Late Adult-Onset Myopia(>40 yrs of age)

Congenital myopia

ü  Frequently seen in

ü  Premature babies

ü  Marfan’s syndrome

ü  Homocystinuria

ü  Increase in axial length

ü  Increase inOverall globe size

ü  Since birth, diagnosed at age 2-3 years

ü  If unilateral, as anisometropia, may develop amblyopia, strabismus

ü  Usually 8-10 D, remain constant

ü  Bilateral- difficulty in distant vision, hold things very close

Ø  Associated conditions

ü  Convergent squint

ü  Cataract

ü  Microphthalmos

ü  Aniridia

ü  Megalocornea

ü  Congenital Separation of retina

Management

Ø    Early Correction is desirable

Ø    Retinoscopy under full cycloplegia

Ø    Early full correction desirable

Ø   Poor prognosis

± Simple / developmental myopia

ü  Physiological error not associated with any disease of the eye

Ø  Etiology :

ü  Normal biological variation in development of eye

ü  Inheritence

Ø  Associated factors

ü  Role of diet

ü  Theory of excessive near work

Ø  Clinical picture

ü  Rarely present at birth

ü  Rather born hypermetropic, become myopic

ü  Begins at 7-10 years, stabilizing around mid teens

ü  Usually around 5D, never exceeds 8D

Ø  Symptoms

ü  Poor vision for distance

ü  Asthenopic symptoms develop due to dissociation between accommodation and convergence

q  Convergence weakness, exophoria, suppression

q  Excessive accommodation inducing ciliary spasm and artificially increasing the amount of myopia

ü  Psychological outlook

Ø  Signs

ü  Large and prominent

ü  Deep AC

ü  Large, sluggishly reacting pupils

ü  Normal fundus, rarely crescent

ü  Usually doesn't exceed 6-8D

± Retinoscopy under full cycloplegia

± Pathological / degenerative / progressive myopia

ü  Rapidly progressive associated with degenerative changes in the eye

Ø  Etiology

ü  Rapid axial growth of the eyeball outside the normal biological variations of development

ü  Role of heredity

ü  Role of general growth process

     Genetic factors        General growth process

              

                More growth of retina

              Stretching of sclera 

              Increased axial length

              Degeneration of choroid

               Degeneration of retina

              Degeneration of vitreous

Symptoms

ü  Defective vision

ü  Muscae volitantes / floating black opacities

Signs

• EYE Large, prominent eyes simulating exophthalmos
• CORNEA large
• ANTERIOR CHAMBER deep
• LENS show opacities at the posterior pole due to aberration of lenticular metabolism and due to overstretching anterior dislocation may also occur
• VITEROUS degeneration,viterous liquefication,vitreous detachment present as WEISS REFLEX
• SCLERA thinning resulting in formation of STAPHYLOMA
• VISUAL FIELD DEFECTS show Contraction and in some ring scotomas present
• DISC

ü   Large in size

ü  Myopic Crescent on the temporal side of the disc

ü  Choroidal Crescent

ü  Supertraction of the retina

ü  Inverse myopia Myopic crescent situated nasally and supertraction of the retina temporally

ü  called as INVERSE CRESCENT

ü  Peripapillary Atrophy

 

• MACULA

   Foster-Fuchs fleck

Ø  RETINAL DETACHMENT

Ø  POSTERIOR  STAPHYLOMA

Ø  RETINAL HOLES

Ø  TESSELATED FUNDUS

            Treatment

 Optical treatment

ü  Appropriate concave lenses

ü  Minimum acceptance providing maximum vision

± Guidelines

 

       

LOW DEGREES OF MYOPIA (Up to -6D)

Ø   IN YOUNG SUBJECTS

    Defect should never be overcorrected and advised for constant use to avoid squinting and develop a normal ACCOMMODATION-CONVERGENCE reflex

Ø  IN ADULTS

    Receiving spectacle for the first time,have the ciliary muscle that are unaccostomed to accommodate efficiently so that lens of slightly lower power(1 or 2 D) may be prescribed for reading,especially if engaged in to any greater extent.Above the age of 40 years,when accommodation fails physiologically, a weaker glass for near work is essential

       ADVANTAGES OF SPECTACLES

Ø  Economical

Ø  Allow incorporation of prism, bifocals, pal which can be used for the management of esophoria or any accommodative disorders accompanying myopia

Ø  Spectacles require less accommodation than contact lens for myopia that likelihood of accommodative asthenopia or near point blur in patients approaching presbyopia may be less

DISPENSING SPECTACLES IN HIGH MYOPIA

± High-index lens materials

± Lighter lens materials

± Reduced eye size of selected frames

± Minus lenticular lens designs

 ADVANTAGES OF CONTACT LENS

± Contact lens provides cosmesis

± Large retinal image size and slightly better visual acuity in severe myopia

SURGICAL TREATMENT

ü  Epikeratophakia

ü  RK

ü  PRK

ü  ISCR

ü  Phakic IOL’S

ü  LASIK

Photorefractive  Keratectomy- (PRK)

± Involves direct laser ablation of corneal stroma after removal of corneal epithelium mechanically or using a laser beam.

± Done using Excimer laser

± MUNNERLYN EQN: depth of ablation (micrometer)=[diameter of optical zone(mm)]² × 1/3power(Diopter)

± For myopic a large amount of ablation is done in central cornea than in the periphery.

± Give good results for -2D to -6D of myopia

LASIK- Laser Assisted In situ Keratomileusis

± Method:Anterior flap of cornea is lifted with a keratome and excimer laser is used to sculpt the stromal bed to change the refractive error of eye

± Corrects 0.5 to 12D of myopia and upto 8D of astigmatism

± Guidelines:Age more than 18yrs

                     BCVA better than 6/12

                     Stable refraction for last 1yr

                     Absence of corneal disease & ectasia

± Note:

± (1) In no case the residual bed thickness after the ablation should measure 250microns so as to avoid central corneal ectasia

± (2) Ideally the ablation should be done within 30sec of the preparation of flap

                     

LASEK- Laser subepithelial Keratomileusis

± Indications:

± Low myopia

± Irregular astigmatism

± LASIK complications in contralateral eye

± Thin corneal pachymetry

± Predisposition to trauma

± Glaucoma suspect

± Method:

± Simple inexpensive procedure that involves creation of epithelial flap after exposure to 18% alcohol for 25sec & subsequent replacement of flap after laser ablation

RK- Radial Keratotomy

± It refers to making deep corneal incisions(initially 16,now down to 4) in the peripheral part of cornea leaving about 4mm central optical zone

± The incisions are made almost down to the level of Descemet’s Membrane

± These incisions on healing flatten the central cornea thereby reducing its refractive power

± For low to moderate degree of myopia(-1.5 to     -6D of myopia)

Epikeratophakia

± For high degree of myopia (upto 20D)

± Method:

± The epithelium is removed & then a pocket is fashioned under the edge of the remaining epithelium & into this is inserted the cryolathed donor homograft

± Preserved material can also be used 

NON CORNEAL INTERVENTIONS

± (A) REMOVAL OF CLEAR LENS

± We know that an aphakic eye is strongly hypermetropic

± If an eye with an axial myopia of -24D is deprived of its lens it will become emmetropic without any correcting lens

± Note:

± Whenever surgery on clear lens is contemplated the eye is examined thoroughly for abnormalties like Raised IOP,Vitreous & retinal degeneration etc

 

± (b)Phakic intraocular lenses

± An IOL of appropriate power is implanted inside the eye without touching normal crystalline lens thus without disturbing accomodation

± Method can be used to correct both myopia & hypermetropia

Phakic IOL types:

± PC IOL

± Angle supported IOL

± Iris claw lens

INTRA CORNEAL RING(ICR) IMPLANTATION

± ICR implantation into the peripheral cornea approx.upto 2/3^rd of stromal depth can also be considered for correction of myopia

± It results in a vaulting effect that flattens the central cornea decreasing the myopia

± The procedure has the advantage of being reversible