MYOPIA

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MYOPIA

Myopia results from an eye having excessive refractive power for its axial length. This may be due either to the eye having a relatively long axial length or to increased dioptric power of one or more of the refractive elements.

the term myopia was derived by Galen (131-201 AD) from the words myein ("to close")and ops ("eye"). Galen observed that nearsighted people partially closed their eyes to see better

• Axial myopia

• if the total refractive power of an eye remains constant but the axial length (i.e., the distance from the anterior corneal surface to the retina measured along the visual axis) increases, a myopic shift in refractive error will result

• Refractive myopia

• if the axial length of an eye remains constant but the refractive power of one or more of its optical elements increases, a myopic shift in refractive status will occur.

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

• 1-Rate ofmyopic progression• 2-Anatomical features of myopia• 3-Degree of myopia• 4-Physiological and pathological myopia• 5-Hereditary and environmentally induced myopia• 6-Theory of myopic development• 7-Age of myopia onset

• Classification by Rate of Myopic Progression

• 1-Stationary myopia

• 2-temporarily progressive

• 3-permanently progressive.

• 1-Stationary myopiaStationary myopia

• is generally of 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.

• 2-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

• 3-permanently progressive• Permanently progressive myopia ascends rapidly until

around 25 to 35 years of age, and there afteradvances more slowly.

• Subsequent increases in myopia are said to occur in jumps, rather than in a smooth progression.

• Donders observed that because of pathological condi-tions such as retinal detachment and macular degener-ation, in these cases it was rare at 60 years of age "to find a tolerably useful eye."

• Classification by the Anatomical Features of Myopia

• 1-Axial• whereby the eye is too long for its refractive power

• 2-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

• . Borish further divided refractive myopia into:1-Index myopia, in which one or more of the refractive indices of the media are anomalous.2-Curvature myopia, in which the reduced radius of curvature of one or more refractive surfaces produces increased dioptric power.3-Anterior chamber myopia, in which a decrease in anterior chamber depth increases the refractive power of the eye.

• Classification by the Anatomical Features of Myopia

• Hirsch examined the refractive error of 562 eyes having at least -1.00 D of • myopia in patients between 18 and 60 years of age. He divided the

population into three groups on the basis of the degree of myopia,

• 1-alpha• Using inferential statistics, he determined that the alpha group followed a

normal distribution curve, with a theoretically assumed peak of -0.50 D.

• 2-beta groups

• 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

• 3-gamma groups

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

• Sorsby et al in an investigation of 341 eyes between 20 and 60 years of age, concluded that 95% of refractive errors fell within ±4 D. They also suggested that the etiology of myopia of less than 4 D differed from that myopia exceeding 4 D,

• Classification into

• Physiological• Pathological Myopias

• 1-Physiological myopia

• was denned 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

• 2-Pathological Myopia(malignant or degenerative myopia)

• Duke-Elder and Abrams defined pathological refractive errors 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.

• 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

• Borish further divided refractive myopia into:

• 1-Index myopia• in which one or more of the refractive indices of the media are

anomalous

• .2-Curvature myopia• in which the reduced radius of curvature of one or more refractive

surfaces produces increased dioptric power

• .3-Anterior chamber myopia• in which a decrease in anterior chamber depth increases the

refractive power of the eye.

• Classification Based on Age of Onset

• 1-Congenital myopia:• Myopia is present at birth and persists through infancy .although many children are born with

myopia (low birth weight) lose their myopia during first year of life this classification includes only children whose myopia persist in infancy and present when entering school. prevalence is about 2%.

• 2-Youth-onset myopia:• The onset of myopia occurs between 6 years of age and the early teens .from 2% at 6 years to

20% at 20 years.

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

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

• Other Myopias

• Night Myopia• The phenomenon of increased myopia under low luminance

conditions was first reported in 1789 by the Revere and Nevil Maskelyne 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 changes in chromatic aberration may also be involved in this myopic shift.

• Pseudomyopia

• Pseudomyopia has been defined as a reversible form of myopia that results from a spasm of the ciliary muscle.

• The excessive accommodative response produces an apparent myopic shift that will disappear when a cycloplegic agent is administered to produce relaxation of accommodation

• Pathological change in myopic eyes

• 1-Posterior staphyloma (ectasia)

• Axial enlargement ,mechanical stretching of posterior choroids and sclera

• indication of a Posterior staphyloma:• a-optic nerve crescents.• b-myopic cupping of the optic nerve head

• Fundus manifestations in pathologic myopia may include the following

• tilting of the optic disc

• peripapillary chorioretinal atrophy

• lacquer cracks• spontaneous ruptures of the elastic lamina of Bruch’s membrane that appear yellowish white and are

usually located in the posterior pole with linear or stellate patterns FA may be useful in detecting subtle lacquer cracks

• isolated, round, deep subretinal hemorrhages• that clear spontaneously and are usually due to the occurrence or extension of a lacquer crack and not to

CNV

• Förster-Fuchs spotsdark spots due to subretinal or intraretinal RPE hyperplasia, presumably developing in response to a small CNV that does not progress

• posterior staphylomalocalized ectasia of the sclera, choroid, and RPE

• elongation and atrophy of the ciliary body

• gyrate areas of atrophy of the RPE and choroid

• cystoid, paving-stone, and lattice degeneration

• thinning or hole formation in the peripheral retina

• thinning and rearrangement of the collagen layers of the sclera

• Some unfortunate consequences of myopia

• 1-chorioretinal degeneration• 2-retinal detachment• 3-glaucoma• 4-cataract

Myopia and Visual Acuity

Uncorrected VA Refractive Error (D)

20/30 0.50

20/40 0.75

20/60 1.00

20/80 1.50

20/120 2.00

20/200 2.50

Pathologic Myopia

Eyes with pathologic myopia have progressive elongation of the eye, thus creating a propensity for thinning of the RPE and choroid

The spherical equivalents of an eye with high myopia are more than -6.00 D, or an axial length greater than 26.5 mm, whereas patients with pathologic myopia are more than —8.00 D, or an

axial length greater than 32.5 mm.

Correction of myopia1-minus glass2-contact lens

Management

• Divergent or minus lenses in

• spectacles

• contact lenses

• Refractive surgery

• Contact lenses

– Soft contact lenses– Rigid gas permeable contact lenses– Ortho-keratology contact lenses

Management

• Surgery– Radial keratotomy (RK)

• Post-surgical complications– Hyperopic shift– Diurnal fluctuations of refractive error and visual acuity– Glare– Corneal scarring

Management

• Refractive surgery– Photorefractive keratectomy (PRK)

• Technique– High energy photons from an excimer laser are used to

photoablate a small amount of tissue from the anterior corneal surface and sculpt the cornea

Management

• Refractive surgery– Photorefractive keratectomy (PRK)

• Post-surgical complications– Eye pain– Corneal disruption– Blurry vision– Prolonged duration of drug use– Greater time needed for eyesight recovery

Management

• Refractive surgery– Laser in-situ keratomileusis (LASIK)

• Technique– A microkeratome is used to cut a flap of superficial corneal

tissue– The flap is lifted back, exposing the underlying corneal stroma – The excimer laser is used to perform tissue removal in the

corneal stroma– The flap is repositioned in its original position and adheres to

the underlying corneal stroma without the need for sutures

Management

• Refractive surgery– Laser in-situ keratomileusis (LASIK)

• Post-surgical complications– Severe dry eye syndrome– Poor night vision– Reduced contrast sensitivity– Astigmatism

Management

• Refractive surgery– Points to consider after the surgical procedure

• Use eyedrops, as prescribed by the ophthalmologist• Be careful not to allow chlorinated water , shampoo, and sweat to

come into contact with the eyes• Wear sunglasses when outside to protect the eyes from sunlight• Remember to go back to the hospital for regular follow up visits

with the ophthalmologist and to get eyedrops

Pathological Conditions Associated with Myopia

• Cataract– Nuclear cataract causes a myopic shift

Pathological Conditions Associated with Myopia

• Diabetes– High blood glucose levels cause increased sorbital

levels in the lens– Water rushes in and dilutes the sorbital in the lens– Lens bulges and results in a myopic shift

Pathological Conditions Associated with Myopia

• Marfan’s syndrome– Suspensory ligaments break– Subluxation of lens occurs superiorly and

temporally and results in a very high increase in myopia