Refractive errors

Refractive errors

Measuring visual acuity
Snellen chart
Preferential looking
Pattern visually evoked potential
Optokinetic nystagmus
Kay picture
Sheridan Gardiner
Keeler logmar test

Myopia

An optical condition in which parallel rays of light from object are brought to focus in front of retina . The refractive error is correctable with a concave negative lens.

Classification

1- Axial myopia . The globe is too long relative to its dioptric power , more than 26 mm.
2- Refractive myopia . The dioptric power of the globe is too high relative to its length


Types
1- Physiological myopia (common)
predominantly axial or refractive

2- pathological myopia

Degenerative myopia myopia associated with numerous ocular signs
Index myopia lenticular ( myopia due to pathological increased lens power ( nuclear sclerotic cataract )
or corneal ( myopia due to pathologically increase corneal power ( e.g keratoconus
Risk factors

Enviromental

Higher level of education
Near work
Intelligence
Prematurity
Low birth weight
Family history

Genetic

AD
AR
Sex linked inheritance pattern


Nutritional

Clinical features

Physiological myopia
Apart from the refractive error usually less than -6.00D there may be no signs because the condition is only due to mismatch between the refractive power of the globe and its length .Both factors of which lie within their normal distribution curves.

Degenerative myopia

Disc : myopic crescent , usually temporal but can be encircle disc white exposed sclera , pigmented exposed choroid
Pallor
Apparent enlargement
T sign visible bifurcation of central retinal vessels

Hypermetropia

In the hypermetropic eye the second principal focus lies behind the retina.

Axial hypermetropia : eye is short relative to its focal power .
Refractive hypermetropia : the refractive power of the eye is inadequate

Aphakia is extreme example of refractive hypermetropia.


Phakic patient can overcome some or all their of their hypermetropia by using accommodation for distance vision .they then have to exercise extra accommodation for near vision .
Because the amplitude of accommodation decline with age ,these patients require reading glasses at ayounger age than emmetropic patients .

classification

Manifest hypermetropia is defined as strongest convex lens correction accepted for clear distance vision .
Latent hypermetropia is the reminder of hypermetropia which is masked by by ciliary tone and involuntry accommodation . This may account for several dioptres especially in children for whom cycloplegic refraction is necessary to ascertain the full magnitude of refractive error.
facultative Hypermeropia which can overcome by accommodation while
absolute hypermetropia in excess of amplitude of accommodation is called absolute .

Astigmatism

The refractive power of the astigmatic eye varies in different meridians. The image formed as aSturms conoid .
regular astigmatism :If the principal meridians are at 90 to each other.
oblique astigmatism: If the principal meridians are at 90 to each other but dont lie at or near 90 and 180
irregular astigmatism: if the principal meridians are not at 90 to each other, and this is cant corrected by spectacles

Compound hypermetropic astigmatism : rays in all meridians come to focus behind the retina .
Simple hypermetropic asigmatism : rays in one meridian focus on the retina , the other focus lie behind the retina .
Mixed astigmatism: one line focus lies infront of the retina , the other behind the retina .
Simple myopic astigmatism : one line focus lies on the retina , the other focus lies infront the retina
Compound myopic astigmatism : rays in all meridians come to focus infront the retina ,


Refraction

History

Age
Visual symptoms
Occupational
General health and medication
Past medical and ophthalmic history

Objective refraction

Record monocular vision
Perform cover test for distance
Measure IPD from limbus to limbus
Fit trial frame and place retinoscopy working distance + 1.5 for 0.66m.
Objective refraction
Instruct the pt to fix on non accommodative target
Neuralize the reflex in the R eye by placing spherical lenses infront cell of trial frame ( with movememt require addition of plus sphere and against movement minus sphere)
If there is astigmatism present,then neutralize one meridian with sphere ,the other place cylinder in trial frame with the axis in the same direction as long axis of streak light ( remember the long axis of streak is at right angles to the power meridian being measured

Recording retinoscope findind

Across is drawn in the orientation of the principal meridians and the angle of one meridian is marked , the dioptric value of the point of reflex reversal is marked on each meridian and the working distance recrorded
When transposing into alens prescription corrected for working distance , remember that the axis of any cylinder lies at 90 to the angle of the power line as example


Subjective refraction

Cross cylinder

Duchrome test

Refractive surgery

Correction of myopia
Radial keratotomy
Photorefractive keratectomy
Laser in situ keratomileusis
Intrastromal plastic ring
Clear lens extraction
Phakic posterior champer IOL
P hakic anterior champer IOL

Correction of hypermetropia

PRK
LASIK
Laser thermal keratoplasty


Correction of asigmatism
Arcute keratotomy
PRK
LASIK
Toric IOL implant

Lasik

Correct hypermetropia up to 4D,astigmatism up to 5 Dand myopia up to 12 D
To prevent corneal ectasia aresidual corneal base of 250 mm thikness remain after flap has been cut and tissue ablated ,
the amount of tissue removed and total treatment is therefore limited by original corneal thickness ( on pachymetry )
1- suction ring is applied to globe which raised IOP to over 65 mmhg this may temporary occlude CRA and extinguish vision
2- the ring centred on the cornea and provides aguide track into which automated microkeratome is inserted
3- the keratome is mechanically advance across the cornea to create very thin flap which is reflected
4- suction is releasd and bed at some point is treated with the excimer laser as for PRK
5-the flap reposition and allowed to settle undisturbed for 30 second













 PAGE \* MERGEFORMAT 7

Produced by
Ahmed I. Alduri