Errors Of Refraction Flashcards
(43 cards)
Refraction is the ___________ that light rays undergo as they pass through _____ of varying _______.
change in direction
media
densities
Refraction in the eye takes place mainly at the ________ and the _______.
cornea
Lens
The aim of refraction in the eye is to ____________________ so that a clear image ,of the object of regard ,can be formed on the ________.
focus light rays entering the eye
retina
A eye with normal refraction is said to be _______________.
emmetropic
Eyes with refractive errors are said to be ____________.
ammetropic
_________ is a leading cause of low vision and a cause of blindness.
Refractive error
Dimensions of an adult eyeball
Anteroposterior diameter ________ mm Horizontal diameter ________ mm
Vertical diameter ________ mm
Circumference ________ mm
Volume ________ ml
Weight ________ gm
Anteroposterior diameter 23- 25 mm Horizontal diameter 23.5 mm
Vertical diameter 23 mm
Circumference 73.5 mm
Volume 6.5 ml
Weight 7 gm
Refractive index (R.I) of the lens: _____
Refractive index (R.I) of the cornea : ______
1.42
1.36
The total dioptric power of the human eye is __________.
The cornea and lens are the major refracting surfaces in the eye.
Cornea contributes ________ of these(_____D)
60D
two thirds
40
The Cornea ,with refractive index of ~ ______, has the dual purpose of ________ the eye and ____________ as it enters the eye.
After light passes through the cornea, a portion of it passes through the _______ and from there enters the ___________.
1.38
protecting
refracting light ; pupil; crystalline lens
The crystalline lens, which has a refractive index of ~_______ , is able to change its ______ and thus serves to ________ the vision process.
The lens is attached to the _________ which relax and contract in order to change the shape of the lens.
By carefully adjusting the lens shape, the ciliary muscles assist the eye in producing an image on the retina.
1.40; shape
fine-tune
ciliary muscles
The retina contains the ________ that serve the task of detecting the _______ and the ___________ of the incoming light.
These send nerve impulses to the ___________ , the ________ cranial nerve, which convey the message to the brain for interpretation.
photoreceptors; intensity
frequency; optic nerve
second
There are three main types of ametropia;
_________,__________,__________
Myopia. Hypermetropia. Astigmatism.
MYOPIA
Common error of refraction, also known as __________ or ___________ in which parallel rays of light coming from infinity are focused _____________ when accommodation is at rest. ______ objects are seen clearly, but objects _________ are blurry.
short sightedness or near sightedness
in front of the retina
Near
farther away
Most common refractive error is???
Myopia
Myopia: epidemiology
About 70-90% in some ______ countries and about 10-20% in _______ countries.
Asian
African
Aetiology and Risk factors for MYOPIA
Risk factors include a positive family history , frequent _____ work.
_____eased axial length (_____ eyeball). Increased ____________
near
Incr; Long
corneal curvature
Aetiology and Risk factors for MYOPIA
Risk factors include a positive family history , frequent _____ work.
_____eased axial length (_____ eyeball). Increased ____________
near
Incr; Long
corneal curvature
Classification of MYOPIA
Based on Aetiology:
Axial- due to _____ease in ___________ of the eye in relation to its refractive components e.g physiological _____ eyeball, ____________ , ________________ surgery
incr; antero-posterior diameter
large; buphthalmos
post-scleral buckle
Classification of MYOPIA
Based on Aetiology
Curvature- due to an ______ease in the curvature of the ______ or one or both surfaces of the ______ e. g _________,_________, or _____________
Index- due to an ____ease in the ________ of the lens e.g ___________ or ___________
incr; cornea
lens
keratoconus, anterior lenticonus, or posterior lenticonus
incr; refractivity
nuclear sclerosis or poorly controlled DM
Classification of MYOPIA
Based on clinical appearance:
Simple – normal overshooting of eye development, usually below ____D, onset usually after ________, normal visual acuity after __________ and __________________ changes
Pathological ( progressive, degenerative)- definite pathological entity, usually above -6D, onset usually before 4years, visual acuity not normal after correction and fundal degenerative changes present
-6; 4years
correction; no degenerative fundal
Classification of MYOPIA
Based on degree:
Low : ______________
Moderate : ______________
High : ______________
Based on age at onset:______,________,______
Low : less than -3.00D
Moderate :-3.00D to -6.00D
High : >-6.00D
Infantile, Youth,Adult
Signs and symptoms of MYOPIA
_______ distance vision
_________
__________ retina
Sub-retinal ___________
_______ spot
Choroidal _____________
Blurry; Floaters
Thin pale
haemorrhages; Fuchs
neovascularization
COMPLICATIONS of MYOPIA
Posterior _______________
________ _____________
Primary _______________
Staphyloma
__________
Vitreous detachment
Retinal detachment
open angle glaucoma
Visual loss
