Module 5 Flashcards
What is an abnormal refractive condition of the eye in which images fail to focus on the retina?
Ametropia
What are the 3 types of ametropia?
Hyperopia
Myopia
Astigmatism
What is myopia?
Nearsightedness
A condition in which parallel rays of light come to focus at a point just in front of the retina in the unaccomodated eye
What are the 3 types of myopia?
Axial - eyeball too long
Curvature - shorter than avg radius of curvature of cornea, more than avg power to retina
Index - caused by diabetes (lens swells) and cataract forms
What is hyperopia?
A condition of ametropia in which sharp image forms posterior to the fovea, indicating presence of inadequate positive vergence (+ power) and requiring plus lenses for correction
What are the types of hyperopia?
Axial- eyeball is too short
Curvature - greater than avg radius of curvature of cornea, less than avg power to retina
What is a refractive error that prevents the light rays from coming to a single focus on the retina because of different degrees of refraction in the various meridians of the eye?
Astigmatism
What are the types of astigmatism?
Simple hyperopic Compound hyperopic Simple myopic Compound hyperopic Mixed
What happens in simple hyperopic astigmatism?
Vertical rays are focused on the retina
Horizontal rays are focused behind the retina
What happens in compound hyperopic astigmatism?
Both focal points fall behind the retina
What happens in simple myopic astigmatism?
Vertical rays are focused on the retina
Horizontal rays are focused in front of the retina
What happens in compound myopic astigmatism?
Both focal points lie in front of the retina
What happens in mixed astigmatism?
Vertical rays come to a focus behind the retina
Horizontal rays focus in front of the retina
What does clarity of retinal image depend upon?
Refractive surface and media (tear film, aqueous, lens, vitreous)
What does cornea contribute in the accommodation process?
About 43 D
What does the lens contribute in the accommodation process?
About 15 D
What is the total power of the eye?
60 D
What is the change in the focusing power of the crystalline lens?
Accommodation effort
Accommodation response
What is accommodation effort?
Ciliary muscles relax
Zonules relax and
Crystalline lens changes shape
What is accommodation response?
Increase in the focusing power of the crystalline lens due to an increase in convexity of the anterior surface of the crystalline lens to create sharp images
What is the accommodation process for viewing a distance object?
Ciliary muscles relax
Zonules are stretched
Lens thins
What is the accommodation process for viewing near objects?
Ciliary muscles contract
Zonules relax
Lens thicken
What is a loss of the amplitude of accommodation?
Presbyopia
What happens with presbyopia?
Lens hardens
Atrophy of ciliary muscles
Loss of muscle tone
What is maximum accommodation?
+14 D age 8
What is accommodation loss scale?
Between 8 and 40 1 D every 4 years
Between 40 and 48 1.5 D every 4 years
Between 48 and 72 0.5 D every 4 years
What are simple refractive errors determined by?
Refractive power of cornea and lens
Length of eyeball
What are pathological errors determined by?
Degenerative changes in the choroid and retina
Excessive growth of the eyeball in all dimensions
What is a normal eye in which all of the rays of light from a distant fixed object are imaged sharply on the retina without the necessity of any accommodative effort.
Emmetropia
