Refractive Errors Flashcards
What is a refractive error?
A refractive error occurs when the shape of the eye prevents light from focusing directly on the retina to form a clear image
Refractive media of the human eye?
Human eye has 4 refractive media
1. cornea
2. aqueous humor
3. lens
4. vitreous humor
- Normal size, shape and curvature of these is critical in achieving clear vision
- Anterior surface of the cornea does the maximum refraction
- Total refractive power of the eye is 63 D
Emmetropia?
light is focused onto the retina
Ametropia?
an abnormal refractive eye condition in which images fail to focus upon the retina
- e.g. myopia, hyperopia, astigmatism
What is myopia?
- Light rays focus in front of the retina
- Difficulty in seeing distant objects clearly (Nearsightedness)
Causes of myopia?
- Mainly caused by increase in axial length of the eye ball
- Increased curvature of the cornea
- Increased refractive index of the eye
Risk factors for myopia?
- A family history of myopia
- Prolonged near work activities
e.g. reading, screen time - Low outdoor activity
- especially in childhood
Types of myopia?
- Congenital myopia: present at birth, progressive, severe
- Simple myopia: typically develops in childhood, correctable with glasses
- Degenerative myopia: severe, with an increased risk of complications
Management of myopia?
- Concave lenses in the form of glasses or contact lenses
- Refractive surgery
Complications of myopia?
- Retinal detachment
- Open angle glaucoma
- Macula hole
What is hypermetropia?
- Light focuses beyond the retina
- Blurred near vision and clear far vision
- Causes discomfort and strain on eyes - headaches and eye fatigue
Causes of hypermetropia?
- Smaller axial length of the eye
- Flatter cornea
- Decreased refractive index
Risk factors for hypermyopia?
- Genetic predisposition
- Prematurity and low birth weight
- Maternal smoking during pregnancy
Management of hypermyopia?
- Appropriate convex lenses in the form of spectacles or contact lenses
- Refractive corneal surgery
Complications of hyper myopia?
- Closed angle glaucoma
- Early onset of presbyopia
- Amblyopia
What is Astigmatism?
- Refraction varies in different meridian of the eye
- Ray of light entering the eye can not converge to a point focus but form focal lines
- Usually due to unequal curvature of the cornea
- Blurred vision at all distances
- Corrected by toric lenses or surgery
- Early onset increases risk for ambylopia
What is presbyopia?
- Loss of accommodative ability
- Usually in older patients, from age of 40
- Difficulty reading small prints, patients usually hold reading material at a distance
- Corrected by concave lenses
Other refractive error manifestations?
- Pain in the eyes
- Headache
- Strabismus
Assessment of refractive errors?
- Objective
- Retinoscope
- autorefractor - Subjective
- Different lenses put in front of patient until clearest vision
Refractive anomalies in children?
Visual system still developing in early childhood (up to six years)
- The visual system needs visual signals to mature
- Any interruption of visual signals lead to irreversible visual impairment (amblyopia)
