Refractive Error Flashcards
The lens
Made of elongated lens fibres wrapped around a nucleus, and enclosed within an elastic capsule
Supported on the zonules connected to the cillary bodies which stretch it to regulate its curvature and refractive power
Cornea
Allows light to the retina and does the majority of the refraction
This both due to its curved shape and the fact that its refractive index is greater than air
Emmetropia
This is where the refractive system of the eye works well and produces a clear image on the retina (normal sight)
This is determined by three structures - the cornea, the lens and the axial length of the eyeball
If not the person is termed ‘ametropic’
Accommodation
This is the process of increasing the refractive power of the eye to focus the more divergent light rays coming from closer objects
The cillary body relaxes allowing the lens to contract and become stronger
Pupils will also constrict
Presbyopia
The process of the lens losing elasticity with increasing age
This reduces its ability to change shape making it harder to focus on near objects
This causes people to require reading glasses (convex lens)
Affects hypermetropic people earlier
Hypermetropia
Long sightedness, is where the image is focused behind the retina because of too weak refractive power of the eye
This means that distance vision is ok, but near vision is poor and is corrected by a positive, convex lens
Hypermetropic people will have a small axial length of their eyes
Myopia
Short sighted, where the image is focused in front of the retina, because of too powerful refractive power of the eyes - ok for near objects but cannot see distance
This requires a negative, concave lens to correct
Myopic people will tend to have a long eyeball
Astigmatism
This occurs when the eyeball has different refractive power at different points, and can occur along side hyper- or myopia
This occurs when light in different planes focus at different points, thus the vertical and horizontal bars of a cross cannot be in focus simultaneously
Astigmatism is corrected with
A tonic lens which has different corrective powers in different meridians (planes)
This allows for light in the vertical and horizontal planes to be brought to a single sharp focus
Keratoconus
A bilateral acquired collagen disorder with peak onset in the late twenties
The cornea thins and distorts causing distortions
Clinical signs of keratoconus
Increasing astigmatism/myopia + bulging lower lid on down gaze
Abnormal retinoscopy reflex - appears like an oil droplet, and shows scissors movement
Central corneal thinning and conical shape
Vertical striae in the corneal stroma
Keratoconus is associated with
Down’s syndrome
Ehlers-danlos syndrome
Marian’s syndrome
Keratoconus management
Rigid contact lens - often better than spectacles
Surgery is contraindicated
Can require corneal grafting at late stages
Regular monitoring is required
Bifocal lenses
Correct for both near and far vision by having different powers in the top and bottom halves
The top is usually for distance and the bottom for near
Varifocal lenses
Similar to bifocals but have a gradually changing power from top to bottom
