Flashcards in Clinical optics Deck (12):
Definition: > Age→ Lens loose elasticity→ < Accommodation power
- > 40 Years
- Trouble focus on near objects
- Near vision worse in poor light and small print
- Hold things further away for normal vision
- Convex lens: Bifocals or reading glasses
Refraction errors: Ametropia (3)
Ametropia: Imbalance between refractive power and axial length of eye. Could be normal but > is pathological
Physiological vs. pathological
Definition: Refractive power of anterior segment too strong for axial length of eye→ Parallel rays focused in front of retina
Myopia cannot be compensated for by accommodation, and so does not produce symptoms of eye strain or strabismus. It usually manifests only as a decrease in vision.
- Unusually long axial length in eye with normal refractory power→ Prominent eyes
- Unusually strong refractive power and normal axial length
- < Grade Myopia
- Slowly progressive until adulthood→ accelerates→ static
- Corrected visual acuity usually remains normal.
- > Grade myopia.
- Rapidly progressive during adolescence→ Slow progression during adulthood.
- Associated: myopic retinal degeneration →permanent visual loss and is associated with > incidence rhegmatogenous retinal detachment.
- Poor distance vision
- Good near vision.
- Concave spectacle or contact lens.
- Refractive surgery.
Definition: Rrefractive power of anterior segment insufficient for the axial length of the eye→ Parallel rays focused behind the retina.
Hypermetropia can be partially or completely compensated for by accommodation, and as a result can produce symptoms of eye strain or strabismus as well as a decrease in vision.
- Unusually short axial length in an eye with a relatively normal refractive power.
- Unusually weak refractive power and a relatively normal axial length.
- Better distance vision than near vision.
- Accommodation > refractive power of the eye.
- Can compensate partially or completely, but excessive use of the accommodation mechanism may produce problems:
1. Eye strain: headache or eye discomfort, which increases with visual tasks.
2. Convergent strabismus: accommodation is part of the near reflex which include convergence.
- Convex spectacle or contact lens is necessary only in the presence of one or more of the following:
1. Subnormal visual acuity: Accommodation is unable to compensate.
2. Symptoms of eye strain: Excessive use of accommodation in order to compensate
3. Strabismus: Compensatory accommodation induces excessive convergence.
- Selected patients are suitable for refractive surgery
Definition: Corneal curvature is not the same in all directions→ Clear retinal image not formed.
Normal cornea has same curvature and thus the same refractive power in all directions.
- Blurred vision at all distances dependent on degree
- Eye strain may occur: Headache , eye discomfort
- Astigmatic spectacle or contact lens, which also has different curvatures along different axes.
- Refractive surgery.
Definition: Refractive errors can be corrected surgically by changing either the curvature of the cornea, or the power of the lens.
- Surgery to correct myopia flattens the central cornea.
- < Refractive power so that light from distant objects, which was focused in front of the retina before the surgery, is focused on the retina after the surgery.
- Similarly, surgery to correct hypermetropia steepens the central cornea→ circular gutter is created in the midperiphery, creating an unnatural surface shape and a lower success rate.
1. RLE (refractive lens exchange): Lens removed and replaced with synthetic lens of appropriate power
2. RK (radial keratotomy): Midperipheral radial corneal incisions produce central corneal flattening and so correct myopia
3. AK (astigmatic keratotomy): Midperipheral transverse corneal incisions flatten the cornea selectively along one axis and so correct astigmatism.
4. PRK (photorefractive keratectomy): Corneal epithelium removed and laser used to reshape stromal bed. (epithelium takes several days to regenerate, making visual recovery relatively slow and uncomfortable)
5. LASIK (laser assisted in situ keratomileusis): Similar to PRK, except that the laser reshaping is done beneath a corneal flap, consisting of epithelium and thin layer stroma, created using laser or mechanical incision. (Visual recovery < 12 hr, painless)
6. Intrastromal procedures: Corneal shape changed by intrastromal laser→ intrastromal lens shaped by laser then mechanically removed through small laser incision to flatten the cornea.
Determination of refractive error
- Vision better near than at distance: Myopic
- Vision better at distance than near: Hypermetropia
- Vision equally poor at distance and near: Astigmatic.
Definition: < visual acuity, repeating the measurement with a pinhole will give an indication of whether the visual loss is organic or refractive in origin.
1. Pinhole diminishes the effect of refractive errors, and so improves the visual acuity when a refractive error is present.
2. Degree of improvement is unpredictable, and the visual acuity with pinhole may be better or worse than the visual acuity with an optimal spectacle correction.
- Visual acuity improves with Pinhole→ Loss is at least partially due to a refractive error.
- Visual acuity does not improve with pinhole → loss due to an organic defect.
- PH may not improve the VA as much as an optimal spectacle correction → PH VA may not be 6/6 even if the VA with spectacles is 6/6.
PH may improve the VA to better than what is possible with an optimal spectacle correction→ PH VA may be 6/6 even when spectacles cannot improve the visual acuity beyond say 6/9.
Definition: Bending of light as it cross interface between 2 media of different optical densities
Refraction of the eye:
Light entering eye→ reflected by cornea (> refractive power)→ refracted by lens (< refractive power)
Definition: Perfect refraction condition of the eye→ Refractive power of eye and length of eyeball matched→ parallel rays of light from distant object are focused on retina when ciliary mm relaxed→ Clear image of object on retina