Visual Problems: Problems with Structures of the Eye

Nota:

• Should talk about focusing light on the retina before mentioning the visual problems
• Refraction: necessary to focus light rays

1. Cornea
   1. Astigmatism
      1. Define: A visual defect caused by the unequal curving of the cornea (it is not spherical)

         Nota:

         • It can be one or more of the refractive surfaces of the eye  - But it's usually the cornea
      2. Test: Fan chart for astigmatism
         1. Some lines [horizontal or vertical] might appear sharp, while others appear out of focus
            1. Solution: lenses that have different amounts of focusing power (focal points) in the horizontal and vertical planes
   2. Highly curved
      1. Powerful refractive surface in the eye

         Nota:

         • Has a higher refractive index than air
2. Aqueous Humor
   1. Glaucoma
      1. Lose your peripheral vision
      2. Caused by high pressure of the watery fluid
   2. Help refract light
3. Crystalline Lens

   Nota:

   • Create a table between myopia and hyperopia
   1. Emmetropia
      1. The condition of having no refractive error

         Nota:

         • In other words, the refractive power of the eye is perfectly matched to the length of the eyeball
   2. Presbyopia
      1. A condition where you have a decreased ability to focus on near objects
         1. Aka. loss of near vision due to insufficient accommodation
      2. Means "old sight"
         1. it's inevitable

            Nota:

            • Because lens become sclerotic (harder) & the capsule that encircles the lens (enabling it to change shape) loses its elasticity.
   3. Myopia
      1. Short-sighted or nearsightedness

         Nota:

         • Remember the Imperssionist painter (Cezanne) - the image from far away looks blurry.
      2. When light entering the eye is focused in front of the retina; and when the eyeball is too long for the optics
         1. Therefore distant objects cannot be seen sharply
      3. Solution: Negative (minus) lenses

         Nota:

         • They diverse the light rays before they enter the eye
   4. Hyperopia

      Nota:

      • Where accommodation is needed to see near objects clearly, 
      1. Long-sighted or farsightedness
      2. When light entering the eye is focused BEHIND the retina; and the eyeball is too short for the optics
      3. Solution: Positive (plus) lenses

         Nota:

         • They converge the light rays before they enter the eye
   5. Cataracts
      1. An opacity (loss of transparency) of the crystalline lens.
         1. Caused by the irregularity (unevenness) of the crystallins

            Nota:

            • Crystallins: are a class of proteins that make up the lens
      2. Consequence: interferes with vision

         Nota:

         • by absorbing and scattering more light than the normal lens does
      3. Forms of cataracts
         1. Congenital cataracts

            Nota:

            • cataracts present at birth
         2. Through old age

            Nota:

            • Most are discovered after the age of 50. 
            1. The prevalence of cataracts increases with age

               Nota:

               • By age 70 almost everyone has some loss of transparency
   6. Alter the refractive power by changing its shape
   7. Accommodation
      1. The process by which the eye changes its focus

         Nota:

         • Therefore altering its refractive power
         • The fatter the lens is, the more power it has.
         1. Because the lens changes shape

            Nota:

            • [There's a table on pg 3 of my notes] When the lens becomes fatter -> focus on near objects
            1. Accomplished through contraction of the cilliary msucle

               Nota:

               • Lens are attached to the ciliary muscle through tiny fibres (“zonules of Zinn”)
            2. Goal: to bring objects into focus
4. Retina
   1. Age-Related Macular Degeneration
      1. There are two forms of it
         1. Wet Form

            Nota:

            • One that involves blood
         2. Dry form

            Nota:

            • One that involves rods and cones; not blood vessels
      2. A disease associated with aging that affects the macula
         1. "age-related"
            1. usually occurs in older adults, starting at the age of 50
         2. Macula
            1. is the central part of the retina;
               1. That includes the fovea
                  1. Fovea
                     1. Contains the highest concentration of cones
                        1. Has no rods
                     2. Is the portion of the retina that
                        1. Produces the highest visual acuity; and
                           1. Therefore "sees" the world most clearly
                              1. E.g. Able to identify objects, read and to inspect fine detail
                        2. Serves as the point of fixation
                     3. A dented region in the retina; behind the pupil
                        1. a small pit near the centre of the macula
               2. That has a high concentration of CONES
      3. Consequence: A blind spot in the visual field (due to a loss of cones in the macula

         Nota:

         • Called a scotoma
         1. Therefore, AMD destroys sharp central vision

            Nota:

            • Thus, making it difficult to read, drive and recognise faces.
   2. Retinitis pigmentosa
      1. A hereditary disease

         Nota:

         • Commonly runs in families
         1. can be caused by defects in a number of different genes
      2. A progressive degeneration of the retina

         Nota:

         • Specifically the photoreceptors and the pigment epithelium
      3. Consequence: on night and peripheral vision
         1. Common form: Rods get affected before cones
         2. Affecting peripheral: shrinks the visual field
            1. Therefore, many people may not notice the onset of RP at first