P/P - Vision Flashcards
visual acuity
the sharpness or clarity of vision
how is visual acuity tested?
Snellen chart
what is the Snellen chart?
- chart that has several lines of letters
- number above each line refers to distance from which a person with normal vision would be able to read clearly the letters on the line
the Snellen chart is always viewed from what distance?
6 metres
what is the Snellen fraction?
viewing distance (m) / number (distance) marked on test type line
what does visual acuity depend on?
ability of the cornea to refract or bend light, shape of the lens, sensitivity of retina, and ability of brain to interpret the info.
during the Snellen test, what happens if the fraction is lower that 6/6?
diverging spehrical lenses of increasing dioptres are placed in front of the eye, ideally until the 6 line can be read.
what is the Snellen tet used to test?
visual acuity
the red and green duochrome rings are used to test for what during the Snellen test?
the accuracy of a correct by making use of chromatic abberation
what is an astigmatism caused by?
uneven curvature of the cornea causing the light to be focused at more than one place in the eye.
can cause a blurring of vision
when should astigmatism be tested for?
once refractive errors have been corrected, then each eye should be tested separately
what chart is used to test for astigmatisms?
fan/Landolt’s chart
the classification of colour blindness is based upon what?
Young-Helmholtz trichromacy theory
what is a perimeter?
instrument that is used to measure visual fields
how does the ANS change the pupil in bright conditions?
- acetylcholine is released from PS nerves
- binds to muscarinic (M3) receptors
- cayses circular muscles of iris to contract
- decreases pupil diameter
how does the ANS change the pupil in low light conditions?
- noradrenaline released from sympathetic nerve endings
- binds to alpha-adrenoceptors
- radial muscles of iris contract
- increases pupil diameter
contraction of the ciliary muscles to focus on near objects, is controlled by what?
PNS and acetylcholie
what receptors are found in the dilator pupillae of the iris?
adrenoceptors
what receptors are found in the sphincter pupillae of the iris?
muscarinic receptors
why would dilating the pupil be helpful in opthalmoscopy?
allows more extensive examination of the retina and structures like macula and optic nerve
mydriasis
dilation of the pupil
what medicines are typically used to induce mydriasis?
tropicamide
phenylephrine
what is the mechanism of action of tropicamide?
- muscarinic (M3) antagonist
- stops ACh binding to M3 receptors
- stops circular muscle contraction
- stops pupil constriction
what is the mechanism of action of phenylephrine?
- alpha-adrenoceptor agonist
- binds to receptors and causes radial muscles of iris to contract
- increases pupil size
what % of tropicamide is standard?
0.5%
what % of phenylephrine is standard?
2.5%
myopia
pupil constriction
why might inducing myopia be useful?
reduces intraocular pressure, eg as a treatment for closed-angle glaucoma
what % is the standard does of pilocarpine?
2%
what medicines can be used to induce myopia?
pilocarpine
