Lab Flashcards
Presbyopia
Denaturation of lens proteins causes a reduction in accommodation hence the near point (the nearest point of distinct vision) recedes throughout life
loss of transparency and increasing hardness of the lens
Visual acuity
degree to which the details and contours of objects can be perceived
It is defined in terms of the shortest distance by which two lines can be separated and still perceived as two lines
The normal eye sees two point sources of light as separate if they lie more than 2 microns apart on the fovea
Outside the fovea the visual acuity becomes progressively poorer decreasing to tenfold as the retinal periphery is reached
SNELLEN CHART
Average diameter of cones in the fovea
1.5 microns
How far apart must the centres of 2 spots of light be to be distinguished separately
Since the spot of light has a bright centre point and shaded edges a person can distinguish two separate points if their centres are approximately 2 microns apart on the retina (which is slightly greater than the width of a foveal cone)
Maximal visual acuity =>
A person with maximum visual acuity looking at two bright pinpoint spots of light from a distance of 10 metres can just barely distinguish them as two separate entities when they are 1.5 to 2 millimetres apart
Diagram of visual acuity
how is visual acuity expressed
As a ratio between the distance at which the subject can see the letters and that at which a subject with normal vision can see the letters
when to move subject when measuring acuity
Acuities worse than 6/60 - move subject to 4m from the chart
What measurement must be taken when analysing visual acuity
Width of smallest letter visible from 6m (in mm)
Calculation for visual acuity
Optic disc/papilla
Region of retina corresponding to confluence of optic nerve fibres as they pass out of the eye
region contains no photoreceptors and hence creates a blind spot in the visual field
Calculation for blind spot
visual field
that portion of the environment seen through that eye by the peripheries of the retina
This field would be circular but for the geography of the face i.e. the nose and the eyebrows limit the field of vision for each eye
where does the optic disc lie
The optic nerve and optic disc are on the nasal side of the eye relative to the fovea (centre of vision)
However when the visual field from both eyes is summated a larger field of vision is attained
What information does direct ophthalmoscopy tell us
Consists of looking at the fundus of the eye with a suitable light
yields information about diseases within the eye (e.g. cataract or vitreous haemhorrhage), in the cardiovascular system (i.e. hypertensive retinopathy) and in the central nervous system (i.e. papilloedema as seen in increased intracranial pressure)
Structure of ophthalmoscope
The light is focused on a mirror set at the top of the instrument so that the beam is deflected through at a right angle
The observer looks through a small hole in the mirror and shines a light into the subject’s eye
The ophthalmoscope has 30 lenses varying in focal length
Lenses marked in green text are convex and positive and are used to correct for a hypermetropic subject or examiner
Lenses with text coloured red are concave and negative and are used to correct for myopic examiner or subject
Red reflex
sparkly orange red colour
caused by light reflecting off of the retina and is the same phenomenon that produces red eyes in flash photographs
sometimes the translucent structures that allow light to pass unimpeded from outside the eye to the retina become opacified and the red reflex is lost
=> cataracts (clouding of lens)
atherosclerosis - change in retina
change in appearance of arteries
copper wiring
hypertension - changes in retina
alterations in the arterial-venous crossing pattern
a-v nicking
(vessels more obvious in superior and inferior aspects of retina, with relative sparing of temporal and medial regions)
BVs around optic disc
Less branching of BVs
point at which the vessels enter the retina along with the head of the optic nerve. The edges of this round disc are sharp and well defined in the normal state. It should be a bit more yellow/orange when compared to the rest of the retina
Optic cup
distinct circular area from which the blood vessels actually emerge
Where is the disc located
disc is not located in the exact centre of the retina but rather towards its medial/nasal aspect
Macula
region located lateral to the optic disc
It looks somewhat darker than the rest of the retina and, as opposed to the disc, has no distinct borders
The macula provides the sharpest vision
normally two disc widths temporal of the optic disc; it looks like a small dull red patch
