SM, a 47-year-old male, presented to an optometrist for an eye test. He had a pa
ID: 3518136 • Letter: S
Question
SM, a 47-year-old male, presented to an optometrist for an eye test. He had a past history of a BB gun injury to the left eye at the age of 12 for which he never sought any medical evaluation Upon testing, his visual acuity with a reading card was 6/9 (equivalent distance visual acuity) in both the eyes. The visual field test showed that his left peripheral vision was impaired. Examination of the retina showed a large scar from the gun injury in the peripheral nasal retina in the left eye; the maculae in both the eyes were healthy a) What is accommodation of the lens? What controls accommodation of the lens and explain how it does this. What are refractive errors? Explain what type of refractive error SM has. (3 marks) (2 marks) b) What is the basis of central and peripheral vision? c) What is a visual field? How do stimuli from the visual field excite the retina in each eye? Explain with the help of a diagram. What is the basis of depth perception in vision? (2 marks) d) What is the likely cause of impairment of SM's left peripheral vision and why?Explanation / Answer
a) Accommodation is a process by which eyes focuses the object or image of the object located at distant location or near location or when the distant varies.
In the process,eye changes refractive power by altering shape of lens.
Contraction of ciliary muscles control the accommodation of lens,it shortens ciliary rings and move towards the equator of lens,this causes tension in lens and changes in its shape.
Refractive error occurs due toloss of power of accommodation as eye cannot focus the object due to which image obtained is blurred .
Types: 1) myopia 2) hyperopia 3) astigmatism 4) presbyopia
SM has myopia as he is unable to focus distant object and seeing objects at 6 metres what normal people sees at 9 metres.
B) central vision is sharp and it is vertical that is vision in straight line.
It helps in estimation of the objects,its distance and details.
It covers only 3 degrees of visual field ,rest is covered by peripheral vision.
Peripheral vision also called as side visionaccompanies central vision,it is not sharp but it is sensitive towards light and motion and thus helps detect the motion of objects especially of sides.
C) visual field or field of vision refers to the amount of the object or activity of the objects seen in side vision when eye focuses on central point.
Light falls on the cornea,travels through lens amd undergo refraction then falls on retina.Retina sense this stimulus and change this light energy into electrical signals or impulses.
These impulses travel through optic nerve then to optic chiasma where nerve fibres decussate,that means left becomes right and right becomes left.They reach to visual cortex where processing of the signals takes place.
Depth of perception means ability to perceive distance of an object or ability to visualise the 3D object.
d)It is due to gun injury.
Vision problem persists longer due to gun injury.Ocular tissues are soft and prone to injury and takes alot of time in healing process.
Here the damage in the nasal area caused loss of peripheral visiom upto 60 degrees.
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