SM, a 47-year-old male, presented to an optometrist for an eye test. He had a pa
ID: 3520521 • 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)
b) What is the basis of central and peripheral vision? (2 marks)
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?
(1 marks)
Requirement : I need the answer with reference and has to be original not copied work, must pass turnitin.
Explanation / Answer
a) Accomodation of the lens is the phenomenon by which the curvature of the anterior surface of the lens of an emmetropic (normal) eye is increased to accomodate the diverging rays falling on it so as to bring them to a focus at a single point.
(Note: without convergence only parallel rays falling on the lens would get focussed and not diverging ones)
Factors controlling accomodation of the lens:
??When a cell undergoes accomodation, the cilliary muscle contracts bringing the cilliary body closer to the lens thereby slackening the zonules holding the lens with the cilliary body, so the lens capsule is now relieved of tension because of the slackened zonules and is free to mould the lens into a shape facilitating accomadation of the diverging rays.
SM has presbyopia as he is middle aged and there is a difficulty in near vision as his visual acuity reads 6/9 . This occours as with age the elasticity of lens and lens capsule decreases as does the contracting power of the cilliary muscle.
b) The central vision of an eye depends on the health of the macula of retina of that eye whereas the peripheral retina is responsible for peripheral vision. Here, due to the childhood BB gun injury there is impairment of peripheral vision of affected eye but as macula is health so central vision is spared of any disturbance.
c) When we focus our eyes on a singular specific object, our eyes also perceive the surroundings of that object to a certain extent. The entire extent of this peripheral vision other than our central object perceived by us is our visual field.
Stimuli from visual field excite retina by Wald's Visual Cycle. It works as followes:
All trans retinal + Rhodopsin. Stimuli Goes to Inner Nuclear layer of retina ( Bipolar cell with primary neuron)
From primary neuron the stimuli goes to Ganglion cell layer ( secondary neuron)
From there it goes to nerve fibre layer. Which in turn leads to the optic nerve which transmits the signals to the brain.
( Kindly put the above steps in a flowchart pattern to form a diagram, there is no other specific diagram as such)
Binocular vision is the basis of depth perception in vision.
d) The likely cause of impairment of SM's left peripheral vision is the scar on the retina from the BB gun injury in his childhood leading to loss of retinal tissue. Hence, light falling on damaged retina is not processed as Wald's Visual Cycle is not functional here thereby leading to a loss of vision.
Refernce : Pearsons's Diseases of the Eye (20th edition)
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